"Guessing before proving! Need I remind you that is how all important discoveries have been made?" - Henri Poincare
An advertisement-free blog about motorcycling, Northern life, home improvement, power plants, submarines, nuclear stuff, and a few other off-beat subjects. Comments are welcome. (Image is the Selway Crags in Northern Idaho)
Showing posts with label Moderator. Show all posts
Showing posts with label Moderator. Show all posts
Tuesday, September 12, 2023
Neutron Activation, N-16, and pools
Wednesday, January 29, 2014
K-27, Project 645 (645 Кит-ЖМТ)
The Soviet and US Navies operated a large number of submarines, each generation improving in quality. A silent covert game of cloak and dagger took place beneath the waves that most people were completely unaware of.
The Soviets at one time had the largest fleet of submarines in the world. In many respects, advanced Soviet submarines were superior to their US cold war counterparts. Several Soviet submarine designs could dive to a greater depth than a standard US MK 48 torpedo!
US submarines (from the arrival of Thresher) had held the advantage of stealth, and superior sonar equipment. Soviet submarines, in contrast, held the advantage of survivability (due to double-hull construction and huge amounts of reserve bouyancy), weapon-carrying capacity, greater depth and top speed.
The US experimented with a variety of reactor/propulsion designs, but only one used a liquid-metal cooled reactor:
USS Seawolf (SSN-575) was the second US submarine (after USS Nautilus), and the only US submarine to have a liquid metal cooled reactor. The reactor was cooled using liquid sodium, which of course would be problematic for the crew if it ever leaked. Seawolf also had steam superheaters, for added efficiency. These were also problematic, and thus were seldom in service. Because liquid metal is much more efficient than water at removing core heat, the propulsion plant was only 40% the size of Nautilus'. Seawolf was eventually converted to a more typical S2W pressurized water reactor (PWR) with a saturated steam plant. PWR and saturated steam plants in US submarine design continues to this day.
The Soviets' emphasis on submarine speed, depth and power of course led to more propulsion designs that used liquid metal cooled reactors. Soviet reactors of this type used a Lead-Bismuth coolant that was far less hazardous than liquid sodium, at least from a fire hazard standpoint. From a power-weight (and size) standpoint, the liquid metal cooled reactor is far superior to a light water cooled reactor. From a safety standpoint, not so great.
Recall that liquid metal cooled reactors are Fast neutron reactors, or sometimes intermediate speed reactors. All liquid metal cooled reactors have a positive void coefficient of reactivity. That means that if the coolant inadvertantly boils in the core, reactor power will increase. Which will boil more metal, and increase power even more. This happens rapidly, and core damage (meltdown!) is fairly common with this type of reactor.
So with that background, lets talk about the Soviet submarine K-27, or Projekt 645.
The Soviet's first class of nuclear attack submarines was called the November class. They used dual 70 Megawatt PWR reactors for propulsion. 13 of these were built before technology allowed creation of superior designs. Even so, they were superior to the USS Nautilus, in speed, depth, and stealth. One could also argue that Nautilus was really an experiment to prove that nuclear propulsion could work on a submarine, rather than a true nuclear attack submarine, however, and not be wrong.
Profile of a November-Class Submarine:
Back to K-27. This was a unique single-ship design by the Soviets, just as Seawolf was for the US Navy. K-27 was a November-Class submarine with a unique power plant. Rather than two 70 Megawatt PWRs, the Soviets used two VT-1 liquid metal cooled reactors, with an output of 73 MW. The advantage of smaller footprint and weight of the metal-cooled reactors allowed more weapons to be carried.
She was laid down on June 1958 and launched in April 1962. She was commissioned October 1963 after full-scale builders sea trials and official tests. She performed well (although with heavy maintenance for the new metal-cooled reactors) until a reactor accident in the port (left) reactor happened in May 1968.
The ship was making a full speed submerged run, when a reactor automatic control rod withdrew itself. Boiling occured, and reactor power plummeted from 83% to 7% in about 90 seconds, as the core melted. Unfortunately for the crew, poor decisions made after the initial accident would cost many of them their lives.
The main purpose of cladding U-235 in a reactor with Zircaloy or Stainless steel is to keep the highly radioactive freshly split atoms from getting into the coolant and spreading. When the fuel assemblies melt down, these radioactive atoms mix in the coolant, and get outside the heavily shielded reactor vessel.
Unknown to the crew, the captain had the radiation alarms disabled. Radioactive gases were released from the fuel, which the crew were exposed to. Another captain might have surfaced the ship and ventilated it with the massive air blowers all submarines are equipped with. The ship limped home on the starboard reactor and was laid up for several years. Five sailors who worked in the propulsion plant died within a week of the accident, while 30 more died between 1968 and 2003. Quite a high death rate for a crew of young, healthy men.
K-27 was brought into shipyard, and the starboard reactor coolant was kept liquid by steam piped in at the shipyard while the radioactivity in the port side reactor died down. In 1973 the decision was made that repairing or replacing the reactor in the aging ship was not worthwhile, and the ship was decomissioned in February 1979.
Her disposal was... interesting. Rather than remove the melted down mess that remained of the port side reactor, the Soviets decided to fill her reactor compartment with a solidifying agent. Next they towed her, not out to sea, but very close to land. In 1982 they sunk her in just 100 ft of water, just offshore of Novaya Zemlya. Google Earth Coordinates Here
She didn't want to sink, however, so they ended up having to ram her.
There is now a great deal of urgency in re-floating K-27 and removing her radioactive coolant system and fuel. This is an environmental hazard that will eventually become a serious problem, and quite close to shore. Where it was disposed of is the Island of Novaya Zemlya, a harsh glacier-scoured island that has been a nuclear testing and dumping ground for generations.
Interestingly there is equipment available to de-fuel this unique ship that was used on many other liquid-metal cooled ships at the end of the cold war. However, this now-unused de-fueling equipment will not remain in optimum condition forever, so the race is on. Hopefully someone is interested in recovering this ship before it becomes a big environmental mess.
The Soviets at one time had the largest fleet of submarines in the world. In many respects, advanced Soviet submarines were superior to their US cold war counterparts. Several Soviet submarine designs could dive to a greater depth than a standard US MK 48 torpedo!
US submarines (from the arrival of Thresher) had held the advantage of stealth, and superior sonar equipment. Soviet submarines, in contrast, held the advantage of survivability (due to double-hull construction and huge amounts of reserve bouyancy), weapon-carrying capacity, greater depth and top speed.
The US experimented with a variety of reactor/propulsion designs, but only one used a liquid-metal cooled reactor:
USS Seawolf (SSN-575) was the second US submarine (after USS Nautilus), and the only US submarine to have a liquid metal cooled reactor. The reactor was cooled using liquid sodium, which of course would be problematic for the crew if it ever leaked. Seawolf also had steam superheaters, for added efficiency. These were also problematic, and thus were seldom in service. Because liquid metal is much more efficient than water at removing core heat, the propulsion plant was only 40% the size of Nautilus'. Seawolf was eventually converted to a more typical S2W pressurized water reactor (PWR) with a saturated steam plant. PWR and saturated steam plants in US submarine design continues to this day.
The Soviets' emphasis on submarine speed, depth and power of course led to more propulsion designs that used liquid metal cooled reactors. Soviet reactors of this type used a Lead-Bismuth coolant that was far less hazardous than liquid sodium, at least from a fire hazard standpoint. From a power-weight (and size) standpoint, the liquid metal cooled reactor is far superior to a light water cooled reactor. From a safety standpoint, not so great.
Recall that liquid metal cooled reactors are Fast neutron reactors, or sometimes intermediate speed reactors. All liquid metal cooled reactors have a positive void coefficient of reactivity. That means that if the coolant inadvertantly boils in the core, reactor power will increase. Which will boil more metal, and increase power even more. This happens rapidly, and core damage (meltdown!) is fairly common with this type of reactor.
So with that background, lets talk about the Soviet submarine K-27, or Projekt 645.
The Soviet's first class of nuclear attack submarines was called the November class. They used dual 70 Megawatt PWR reactors for propulsion. 13 of these were built before technology allowed creation of superior designs. Even so, they were superior to the USS Nautilus, in speed, depth, and stealth. One could also argue that Nautilus was really an experiment to prove that nuclear propulsion could work on a submarine, rather than a true nuclear attack submarine, however, and not be wrong.
Profile of a November-Class Submarine:
Back to K-27. This was a unique single-ship design by the Soviets, just as Seawolf was for the US Navy. K-27 was a November-Class submarine with a unique power plant. Rather than two 70 Megawatt PWRs, the Soviets used two VT-1 liquid metal cooled reactors, with an output of 73 MW. The advantage of smaller footprint and weight of the metal-cooled reactors allowed more weapons to be carried.
She was laid down on June 1958 and launched in April 1962. She was commissioned October 1963 after full-scale builders sea trials and official tests. She performed well (although with heavy maintenance for the new metal-cooled reactors) until a reactor accident in the port (left) reactor happened in May 1968.
The ship was making a full speed submerged run, when a reactor automatic control rod withdrew itself. Boiling occured, and reactor power plummeted from 83% to 7% in about 90 seconds, as the core melted. Unfortunately for the crew, poor decisions made after the initial accident would cost many of them their lives.
The main purpose of cladding U-235 in a reactor with Zircaloy or Stainless steel is to keep the highly radioactive freshly split atoms from getting into the coolant and spreading. When the fuel assemblies melt down, these radioactive atoms mix in the coolant, and get outside the heavily shielded reactor vessel.
Unknown to the crew, the captain had the radiation alarms disabled. Radioactive gases were released from the fuel, which the crew were exposed to. Another captain might have surfaced the ship and ventilated it with the massive air blowers all submarines are equipped with. The ship limped home on the starboard reactor and was laid up for several years. Five sailors who worked in the propulsion plant died within a week of the accident, while 30 more died between 1968 and 2003. Quite a high death rate for a crew of young, healthy men.
K-27 was brought into shipyard, and the starboard reactor coolant was kept liquid by steam piped in at the shipyard while the radioactivity in the port side reactor died down. In 1973 the decision was made that repairing or replacing the reactor in the aging ship was not worthwhile, and the ship was decomissioned in February 1979.
Her disposal was... interesting. Rather than remove the melted down mess that remained of the port side reactor, the Soviets decided to fill her reactor compartment with a solidifying agent. Next they towed her, not out to sea, but very close to land. In 1982 they sunk her in just 100 ft of water, just offshore of Novaya Zemlya. Google Earth Coordinates Here
She didn't want to sink, however, so they ended up having to ram her.
K-27 refusing to be scuttled:
There is now a great deal of urgency in re-floating K-27 and removing her radioactive coolant system and fuel. This is an environmental hazard that will eventually become a serious problem, and quite close to shore. Where it was disposed of is the Island of Novaya Zemlya, a harsh glacier-scoured island that has been a nuclear testing and dumping ground for generations.
Interestingly there is equipment available to de-fuel this unique ship that was used on many other liquid-metal cooled ships at the end of the cold war. However, this now-unused de-fueling equipment will not remain in optimum condition forever, so the race is on. Hopefully someone is interested in recovering this ship before it becomes a big environmental mess.
Sunday, October 20, 2013
TRIGA - an amazingly safe nuclear research reactor
So far we have talked about natural reactors, fast neutron reactors, plutonium production reactors, and thermal power plant reactors.
There is one cool reactor that I would be remiss in discussing, and that is the TRIGA reactor. I worked at a facility where two of these were located, and was licensed to operate them both. One was the very first TRIGA reactor ever built, rated at 250 KW (thermal), and the other was a MK IV model, rated at 1.5 MW (thermal). These reactors are swimming-pool reactors, and so they don't generate steam or electrical power using steam turbines.
In fact TRIGA stands for (T)raining, (R)esearch, (I)sotope production, (GA) General Atomic, the manufacturer.
In the late 1950's there was a desire to promote "Atoms for Peace". This was Eisenhower's attempt to invoke the power of the atom for peaceful purposes. The world was understandably horrified by the images of Hiroshima and Nagasaki, as well as the possibility that the budding cold war might turn into a hot war at some point.
The invention of TRIGA reactors went a long way toward fulfilling that vision. Unlike other reactors, TRIGA reactors have a solid moderator that is cast into the fuel itself. Therefore there is a homogenous blend of solid moderator and fuel. The moderator is Zirconium Hydride, and as you would expect, the hydrogen atoms do the moderating.
Because about 60% of the moderator is solid and homogenous with the fuel, this reactor has what is known as a "prompt negative temperature coefficient of reactivity". In other words, the very instant a runaway nuclear reaction begins and starts causing fuel temperature to increase, the solid moderator temperature also instantly increases, which in turn reduces the available thermal neutrons. This provides a VERY rapid damping of the runaway nuclear reaction.
If you recall in an earlier post, it is important (in all reactors except TRIGA) to never allow the reactor to be critical on prompt neutrons alone, because each generation of neutrons only last 10^-14 seconds. There is no way to control a reaction that proceeds so quickly, so the small fraction of delayed neutrons are what allow us to control reactors. Chernobyl, SL-1, and the Borax Experiment were each prompt criticality events that ended badly.
TRIGA reactors though, can easily tolerate a prompt critical event. Doing this is called "Pulsing" the reactor. Any reactor can be pulsed, but only a TRIGA can do it more than once ;) In fact, the record reactivity insertion into any reactor was TRIGA, at 5.22 times the value needed to be prompt critical. Because the moderator heats up as rapidly as the fuel, it shuts the reactor down just as soon as heat is generated, in a few thousandths of a second, without operator intervention.
TRIGA reactors ended up being sold around the world. Being low-power, they weren't practical for making weapons, and the solid UZrH moderator was incredibly difficult to extract from the fissionable fuel, so using the fuel for making weapons was not possible. Even so, currently manufactured TRIGA fuel has been reduced from 20% U-235 down to 7% to prevent proliferation.
Here is a video of a TRIGA reactor being pulsed to 2.5 x prompt criticality. Any other reactor would vaporize the fuel and create a steam explosion, blowing water upwards out of the tank!
Below is an image of a TRIGA reactor at the bottom of the pool, while not in operation. As you can see, it's quite simple. The fuel elements rest on a bottom grid plate, and are kept vertical by the upper grid plate. The fuel can be grabbed by a long-handled pole with a ball-type coupler at the end, similar to how modern hydraulic couplings work.
The rods sticking down into the core are just aluminum shafts that connect the drive motors to the control rods (which are partially out of the core).
The inner ring around the core is a lazy susan. Samples can be dropped into a number of holders in this dry ring. When the reactor is in operation, the ring rotates to ensure each sample is exposed to equal amounts of neutron flux. This is useful for performing neutron activation analysis on several samples at once.
The outer ring is a graphite reflector/moderator, which reduces the amount of fuel needed. The cans outside the reflector are neutron detectors, for determining what power level the reactor is at. The little lanyard at the bottom is attached to a neutron source (usually Americium/Beryllium). This makes sure there are enough neutrons available to start the reactor up. Also its a daily test to pull it and stick it next to each neutron detector and make sure they work OK before you start the reactor up.
There is one cool reactor that I would be remiss in discussing, and that is the TRIGA reactor. I worked at a facility where two of these were located, and was licensed to operate them both. One was the very first TRIGA reactor ever built, rated at 250 KW (thermal), and the other was a MK IV model, rated at 1.5 MW (thermal). These reactors are swimming-pool reactors, and so they don't generate steam or electrical power using steam turbines.
In fact TRIGA stands for (T)raining, (R)esearch, (I)sotope production, (GA) General Atomic, the manufacturer.
In the late 1950's there was a desire to promote "Atoms for Peace". This was Eisenhower's attempt to invoke the power of the atom for peaceful purposes. The world was understandably horrified by the images of Hiroshima and Nagasaki, as well as the possibility that the budding cold war might turn into a hot war at some point.
The invention of TRIGA reactors went a long way toward fulfilling that vision. Unlike other reactors, TRIGA reactors have a solid moderator that is cast into the fuel itself. Therefore there is a homogenous blend of solid moderator and fuel. The moderator is Zirconium Hydride, and as you would expect, the hydrogen atoms do the moderating.
Because about 60% of the moderator is solid and homogenous with the fuel, this reactor has what is known as a "prompt negative temperature coefficient of reactivity". In other words, the very instant a runaway nuclear reaction begins and starts causing fuel temperature to increase, the solid moderator temperature also instantly increases, which in turn reduces the available thermal neutrons. This provides a VERY rapid damping of the runaway nuclear reaction.
If you recall in an earlier post, it is important (in all reactors except TRIGA) to never allow the reactor to be critical on prompt neutrons alone, because each generation of neutrons only last 10^-14 seconds. There is no way to control a reaction that proceeds so quickly, so the small fraction of delayed neutrons are what allow us to control reactors. Chernobyl, SL-1, and the Borax Experiment were each prompt criticality events that ended badly.
TRIGA reactors though, can easily tolerate a prompt critical event. Doing this is called "Pulsing" the reactor. Any reactor can be pulsed, but only a TRIGA can do it more than once ;) In fact, the record reactivity insertion into any reactor was TRIGA, at 5.22 times the value needed to be prompt critical. Because the moderator heats up as rapidly as the fuel, it shuts the reactor down just as soon as heat is generated, in a few thousandths of a second, without operator intervention.
TRIGA reactors ended up being sold around the world. Being low-power, they weren't practical for making weapons, and the solid UZrH moderator was incredibly difficult to extract from the fissionable fuel, so using the fuel for making weapons was not possible. Even so, currently manufactured TRIGA fuel has been reduced from 20% U-235 down to 7% to prevent proliferation.
Here is a video of a TRIGA reactor being pulsed to 2.5 x prompt criticality. Any other reactor would vaporize the fuel and create a steam explosion, blowing water upwards out of the tank!
Below is an image of a TRIGA reactor at the bottom of the pool, while not in operation. As you can see, it's quite simple. The fuel elements rest on a bottom grid plate, and are kept vertical by the upper grid plate. The fuel can be grabbed by a long-handled pole with a ball-type coupler at the end, similar to how modern hydraulic couplings work.
The rods sticking down into the core are just aluminum shafts that connect the drive motors to the control rods (which are partially out of the core).
The inner ring around the core is a lazy susan. Samples can be dropped into a number of holders in this dry ring. When the reactor is in operation, the ring rotates to ensure each sample is exposed to equal amounts of neutron flux. This is useful for performing neutron activation analysis on several samples at once.
The outer ring is a graphite reflector/moderator, which reduces the amount of fuel needed. The cans outside the reflector are neutron detectors, for determining what power level the reactor is at. The little lanyard at the bottom is attached to a neutron source (usually Americium/Beryllium). This makes sure there are enough neutrons available to start the reactor up. Also its a daily test to pull it and stick it next to each neutron detector and make sure they work OK before you start the reactor up.
Saturday, October 19, 2013
Odd Quirks About Nuclear Reactors - Criticality Accidents
One quirky thing about nuclear reactors: If you are not careful - particularly with Plutonium - it is possible to create a reactor unintentionally. That is, you can inadvertently assemble enough fissile material to start a chain reaction, outside of the safe confinement of a shielded reactor vessel.
Monday, October 14, 2013
Unusual Reactors - Fast Neutron Reactors
In earlier posts I alluded to Fast Neutron Reactors. Here and Here.
In a Fast Neutron Reactor, there is no moderator. Neutron speed is allowed to remain at 49 million miles per hour. Because the neutrons are traveling so fast, the likelihood of them interacting with a Uranium-235 nucleus and causing a fission is very much reduced. To counter this, the enrichment of the fuel must be increased, typically to 20-30% U-235.
In a Fast Neutron Reactor, there is no moderator. Neutron speed is allowed to remain at 49 million miles per hour. Because the neutrons are traveling so fast, the likelihood of them interacting with a Uranium-235 nucleus and causing a fission is very much reduced. To counter this, the enrichment of the fuel must be increased, typically to 20-30% U-235.
Sunday, October 13, 2013
Nuclear Reactors and Nuclear Weapons - There is a difference!
I know in a lot of people's minds, there is a notion that a nuclear power plant could detonate like a nuclear weapon. It's not possible, and I will use the previous few posts to explain why that is.
Recall that neutrons from fission have a very high energy level, about 2 MeV, which equates to a speed of 45 million miles per hour. Neutrons traveling this fast do not interact with Uranium atoms as often as they do after they have been moderated (slowed down) to thermal speed, about 4900 miles per hour.
Nuclear weapons require an absolutely uncontrolled fission reaction. The more fissions that occur before the core is vaporized, the better. This requires that the entire thing take place in microseconds, which also means the neutrons must be fast neutrons. It also requires a VERY dense concentration of fissile material, because fast neutrons don't interact well with atoms.
There are some other techniques used to improve nuclear weapon performance. A neutron reflector is one. It bounces fission neutrons back into the reaction, and makes more fissions possible before the whole thing vaporizes. Another trick is using a tamper - a thick casing to hold the exploding device together for a couple more microseconds, allowing a couple more generations of neutrons to build up.
With a weapon, if the neutrons were thermalized, it would slow down the chain reaction to the point that the uranium or plutonium device would melt or vaporize, rather than detonate. AKA, a "fizzle".
On the other hand, nearly all nuclear reactors are "thermal" reactors, meaning the neutrons are slowed down. This provides fine control of the nuclear reaction, but more importantly allows us to use lower enrichment of Uranium than would be possible in a Fast Neutron Reactor. Fast neutron reactors require enrichment that would be appropriate for weapons, which is why nobody really likes them.
Typical Thermal Neutron Reactors, such as those in power plants, use just 3-5% enriched Uranium, which is too low for a nuclear explosion of the weapons type.
What can occur in a thermal reactor though is a runaway chain reaction that creates enough heat to vaporize metal in the core and create a steam explosion. All nuclear reactors rely on "delayed neutrons" to achieve control. Most fission neutrons are released within 10^-14 seconds following a fission. These neutrons are called "prompt neutrons". That reaction speed is far too fast for humans or electronics to prevent a runaway chain reaction.
Fortunately for us, a small fraction of neutrons are produced by the neutron-rich fission fragments of the split atoms. So... an atom fissions (splits) and releases 2-3 prompt neutrons. Some of the split nuclei decay later and release more neutrons. These are the "delayed neutrons", which can be generated up to 55 seconds after the initial fission event.
The key to controlling a nuclear reactor is to never allow the reactor to be critical on Prompt Neutrons alone. If a reactor does become critical on Prompt Neutrons, the condition is called "Prompt Critical". This is a dangerous uncontrolled reaction, which led to the violent explosions at both Chernobyl and SL-1. It was also done intentionally for the SPERT experiment.
Each of these reactors experienced a prompt criticality event, just like a nuclear weapon. However, because the neutrons were thermalized, which greatly slows down the increase of neutrons, none of these reactors detonated with a nuclear weapon type of blast.
Here is some interesting footage of the Prompt Critical SPERT reactor experiment:
Below are two accounts of the account of the prompt critical SL-1 accident, where the first operator fatalities took place. The first is a YouTuber overview, and the second is an old newsreel that goes into greater depth.
And lastly, Chernobyl, nicely explained by Scott Manley. I like to flatter myself that maybe he read this post before making that video :) Chernobyl was the worlds biggest Dirty Bomb.
Saturday, October 12, 2013
Nuclear Weapons - Plutonium Production
After the CP-1 Reactor successfully proved the concept of a self-sustaining fission reaction, the US accelerated its development of nuclear weapons. The Manhattan Project took a two-pronged approach to generating enough material to build the first bomb: Uranium Enrichment and Plutonium Production. This post is about Plutonium Production.
The post about Uranium Enrichment is linked here.
The post about Uranium Enrichment is linked here.
Friday, October 04, 2013
The CP-1 reactor
The first man-made nuclear reactor was built under the grandstand of Stagg Field, at the University of Chicago.
The first reactor was called a "pile". That may have been wartime jargon to hide its true nature from enemy spies, and it may have been descriptive. In fact, it was a pile of uranium and graphite blocks. It's designation was CP-1, or Chicago Pile #1. CP-1 was part of the Manhattan Project, the US government's secret WW2 program to rush a nuclear weapon into production. Its importance cannot be overstated.
The neutron had been discovered in 1932 by an Englishman named Chadwick. In 1938, Lise Meitner, Otto Hahn, and Fritz Strassman collaborated to recognize and report on the fission of Uranium by neutrons. By 1939 it was understood that excess neutrons from a split atom might be able create a self-sustaining reaction. The next step was inevitable: Make it happen.
Below: The experimental setup that led to the discovery of nuclear fission.
Thus CP-1 was born - an industrial scale-up of what had previously been bench-top experiments in laboratories, with the intent of proving the possibility of a self-sustaining fission chain reaction. The techniques for increasing enrichment of Uranium had not yet been invented. Therefore CP-1 used very pure refined metallic naturally occurring Uranium.
Since CP-1 was fueled by natural uranium, it was not possible to use water as a moderator. Light water absorbs too many neutrons. This would prevent a reactor with such a low percentage of U-235 to begin a chain reaction. Light water is a good moderator, but it is also a mild neutron poison.
For this reason, graphite was chosen for the moderator, as it absorbs neutrons 100x less often than water does Graphite is therefore a superior moderator, because it slows down neutrons while preventing their loss via absorption.
To create CP-1, several other laboratory-level experiments also had to be ramped up to industrial levels. At that point in time, the amount of pure metallic Uranium in the world was measured in grams. Further complicating issues was the fact that nobody had ever bothered segregating trace amounts of Boron (A very powerful neutron poison) from the Carbon that the graphite moderator was made from. After enough Uranium metal had been obtained, construction began. Small cylinders of Uranium were interspersed between large blocks of Boron-free graphite.
Enrico Fermi was in charge of the construction of the reactor. Fermi was a brilliant physicist, and was also probably the only man in the world who could have assembled and controlled a nuclear reactor at that time.
Below, a rendering of the CP-1 reactor. The three cylindrical things dangling from a cable are neutron detectors. The man standing is manipulating a control rod, which when removed, stopped absorbing neutrons and allowed a self-sustaining reaction to occur.
From Wiki:
However all these issues were overcome. On 2 December 1942, CP-1 was ready for a demonstration. Before a group of dignitaries, George Weil worked the final control rod while physicist Enrico Fermi carefully monitored the neutron activity. The pile "went critical" (reached a self-sustaining reaction) at 15:25. Fermi shut it down 28 minutes later.
Unlike most reactors that have been built since, CP-1 had no radiation shielding and no cooling system of any kind. Fermi had convinced Arthur Compton that his calculations were reliable enough to rule out a runaway chain reaction or an explosion. But, as the official historians of theAtomic Energy Commission noted, the "gamble" remained in conducting "a possibly catastrophic experiment in one of the most densely populated areas of the nation!"
With the proof at hand that a nuclear reaction could be made self-sustaining, the Manhattan Project would rapidly move forward toward a nuclear weapon.
The first reactor was called a "pile". That may have been wartime jargon to hide its true nature from enemy spies, and it may have been descriptive. In fact, it was a pile of uranium and graphite blocks. It's designation was CP-1, or Chicago Pile #1. CP-1 was part of the Manhattan Project, the US government's secret WW2 program to rush a nuclear weapon into production. Its importance cannot be overstated.
The neutron had been discovered in 1932 by an Englishman named Chadwick. In 1938, Lise Meitner, Otto Hahn, and Fritz Strassman collaborated to recognize and report on the fission of Uranium by neutrons. By 1939 it was understood that excess neutrons from a split atom might be able create a self-sustaining reaction. The next step was inevitable: Make it happen.
Below: The experimental setup that led to the discovery of nuclear fission.
Thus CP-1 was born - an industrial scale-up of what had previously been bench-top experiments in laboratories, with the intent of proving the possibility of a self-sustaining fission chain reaction. The techniques for increasing enrichment of Uranium had not yet been invented. Therefore CP-1 used very pure refined metallic naturally occurring Uranium.
Since CP-1 was fueled by natural uranium, it was not possible to use water as a moderator. Light water absorbs too many neutrons. This would prevent a reactor with such a low percentage of U-235 to begin a chain reaction. Light water is a good moderator, but it is also a mild neutron poison.
For this reason, graphite was chosen for the moderator, as it absorbs neutrons 100x less often than water does Graphite is therefore a superior moderator, because it slows down neutrons while preventing their loss via absorption.
To create CP-1, several other laboratory-level experiments also had to be ramped up to industrial levels. At that point in time, the amount of pure metallic Uranium in the world was measured in grams. Further complicating issues was the fact that nobody had ever bothered segregating trace amounts of Boron (A very powerful neutron poison) from the Carbon that the graphite moderator was made from. After enough Uranium metal had been obtained, construction began. Small cylinders of Uranium were interspersed between large blocks of Boron-free graphite.
Enrico Fermi was in charge of the construction of the reactor. Fermi was a brilliant physicist, and was also probably the only man in the world who could have assembled and controlled a nuclear reactor at that time.
Below, a rendering of the CP-1 reactor. The three cylindrical things dangling from a cable are neutron detectors. The man standing is manipulating a control rod, which when removed, stopped absorbing neutrons and allowed a self-sustaining reaction to occur.
From Wiki:
However all these issues were overcome. On 2 December 1942, CP-1 was ready for a demonstration. Before a group of dignitaries, George Weil worked the final control rod while physicist Enrico Fermi carefully monitored the neutron activity. The pile "went critical" (reached a self-sustaining reaction) at 15:25. Fermi shut it down 28 minutes later.
Unlike most reactors that have been built since, CP-1 had no radiation shielding and no cooling system of any kind. Fermi had convinced Arthur Compton that his calculations were reliable enough to rule out a runaway chain reaction or an explosion. But, as the official historians of theAtomic Energy Commission noted, the "gamble" remained in conducting "a possibly catastrophic experiment in one of the most densely populated areas of the nation!"
With the proof at hand that a nuclear reaction could be made self-sustaining, the Manhattan Project would rapidly move forward toward a nuclear weapon.
Thursday, September 26, 2013
Unusual Reactors - A reactor in nature!
So far, I've discussed BWRs and PWRs fueled with Low-Enrichment U-235 (typically 5% enrichment)
Lets talk about some other reactors. How about a naturally occurring reactor that once occured in nature in a seam of Uranium?
I will simply copy and paste bits from the Wikipedia article linked above, since I cannot improve on it in any way.
From Wiki:
A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions have occurred. This can be examined by analysis of isotope ratios. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa, by French physicist Francis Perrin. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kazuo Kuroda.
The conditions found were very similar to what was predicted.
Oklo is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of power output during that time
My aside: This enought to power about 1000 light bulbs of 100 Watts each.
In May 1972 at the Pierrelatte uranium enrichment facility in France, routine mass spectrometry comparing UF6 samples from the Oklo Mine, located in Gabon, Central Africa, showed a discrepancy in the amount of the U-235 isotope. Normally the concentration is 0.720% while these samples had only 0.717%, a significant difference.
This discrepancy required explanation, as all uranium handling facilities must meticulously account for all fissionable isotopes to assure that none are diverted for weapons purposes. Thus the French Commissariat à l'énergie atomique (CEA) began an investigation. A series of measurements of the relative abundances of the two most significant isotopes of the uranium mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a U-235 concentration as low as 0.440%.
This loss in U-235 is exactly what happens in a nuclear reactor. A possible explanation therefore was that the uranium ore had operated as a natural fission reactor. Other observations led to the same conclusion, and on September 25, 1972, the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.
Geological Situation in Gabon leading to natural nuclear fission reactors
1. Nuclear reactor zones
2. Sandstone
3. Ore layer
4. Granite
Here are a couple of pictures I found on the web of the Oklo reactors.
Lets talk about some other reactors. How about a naturally occurring reactor that once occured in nature in a seam of Uranium?
I will simply copy and paste bits from the Wikipedia article linked above, since I cannot improve on it in any way.
From Wiki:
A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions have occurred. This can be examined by analysis of isotope ratios. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa, by French physicist Francis Perrin. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kazuo Kuroda.
The conditions found were very similar to what was predicted.
Oklo is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of power output during that time
My aside: This enought to power about 1000 light bulbs of 100 Watts each.
In May 1972 at the Pierrelatte uranium enrichment facility in France, routine mass spectrometry comparing UF6 samples from the Oklo Mine, located in Gabon, Central Africa, showed a discrepancy in the amount of the U-235 isotope. Normally the concentration is 0.720% while these samples had only 0.717%, a significant difference.
This discrepancy required explanation, as all uranium handling facilities must meticulously account for all fissionable isotopes to assure that none are diverted for weapons purposes. Thus the French Commissariat à l'énergie atomique (CEA) began an investigation. A series of measurements of the relative abundances of the two most significant isotopes of the uranium mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a U-235 concentration as low as 0.440%.
This loss in U-235 is exactly what happens in a nuclear reactor. A possible explanation therefore was that the uranium ore had operated as a natural fission reactor. Other observations led to the same conclusion, and on September 25, 1972, the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.
Geological Situation in Gabon leading to natural nuclear fission reactors
1. Nuclear reactor zones
2. Sandstone
3. Ore layer
4. Granite
My aside: Here is where the reactor physics gets interesting!
The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the reaction. After cooling of the mineral deposit, the water returned and the reaction started again. These fission reactions were sustained for hundreds of thousands of years, until a chain reaction could no longer be supported.
Fission of uranium normally produces five known isotopes of the fission-product gas xenon; all five have been found trapped in the remnants of the natural reactor, in varying concentrations. The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down to complete a 3-hour cycle.
A key factor that made the reaction possible was that, at the time the reactor went critical 1.7 billion years ago, the fissile isotope U-235 made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 97% was non-fissile U-238.) Because U-235 has a shorter half life than U-238, and thus decays more rapidly, the current abundance of U-235 in natural uranium is about 0.7%. A natural nuclear reactor is therefore no longer possible on Earth without moderation using heavy water or graphite.
The Oklo uranium ore deposits are the only known sites in which natural nuclear reactors existed. Other rich uranium ore bodies would also have had sufficient uranium to support nuclear reactions at that time, but the combination of uranium, water and physical conditions needed to support the chain reaction was unique to the Oklo ore bodies.
Fission of uranium normally produces five known isotopes of the fission-product gas xenon; all five have been found trapped in the remnants of the natural reactor, in varying concentrations. The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down to complete a 3-hour cycle.
A key factor that made the reaction possible was that, at the time the reactor went critical 1.7 billion years ago, the fissile isotope U-235 made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 97% was non-fissile U-238.) Because U-235 has a shorter half life than U-238, and thus decays more rapidly, the current abundance of U-235 in natural uranium is about 0.7%. A natural nuclear reactor is therefore no longer possible on Earth without moderation using heavy water or graphite.
The Oklo uranium ore deposits are the only known sites in which natural nuclear reactors existed. Other rich uranium ore bodies would also have had sufficient uranium to support nuclear reactions at that time, but the combination of uranium, water and physical conditions needed to support the chain reaction was unique to the Oklo ore bodies.
Here are a couple of pictures I found on the web of the Oklo reactors.
Wednesday, September 25, 2013
Moderators, Neutrons, and Enrichment, oh my!
I want to discuss a few other reactor designs besides the typical US power reactors, the PWR and BWR types. Before I do that, it's important to understand a few things that force us down certain paths in designing reactor cores.
We need to understand the relative abundances of U-235 and U-238, why we use neutrons to split atoms, and also how a moderator works, in a physical sense.
First off, about Uranium. According to Wikipedia, naturally occurring Uranium consists of three major isotopes: U-238 (99.28%), U-235 (0.71%), and U-234 (0.0054%). The stuff we need for fission (U-235) is less than 1% of naturally occurring Uranium.
There is an entire industry based around the enrichment of Uranium. The process of enrichment is as fascinating as it is tedious. I might do a post on that process at some point, but for now it will suffice to point out that it is difficult to sustain a fission chain reaction using only naturally occurring Uranium. For this reason, in most reactors, the concentration of U-235 in U-238 is increased, or enriched.
A fission chain reaction must be accomplished by freeing up large numbers of neutrons from the nucleus of a parent atom. Why neutrons? A couple of reasons. Since neutrons have no charge, they are not repelled by the positive charge of a nucleus. Thus they can wander at will through any part of an atom. Neutrons can also pass through many materials other than Uranium, as if that material weren't there. Lastly, since fission produces additional neutrons, it can become self-sustaining.
When a fission occurs, 2-3 neutrons are ejected, but they are moving at 2MeV, which is about 20% of light speed. At this speed, they CAN interact with another Uranium nucleus and cause a fission, but this is unlikely. In order to improve the likelihood of a fission, there are two things we can do: Increase the number of U-235 atoms in the core, or moderate (slow down) the neutrons.
In practice, both methods (moderation and enrichment) are usually used. Reactors using no moderator, and containing a core of highly enriched U-235 have been built and operated. Since there is no moderator slowing the neutrons down, these type of reactors are called "Fast Neutron Reactors".
The vast majority of reactors are "Thermal Neutron Reactors". The reasons most reactors use thermal neutrons are two-fold. Fast Reactors require fuel enriched above 20%, which can potentially be diverted and used for a nuclear weapon. Additionally Fast Reactors have poor safety characteristics from a reactor physics standpoint, and not surprisingly, have an abysmal safety record. Of the handful of Fast Reactors built, several have suffered meltdowns.
More on moderators though. The purpose of moderation is to slow neutrons down to the point where they are at thermal equilibrium with the surrounding material. The way to accomplish this is by allowing them to impact (or "scatter") against atoms with low mass, and low neutron absorption. If a neutron is absorbed by the moderator, then it is no longer available to split a Uranium atom.
Think of the neutron as a ping-pong ball fired from a cannon. To remove the most energy by collision, ideally it should run into another ping-pong ball, which will recoil and remove some speed. If it collides with a heavy nucleus, like steel, it would be like our ping pong ball bounced off a boulder. It would not lose as much energy to the boulder.
Water is a decent, but not great, moderator. It has two hydrogen atoms for neutrons to scatter against, and it is reasonably dense. The oxygen molecule has the ability to scatter a neutron, but it due to its mass (16) it would take many more oxygen atom collisions to accomplish the task. Water is also plentiful and cheap. This is why in most applications, water is used as a moderator.
Water though, is not an ideal neutron moderator, and here is why: Both the hydrogen and oxygen atoms have a small, but noticeable possibility of absorbing a neutron, thus removing it from the process and making it unavailable to cause a fission.
There are other moderators that are superior to light water from the standpoint of neutron absorption. Even though a Fast neutron may require a larger number of collisions to reach thermal equilibrium, the lower likelihood of being absorbed make other materials superior moderators.
Moderator Number of collisions Likelihood of neutron absorption %
Hydrogen 18 0.3326
Dueterium 25 0.000519
Beryllium 86 0.0076
Carbon 114 0.0035
Oxygen 150 0.00019
Uranium 2152 7.57
Dueterium (in the form of heavy water) is probably the best moderator. This is water with hydrogen atoms that already have one neutron and one proton. Normal hydrogen atoms have just one proton, and occasionally capture one, taking it out of use for the reactor. Dueterium hydrogen atoms have already absorbed a neutron, and are highly unlikely to absorb a second one.
Dueterium is found in nature, but like U-235, is not abundant. It also needs to be enriched at great difficulty and expense to be used as a moderator. However, it is such an efficient moderator that a reactor can be operated with Uranium fuel that has not been enriched.
Beryllium is another excellent moderator, with very little likelihood of absorbing a neutron. Its disadvantages are expense and high toxicity.
Carbon is an excellent moderator, even with the large number of collisions required, its low neutron absorption, and low cost make it a practical moderator, even if it is not a good material to remove heat. A graphite-moderated reactor can also sustain a chain reaction with Uranium that has not been enriched.
The other two items on the list, Oxygen and Uranium are not moderators at all. They just show how increasing the mass of the target atom increases the number of collisions required to moderate a neutron. However on the right hand column, notice the likelihood of absorption for Naturally Occurring Uranium. It does love to vacuum up neutrons :)
In the next post, using what we have learned here, I will describe a few unusual reactors that have been built, either for testing purposes, or for breeding additional nuclear fuel, for nuclear weapons production, and how a nuclear weapon itself works.
We need to understand the relative abundances of U-235 and U-238, why we use neutrons to split atoms, and also how a moderator works, in a physical sense.
First off, about Uranium. According to Wikipedia, naturally occurring Uranium consists of three major isotopes: U-238 (99.28%), U-235 (0.71%), and U-234 (0.0054%). The stuff we need for fission (U-235) is less than 1% of naturally occurring Uranium.
There is an entire industry based around the enrichment of Uranium. The process of enrichment is as fascinating as it is tedious. I might do a post on that process at some point, but for now it will suffice to point out that it is difficult to sustain a fission chain reaction using only naturally occurring Uranium. For this reason, in most reactors, the concentration of U-235 in U-238 is increased, or enriched.
A fission chain reaction must be accomplished by freeing up large numbers of neutrons from the nucleus of a parent atom. Why neutrons? A couple of reasons. Since neutrons have no charge, they are not repelled by the positive charge of a nucleus. Thus they can wander at will through any part of an atom. Neutrons can also pass through many materials other than Uranium, as if that material weren't there. Lastly, since fission produces additional neutrons, it can become self-sustaining.
When a fission occurs, 2-3 neutrons are ejected, but they are moving at 2MeV, which is about 20% of light speed. At this speed, they CAN interact with another Uranium nucleus and cause a fission, but this is unlikely. In order to improve the likelihood of a fission, there are two things we can do: Increase the number of U-235 atoms in the core, or moderate (slow down) the neutrons.
In practice, both methods (moderation and enrichment) are usually used. Reactors using no moderator, and containing a core of highly enriched U-235 have been built and operated. Since there is no moderator slowing the neutrons down, these type of reactors are called "Fast Neutron Reactors".
The vast majority of reactors are "Thermal Neutron Reactors". The reasons most reactors use thermal neutrons are two-fold. Fast Reactors require fuel enriched above 20%, which can potentially be diverted and used for a nuclear weapon. Additionally Fast Reactors have poor safety characteristics from a reactor physics standpoint, and not surprisingly, have an abysmal safety record. Of the handful of Fast Reactors built, several have suffered meltdowns.
More on moderators though. The purpose of moderation is to slow neutrons down to the point where they are at thermal equilibrium with the surrounding material. The way to accomplish this is by allowing them to impact (or "scatter") against atoms with low mass, and low neutron absorption. If a neutron is absorbed by the moderator, then it is no longer available to split a Uranium atom.
Think of the neutron as a ping-pong ball fired from a cannon. To remove the most energy by collision, ideally it should run into another ping-pong ball, which will recoil and remove some speed. If it collides with a heavy nucleus, like steel, it would be like our ping pong ball bounced off a boulder. It would not lose as much energy to the boulder.
Water is a decent, but not great, moderator. It has two hydrogen atoms for neutrons to scatter against, and it is reasonably dense. The oxygen molecule has the ability to scatter a neutron, but it due to its mass (16) it would take many more oxygen atom collisions to accomplish the task. Water is also plentiful and cheap. This is why in most applications, water is used as a moderator.
Water though, is not an ideal neutron moderator, and here is why: Both the hydrogen and oxygen atoms have a small, but noticeable possibility of absorbing a neutron, thus removing it from the process and making it unavailable to cause a fission.
There are other moderators that are superior to light water from the standpoint of neutron absorption. Even though a Fast neutron may require a larger number of collisions to reach thermal equilibrium, the lower likelihood of being absorbed make other materials superior moderators.
Moderator Number of collisions Likelihood of neutron absorption %
Hydrogen 18 0.3326
Dueterium 25 0.000519
Beryllium 86 0.0076
Carbon 114 0.0035
Oxygen 150 0.00019
Uranium 2152 7.57
Dueterium (in the form of heavy water) is probably the best moderator. This is water with hydrogen atoms that already have one neutron and one proton. Normal hydrogen atoms have just one proton, and occasionally capture one, taking it out of use for the reactor. Dueterium hydrogen atoms have already absorbed a neutron, and are highly unlikely to absorb a second one.
Dueterium is found in nature, but like U-235, is not abundant. It also needs to be enriched at great difficulty and expense to be used as a moderator. However, it is such an efficient moderator that a reactor can be operated with Uranium fuel that has not been enriched.
Beryllium is another excellent moderator, with very little likelihood of absorbing a neutron. Its disadvantages are expense and high toxicity.
Carbon is an excellent moderator, even with the large number of collisions required, its low neutron absorption, and low cost make it a practical moderator, even if it is not a good material to remove heat. A graphite-moderated reactor can also sustain a chain reaction with Uranium that has not been enriched.
The other two items on the list, Oxygen and Uranium are not moderators at all. They just show how increasing the mass of the target atom increases the number of collisions required to moderate a neutron. However on the right hand column, notice the likelihood of absorption for Naturally Occurring Uranium. It does love to vacuum up neutrons :)
In the next post, using what we have learned here, I will describe a few unusual reactors that have been built, either for testing purposes, or for breeding additional nuclear fuel, for nuclear weapons production, and how a nuclear weapon itself works.
Sunday, September 22, 2013
Nuclear Reactors - How they work (part 2)
Previously (part 1) we had a Pressurized Water Reactor (PWR) which was critical and adding heat to the coolant/moderator. After the reactor reaches the point where it is generating heat, and the moderator becomes less effective, it becomes neccessary to withdraw the control rods again to increase power output.
Since this is a PWR, we are not supposed to have large-scale boiling in the core. Steam doesn't transfer heat as readily as water does, and in a PWR, in-core boiling is a bad, bad thing. For this reason the pressure in the primary coolant loop must be kept well above the boiling point of water for the temperature found in the core. How do we do that? We have a pressurizer!
The pressurizer does three things: First, it acts as a surge volume for the primary coolant system by maintaining a compressible steam-filled void space. Secondly, it keeps the system pressure high enough that boiling cannot occur in the core. This is accomplished by keeping the pressurizer vessel half full of water, and adding enough electrical heaters that the temperature in the pressurizer is kept higher than that of the core. All boiling will occur in the pressurizer, not in the core.
Lastly, the pressurizer controls the primary coolant system pressure. As more electric heaters are energized in the pressurizer vessel, saturation conditions increase, causing pressure to rise. To reduce primary pressure, a shower spray head at the top of the vessel can spray slightly cooler water into the steam, condensing a portion of the steam and reducing pressure. It's quite a simple and elegant process.
Below is a simplified diagram of the primary and secondary systems of a PWR. The radioactive primary coolant (Red) is contained in a closed loop. The reactor core is the heat source, the control rod drive mechanism positions the control rods to increase/decrease reactor power, the primary pump circulates the primary coolant, and pressurizer serves as a static surge volume.
The steam generator segregates the radioactive primary coolant from the steam/water system, but allows heat to be transferred out of the primary coolant loop and generate steam for the turbine.
Pressure in the primary system is increased by energizing submerged electrical heaters in the pressurizer, and reduced by spraying a small slip-stream of primary coolant from the reactor coolant pump through a spray head at the top of the pressurizer. This cools and condenses some of the steam in the pressurizer, reducing system pressure.
Temperature is gradually increased by withdrawing the control rods. Remember in the previous post that the neutron population would increase with no control rod movement if the reactor were slightly supercritical? That is no longer the case when power level is high enough for the reactor to generate heat. After the core begins heating and the "negative temperature coefficient of reactivity" kicks in, withdrawing the control rods increases power, followed by a smaller drop in power as the primary coolant heats up. So control rods are withdrawn in a series bumps, and the coolant temperature is slowly increased.
The primary coolant pump circulates primary coolant (very pure water) between the reactor core and the steam generator. The reactor core adds heat created by fission. The steam generator removes heat by boiling secondary water in a heat exchanger. Thus, primary coolant is radioactive, but does not boil. Secondary water is non-radioactive, and does boil.
In a PWR, excess fuel is loaded, more than what is required to bring the reactor up to power. This is done so that the reactor can continue to run as the U-235 is expended, and fission products build up. Many of the atoms that are created by fission are strong neutron absorbers (poisons), and eventually result in the inability of the reactor to maintain full power. This is in spite of excess U-235 added at during refueling. Only about 3% of the U-235 is used up before the accumulation of poison halts reactor operation. This is why fuel recycling is so desirable from an industry standpoint.
Commercial PWRs are unique in that they add boric acid to the primary coolant. Boron is a neutron absorber. With the addition of boron to the coolant, the control rods may be entirely withdrawn from the core, using only boron to control neutron population. This is desirable because in the vicinity of control rods, neutron population, and fissions will be depressed. This forces other regions of the core to run hotter for a given power level. With control rods out of the core, neutron distribution is more consistent and hot spots due to uneven neutron flux are less likely to develop.
At a certain point, on the secondary side of the steam generator, steam is at an adequate pressure to begin warming up the steam turbine. This also has an interesting impact on reactor power. As we create steam on the secondary side of the steam generator, we are also removing heat from the primary coolant. This slightly cooler primary coolant returns to the reactor. Since it is now more dense, it is a better moderator, and more neutrons stay in the core to cause fissions. Reactor power now increases along with steam demand. This is a very desirable feedback loop, because now there is little need to adjust the control rods. We allow the steam turbine load to determine what power level the reactor will operate at.
The remainder of the steam plant operates in a very similar manner to a fossil-fueled plant. One difference: Since it is not possible to superheat the steam with this process, steam dryers are added between different stages on the steam turbine, to prevent mist from damaging the lower pressure sections.
Since this is a PWR, we are not supposed to have large-scale boiling in the core. Steam doesn't transfer heat as readily as water does, and in a PWR, in-core boiling is a bad, bad thing. For this reason the pressure in the primary coolant loop must be kept well above the boiling point of water for the temperature found in the core. How do we do that? We have a pressurizer!
The pressurizer does three things: First, it acts as a surge volume for the primary coolant system by maintaining a compressible steam-filled void space. Secondly, it keeps the system pressure high enough that boiling cannot occur in the core. This is accomplished by keeping the pressurizer vessel half full of water, and adding enough electrical heaters that the temperature in the pressurizer is kept higher than that of the core. All boiling will occur in the pressurizer, not in the core.
Lastly, the pressurizer controls the primary coolant system pressure. As more electric heaters are energized in the pressurizer vessel, saturation conditions increase, causing pressure to rise. To reduce primary pressure, a shower spray head at the top of the vessel can spray slightly cooler water into the steam, condensing a portion of the steam and reducing pressure. It's quite a simple and elegant process.
Below is a simplified diagram of the primary and secondary systems of a PWR. The radioactive primary coolant (Red) is contained in a closed loop. The reactor core is the heat source, the control rod drive mechanism positions the control rods to increase/decrease reactor power, the primary pump circulates the primary coolant, and pressurizer serves as a static surge volume.
The steam generator segregates the radioactive primary coolant from the steam/water system, but allows heat to be transferred out of the primary coolant loop and generate steam for the turbine.
Pressure in the primary system is increased by energizing submerged electrical heaters in the pressurizer, and reduced by spraying a small slip-stream of primary coolant from the reactor coolant pump through a spray head at the top of the pressurizer. This cools and condenses some of the steam in the pressurizer, reducing system pressure.
Temperature is gradually increased by withdrawing the control rods. Remember in the previous post that the neutron population would increase with no control rod movement if the reactor were slightly supercritical? That is no longer the case when power level is high enough for the reactor to generate heat. After the core begins heating and the "negative temperature coefficient of reactivity" kicks in, withdrawing the control rods increases power, followed by a smaller drop in power as the primary coolant heats up. So control rods are withdrawn in a series bumps, and the coolant temperature is slowly increased.
The primary coolant pump circulates primary coolant (very pure water) between the reactor core and the steam generator. The reactor core adds heat created by fission. The steam generator removes heat by boiling secondary water in a heat exchanger. Thus, primary coolant is radioactive, but does not boil. Secondary water is non-radioactive, and does boil.
In a PWR, excess fuel is loaded, more than what is required to bring the reactor up to power. This is done so that the reactor can continue to run as the U-235 is expended, and fission products build up. Many of the atoms that are created by fission are strong neutron absorbers (poisons), and eventually result in the inability of the reactor to maintain full power. This is in spite of excess U-235 added at during refueling. Only about 3% of the U-235 is used up before the accumulation of poison halts reactor operation. This is why fuel recycling is so desirable from an industry standpoint.
Commercial PWRs are unique in that they add boric acid to the primary coolant. Boron is a neutron absorber. With the addition of boron to the coolant, the control rods may be entirely withdrawn from the core, using only boron to control neutron population. This is desirable because in the vicinity of control rods, neutron population, and fissions will be depressed. This forces other regions of the core to run hotter for a given power level. With control rods out of the core, neutron distribution is more consistent and hot spots due to uneven neutron flux are less likely to develop.
At a certain point, on the secondary side of the steam generator, steam is at an adequate pressure to begin warming up the steam turbine. This also has an interesting impact on reactor power. As we create steam on the secondary side of the steam generator, we are also removing heat from the primary coolant. This slightly cooler primary coolant returns to the reactor. Since it is now more dense, it is a better moderator, and more neutrons stay in the core to cause fissions. Reactor power now increases along with steam demand. This is a very desirable feedback loop, because now there is little need to adjust the control rods. We allow the steam turbine load to determine what power level the reactor will operate at.
The remainder of the steam plant operates in a very similar manner to a fossil-fueled plant. One difference: Since it is not possible to superheat the steam with this process, steam dryers are added between different stages on the steam turbine, to prevent mist from damaging the lower pressure sections.
Saturday, September 21, 2013
Nuclear Reactors - How they work (part 1)
I've always been fascinated by nuclear reactors. Even as a child, I was mystified by the blue glowing water I would see in the National Geographic Magazine. The whole notion of releasing enormous amounts of energy by gathering certain materials together in a small area is a bit mind-boggling.
So how does a reactor work? First, a little chemistry, then a little physics.
As an example of how nuclear energy is created, we can look at an atom of Helium-4, which contains 2 protons, 2 neutrons, and 2 electrons.
Neutrons (blue) have no electrical charge. Protons (red) have a +1 charge. Electrons (yellow) orbit the atom at a pretty far distance and have a -1 charge. Electrons and protons (and therefore charge) are balanced in typical atoms.
Because protons have a +1 charge, they repel each other. The repulsive electrostatic force is squared every time the distance is cut in half. The closer these protons get to each other, the more they want to fly apart! This is similar to pushing powerful magnets together with both north poles together. However nature provides a way to hold these repelling protons together in spite of themselves :)
Back now to our physics...
The atomic mass of each component:
proton = 1.007766612
neutron = 1.00864160
electron = .000548892490
OK, funky numbers with lots of decimal places. Bear with me here. This is important.
Mass of all these individual components added together in a Helium Atom = 4.03391420898
Yet the actual measured mass of a Helium Atom is only = 4.002602 What the HECK!!!!???
These numbers should be the same!
There is a difference in the atomic mass of 0.03131220898 between the individal components and the assembled atom.
This difference in the calculated mass and the actual mass is called the "mass defect". Holding two positively-charged protons very close together requires a great deal of energy. It also requires an incredibly powerful short-range force, called the nuclear force. The nuclear force is so strong that it can overcome the repulsion of particles with like charges. However, it requires so much energy to bind a nucleus together against the repulsive electrostatic force that there is a noticable change in the mass of the atom and its components.
Here is where we get to Einstein's awesome realization: Mass and energy are equivalent and interchangeble.
The difference in mass of an assembled Helium atom and its components is due to the large amount of energy required to hold that atom's nucleus together against the repulsive force of those positively charge protons that are trying to repel each other. E=mc^2 is the equation that explains how much energy was needed to assemble the Helium atom from its components. It's also how much energy would be released if they were taken apart again. This energy is called Binding Energy.
Binding energy is the same exact thing as the mass defect, only from the viewpoint of energy, rather than mass. E=mc^2 is the mathematical conversion between the two states (matter and energy).
To better understand how much energy, E we are talking about, we need to look on the right side of the equation. The m (or mass) is a very tiny number, 0.03131220898. However c is the speed of light, and that number is squared. So the speed of light times the speed of light is a very, very, large number. The energy that would be released if we break apart one helium nucleus into its components would be 28.3 Million Electron Volts. To put this in perspective, an X-ray has energy at 40-60 thousand Electron volts, while visible light has energy at 2-6 electron volts.
Unfortunately for our energy needs, Helium atoms are notoriously stable, and almost impossible to break apart. However, nature has provided us with certain atoms that will split apart quite readily under the correct conditions.
It turns out that certain high-mass atomic nuclei will split into two smaller fragments after absorbing (or capturing) a neutron. More importantly, when these atoms split, they release many more neutrons, allowing a continuous reaction (or chain reaction) to be sustained. Neutrons are the best subatomic particle to create large numbers of fissions, because they have no charge, and readily interact with the nucleus of an atom. Protons have a positive charge, and therefore are repelled by the positively charged nuclei of atoms.
Below to the left, a neutron is absorbed by a fissionable nucleus. The nucleus splits into two smaller fragments, three additional neutrons, and a gamma ray. This is a pretty typical result of fission.
Atoms that will split (or fission) after capturing a neutron are called "fissionable". There are a large number of atoms that will fission. However not just any fissionable material will work in a reactor.
To assemble a reactor core we need to find fissionable material with some specific properties:
To understand the difference between fissionable and fissile, we need to understand about neutron temperature. When a neutron is ejected from a split atom, it has enormous energy, about 2 Million Electron Volts (or 2 MeV).
Physically, this is the speed of the neutron. A 2 MeV neutron is traveling at about 45 million miles/hr. Neutrons moving this fast don't often linger to induce fission in other atoms. Statistically they are likely to leave the reactor core before an interaction occurs. For this reason it is desirable to slow them down. Neutrons are slowed by allowing them to bounce off lighter atoms and transfer their kinetic energy to the atoms. This process is called "moderation", and slows the neutrons down to what is called "thermal" energy, .025 eV, or about 4900 miles/hr. Depending on the mass of the moderating material, it may take from 10-2500 impacts to reduce the speed of a fast neutron from fission to thermal equillibrium.
Below is a table showing neutron energy (or straight line speed) of a neutron with its description.
It is at thermal energy, speed, or temperature (pick one), that neutrons will more readily be absorbed by a nucleus and lead to another fission.
Now that we understand the process of neutron moderation, we can return to our need for Fissile atoms. Fissile atoms are a small subset of fissionable atoms, that will readily split after absorbing a thermal neutron. The list of candidates of fissile materials is small. Uranium 233 and 235, Plutonium 239, and 241. Only Uranium 235 is naturally occuring, while the others must be produced by excess neutrons from other materials inside a breeder reactor.
In another post I will describe other types of reactors, but for now we will stay with the basic thermal reactor, which is fueled with 5% U-235, both cooled and moderated by normal (light) water.
When an atom splits, it creates two highly radioactive atoms that must be contained, and not allowed to get into the coolant/moderator. Otherwise that water will become very contaminated with radwaste. For this reason, the uranium fuel is surrounded by zircaloy cladding. Below is a photo of people inspecting new fuel assemblies for defects in the cladding.
The energy released by splitting or fission of a single U-235 atom is about 200 MeV, most of which is in the form of kinetic energy of the two new atoms. They are moving at high speed, but due to their mass and charge, they quickly come to rest among other atoms of fuel. The energy these atom fragments lose as they come to rest is converted to heat.
Back to the reactor core. Water surrounds an array of these fuel assemblies, which is called the "core". It takes a certain amount of fissile U-235 packed closely together, and moderated neurtons for a continuous reaction to take place. Below is a top-down view of a generic reactor core.
The reaction is started by withdrawing control rods. Control rods contain strong absorbers of thermal neutrons. These neutron absorbers are also known as "poisons", in that they slow or stop the nuclear fission process. Typically a control rod contains cadmium, boron (or a borated alloy) for neutron control.
"Keff" is the number reactor physicists use to determine whether the reactor core is shutdown, steady-state, or starting up. It's the neutron multiplier. Keff is the likelihood that a single neutron will make it from birth to cause a fission in another Uranium atom. Some of the other possibilities are that a neutron will escape from the reactor core, be absorbed in some material other than Uranium, or be absorbed by a Uranium atom, but not cause a fission.
If Keff is < 1.0 the reactor is "subcritical". If Keff is = 1.0 the reactor is "critical", and if the Keff is >1.0 the reactor is "supercritical". Nothing in any of these terms should be fear-inducing, they just describe a core reactivity condition for sustaining (or changing) the neutron population.
So... what happens when the control rods are removed? I will skip the mathematics this time and give a simple description. Neutrons are always present in U-235, because one of the natural decay modes for U-235 is spontaneous fission. However reactors contain a strong artificial neutron source to ensure there are plenty of neutrons to get things going. Think of the installed neutron source as sort of a pilot light for a gas furnace.
When the control rods are removed partially, neutron population begins to increase for two reasons. The neutrons are no longer being absorbed in the poison, and so they begin splitting U-235 atoms, which generates even more neutrons. The neutron population increases, even if Keff is less than 1, for the above reasons.
Eventually though, if the control rods have been removed far enough, neutrons from new fissions will equal the losses, and Keff will be 1. The reactor will then be critical. However it won't be generating any heat. Reactors typically are critical at an output of about 5 watts. About the power required by a single "night light" bulb. Power must be increased by a factor of 10^8 to get to 500 Megawatts.
After the reactor is critical, the neutron population must increase a great deal before any noticeable heat is generated. So the rods are pulled out a little more, and each generation of neutrons has a few million more than the previous generation. With this small increase in reactivity, there is no further need to withdraw the control rods. Neutron multiplication will increase on its own.
Eventually the reactor reaches a point at which heat is generated, and this is where things get interesting. The moderator (cooling water surrounding the fuel assemblies) heats up, and becomes less dense. It is therefore less effective at slowing down neutrons.
Because the moderator is less effective as temperature increases, more neutrons escape the core, which reduces neutron population, and reactor power stops rising. We therefore have a heat-induced dampening effect on reactor power. Its technical term is "negative temperature coefficient of reactivity". Basically, more heating tends to reduce reactor power. This is a desirable thing, because it prevents a runaway nuclear reaction. Unfortunately not all reactors have this characteristic :(
So far, so good. We have a critical nuclear reactor (Keff = 1.0) which is adding heat to the coolant!
I think that now is a good place to stop and think about the next post. My brain hurts :)
Part 2 is here.
So how does a reactor work? First, a little chemistry, then a little physics.
As an example of how nuclear energy is created, we can look at an atom of Helium-4, which contains 2 protons, 2 neutrons, and 2 electrons.
Neutrons (blue) have no electrical charge. Protons (red) have a +1 charge. Electrons (yellow) orbit the atom at a pretty far distance and have a -1 charge. Electrons and protons (and therefore charge) are balanced in typical atoms.
Because protons have a +1 charge, they repel each other. The repulsive electrostatic force is squared every time the distance is cut in half. The closer these protons get to each other, the more they want to fly apart! This is similar to pushing powerful magnets together with both north poles together. However nature provides a way to hold these repelling protons together in spite of themselves :)
Back now to our physics...
The atomic mass of each component:
proton = 1.007766612
neutron = 1.00864160
electron = .000548892490
OK, funky numbers with lots of decimal places. Bear with me here. This is important.
Mass of all these individual components added together in a Helium Atom = 4.03391420898
Yet the actual measured mass of a Helium Atom is only = 4.002602 What the HECK!!!!???
These numbers should be the same!
There is a difference in the atomic mass of 0.03131220898 between the individal components and the assembled atom.
This difference in the calculated mass and the actual mass is called the "mass defect". Holding two positively-charged protons very close together requires a great deal of energy. It also requires an incredibly powerful short-range force, called the nuclear force. The nuclear force is so strong that it can overcome the repulsion of particles with like charges. However, it requires so much energy to bind a nucleus together against the repulsive electrostatic force that there is a noticable change in the mass of the atom and its components.
Here is where we get to Einstein's awesome realization: Mass and energy are equivalent and interchangeble.
The difference in mass of an assembled Helium atom and its components is due to the large amount of energy required to hold that atom's nucleus together against the repulsive force of those positively charge protons that are trying to repel each other. E=mc^2 is the equation that explains how much energy was needed to assemble the Helium atom from its components. It's also how much energy would be released if they were taken apart again. This energy is called Binding Energy.
Binding energy is the same exact thing as the mass defect, only from the viewpoint of energy, rather than mass. E=mc^2 is the mathematical conversion between the two states (matter and energy).
To better understand how much energy, E we are talking about, we need to look on the right side of the equation. The m (or mass) is a very tiny number, 0.03131220898. However c is the speed of light, and that number is squared. So the speed of light times the speed of light is a very, very, large number. The energy that would be released if we break apart one helium nucleus into its components would be 28.3 Million Electron Volts. To put this in perspective, an X-ray has energy at 40-60 thousand Electron volts, while visible light has energy at 2-6 electron volts.
Unfortunately for our energy needs, Helium atoms are notoriously stable, and almost impossible to break apart. However, nature has provided us with certain atoms that will split apart quite readily under the correct conditions.
It turns out that certain high-mass atomic nuclei will split into two smaller fragments after absorbing (or capturing) a neutron. More importantly, when these atoms split, they release many more neutrons, allowing a continuous reaction (or chain reaction) to be sustained. Neutrons are the best subatomic particle to create large numbers of fissions, because they have no charge, and readily interact with the nucleus of an atom. Protons have a positive charge, and therefore are repelled by the positively charged nuclei of atoms.
Below to the left, a neutron is absorbed by a fissionable nucleus. The nucleus splits into two smaller fragments, three additional neutrons, and a gamma ray. This is a pretty typical result of fission.
Atoms that will split (or fission) after capturing a neutron are called "fissionable". There are a large number of atoms that will fission. However not just any fissionable material will work in a reactor.
To assemble a reactor core we need to find fissionable material with some specific properties:
- Some atoms which are fissionable don't readily absorb neutrons, so they won't work.
- There needs to be a sufficient supply. If the fissionable material is rare, it cannot be used.
- The fissionable atom must produce 2 neutrons when spit, or a reaction cannot be maintained.
- The fissionable atoms must have a long half-life, and not decay to some other element while in the reactor.
To understand the difference between fissionable and fissile, we need to understand about neutron temperature. When a neutron is ejected from a split atom, it has enormous energy, about 2 Million Electron Volts (or 2 MeV).
Physically, this is the speed of the neutron. A 2 MeV neutron is traveling at about 45 million miles/hr. Neutrons moving this fast don't often linger to induce fission in other atoms. Statistically they are likely to leave the reactor core before an interaction occurs. For this reason it is desirable to slow them down. Neutrons are slowed by allowing them to bounce off lighter atoms and transfer their kinetic energy to the atoms. This process is called "moderation", and slows the neutrons down to what is called "thermal" energy, .025 eV, or about 4900 miles/hr. Depending on the mass of the moderating material, it may take from 10-2500 impacts to reduce the speed of a fast neutron from fission to thermal equillibrium.
Below is a table showing neutron energy (or straight line speed) of a neutron with its description.
| Neutron energy | Energy range |
|---|---|
| 0.0 eV-0.025 eV | Cold neutrons |
| 0.025 eV | Thermal neutrons |
| 0.025 eV-0.4 eV | Epithermal neutrons |
| 0.4 eV-0.6 eV | Cadmium neutrons |
| 0.6 eV-1 eV | EpiCadmium neutrons |
| 1 eV-10 eV | Slow neutrons |
| 10 eV-300 eV | Resonance neutrons |
| 300 eV-1 MeV | Intermediate neutrons |
| 1 MeV-20 MeV | Fast neutrons |
| > 20 MeV | Relativistic neutrons |
It is at thermal energy, speed, or temperature (pick one), that neutrons will more readily be absorbed by a nucleus and lead to another fission.
Now that we understand the process of neutron moderation, we can return to our need for Fissile atoms. Fissile atoms are a small subset of fissionable atoms, that will readily split after absorbing a thermal neutron. The list of candidates of fissile materials is small. Uranium 233 and 235, Plutonium 239, and 241. Only Uranium 235 is naturally occuring, while the others must be produced by excess neutrons from other materials inside a breeder reactor.
In another post I will describe other types of reactors, but for now we will stay with the basic thermal reactor, which is fueled with 5% U-235, both cooled and moderated by normal (light) water.
When an atom splits, it creates two highly radioactive atoms that must be contained, and not allowed to get into the coolant/moderator. Otherwise that water will become very contaminated with radwaste. For this reason, the uranium fuel is surrounded by zircaloy cladding. Below is a photo of people inspecting new fuel assemblies for defects in the cladding.
The energy released by splitting or fission of a single U-235 atom is about 200 MeV, most of which is in the form of kinetic energy of the two new atoms. They are moving at high speed, but due to their mass and charge, they quickly come to rest among other atoms of fuel. The energy these atom fragments lose as they come to rest is converted to heat.
Back to the reactor core. Water surrounds an array of these fuel assemblies, which is called the "core". It takes a certain amount of fissile U-235 packed closely together, and moderated neurtons for a continuous reaction to take place. Below is a top-down view of a generic reactor core.
The reaction is started by withdrawing control rods. Control rods contain strong absorbers of thermal neutrons. These neutron absorbers are also known as "poisons", in that they slow or stop the nuclear fission process. Typically a control rod contains cadmium, boron (or a borated alloy) for neutron control.
"Keff" is the number reactor physicists use to determine whether the reactor core is shutdown, steady-state, or starting up. It's the neutron multiplier. Keff is the likelihood that a single neutron will make it from birth to cause a fission in another Uranium atom. Some of the other possibilities are that a neutron will escape from the reactor core, be absorbed in some material other than Uranium, or be absorbed by a Uranium atom, but not cause a fission.
If Keff is < 1.0 the reactor is "subcritical". If Keff is = 1.0 the reactor is "critical", and if the Keff is >1.0 the reactor is "supercritical". Nothing in any of these terms should be fear-inducing, they just describe a core reactivity condition for sustaining (or changing) the neutron population.
So... what happens when the control rods are removed? I will skip the mathematics this time and give a simple description. Neutrons are always present in U-235, because one of the natural decay modes for U-235 is spontaneous fission. However reactors contain a strong artificial neutron source to ensure there are plenty of neutrons to get things going. Think of the installed neutron source as sort of a pilot light for a gas furnace.
When the control rods are removed partially, neutron population begins to increase for two reasons. The neutrons are no longer being absorbed in the poison, and so they begin splitting U-235 atoms, which generates even more neutrons. The neutron population increases, even if Keff is less than 1, for the above reasons.
Eventually though, if the control rods have been removed far enough, neutrons from new fissions will equal the losses, and Keff will be 1. The reactor will then be critical. However it won't be generating any heat. Reactors typically are critical at an output of about 5 watts. About the power required by a single "night light" bulb. Power must be increased by a factor of 10^8 to get to 500 Megawatts.
After the reactor is critical, the neutron population must increase a great deal before any noticeable heat is generated. So the rods are pulled out a little more, and each generation of neutrons has a few million more than the previous generation. With this small increase in reactivity, there is no further need to withdraw the control rods. Neutron multiplication will increase on its own.
Eventually the reactor reaches a point at which heat is generated, and this is where things get interesting. The moderator (cooling water surrounding the fuel assemblies) heats up, and becomes less dense. It is therefore less effective at slowing down neutrons.
Because the moderator is less effective as temperature increases, more neutrons escape the core, which reduces neutron population, and reactor power stops rising. We therefore have a heat-induced dampening effect on reactor power. Its technical term is "negative temperature coefficient of reactivity". Basically, more heating tends to reduce reactor power. This is a desirable thing, because it prevents a runaway nuclear reaction. Unfortunately not all reactors have this characteristic :(
So far, so good. We have a critical nuclear reactor (Keff = 1.0) which is adding heat to the coolant!
I think that now is a good place to stop and think about the next post. My brain hurts :)
Part 2 is here.
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