"Do not try to seem wise to others. If you want to live a wise life, live it on your own terms and in your own eyes." - Epictetus
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 Chain Reaction. Show all posts
Showing posts with label Chain Reaction. Show all posts
Friday, May 07, 2021
Curio idea: CP-1
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
Boiling Water Reactors
Boiling Water Reactors (BWRs) are a fairly common design in electrical generation. The advantage a BWR has over a Pressurized Water Reactor (PWR) is mainly the up-front cost.
The primary coolant loop in a PWR is kept at about 2200 psig (155 bar) to prevent boiling in the core. This means the reactor vessel, pressurizer, and steam generators as well as all primary loop components, must be very robust. In addition, they must be made from (or lined with) corrosion resistant materials, due to the addition of boric acid for reactivity control.
With a Boiling Water reactor, the primary coolant loop is also the steam loop. There are no steam generators or pressurizer. The operating pressure is about half that of a PWR, and no boric acid is used. Thus a boiling water reactor can be built with less cost. A BWR also has higher thermal efficiency than a PWR.
Below is a diagram of a BWR vessel. While less expensive to build than a PWR, these are still pretty complex reactors.
Below is a simplified overall diagram of an entire BWR steam/water circuit.
A few things you will notice that are different from a pressurized water reactor:
.
The primary coolant loop in a PWR is kept at about 2200 psig (155 bar) to prevent boiling in the core. This means the reactor vessel, pressurizer, and steam generators as well as all primary loop components, must be very robust. In addition, they must be made from (or lined with) corrosion resistant materials, due to the addition of boric acid for reactivity control.
With a Boiling Water reactor, the primary coolant loop is also the steam loop. There are no steam generators or pressurizer. The operating pressure is about half that of a PWR, and no boric acid is used. Thus a boiling water reactor can be built with less cost. A BWR also has higher thermal efficiency than a PWR.
Below is a diagram of a BWR vessel. While less expensive to build than a PWR, these are still pretty complex reactors.
Below is a simplified overall diagram of an entire BWR steam/water circuit.
A few things you will notice that are different from a pressurized water reactor:
- The top of the reactor vessel is used as a steam drum, to separate water and steam.
- Because the top of the reactor is a steam drum, control rods are moved to the bottom.
- Inside the reactor vessel are circulation pumps (#4). More on these later.
- Steam directly from the reactor core flows through the entire steam system.
- Not shown in the above image are the feedwater and steam shutoff valves. In the event of a steam line rupture, it is important to keep water in the core, and also to prevent the release of radioactive steam outside of the containment structure.
- Note the shielding required around the entire steam system.
- The control rods and circulating pumps penetrate the bottom of the reactor vessel. Should a leak develop there, it can drain all the water from the reactor vessel. Since the fuel must be kept submerged in water (even for several years after use), this is a potential hazard.
- Water circulating in a power plant always contains minor levels of corrosion - flecks of iron from valves and pumps, and magnetite that falls from the inside of piping as temperatures expand and contract the pipe. These corrosion particles can be carried through the reactor core and absorb a neutron. They can become very radioactive, and then settle outside the reactor core. This material is called CRUD, and is only detectable with a radiation detector.
- In a PWR, CRUD is all contained within the primary loop and inside the containment structure. In a BWR, CRUD is everywhere within the steam system - the steam and drain lines, the turbine, the condenser, the condensate pumps, and feedwater pumps.
- There is an important nuclear reaction that occurs that involves the Oxygen in water molecules, H2O. Oxygen-16, when hit with a high energy neutron, can absorb the neutron and eject\ a proton. It then becomes radioactive Nitrogen-16. This is a nasty isotope of nitrogen, mainly because it decays by emitting a gamma of 7.37 MeV. That's a VERY hot gamma ray. Nitrogen-16 has a half-life of 7.1 seconds, so that means it will be emitting these hot gamma rays while it is passing through the steam turbine, condenser, and settling in the hotwell. Therefore massive shielding is required from all steam and feedwater systems during operation.
- Due to the short half-life of Nitrogen-16, it quickly decays away. Shortly after the reactor is shutdown, the only serious radiological hazard in the steam system is due to CRUD, which is composed of metals that have longer half-lives.
BWRs are a little more complex than PWRs from a reactor control standpoint. A BWR, by its very description, has significant in-core boiling - primarily in the upper portion. However a BWR, like the PWR, uses water as a moderator. When water changes to steam, its neutron moderating properties change drastically - 1/1000 drop in density equates to about 1/1000 drop in moderating ability.
With the phase change from water to steam there is an ongoing change in moderation and thermal neutron population, and therefore in the fission process. It is therefore important to monitor neutron levels (flux) at many more points than would be necessary in a BWR, to ensure even power distribution. Fuel burn-up (both axial and radial) is also difficult to calculate for the top end of a BWR, due to the dynamic two-phase activity in the core.
Interestingly, like the PWR, the BWR control rods are out of the core at full power, and reactivity is controlled by using the circulation pumps. When the pumps are run at high speed, boiling occurs higher up in the core, and so more moderator is present, and more neutrons are available to create fissions. Similarly, if reactor power needs to be reduced, the recirculation pumps can be slowed down, and boiling occurs lower in the core. More steam in the core means less neutrons will be thermalized, and so fewer neutrons will be available to cause fission.
Lastly, BWR cores have to be somewhat larger than PWR cores for a given output, and that is mainly due to the fact that the upper part of a BWR core isn't generating much power, because steam is a poor moderator.
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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