I want to build off of the previous post by discussing some key elements of nuclear energy related to one type of radiation: neutron radiation. Neutron radiation is the key type of radiation that makes nuclear energy possible, and it's good to know how. I'm then going to discuss two ways in which this understanding impacts the safety of nuclear power plants, and I will end with a brief discursion into some problems with theoretical future nuclear fusion plants.
Fissile Fuel
What it is
I said earlier that neutron radiation is able to penetrate into materials because it ignores the electron cloud around atoms; it only stops if it hits an atom's nucleus, and when it does it can either bounce off, be absorbed into the nucleus (thus changing the atom to be another element one higher on the periodic table), or split the nucleus up--nuclear fission. I also said that neutron radiation is created by the breakup of nuclei into components, which releases a mix of nuclear output including energetic neutrons.
So you can see that neutron radiation both causes fission and is caused by fission. This is the key fact that makes nuclear energy possible, because if you balance things just right, you can create a self sustaining reaction, where your fuel is undergoing fission continuously, as some atoms break apart, releasing neutrons which cause neighboring atoms to break apart, and so on and so forth.
Conceptually, creating a self-sustaining nuclear fission reaction is very simple. All you have to do is find enough naturally occurring unstable material that spontaneously decays at a rapid enough rate and bring it physically together. The physical proximity of unstable material in a dense enough arrangement is enough to fire off a self-sustaining reaction.
How it's made
Originally
Getting this material is difficult because it doesn't exist in dense enough arrangements in nature for a self-sustaining reaction to occur--which should be obvious, if you think about it, because if such a reaction *did* occur in nature, it would burn itself out in short order and cease to exist.
In order to get enough material that can cause a self-sustaining reaction, you have to take advantage of the fact that unstable isotopes of Uranium weight slightly more than stable isotopes of Uranium--because they have more neutrons per atom. So, theoretically, it's quite simple to separate unstable Uranium from stable: just dissolve mined Uranium and put it in a high-speed centrifuge. The heavier isotopes of Uranium will settle to the bottom of the result if you do this fast enough and for long enough.
In practice, this is much more easily said than done, and the engineering know-how in order to do this sort of refinement properly is a critical "controlled" secret that we try to keep from being common knowledge. So if you hear about negotiations with Iran or some other state that has ambitions to become a nuclear power and you hear about "centrifuges", this is what is being discussed.
An important distinction to keep in mind here is that the density of neutron active material that is required to run a nuclear power plant is not as great as the density required to get fuel that will explode in a nuclear bomb. Getting the explosive chain reaction is a step above the difficulty of getting a self-sustaining chain reaction. However, the technological step you need to get weapons-grade material is *not that high* above the step you need to get power-plant grade material.
In a power plant
Another way in which fissile fuel can be manufactured, however, is in a nuclear power plant (if designed just right). Because these things generate constant neutron radiation and because atoms hit with a neutron sometimes absorb the neutron rather than splitting, nuclear power plants can be configured in such a way as to generate unstable isotopes that can be used for nuclear fuel (or nuclear weapons). Such things are called "breeder plants", and they do have to be specially designed to work in this way: you can't just take any old nuclear power plant and roast your Uranium over the nuclear fire and get weapon's grade Plutonium out.
However, it must be said that from a distance, it is difficult to tell the difference between a breeder plant that is creating more fuel for nuclear power and one that is creating material suitable for a nuclear bomb. Hence, there is a lot of concern when a state with nuclear ambitions says they are just looking to get into nuclear power plants. I'll be getting into this in more detail later, but for now you have to realize that this is a concern.
Some safety considerations based on the nature of fissile fuel
Reactor core composition and types of core failures
Nuclear cores are made so that their effective neutron density--that is, how much fissile material is being exposed to neutrons at any given moment and hence how much fission is happening at that moment--is controllable. This happens in several key ways (warning: painful oversimplification follows!):
- The physical configuration of the core. Most nuclear cores have control rods which are made of a material that is called a moderator, which is a material that has the property of impeding neutron radiation. In a typical configuration, the rods are arranged over the core, which has holes to receive the rods. If the rods are completely removed, the core has enough reactive material density to "go critical" and have a self-sustaining reaction. If the rods are inserted, however, the reaction slows down. If they are inserted all the way, the reaction is no longer self sustaining and will die off. You can control the power output of the reactor core by controlling the height of the control rods.
- Liquid coolant / moderator. In addition to the control rods, most reactor designs also have a liquid coolant constantly flowing around the core. This serves, at the minimum, the purpose of cooling down the core so that it doesn't physically melt, as well as taking away the heat energy to be turned into energy. Depending on the design, this liquid will often also play some moderating role on the reaction.
Every core meltdown, therefore, depends on something disturbing the necessary balance between reactive material density, presence of moderating material, and coolant flowing through the core. So, for example, if for some reason you have a failure in the mechanism that injects the control rods into the core, the reaction can be stuck in "on" mode and start heating up out of control.
Active vs. Passive
When evaluating the possible outcomes of a failure with a nuclear core, you have to look at the possible ways in which the mechanisms that maintain this balance can fail. The key distinction here is, are these mechanisms active (meaning, some process needs to happen to in order for the moderating influence to be applied) or passive (meaning, something automatically happens to trigger moderation when needed)?
Passive is always to be preferred to active, when possible.
Passive is always to be preferred to active, when possible.
Uncontrolled Reaction Spike vs. Decay Heat Meltdown
Different reactor types have different safety profiles because of having different active vs. passive safety mechanisms protecting against different things. Most people lump together all "meltdown" events they hear about into the same category, but there are massive differences.
The nuclear power plant in Chernobyl was of a type called "graphite moderated", and it did not really have any passive safety mechanisms at all. In particular, there was not a good way to keep the core from undergoing uncontrolled cascading reaction without the rods in specific configurations. There was a portion of the range of motion through which the rods went which actually spiked the reactivity of the core by displacing moderating water. This was known and procedures for avoiding this spike had been figured out in another plant, but the procedures had not been transferred to Chernobyl. This failure compounded with others led to an uncontrolled reaction spike, or (simply put) an explosion. The nuclear reaction got so out of control at a particular spot in the core that it actually blew itself up.
Other reactor designs do not have this same instability. Pressurized Water Reactors (PWR) and Boiling Water Reactors (BWR) both have a passive safety feature that prevents this sort of extreme power excursion. For these reactors, the water moderators are essential to the operation of the core. If the temperature spikes, the water develops steam cavitation bubbles immediately, which increases the moderating effects of the water and dampens the reaction. This happens via physics and not human procedures. Consequently, these reactor designs are quite unlikely to explode due to some core malfunction.
On the other hand, this does not remove the possibility of a meltdown. The reason for this is that even after the self-sustaining nuclear reaction has been halted by the insertion of control rods, the nuclear core is still extremely hot. This is called the decay heat of the reactor--the heat it generates as it gradually cools off. If you want to prevent the core from melting down, you must find a way for it to deal with *this* heat even after the core has been "shut down". In older reactors, even after a core has been shut down it must be *actively* cooled for a substantial time before the reactor is cool enough to be safe.
If a reactor looses cooling for long enough, even if it is "shutdown", it can melt and begin to damage the containment vessel in which it is held. This is how radioactive material was released both in the PWR Three Mile Island power plant, and in the BWR Fukushima disaster. In the case of Three Mile Island, coolant was lost for too long (due to a comedy of errors and accidents), leading to the reactor automatically "tripping" and shutting down the core. Coolant was *still* not being supplied after the automatic shutdown, however, so the core began to melt through the first containment vessel, causing hydrogen explosions and the release of small amounts of radioactive gases.
In the case of Fukushima, the reactors were shut down on purpose as standard practice because of the earthquake. However, when the tsunami hit, all of the generators powering the coolant pumps became submerged and stopped functioning. The shut-down reactors therefore melted, one of which even damaging its containment vessel sufficiently to cause some leakage of radioactive material dissolved in water.
The difference between Chernobyl and the two other accidents was thus the difference between a core explosion and a core meltdown. This made a dramatic difference in the amount, type, and dispersion of radioactive material released.
The similarity in all three of those incidents, however, was that in each case it was active systems that failed to maintain reactor safety: active moderation of the core in the case of Chernobyl and active cooling of a shutdown core in the case of Three Mile Island and Fukushima. Consequently, the focus of nuclear reactor design for a while has been to design systems that are passively safe throughout. I will go into more detail on this in a future post.
For the sake of current events, we should know that the nuclear power plant recently seized by the Russians in Zaporizhzhia is a PWR plant, and consequently has passive protection against runaway reactor scenarios. Therefore, this plant is unlikely to experience an explosive core failure of the type that Chernobyl did, even in the case of complete loss of coolant due to equipment destruction. However, a meltdown and some leakage of radiation is certainly possible.
Spent fuel rods
One more safety consideration presents itself based on what I have just discussed. That is, even after reactor fuel is no longer being used to perpetuate a fission reaction, it is still quite radioactive with the remaining "decay heat". This implies that after fuel can no longer be used in a reactor, it cannot be instantly disposed of. In fact, it takes about three years before spent fuel can cool down sufficiently to be transported for disposal.
How nuclear plants deal with this is something called "Spent Fuel Pools". The spent fuel is just stored in big pools on-site for long enough for the decay heat to die down.
The safety consideration here is that most functional nuclear power plants have radioactive material just sitting around on-site, cooling off in pools. The problem is made worse by the fact that the original intention for all of this spent fuel was to transport it to long-term storage facilities. However, for many decades now, people have actively resisted the creation of these facilities in their own backyards. Constant legal battles and political unpopularity have dramatically limited the construction of suitable long-term storage for spent fuel. This means that many nuclear power plants are storing spent fuel on-site for years and years longer than anyone originally anticipated.
The safety implications are fairly large. There isn't a lot of potential for this spent fuel to accidentally leak, though that was a concern at Fukushima because of the tsunami. However, the potential for misuse by terrorists or bad state actors is quite high. This material is perfect for the creation of a "dirty bomb", which is just radioactive material that you pack with conventional explosives in order to create radioactive fallout without the nuclear explosion.
Again for the sake of current events, the Zaporizhzhia nuclear power plant has six cooling pools with hundreds of tons of spent fuel of varying degrees of radioactivity, now currently controlled by the invading Russian army.
A quick discursion on fusion power
This is a bit out-of-the way, but since we are talking about neutron radiation and nuclear safety, I'm going to take the opportunity here to briefly discuss nuclear fusion. There has been some news recently about new milestones achieved in energy-positive fusion reactions that *might* cause some people to become hopeful that fusion is coming as a clean alternative to nuclear fission before too long.
I am here to dash those hopes now.
I am here to dash those hopes now.
Long-term, I hope that fusion does eventually come through. However, recent small milestones notwithstanding, there are more barriers for fusion to overcome than most people realize. One key barrier has to do with neutron radiation.
Fusion is called "clean" with respect to fission because it does not rely on actinides--the heavy, radioactive metals that are the fuel for nuclear fission. Instead it fuses isotopes of lighter elements into heavier elements. Although this process does not use or create the same nasty materials that fission uses and/or creates, it does create neutrons: a lot of neutrons. In fact, a fusion power plant will produce more neutron radiation, and higher energy neutron radiation than a comparable fission power plant.
As we learned earlier, this *will* make things exposed to it also radioactive. But we also learned that this type of radioactivity is temporary and not of the same long-term danger as fission by-products. So is the neutron radiation produced by a fusion plant a problem? Yes, for at least two reasons.
Cost
A fission reaction is comparably simple to do--all you need is a lump of the right material at the right density, and the reaction will create itself. Consequently, you have a lot of options for how you can design a fission core and still have it work--you can shape it all sorts of ways, dunk it in all sorts of moderating fluids, etc.
Fusion, on the other hand, is incredibly difficult to achieve. To achieve fusion, you need incredibly precise conditions in an incredibly controlled magnetic field and hard vacuum--no moderating liquids allowed in there! The world's leading fusion reactor, ITER, is by some counts the most expensive scientific instrument ever created. Should it ever achieve constant operation, all of that expensive equipment would fairly quickly fry from the radiation that it is creating. (That's why it's not ITER, but it's planned successor that might one day run continuously.)
The problem of continuous operation of fusion has not *at all* been solved--really, fusion scientists would just be happy if they were in the position to be able to worry about that problem, since only tiny bursts of continuous power output have yet been achieved. This is a huge problem, and all proposals to deal with the problem so far have been enormously complex and expensive. Getting fusion *practical* might be a further 30-year problem after we manage to make it merely energy positive.
Proliferation
The other issue with plants that generate neutron radiation is that neutron radiation is what is necessary to breed nuclear fuel. Since fusion plants will generate neutron radiation in abundance, does this mean they could also be used to create weapons-grade Plutonium out of mined Uranium?
Yes. It absolutely does. Fusion plants will be able to work as breeder plants *very* well.
This means that although fusion plants will not need any of the same materials that are used in nuclear weapons as do fission plants, they nonetheless represent a nuclear weapons proliferation challenge.
The bottom-line for fusion is that you can pin no hopes for it to be a viable part of the world's energy mix any time soon, and even when or if it does become viable, we will still need to worry about it enabling nuclear weaponry.
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