Nuclear Power: The Reactor

Jun 2nd, 2008 | By | Category: Nukes

The goal? A controlled fissioning of large nuclei. You’ll need fuel, moderation, coolant, and some control.

Coolant is the easiest to grasp. The fission chain reaction in the nuclear reactor will produce heat. It’s the goal! We need some way to transfer the heat away from the reactor and put it to use. Water is about perfect for this task. Yes, the boiling point of water is pretty low at standard atmospheric pressure. No worries. Just make the reactor pressurized, raising the boiling point of the water, and you have the task done. Most reactors use two circuits, plus a heat exchanger. The high-pressure loop of water goes right into the nuclear reactor. The heat is exchanged to a low-pressure loop of water that boils to steam. The steam is then used to run a turbine, and then an electrical generator. Only the water from the first, high pressure loop, tends to get radioactive.

Ok, fuel. Specifically? An atom with large nuclei (a really big party), unstable enough to be shattered when hit by a neutron (joined by someone screaming away from another party that just broke up) that also releases some neutrons when it splits. The problem is, most of these atoms absorb more neutrons than they release when doing so. To build our chain-reaction, we need something that is always releasing more neutrons than it eats. A handful of elements fit the bill, most commonly Uranium and Plutonium–Uranium is about your best bet, for naturally-occurring atoms. The problem is, it comes in a few versions. U-238 is by far the most common, and it also releases the fewest neutrons when breaking apart; the rarer U-235 and U-234 release far more. Better nuclear fuels release more neutrons when they fission. The more neutrons the atom releases when breaking up, the better the chance one will hit another fuel atom.

Time to mix even more metaphors! We can think about our reactor as a giant pool table. The neutrons are our cue balls. The fuel nuclei are sets of pool balls we want to break—to fission. When our fuel fissions, the sets of balls split roughly into halves, with a few cue balls racing outwards. The problem is, the nuclei are so excited to split up, the neutrons are going way too fast. Just like a cue ball flying off the table before a successful break, the neutrons are gone from our reactor before we can do anything useful. We need some way to slow the neutrons down, keeping them around long enough to actually hit the next fuel nucleus and keep the chain going. What we need are a whole bunch of little rubber balls on the table, bouncing around, for the cue balls to bounce off of–bumper pool!

Enter the moderator—any atom or nucleus off which neutrons can bounce without getting stuck too often. Even if the neutrons—our cue balls—are moving way faster than the moderator, just by bouncing off of the moderator molecules so often, the speed of the two will be eventually matched.

Now our slowed-down neutrons will have a vastly increased chance of hitting another fuel nucleus before leaving our reactor. Ideal moderators are very slippery to neutrons, absorbing none. Regular old distilled water–comprised of two hydrogen atoms (basically a proton) attached to an oxygen–makes a decent moderator. The biggest problem with water as a moderator is all those lonely hydrogen nuclei, tending to stick to our neutrons before they can help fission the next fuel nucleus. Ah hah! If you enrich the water for its heavy variant—in which the hydrogen nuclei are already carrying a neutron, heavy water—things start going far better. Heavy water is an excellent moderator, as there are few spaces for a neutron to stick.

Hey, something nifty! Water is both a good coolant and moderator! No moderator, no chain reaction, right? So, if you use water as your coolant and moderator, your reactor has an intrinsic safety feature. If you lose coolant, you lose moderation and the chain reaction stops. We all live! Thus, almost all nuclear reactors in operation today use water as a coolant and moderator.

Controlling the reactor is all about absorbing up the neutrons before they can continue the chain reaction. If you can insert and remove rods made of some neutron absorbing material, you can control the chain reaction.

But what water, and what fuel? Here’s the trade-off: The better the moderator, the crappier your fuel can be. If you’re using a so-so moderator, your fuel must release many neutrons when fissioning, to account for those lost by sticking to your crappy moderator. Likewise, a really good moderator can make up for a crappy fuel, by keeping around more neutrons. The rarer versions of fuel (U-235 is better than the common U-238) or moderators (heavy water is better than regular water) tend to be the better. Building a reactor typically requires enriching for either the better moderator or the better fuel. With a good enough fuel, you don’t need a moderator at all.

The sensible, Canadian, choice is to enrich the water for the heavy variant–to use one of the best moderators available. Yes, it’s painful and expensive. But water is minimally radioactive (thanks to the very, very rare Tritium-containing super heavy water, where at least one hydrogen is a radioactive proton and two neutrons) and doesn’t explode when piled too high. With heavy water as a moderator, unenriched Uranium, mostly U-238, works just fine.

The American way is to enrich the Uranium for U-235. Yes, even unenriched Uranium is hideously radioactive. Yes, even unenriched Uranium will explode, if you pile up more than the critical mass at which an uncontrollable chain fission reaction will start. A chunk of Japan was nearly destroyed by this sort of accident. On the positive, regular old water is perfectly fine as a moderator and coolant for this sort of fuel. Most reactors around the world are of this design: Uranium enriched for U-235 with pressurized regular water as both a moderator and coolant. USA! USA!

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  7. Your last paragraph here is a little misleading. Yes, pressurized water reactors in the US use enriched uranium as the fuel. However that enrichment is only between 3% and 5% (natural U-238 has an enrichment of approximately 0.7%). The physical size of the reactor core acts as a moderator as the neutrons have such a distance to travel that the neutrons hit the moderator many more times than in a smaller reactor.

    There are reactor designs that use highly enriched uranium but these are not used for commercial power. They are used when very compact reactors are needed (or when getting heavy water to replenish water supply is impossible to get).

    Also, due to reactor design it is not possible for a nuclear explosion to occur. Very specific geometries and designs must be met for a nuclear explosion. Yet, this seems to be what you meant by “Yes, even unenriched Uranium will explode, if you pile up more than the critical mass at which an uncontrollable chain fission reaction will start.” An “uncontrollable chain fission reaction” is not necessarily a nuclear explosion. It takes much more than exceeding critical mass to result in a nuclear explosion. It may result in a steam explosion as the water is heated up to a pressure greater than the system can handle if heat is added too fast. This is called “prompt critical”. There have been a number of prompt criticality accidents, but none have ended up in a nuclear explosion as the heat from the reaction tends to disrupt the geometry and stopping the reaction naturally.

  8. no mention of neutron reflectors in this article.
    also natural water contains a mix of heavy and light, it is not ALL light.

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