What d’ya say – want to design a nuclear reactor? And why not? It’s a slow day, and what else are we going to do?
“You’re going to learn how to heat water about ten degrees.”
When you get down to brass tacks, a nuclear reactor is just a way to heat water – in fact, on my first day at the Navy’s Nuclear Power School, our first instructor came in and said, “Gentlemen (there were no women in the program at the time), over the next six months you’re going to learn how to heat water about ten degrees.” I was underwhelmed – until I learned how much water we would be heating. In any event, whether one is using a boiler, geothermal energy, or almost any other form of energy, power plants work largely the same way – energy is used to heat water to the boiling point, the steam that issues forth spins a turbine attached to a generator that produces electricity, and then the steam is condensed back into water and sent back to the heat source to be heated up and boiled again. The only difference between a nuclear power plant and one powered by fossil fuels is the source of the heat – nuclear fission instead of flame.
On to our drawing board.
In a nuclear reactor, the energy comes from splitting atoms. Each fission creates two radioactive atoms – U-238, for example, might create Cs-137 and Tc-99 plus a few neutrons. Typically these fission fragments are safely contained within the fuel matrix or “rods,” – which means that one of our goals in designing a reactor ought to be keeping temperatures low enough to keep the fuel matrix from melting. For that, we need a coolant.
Luckily, all sorts of materials can carry heat away from the core, one of which is moderately effective, incredibly common, and not too expensive – water. Water isn’t perfect, but it’s good enough to serve as a coolant. And – even better – it slows down (moderates)
, neutrons to make them more effective at causing fission. With water, we get a twofer – both coolant and moderator.
An important concept – moderation
Moderation in a nuclear reactor does not mean quite the same thing we might have heard from our parents.
When uranium fissions, it produces more than just two fission fragments and some gamma rays – it also produces 2-3 neutrons, and those neutrons are moving at quite a clip. Unfortunately, fast neutrons aren’t very effective at causing fission because they’re moving too quickly to be drawn into the nucleus by one of the fundamental forces in nature, the strong nuclear force. A fast neutron will just zip right by a uranium atom without interacting – the same way you might zip past a small town if you’re on the highway. A slow, moderated neutron will sort of mosey on by and has a much better chance of being sucked in and persuaded to stay. The addition of that neutron upsets the delicate balance of energies in the nucleus and causes fission. This series, a fission producing neutrons that produce more fissions, is a nuclear chain reaction, and in a reactor, it’s made possible by neutron moderation. So, we need a moderator….
One of the best ways to slow a neutron is to bounce it off of something that’s about the same mass. Consider a pool table. The table itself weighs far more than the cue ball; that’s why the cue ball bounces off a rail with about the same energy that it hits with. However, strike another ball, and the cue ball loses energy because it transfers some of its energy to the ball that it hit. Similarly, if we bounce a neutron off a hydrogen atom (of which each molecule of water contains two), it will lose energy and slow down. Any material that contains a lot of hydrogen – plastic, water, even tapioca pudding – will make a good moderator. Water is cheap, common, and liquid which makes it preferable to plastic or tapioca.
Water is actually not quite as good at transferring heat as is, say, molten metal, and it’s not as good a moderator as is heavy water (water in which some or all of the hydrogen has been replaced by slightly heavier deuterium) or carbon. But it’s good enough in both of these areas, and the fact that it can do double duty makes it valuable.
Water has an added “safety feature.” If water leaks, we lose our coolant, and core temperature rises, but we also lose our moderator, and less energy and heat are being produced. In other words, the reactor will start to shut down as water levels drop in the core – an advantage to water that other moderators and coolants don’t have. Taking water out of the core is a needlessly dangerous way to control reactor power – it’s better to keep the core covered while controlling power some other way.
Since neutrons are causing the fissions, the best way to control power is to find a way to absorb neutrons before a uranium atom can absorb them – we do that by lowering rods made of a material that absorbs neutrons into the core. As we insert these control rods into the core, they soak up the neutrons; with fewer neutrons flying about, there are fewer fissions, and reactor power begins to drop. And to make shutting down as fast as possible, we’d likely want to design the control rods to absorb neutrons along their entire length so that even partially inserting them would reduce reactor power.
Many other things go into designing a safe nuclear reactor, but we have to assume that there will be an accident at some point. We need to try to find a way to protect the environment if something catastrophic happens and protect the reactor from intrusion or attack from without – something like wrapping the reactor with a meter or two of reinforced concrete. This serves not only to keep attackers out but also to contain the highly radioactive fission products if something goes terribly wrong.
These components, water as moderator and coolant, neutron-absorbing control rods, and a containment structure, are common to virtually all commercial nuclear reactors globally. With one notable exception, the handful of reactors developed in the former Soviet Union that were also intended to produce plutonium for nuclear weapons. One reactor of that sort was the Chernobyl reactor. So, let’s compare the Chernobyl reactor with Western-style reactors in these three areas and see if the differences can help to explain why Chernobyl suffered such a catastrophe and why other reactor designs won’t. We’ll do that in the second part of this article – so stay tuned!