A consortium led by France’s EDF Energy, including Chinese investors, has agreed with the government of the UK on terms for building a pair of new nuclear reactors at Hinkley Point in the southwest of the country, not far from Bristol. If a final investment decision is made some time next year, and the plants are actually built, they will probably be big (about 1600 Megawatts) pressurized water reactors (PWR’s) based on the French company Areva’s EPR design. These are supposed to be (and probably are) safer, more efficient, and more environmentally friendly than earlier designs. In general, I tend to be pro-nuclear. I would certainly feel a lot safer living next to a nuclear plant than a coal plant. However, I’m a bit ambivalent about these new starts. I think we could be a lot smarter in the way we implement nuclear power programs.
Reactors of the type proposed will burn uranium. Natural uranium consists mostly of two isotopes, U235 and U238, and only U235 can be burnt directly in a nuclear reactor. Why? The answer to that question depends on something called “the binding energy of the last neutron.” Think of a neutron as a bowling ball, and the nucleus of a uranium atom as a deep well. If the bowling ball happens to roll into the well, it will drop over the edge, eventually smacking into the bottom, and releasing the energy it acquired due to the acceleration of gravity in the process. The analogous force in the nucleus of a uranium atom is the nuclear force, incomparably greater than the force of gravity, but it acts in much the same way. The neutron doesn’t notice this very short range force until it gets very close to the nucleus, or “lip of the well,” but when it does, it “falls in” and releases the energy acquired in the process in much the same way. This energy is what I’ve referred to above as “the binding energy of the last neutron.”
When this binding energy is released in the nucleus, it causes it to wiggle and vibrate, something like a big drop of water falling through the air. In the case of U235, the energy is sufficient to cause this “liquid drop” to actually break in two, or “fission.” Such isotopes are referred to as “fissile.” In U238, the binding energy of the last neutron alone is not sufficient to cause fission, but the isotope can still actually fission if the neutron happens to be moving very fast when it hits the nucleus, bringing some of its own energy to the mix. Such isotopes, while not “fissile,” are referred to as “fissionable.” Unfortunately, the isotope U235 is only 0.7 percent of natural uranium. Once it’s burnt, the remaining U238 is no longer useful for starting a nuclear chain reaction on its own.
That would be the end of the story as far as conventional reactors are concerned, except for the fact that something interesting happens to the U238 when it absorbs a neutron. As mentioned above, it doesn’t fission unless the neutron is going very fast to begin with. Instead, with the extra neutron, it becomes U239. However, U239 is unstable, and decays into neptunium 239, which further decays into plutonium 239, or Pu239. In Pu239 the binding energy of the last neutron IS enough to cause it to fission. Thus, conventional reactors burn not only U235, but also some of the Pu239 that is produced in this way. Unfortunately, they don’t produce enough extra plutonium to keep the reactor going, so only a few percent of the U238 is “burnt” in addition to the U235 before the fuel has to be replaced and the old fuel either reprocessed or stored as radioactive waste. Even though a lot of energy is locked up in the remaining U238, it is usually just discarded or used in such applications as the production of heavy armor or armor piercing munitions. In other words, the process is something like throwing a log on your fireplace, then fishing it out and throwing it away when only a small fraction of it has been burnt.
Can anything be done about it? It turns out that it can. The key is neutrons. They not only cause the U235 and Pu239 to fission, but also produce Pu239 via absorption in U238. What if there were more of them around? If there were enough, then enough new Pu239 could be produced to replace the U235 and old Pu239 lost to fission, and a much greater fraction of the U238 could be converted into useful energy. A much bigger piece of the “log” could be burnt.
As a matter of fact, what I’ve described has actually been done, in so-called breeder reactors. To answer the question “How?” it’s necessary to understand where all those neutrons come from to begin with. In fact, they come from the fission process itself. When an atom of uranium or plutonium fissions, it releases an average of something between 2 and 3 neutrons in the process. These, in turn, can cause other fissions, keeping the nuclear chain reaction going. The chances that they actually will cause another fission depends, among other things, on how fast they are going. In general, the slower the neutron, the greater the probability that it will cause another fission. For that reason, the neutrons in nuclear reactors are usually “moderated” to slower speeds by allowing them to collide with lighter elements, such as hydrogen. Think of billiard balls. If one of them hits another straight on, it will stop, transferring its energy to the second ball. Much the same thing happens in neutron “moderation.”
However, more neutrons will be produced in each fission if the neutrons aren’t heavily moderated, but remain “fast.” In fact, enough can be produced, not only to keep the chain reaction going, but to convert more U238 into useful fuel via neutron absorption than is consumed. That is the principle of the so-called fast breeder reactor. Another way to do the same thing is to replace the U238 with the more plentiful naturally occurring element thorium 232. When it absorbs a neutron, it eventually decays into U233, which, like U235, is fissile. There are actually many potential advantages to this thorium breeding cycle, such as potentially greater resistance to nuclear weapons proliferation, the ability to run the process at slower average neutron speeds, allowing smaller reactor size and easier control, less production of dangerous, long-lived transuranic actinides, such as plutonium and americium, etc. In fact, if enough neutrons are flying around, they will fission and eliminate these actinides. It turns out that’s very important, because they’re the nastiest components of nuclear waste. If they could be recycled and burned, the amount of residual radiation from the waste produced by operating a nuclear plant for 30 or 40 years could be reduced to a level below that of the original uranium or thorium ore in a matter of only a few hundred years, rather than the many thousands that would otherwise be necessary.
So breeders can use almost all the potential energy in uranium or thorium instead of just a small fraction, while at the same time minimizing problems with radioactive waste. What’s not to like? Why aren’t we doing this? The answer is profit. As things now stand, power from breeder reactors of the type I’ve just described would be significantly more expensive than that from conventional reactors like EPR. EPR’s would use enriched natural uranium, which is still relatively cheap and plentiful. They would require no expensive reprocessing step. Ask an industry spokesman, and they will generally assure you (and quite possibly believe themselves, because self-interest has always had a strong delusional effect) that we will never run out of natural uranium, that the radioactive danger from conventional reactor waste has been grossly exaggerated, and there is no long-term proliferation danger from simply discarding plutonium-laced waste somewhere and letting it decay for several thousand years. I’m not so sure.
Now, I have no problem with profit, and I find Hollywood’s obsession with the evils of large corporations tiresome, but I really do think this is one area in which government might actually do something useful. It might involve some mix of increased investment in research and development of advanced reactor technology, including the building of small demonstration reactors, continued robust support for the nuclear Navy, and eliminating subsidies on new conventional reactors. Somehow, we managed to build scores of research reactors back in the 50’s, 60’s and 70’s. It would be nice if we could continue building a few more now and then, not only for research into breeder technology, but as test beds for new corrosion and radiation resistant materials and fuels, exploration of high temperature gas-cooled reactors that could not only produce electricity but facilitate the production of hydrogen from water and synthetic natural gas from carbon dioxide and coal, both processes that are potentially much more efficient at high temperatures, and even fusion-fission hybrids if we can ever get fusion to work.
We aren’t going to run out of energy any time soon, but there are now over 7 billion people on the planet. Eventually we will run out of fossil fuels, and depending entirely on wind, solar and other renewables to take up the slack seems a little risky to me. Wasting potential fuel for the reactors of the future doesn’t seem like such a good idea either. Under the circumstances, keeping breeder technology on the table as a viable alternative doesn’t seem like a bad idea.