Let’s build a nuclear reactor!

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“You are going to learn how to heat water to about ten degrees.”

When you get to brass tacks, a nuclear reactor is just a way to heat water – in fact, on my first day at the Navy School of Nuclear Power, our first instructor was walked in and said, “Gentlemen (there were no women in the program at the time), over the next six months you are going to learn how to heat water to about ten degrees. I was disappointed – until I found out how much the water we would heat. Either way, whether one uses a boiler, geothermal energy, or almost any other form of energy, power plants work in much the same way – the energy is used to heat water up to the point. At the boiling point, the emitting steam spins an attached turbine. to a generator that produces electricity, then the steam is condensed into water and returned to the heat source to be heated and boiled again. The only difference between a nuclear power plant and a power plant powered by fossil fuels is the source of heat – nuclear fission instead of flame.

On our drawing board.

In a nuclear reactor, energy comes from the division of atoms. Each fission creates two radioactive atoms – U-238, for example, could 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 should be to maintain temperatures low enough to prevent the fuel matrix to melt. For this we need a coolant.

Fortunately, there are all kinds of materials that can remove heat from the core, one of which is moderately efficient, incredibly common, and not too expensive – water. The 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 efficient at causing fission. With water we have two – both coolant and moderator.

An important concept – moderation

Moderation in a nuclear reactor doesn’t mean quite the same thing we might have heard from our parents.

When fissioning uranium, it produces more than two fission fragments and a few gamma rays – it also produces 2-3 neutrons, and these neutrons move at a fairly close rate. Unfortunately, fast neutrons are not very good at causing fission because they move too quickly to be drawn into the nucleus by one of the fundamental forces of nature, the strong nuclear force. A fast neutron will pass right by a uranium atom without interacting – the same way you might fly over a small town if you are on the highway. A slow, moderate neutron will sort of be mosey and has a much better chance of being sucked in and persuaded to stay. The addition of this neutron disrupts 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 is made possible by neutron moderation. So we need a moderator….

One of the best ways to slow down a neutron is to bounce it off something that has roughly the same mass. Consider a pool table. The table itself weighs much more than the cue ball; this is why the cue ball bounces off a rail with about the same energy with which it hits. However, hit another ball and the cue ball loses energy as it transfers some of its energy to the ball it hit. Likewise, if we bounce a neutron off a hydrogen atom (of which each water molecule contains two), it will lose energy and slow down. Any material that has a lot of hydrogen – plastic, water, even tapioca pudding – will make a good moderator. Water is cheap, running, and liquid, which makes it preferable to plastic or tapioca.

Water is actually not as efficient at transferring heat as molten metal, for example, and it is not as good a moderator as heavy water (water in which some or all of the hydrogen was replaced by slightly heavier deuterium) or carbon. But it’s pretty good at both of those things, and the fact that it can do double duty makes it valuable.

The water has an additional “safety device”. If water leaks, we lose our coolant and the core temperature rises, but we also lose our moderator, and less energy and heat are produced. In other words, the reactor will start shutting down when water levels in the core drop – a water benefit that other moderators and coolers don’t have. Removing water from the core is an unnecessarily dangerous way of controlling reactor power – it’s best to keep the core covered while controlling the power in some other way.

Since neutrons are the source of fissions, the best way to control power is to find a way to absorb neutrons before a uranium atom can absorb them – we do this by lowering bars made of a material that absorbs neutrons in the nucleus. When we insert these control bars into the heart, they absorb neutrons; with fewer neutrons flying, there are fewer fissions and the reactor power begins to drop. And to make shutdown as quick as possible, we would probably want to design the control rods to absorb neutrons along their entire length so that even their partial insertion reduces reactor power.

Containment

A lot of other things go into the design of a safe nuclear reactor, but we have to assume that there will be an accident at some point. We have to try to find a way to protect the environment if something catastrophic happens and to protect the reactor from intrusion or attack from the outside – something like wrapping the reactor with a yard or two of reinforced concrete. This not only serves to keep attackers out, but also to contain the highly radioactive fission products if something is wrong at all.

These components, water as a moderator and coolant, neutron absorbing control rods and a containment structure, are common to virtually all commercial nuclear reactors around the world. With one notable exception, the handful of reactors developed in the former Soviet Union were also intended to produce plutonium for nuclear weapons. One such reactor 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 explain why Chernobyl suffered such a disaster and why other reactor designs won’t. We’ll do that in Part 2 of this article – so stay tuned!



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