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 neutron. Show all posts
Showing posts with label neutron. Show all posts
Saturday, February 01, 2014
K-431 (Project 675)
K-431 was an Echo II class submarine. Construction was started in January 1964, and she was commissioned in September of 1965, in a very rapid construction process. The Echo II class was a twin reactor ship which carried cruise missiles for attacking surface ships. Not very pretty.
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.
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.
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