Tag: breeder reactors

Polyanna Pinker’s Power Profundities

Recently Steven Pinker, public intellectual and author of a “history” of the Blank Slate debacle that was largely a fairy tale but at least drew attention to the fact that it happened, has been dabbling in something entirely different. Inspired by the latest UN Jeremiad against climate change, he has embraced nuclear power. In a series of tweets, he has endorsed articles advocating expanded reliance on nuclear power, such as one that recently turned up at Huffpo cleverly entitled “If We’re Going To Save the Planet, We’ve Got To Use the Nuclear Option.” As things now stand, that would be a dangerous, wasteful, and generally ill-advised idea.

I say “as things now stand.” I’m certainly not opposed to nuclear power. I’m just opposed to the way it would be implemented if we suddenly decided to build a bevy of new nukes given current economic realities.  The new reactors would probably look like the AP1000 models recently abandoned in South Carolina. Such reactors would use only a fraction of the available energy in their nuclear fuel, and would produce far larger amounts of long-lived radioactive waste than necessary. They are, however, cheaper than alternatives that could avoid both problems using proven technologies. Given the small number of players capable of coming up with the capital necessary to build even these inferior reactors, there is little chance that more rational alternatives will be chosen until alternative sources of energy become a great deal more expensive, or government steps in to subsidize them. Until that happens, we are better off doing without new nuclear reactors.

As noted above, the reasons for this have to do with the efficient utilization of nuclear fuel, and the generation of radioactive waste.  In nature there is only one potential nuclear fuel – Uranium 235, or U235. U235 is “fissile,” meaning it may fission if it encounters a neutron no matter how slow that neutron happens to be traveling.  As a result, it can sustain a nuclear chain reaction, which is the source of nuclear energy. Unfortunately, natural uranium consists of only 0.7 percent U235. The rest is a heavier isotope – U238. U238 is “fissionable.” In other words, it will fission, but only if it is struck by a very energetic neutron. It cannot sustain a fission chain reaction by itself.  However, if U238 absorbs a neutron, it becomes the isotope U239, which quickly decays to neptunium 239, which, in turn, quickly decays to plutonium 239. Plutonium 239 is fissile. It follows that if all the U238 in natural uranium could be converted to Pu239 in this way, it could release vastly more energy than the tiny amount of U235 alone. This is not possible in conventional reactors such as the AP1000 mentioned above. A certain amount of plutonium is produced and burned in the fuel elements of such reactors, but the amount is very small compared to the amount of available U238. In addition, other transuranic elements, such as americium and curium, which are produced in such reactors, along with various isotopes of plutonium, would remain dangerously radioactive for thousands of years.

These problems could be avoided by building fast breeder reactors. In conventional reactors, neutrons are “thermalized” to low energies, where the probability that they will react with a fuel nucleus are greatly increased. The neutron spectrum in “fast” reactors is significantly hotter but, as a result, more neutrons are produced, on average, in each encounter. More neutrons means that more Pu239 can be produced without quenching the fission chain reaction.  It also means that the dangerous transuranic elements referred to above, as well as long lived fission products that are the source of the most long-lived and dangerous radioactive isotopes in nuclear waste, could be destroyed via fission or transmutation. As a result, the residual radioactivity resulting from running such a nuclear reactor for, say 30 years, would drop below that released into the environment by a coal plant of comparable size in 300 to 500 years, as opposed to the thousands of years it would take for conventional reactors. And, yes, radioactivity is released by coal plants, because coal contains several parts per million each of radioactive uranium and thorium.  Meanwhile, a far higher percentage of the U238 in natural uranium would be converted to Pu239, resulting in a far more efficient utilization of the fuel material.

An even better alternative might be molten salt reactors. In such reactors, the critical mass would be in liquid form, and would include thorium 232 (Th232) in addition to a fissile isotope.  When Th232 absorbs a neutron, it decays into U233, another fissile material.  Such reactors could run at a lower neutron “temperature” than plutonium breeders, and would be easier to control as a result.  The liquid core would also greatly reduce the danger of a nuclear accident. If it became too hot, it could simply be decanted into a holding pan where it would immediately become subcritical. Thorium is more abundant than uranium in nature, so the “fuel” material would be cheaper.

Consider the above in the context of the present. Instead of extracting the vast amounts of energy locked up in U238, or “depleted” uranium, we use it for tank armor and armor piercing munitions. In addition to this incredibly stupid waste of potentially vast energy resources, we dispose of huge amounts of it as “radioactive waste.”  Instead of treasuring our huge stores of plutonium as sources of carbon-free energy, we busy ourselves thinking up clever ways to render them “safe” for burial in waste dumps.  It won’t work.  Plutonium can never be made “safe” in this way. Pu239 has a half-live of about 25,000 years.  It will always be possible to extract it chemically from whatever material we choose to mix it with.  Even if it is “reactor grade,” including other isotopes of plutonium such as Pu240, it will still be extremely dangerous – difficult to make into a bomb, to be sure, but easy to assemble into a critical mass that could potentially result in radioactive contamination of large areas. Carefully monitored breeder reactors are the only way of avoiding these problems.

According to the Huffpo article referenced above,

Doesn’t nuclear power contribute to nuclear weapons proliferation? No. Weapons programs do not depend on civilian nuclear power, which operates under stringent international safeguards.

Really? Will the “stringent international safeguards” last for the 25,000 years it takes for even half the plutonium waste produced by conventional reactors to decay? I would advise anyone who thinks it is impossible to fabricate this waste into a bomb, no matter what combination of isotopes it contains, to take an elementary course in nuclear engineering. The only way to avoid this problem is to burn all the plutonium in breeder reactors.  Predictably, the article doesn’t even mention the incredible wastefulness of current reactors, or the existence of breeder technology.

It’s nice that a few leftist “progressives” have finally noticed that their narrative on nuclear power has been controlled by imbeciles for the last half a century. I heartily concur that nuclear energy is a potent tool for reducing carbon and other greenhouse gas emissions.  I simply suggest that, if we decide to return to nuclear, we either provide the subsidies necessary to implement rational nuclear technologies now, or wait until it becomes economically feasible to implement them.

New Reactors in the UK and the Future of Nuclear Power

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.