Nuclear Power, Thorium, and the Role of Government

Nuclear power is an attractive candidate for meeting our future energy needs.  Nuclear plants do not release greenhouse gases.  They release significantly less radiation into the environment than coal plants, because coal contains several parts per million of radioactive thorium and uranium.  They require far less space and are far more reliable than alternative energy sources such as wind and solar.  In spite of some of the worst accidents imaginable due to human error and natural disasters, we have not lost any cities or suffered any mass casualties, and the horrific “China Syndrome” scenarios invented by the self-appointed saviors of mankind have proven to be fantasies.  That is not to say nuclear power is benign.  It is just more benign than any of the currently available alternatives.  The main problem with nuclear is not that it is unsafe, but that it is being ill-used.  In this case, government could actually be helpful.  Leadership and political will could put nuclear on a better track.

To understand why, it is necessary to know a few things about nuclear fuel, and how it “burns.”  Bear with me while I present a brief tutorial in nuclear engineering.  Nuclear energy is released by nuclear fission, or the splitting of heavy elements into two or more lighter ones.  This doesn’t usually happen spontaneously.  Before a heavy element can undergo fission, an amount of energy above a certain threshold must first be delivered to its nucleus.  How does this happen?  Imagine a deep well.  If you drop a bowling ball into the well, it will cause a large splash when it hits the water.  It does so because it has been accelerated by the force of gravity.  A heavy nucleus is something like a well, but things don’t fall into it because of gravity.  Instead, it relies on the strong force, which is very short range, but vastly more powerful than gravity.  The role of “bowling ball” can be played by a neutron.  If one happens along and gets close enough to fall into the strong force “well,” it will also cause a “splash,” releasing energy as it is bound to the heavy element’s nucleus, just as the real bowling ball is “bound” in the water well until someone fishes it out.  This “splash,” or release of energy, causes the heavy nucleus to “jiggle,” much like an unstable drop of water.  In one naturally occurring isotope – uranium with an atomic weight of 235 – this “jiggle” is so violent that it can cause the “drop of water” to split apart, or fission.

There are other isotopes of uranium.  All of them have 92 protons in their nucleus, but can have varying numbers of neutrons.  The nucleus of uranium 235, or U235, has 92 protons and 143 protons, adding up to a total of 235.  Unfortunately, U235 is only 0.7% of natural uranium.  Almost all the rest is U238, which has 92 protons and 146 neutrons.  When a neutron falls into the U238 “well,” the “splash” isn’t big enough to cause fission, or at least not unless the neutron had a lot of energy to begin with, as if the “bowling ball” had been shot from a cannon.  As a result, U238 can’t act as the fuel in a nuclear reactor.  Almost all the nuclear reactors in operation today simply burn that 0.7% of U235 and store what’s left over as radioactive waste.  Unfortunately, that’s an extremely inefficient and wasteful use of the available fuel resources.

To understand why, it’s necessary to understand something about what happens to the neutrons in a reactor that keep the nuclear chain reaction going.  First of all, where do they come from?  Well, each fission releases more neutrons.  The exact number depends on how fast the neutron that caused the fission was going, and what isotope underwent fission.  If enough are released to cause, on average, one more fission, then the resulting chain reaction will continue until the fuel is used up.  Actually, two neutrons, give or take, are released in each fission.  However, not all of them cause another fission.  Some escape the fuel region and are lost.  Others are absorbed in the fuel material.  That’s where things get interesting.

Recall that, normally, most of the fuel in a reactor isn’t U235, but the more common isotope, U238.  When U238 absorbs a neutron, it forms U239, which quickly decays to neptunium 239 and then plutonium 239.  Now it just so happens that plutonium 239, or Pu239, will also fission if a neutron “falls into its well,” just like U235.  In other words, if enough neutrons were available, the reactor could actually produce more fuel, in the form of Pu239, than it consumes, potentially burning up most of the U238 as well as the U235.  This is referred to as the “breeding” of nuclear fuel.  Instead of just lighting the U235 “match” and letting it burn out, it would be used to light and burn the entire U238 “log.”  Unfortunately, there are not enough neutrons in normal nuclear reactors to breed more fuel than is consumed.  Such reactors have, however, been built, both in the United States and other countries, and have been safely operated for periods of many years.

Plutonium breeders aren’t the only feasible type.  In addition to U235 and Pu239, another isotope will also fission if a neutron falls into its “well” – uranium 233.  Like Pu239, U233 doesn’t occur in nature.  However, it can be “bred,” just like Pu239, from another element that does occur in nature, and is actually more common than uranium – thorium.  I’ve had a few critical things to say about some of the popular science articles I’ve seen on thorium lately, but my criticisms were directed at inaccuracies in the articles, not at thorium technology itself.  Thorium breeders actually have some important advantages over plutonium.  When U233 fissions, it produces more neutrons than Pu239, and it does so in a “cooler” neutron spectrum, where the average neutron energy is much lower, making the reactor significantly easier to control.  These extra neutrons could not only breed more fuel.  They could also be used to burn up the transuranic elements – those beyond uranium on the table of the elements – that are produced in conventional nuclear reactors, and account for the lion’s share of the long-lived radioactive waste.  This would be a huge advantage.  Destroy the transuranics, and the residual radioactivity from a reactor would be less than that of the original ore, potentially in a few hundred years, rather than many thousands.

Thorium breeders have other potentially important advantages.  The fuel material could be circulated through the core in the form of a liquid, suspended in a special “salt” material.  Of course, this would eliminate the danger of a fuel meltdown.  In the event of an accident like the one at Fukushima, the fuel would simply be allowed to run into a holding basin, where it would be sub-critical and cool quickly.  Perhaps more importantly, the United States has the biggest proven reserves of thorium on the planet.

Breeders aren’t the only reactor types that hold great promise for meeting our future energy needs.  High temperature gas cooled reactors would produce gas heated to high temperature in addition to electricity.  This could be used to produce hydrogen gas via electrolysis, which is much more efficient at such high temperatures.  When hydrogen burns, it produces only water.  Such reactors could also be built over the massive oil shale deposits in the western United States.  The hot gas could then be used to efficiently extract oil from the shale “in situ” without the need to mine it.  It is estimated that the amount of oil that could be economically recovered in this way from the Green River Basin deposits in Utah, Wyoming and Colorado alone is three times greater than the oil reserves of Saudi Arabia.

Will any of this happen without government support and leadership?  Not any time soon.  The people who build nuclear reactors expect to make a profit, and the easiest way to make a profit is to build more conventional reactors of the type we already have.  Raise the points I’ve mentioned above, and they’ll simply tell you that there’s plenty of cheap uranium around and therefore no need to breed more fuel, the radioactive danger of transuranics has been much exaggerated, etc., etc.  All these meretricious arguments make sense if your goal is to make a profit in the short run.  They make no sense at all if you have any concern for the energy security and welfare of future generations.

Unless the proponents of controlled fusion or solar and other forms of alternative energy manage to pull a rabbit out of their collective hats, I suspect we will eventually adopt breeder technology.  The question is when.  After we have finally burnt our last reserves of fossil fuel?  After we have used up all our precious reserves of U238 by scattering it hither and yon in the form of “depleted uranium” munitions?  The longer we wait, the harder and more expensive it will become to develop a breeder economy.  It would be well if, in this unusual case, government stepped in and did what it is theoretically supposed to do; lead.

Author: Helian

I am Doug Drake, and I live in Maryland, not far from Washington, DC. I am a graduate of West Point, and I hold a Ph.D. in nuclear engineering from the University of Wisconsin. My blog reflects my enduring fascination with human nature and human morality. View all posts by Helian