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  • Whither Nuclear Power? A Few Comments on Thorium and the End of the “Nuclear Renaissance”

    Posted on September 2nd, 2017 Helian No comments

    About a decade ago there was much talk of a “nuclear renaissance” amid concerns about greenhouse gas emissions and the increasing cost of fossil fuel alternatives.  The Nuclear Regulatory Commission received applications to build no less than 31 new nuclear plants as the price of crude oil spiked to over $140 per barrel.  Now, however, with last month’s decision by SCANA Corp. to abandon the V. C. Summer project, a pair of nukes that had been under construction in South Carolina, nuclear’s future prospects look dim, at least in the United States.  Two plants remain under construction in Georgia but, like the ones abandoned in South Carolina, they are to be AP1000s, designed by Westinghouse.  Westinghouse filed for bankruptcy in March.  Delays and massive cost overruns similar to those that led to the demise of V. C. Summer also afflict the Georgia project, and its future seems doubtful at best.

    In short, the dream of a nuclear renaissance has evaporated.  For the time being, at least, nuclear in the U.S. is no match for more agile competitors like wind, solar, and natural gas.  However, there may be a silver lining to this cloud.  Plants like Westinghouse’s AP1000 waste most of the energy in their nuclear fuel, creating massive amounts of avoidable radioactive waste in the process.  To the extent that it makes sense to build nuclear plants at all, these are not the kind we should be building.  To understand why this is true it is first necessary to acquire some elementary knowledge about nuclear physics.

    The source of the energy produced in the core of nuclear reactors is a nuclear fission chain reaction.  Only one material that exists in significant quantities in nature can sustain such a chain reaction – uranium 235, or U235.  U235 is an isotope of uranium.  Isotopes of a given element consist of atoms with the same number of positively charged protons in their central core, or nucleus.  Like all other isotopes of uranium, U235 has 92.  There are also 143 neutrally charged neutrons, making a total of 235 “nucleons.”  Natural uranium consists of only about 0.7 percent U235.  Almost all the rest is a different isotope, U238, with a nucleus containing 146 neutrons instead of 143.

    When we say that U235 can sustain a nuclear chain reaction, we mean that if a free neutron happens to come within a very short distance of its nucleus, it may be captured, releasing enough energy in the process to cause the nucleus to split into two fragments.  When this happens, more free neutrons are released, that can then be captured by other uranium nuclei, which, in turn, fission, releasing yet more neutrons, and so on.  As noted above, U235 is the only naturally occurring isotope that can sustain such a nuclear chain reaction.  However, other isotopes can be created artificially that can do so as well.  The most important of these are U233 and plutonium 239, or Pu239.  They are important because it is possible to “breed” them in properly designed nuclear reactors, potentially producing more usable fuel than the reactor consumes.  U233 is produced by the reactions following absorption of a neutron by thorium 232, or Th232, and Pu239 by those following the absorption of a neutron by U238.  In other words, we know of three practical types of nuclear fuel; U235, U233 and Pu239.  The first occurs naturally, and the other two can be readily “bred” artificially in nuclear reactors.

    Let’s consider what this means in the case of conventional nuclear reactors like the Westinghouse AP1000.  These are powered by fuel elements that typically are enriched in U235 from the naturally occurring 0.7 percent to from three to five percent.  The remaining 95 to 97 percent of the uranium in these fuel elements is U238.  When the fission process starts, some of the neutrons released are captured by the U238, eventually resulting in the production of Pu239.  Some of this plutonium fissions along with the U235, contributing to the total energy produced by the fuel elements.  However, only a small fraction of the U238 is converted to Pu239 in this way before the fuel is consumed and it becomes necessary to replace the old fuel elements with fresh ones.  In addition to a great deal of U238, these spent fuel elements contain a significant amount of plutonium, as well as other transuranic elements such as americium and curium, which can remain dangerously radioactive for thousands of years.  The “waste” plutonium might even be used to produce a nuclear weapon.

    Obviously, if possible it would be better to extract all the energy locked up in natural uranium rather than just a small fraction of it.  In fact, it is possible, or very nearly so.  Breeder reactors are feasible that could burn nearly all the U238 in natural uranium as well as the U235 by converting it into Pu239.  In the process they could destroy much of the transuranic waste that is the main source of radioactive danger from spent fuel.  In as little as 500 years the residual radioactivity from running a nuclear plant for 30 years could potentially be less than that of the original naturally occurring uranium.  Unfortunately, while all this is scientifically feasible, it is not economically feasible.  It won’t happen without massive government subsidies.  Perhaps such subsidies are warranted in view of the threat of climate change and perhaps not, but, regardless, breeder reactors won’t be built without them.  Since they are really the only types of reactors it makes sense to build, we would probably be better off, at least for the time being, building no reactors at all.  That’s the “silver lining” I referred to above.  Perhaps a time will come when the world runs out of expendable sources of base load electrical power, such as oil, coal and natural gas, and no way has been found to take up the slack with renewables.  In that case, it may once again make economic sense to build breeder reactors.  Until that time, the United States would do well to build up a healthy stockpile of uranium, and put a stop to the stupid, wasteful, and counterproductive use of depleted uranium that could potentially become a source of vast amounts of energy to produce munitions and armor.

    But wait, there’s more!  What about thorium?  Thorium by itself can’t sustain a nuclear chain reaction.  It can, however, be converted into U233 by neutron absorption, and that is an ideal reactor fuel.  Among other things, it generates more neutrons per fission at lower neutron “temperatures” than either Pu239 or U235.  That means that extra neutrons are available to “breed” fuel at those lower temperatures where nuclear reactors are easier to control.  By “temperature” here, we’re referring to the average speed of the neutrons.  The slower they are, the more likely they are to be absorbed by a nucleus and cause fission reactions.  Neutrons are slowed in “moderators,” which can be any number of light types of atoms.  The most common is plain water, consisting of the elements hydrogen and oxygen.  Think of a billiard ball hitting another billiard ball head on.  It comes to a complete stop, transferring its energy to the other ball.  The same thing can happen with neutrons and the proton nucleus of hydrogen atoms, which are of approximately equal mass.  To breed plutonium effectively, reactors must be run at significantly higher neutron temperatures.

    There’s more good news about thorium.  It can be dissolved in various exotic mixtures and breed U233 in a reactor with a liquid instead of a solid core.  This would have a number of advantages.  In the first place, a “meltdown” would be impossible in a core that’s already “melted.”  If the core became too “hot” it could simply be drained into a holding pan to form a subcritical mass that would quickly cool.  It would also be possible to extract waste fission products and introduce fresh fuel, etc., into the core “on the fly.”  As a result the reactor would be able to stay in operation longer between shutdowns for maintenance and refueling.  The necessary technology has already been demonstrated at places like Oak Ridge, Tennessee and Shippingport, Pennsylvania.  Recently, a Dutch team finally began experiments with molten salt technology intended to take up where these earlier experiments left off after a hiatus of more than 40 years.

    Perhaps thorium’s biggest problem is the tendency of its proponents to over-hype its promise.  It even has a founding myth based on bogus claims that thorium technology isn’t dominant in the energy industry today because “it’s much harder to weaponize.”  For example, according to the article about the Dutch experiments linked above, entitled, ‘Safer’ thorium reactor trials could salvage nuclear power,

    But, if it’s so safe and reliable why hasn’t thorium been used all along? Because (unlike uranium) it’s much harder to weaponize. As a result, it’s historically been sidelined by nations in search of both energy and a potential source of weapons-grade plutonium.

    This yarn about a benign source of energy that might have benefited all mankind being torpedoed by evil weaponeers might sound good, but it’s complete nonsense.  Thorium itself can’t be weaponized, because it can’t sustain a nuclear chain reaction on its own.  The sole reason there’s any interest in it at all as a source of nuclear power is the possibility of transmuting it to U233.  Of course, it can’t be used to produce weapons-grade plutonium.  However, there is no better material for making nuclear bombs than U233.  As is the case with Pu239, four kilograms is sufficient to make a nuclear weapon, compared to the 25 kilograms that is a sufficient quantity of U235.  It’s main drawback as a weapons material is the fact that small amounts of U232 are produced along with it in thorium-based reactors, and U232 decays into radioactive daughters that are deadly sources of powerful gamma rays.  However, the amount of U232 produced can be reduced dramatically by cooling the neutron spectrum to a low “temperature.”  In short, thorium could definitely be used to make weapons.  The reason it isn’t the dominant technology for that purpose is the same as the reason it isn’t the dominant technology for producing electric power; it would be significantly more complex and expensive than using natural or slightly enriched uranium as a fuel.  That reason is as valid now as it was in the days of Little Boy and Fat Man.  The “dominant technology” would be the same as it is today whether nuclear weapons had ever been produced or not.

    When it comes to the technology itself, thorium proponents also tend to be coy about mentioning problems that don’t afflict other reactor types.  For example, the materials needed for practical molten salt reactors are extremely corrosive.  There has been progress towards finding a metal that can hold them, but no ideal alloy has yet been found.  This isn’t necessarily a show stopper, but it’s not an insignificant problem, either.  Such material issues have been largely solved for conventional reactors.  If, as would seem to be the case, these are no longer economically competitive with their rivals, then molten salt is pretty much out of the question, at least for the time being.  It’s important to point out that, if breeder reactors ever do become economically feasible again, it will always be necessary to insure that they are secure, and that the materials they produce can’t be diverted for making weapons.  That concern applies to both plutonium and thorium breeders.

    Meanwhile, it might behoove our political leaders to consider the question of why it was once possible to build more than 50 experimental reactors at what is now Idaho National Laboratory alone in a relatively short period of time for a small fraction of what similar reactors would cost today.  Merely negotiating the regulatory hurdles for building a power reactor based on anything as novel as the thorium fuel cycle would take the better part of a decade.  All these hurdles have been put in place in the name of “safety.”  That begs the question of how “safe” we will be if we lack reliable sources of electric energy.  There is a point beyond which excessive regulation itself becomes unsafe.

  • Nuclear Update: Molten Salt, Rugby Balls, and the Advanced Hydrodynamic Facility

    Posted on August 7th, 2015 Helian No comments

    I hear at 7th or 8th hand that the folks at DOE have been seriously scratching their heads about the possibility of building a demonstration molten salt reactor.  They come in various flavors, but the “default” version is a breeder, capable of extracting far more energy from a given quantity of fuel material than current reactors by converting thorium into fissile uranium 233.  As they would have a liquid core, the possibility of a meltdown would be eliminated.  The copious production of neutrons in such reactors would make it possible to destroy the transuranic actinides, such as americium and curium, and, potentially, also some of the most long-lived radioactive products produced in fission reactions.  As a result, the residual radioactivity from running such a reactor for, say, 30 years, would potentially be less than that of the ore from which the fuel was originally extracted in under 500 years, a far cry from the millions commonly cited by anti-nuke alarmists.  Such reactors would be particularly attractive for the United States, because we have the largest proven reserves of thorium on the planet.  Disadvantages include the fact that uranium 233 is a potential bomb material, and therefore a proliferation concern, and the highly corrosive nature of the fluoride and/or chloride “salts” in the reactor core.  More detailed discussions of the advantages and disadvantages may be found here and here.

    The chances that the U.S. government will actually provide the funding necessary to build a molten salt or any other kind of advanced reactor are, unfortunately, slim and none.  We could do such things in the 50’s and 60’s with alacrity, but those days are long gone, and the country seems to have fallen victim to a form of technological palsy.  That’s too bad, because private industry won’t take up the slack.  To the extent they’re interested in nuclear at all, the profit motive rules.  At the moment, the most profitable way to generate nuclear energy is with reactors that simply burn naturally occurring uranium, wasting the lion’s share of the potential energy content, and generating copious amounts of long-lived radioactive waste for which no rational long term storage solution has yet been devised.  In theory, DOE’s national laboratories should be stepping in to take up the slack, doing the things that industry can’t or won’t do.  In reality what they do is generate massive stacks of paper studies and reports on advanced systems that have no chance of being built.  Enough must have accumulated since the last research reactor was actually built at any of the national labs to stretch back and forth to the moon several times.  Oh, well, we can take comfort in the knowledge that at least some people at DOE are thinking about the possibilities.

    Moving right along, as most of my readers are aware, the National Ignition Facility, or NIF, did not live up to its name.  It failed to achieve inertial confinement fusion (ICF) ignition in the most recent round of experiments, missing that elusive goal by nearly two orders of magnitude.  The NIF is a giant, 192 beam laser system at Lawrence Livermore National Laboratory (LLNL) that focuses all of its 1.8 megajoules of laser energy on a tiny target containing deuterium and tritium, two heavy isotopes of hydrogen.  Instead of generating energy by splitting or “fissioning” heavy atoms, the goal is to get these light elements to “fuse,” releasing massive amounts of energy.

    Actually, the beams don’t hit the target itself.  Instead they’re focused through two holes in the ends of a tiny cylinder, known as a hohlraum, that holds a “capsule” of fuel material mounted in its center.  It’s what’s known as the indirect drive approach to ICF, as opposed to direct drive, in which the beams are focused directly on a target containing the fuel material.  When the beams hit the inside walls of the cylinder they generate a burst of x-rays.  These are what actually illuminate the target, causing it to implode to extremely high densities.  At just the right moment a “hot spot” is created in the very center of this dense, imploded fuel material, where fusion ignition begins.  The fusion reactions create alpha particles, helium nuclei containing two neutrons and two protons, which then smash into the surrounding “cold” fuel material, causing it to ignite as well, resulting in a “burn wave,” which spreads outward, igniting the rest of the fuel.  For this to happen, everything has to be just right.  The most important thing is that the implosion be almost perfectly symmetric, so that the capsule isn’t squished into a “pancake,” or squashed into a “sausage,” but is very nearly spherical at the point of highest density.

    Obviously, everything wasn’t just right in the recently concluded ignition experiments.  There are many potential reasons for this.  Material blowing off the hohlraum walls could expand into the interior in unforeseen ways, intercepting some of the laser light and/or x-rays, resulting in asymmetric illumination of the capsule.  So-called laser/plasma interactions with abstruse names like stimulated Raman scattering, stimulated Brillouin scattering, and two plasmon decay, could be more significant than expected, absorbing laser light so as to prevent symmetric illumination and at the same time generating hot electrons that could potentially preheat the fuel, making it much more difficult to implode and ignite.  There are several other potential failure mechanisms, all of which are extremely difficult to model on even the most powerful computers, especially in all three dimensions.

    LLNL isn’t throwing in the towel, though.  In fact, there are several promising alternatives to indirect drive with cylindrical hohlraums.  One that recently showed promise in experiments on the much smaller OMEGA laser system at the University of Rochester’s Laboratory for Laser Energetics (LLE) is substitution of “rugby ball” shaped targets in place of the “traditional” cylinders.  According to the paper cited in the link, these “exhibit advantages over cylinders, in terms of temperature and of symmetry control of the capsule implosion.”  LLNL could also try hitting the targets with “green” laser light instead of the current “blue.”  The laser light is initially “red,” but is currently doubled and then tripled in frequency by passing it through slabs of a special crystal material, shortening its wavelength to the shorter “blue” wavelength, which is absorbed more efficiently.  However, each time the wavelength is shortened, energy is lost.  If “green” light were used, as much as 4 megajoules of energy could be focused on the target instead of the current maximum of around 1.8.  If “green” is absorbed well enough and doesn’t set off excessive laser/plasma interactions, the additional energy just might be enough to do the trick.  Other possible approaches include direct drive, hitting the fuel containing target directly with the laser beams, and “fast ignitor,” in which a separate laser beam is used to ignite a hot spot on the outside of the “cold,” imploded fuel material instead of relying on the complicated central hot spot approach.

    Regardless of whether ignition is achieved on the NIF or not, it will remain an extremely valuable experimental facility.  The reason?  Even without ignition it can generate extreme material conditions that are relevant to those that exist in exploding nuclear weapons.  As a result, it gives us a significant leg up over other nuclear weapons states in an era of no nuclear testing by enabling us to field experiments relevant to the effects of aging on the weapons in our stockpile, and suggesting ways to insure they remain safe and reliable.  Which brings us to the final topic of this post, the Advanced Hydrodynamic Facility, or AHF.

    The possibility of building such a beast was actively discussed and studied back in the 90’s, but Google it now and you’ll turn up very little.  It would behoove us to start thinking seriously about it again.  In modern nuclear weapons, conventional explosives are used to implode a “pit” of fissile material to supercritical conditions.  The implosion must be highly symmetric or the pit will “fizzle,” failing to produce enough energy to set off the thermonuclear “secondary” of the weapon that produces most of the yield.  The biggest uncertainty we face in maintaining the safety and reliability of our stockpile is the degree to which the possible deterioration of explosives, fusing systems, etc., will impair the implosion of the pit.  Basically, an AHF would be a system of massive particle accelerators capable of generating bursts of hard x-rays, or, alternatively, protons, powerful enough to image the implosion of the fission “pit” of a nuclear weapon at multiple points in time and in three dimensions.  Currently we have facilities such as the Dual-Axis Radiographic Hydrodynamic Test Facility (DARHT) at Los Alamos National Laboratory (LANL), but it only enables us study the implosion of small samples of surrogate pit material.  An AHF would be able to image the implosion of actual pits, with physically similar surrogate materials replacing the fissile material.

    Obviously, such experiments could not be conducted in a conventional laboratory.  Ideally, the facility would be built at a place like the Nevada Test Site (NTS) north of Las Vegas.  Experiments would be fielded underground in the same way that actual nuclear tests were once conducted there.  That would have the advantage of keeping us prepared to conduct actual nuclear tests within a reasonably short time if we should ever be forced to do so, for example, by the resumption of testing by other nuclear powers.  With an AHF we could be virtually certain that the pits of the weapons in our arsenal will work for an indefinite time into the future.  If the pit works, we will also be virtually certain that the secondary will work as well, and the reliability of the weapons in our stockpile will be assured.

    Isn’t the AHF just a weaponeer’s wet dream?  Why is it really necessary?  Mainly because it would remove once and for all any credible argument for the resumption of nuclear testing.  Resumption of testing would certainly increase the nuclear danger to mankind, and, IMHO, is to be avoided at all costs.  Not everyone in the military and weapons communities agrees.  Some are champing at the bit for a resumption of testing.  They argue that our stockpile cannot be a reliable deterrent if we are not even sure if our weapons will still work.  With an AHF, we can be sure.  It’s high time for us to dust off those old studies and give some serious thought to building it.

    ICF

  • Nuclear Power and the Anti-Science Ideology of the “Progressive” Left

    Posted on February 22nd, 2015 Helian No comments

    The ideological Left is fond of accusing the Right of being “anti-science.”  The evidence often comes in the form of Exhibit A (climate denialism) and Exhibit B (Darwin denialism).  True, these maladies are encountered more frequently on the Right than on the Left.  As it happens, however, there are also scientific allergies on the Left, and there is little question that they have been a great deal more damaging than their conservative analogs.  The best example is probably the Blank Slate debacle.  In order to prop up leftist shibboleths, denial of the very existence of human nature was enforced for more than half a century.  The effect on the behavioral sciences, and with them the self-knowledge critical to our very survival, was devastating.  “Scientific” Marxism-Leninism is another obvious example.  However, when it comes to scientific allergies, the Left’s irrational and often fanatical opposition to nuclear power may turn out to be the most damaging of all.

    Those who seek to alarm us about rising CO2 levels in the atmosphere, and yet reject the most effective technology for bringing them under control, are not serious.  They are mere poseurs.  Thanks to these anti-science attitudes on the Left, dozens of dirty, coal-fired power plants will be built in Germany alone to replace the baseload generating capacity once provided by nuclear reactors.  The situation is no better in the U.S.  Both countries have developed some of the most advanced, not to mention safest, nuclear technologies known to man, and yet both, hamstrung by opposition coming from the Left of the political spectrum, have abdicated the responsibility to apply that knowledge.  Instead, they are exporting it – to China.

    As I write this, we are helping China to build a novel type of reactor that combines molten salt technology developed in the United States with a version of the “pebble” type fuel pioneered by the Germans.  Approved in 2011, the original target completion date of 2015 has now slipped to 2020, but both goals would be out of the question in the byzantine regulatory atmosphere of the 21st century United States.  U.S. knowhow will also be used to build the novel “traveling wave” reactor design favored by Bill Gates – also in China.  The Chinese are also actively pursuing the high temperature gas-cooled reactor (HTGR) technology that was proposed for the ill-fated Next Generation Nuclear Plant (NGNP), further development of which was recently cancelled in the United States.

    I certainly have nothing against China building advanced reactors using technology that was developed elsewhere.  It’s good that the knowledge in question is being applied at least somewhere on the planet.  However, I find it unfortunate that we no longer have the leadership, vision, or political will to do so ourselves.  It was not always so.  The U.S. commissioned the world’s first nuclear powered submarine, the U.S.S. Nautilus, in 1954, little more than a decade after the successful demonstration of the first self-sustaining nuclear chain reaction at the University of Chicago.  More than 50 experimental nuclear reactors were built at what is now Idaho National Laboratory (INL) in a period of about two decades stretching from the 50’s to the mid-70’s.  None has been built since.  The situation is similar at Oak Ridge National Laboratory (ORNL), site of the world’s first molten salt reactor.  Instead of working, next generation reactors, INL, ORNL, and the rest of the U.S. national laboratories now turn out only paper studies – gigantic mounds of them – in quantities that would probably stretch to the moon and back by now.  The chances that any of them will ever be usefully applied in this country are slim and none.

    The technologies in question are not mere incremental improvements over the conventional nuclear power plants that now produce almost all the world’s nuclear power.  They have the demonstrated capacity to extract more than an order of magnitude more energy out of a given quantity of mined fuel material than conventional designs.  They can burn the long-lived radioactive actinides and other hazardous isotopes produced in nuclear fission that represent the most dangerous types of radioactive waste, reducing the residual radioactivity from operation of a nuclear plant to a level less than that of the original uranium ore is less than 500 years – a far cry from the millions of years often cited by hysterical anti-nukers.  Under the circumstances, it is worth taking note of where the opposition that stopped the development and application of these technologies in the past, and continues to do so today, is coming from.

    The regulatory nightmare that has brought the continued development of these technologies in the United States to a virtual standstill is primarily the legacy of the “progressive” Left.  The anti-nuclear zealots on that side of the political spectrum cling to bogus linear no-threshold models of radioactive hazard, grotesquely exaggerated horror stories about the supposed impossibility of dealing with nuclear waste, and a stubborn cluelessness about the dangers of the alternative coal and other fossil-fired technologies that their opposition to nuclear will inevitably continue to promote in spite of all their strident denials.  These are facts that it would be well to keep in mind the next time you hear the Left calling the Right “anti-science,” or, for that matter, the next time you hear them pontificating about their deep commitment to the fight against global warming.

  • China Bets on Thorium Reactors

    Posted on March 21st, 2014 Helian 2 comments

    According to the South China Morning Post (hattip Next Big Future),

    The deadline to develop a new design of nuclear power plant has been brought forward by 15 years as the central government tries to reduce the nation’s reliance on smog-producing coal-fired power stations.  A team of scientists in Shanghai had originally been given 25 years to try to develop the world’s first nuclear plant using the radioactive element thorium as fuel rather than uranium, but they have now been told they have 10, the researchers said.

    I have to admit, I feel a little envious when I read things like that.  The Chinese government is showing exactly the kind of leadership that’s necessary to guide the development of nuclear power along rational channels, and it’s a style of leadership of which our own government no longer seems capable.

    What do I mean by “rational channels?”  Among other things, I mean acting as a responsible steward of our nuclear resources, instead of blindly wasting them , as we are doing now.  How are we wasting them?  By simply throwing away the lion’s share of the energy content of every pound of uranium we mine.

    Contrary to the Morning Post article, thorium is not a nuclear fuel.  The only naturally occurring nuclear fuel is uranium 235 (U235).  It is the only naturally occurring isotope that can be used directly to fuel a nuclear reactor.  It makes up only a tiny share – about 0.7% – of mined uranium.  The other 99.3% is mostly uranium 238 (U238).  What’s the difference?  When a neutron happens along and hits the nucleus of an atom of U235, it usually fissions.  When a neutron happens along and hits the nucleus of an atom of U238, unless its going very fast, it commonly just gets absorbed.  There’s more to the story than that, though.  When it gets absorbed, the result is an atom of U239, which eventually decays to an isotope of plutonium – plutonium 239 (Pu239).  Like U235, Pu239 actually is a nuclear fuel.  When a neutron hits its nucleus, it too will usually fission.  The term “fissile” is used to describe such isotopes.

    In other words, while only 0.7% of naturally occurring uranium can be used directly to produce energy, the rest could potentially be transmuted into Pu239 and burned as well.  All that’s necessary for this to happen is to supply enough extra neutrons to convert the U238.  As it happens, that’s quite possible, using so-called breeder reactors.  And that’s where thorium comes in.  Like U238, the naturally occurring isotope thorium 232 (Th232) absorbs neutrons, yielding the isotope Th233, which eventually decays to U233, which is also fissile.  In other words, useful fuel can be “bred” from Th232 just as it can from U238.  Thorium is about three times as abundant as uranium, and China happens to have large reserves of the element.  According to current estimates, reserves in the U.S. are much larger, and India’s are the biggest on earth.

    What actually happens in almost all of our currently operational nuclear reactors is a bit different.  They just burn up that 0.7% of U235 in naturally occurring uranium, and a fraction of the Pu239 that gets bred in the process, and then throw what’s left away.  “What’s left” includes large amounts of U238 and various isotopes of plutonium as well as a brew of highly radioactive reaction products left over from the split atoms of uranium and plutonium.  Perhaps worst of all, “what’s left” also includes transuranic actinides such as americium and curium as well as plutonium.  These can remain highly radioactive and dangerous for thousands of years, and account for much of the long-term radioactive hazard of spent nuclear fuel.  As it happens, these actinides, as well as some of the more dangerous and long lived fission products, could potentially be destroyed during the normal operation of just the sort of molten salt reactors the crash Chinese program seeks to develop.  As a result, the residual radioactivity from operating such a plant for, say, 40 years, could potentially be less than that of the original uranium ore after a few hundreds of years instead of many thousands.  The radioactive hazard of such plants would actually be much less than that of burning coal, because coal contains small amounts of both uranium and thorium.  Coal plants spew tons of these radioactive elements, potentially deadly if inhaled, into the atmosphere every year.

    Why on earth are we blindly wasting our potential nuclear energy resources in such a dangerous fashion?  Because it’s profitable.  For the time being, at least, uranium is still cheap.  Breeder reactors would be more expensive to build than current generation light water reactors (LWRs).  To even start one, you’d have to spend about a decade, give or take, negotiating the highly costly and byzantine Nuclear Regulatory Commission licensing process.  You could count on years of even more costly litigation after that.  No reprocessing is necessary in LWRs.  Just quick and dirty storage of the highly radioactive leftovers, leaving them to future generations to deal with.  You can’t blame the power companies.  They’re in the business to make a profit, and can’t continue to operate otherwise.  In other words, to develop nuclear power rationally, you need something else in the mix – government leadership.

    We lack that leadership.  Apparently the Chinese don’t.

     

    Thorium metal

    Thorium metal

  • Accelerator-Driven Thorium Reactors, or An Easy Way to Eliminate Surplus Population

    Posted on September 10th, 2013 Helian 7 comments

    The Daily Telegraph has just taken thorium wowserism to a whole new level.  According to the title of an article penned by International Business Editor Ambrose Evans-Pritchard, Obama could kill fossil fuels overnight with a nuclear dash for thorium.  Continuing in the same vein, the byline assures us that,

    If Barack Obama were to marshal America’s vast scientific and strategic resources behind a new Manhattan Project, he might reasonably hope to reinvent the global energy landscape and sketch an end to our dependence on fossil fuels within three to five years.

    And how is this prodigious feat to be accomplished?  Via none other than Nobel laureate Dr. Carlo Rubbia’s  really bad idea for building accelerator-driven thorium reactors.  It would seem that Dr. Rubbia has assured the credulous Telegraph editor that, “a tonne of the silvery metal produces as much energy as 200 tonnes of uranium.”  This egregious whopper is based on nothing more complicated than a comparison of apples and oranges.  Thorium by itself cannot power a nuclear reactor.  It must first be converted into the isotope uranium 233 via absorption of a neutron.  Natural uranium, on the other hand, can be used directly in reactors, because 0.7 percent of it consists of the fissile isotope uranium 235.  In other words, Rubbia is comparing the energy potential of thorium after it has been converted to U233 with the energy potential of only the U235 in natural uranium.  The obvious objection to this absurd comparison is that the rest of natural uranium is made up mostly of the isotope U238, which can also absorb a neutron to produce plutonium 239, which, like U233, can power nuclear reactors.  In other words, if we compare apples to apples, that is, thorium after it has been converted to U233 with U238 after it has been converted to Pu239, the potential energy content of thorium and uranium is about equal.

    As it happens, the really bad news in the Telegraph article is that,

    The Norwegian group Aker Solutions has bought Dr. Rubbia’s patent for an accelerator-driven sub-critical reactor, and is working on his design for a thorium version at its UK operation.

    In fact, Aker has already completed a conceptual design for a power plant.  According to Aker project manager Victoria Ashley, the group needs a paltry $3 million, give or take, to build the first one, and another $150 million for the test phase to follow.  Why is that disturbing news?  Because the U233 produced in these wonderful new reactors will be ideal for producing nuclear weapons.

    In fact, it will be even better than the “traditional” bomb materials; highly enriched uranium (HEU) and weapons grade plutonium.  The explosion of a nuclear device is produced by assembling a highly supercritical mass of fissile material, and then introducing a source of neutrons at just the right moment, setting off a runaway chain reaction.  The problem with plutonium is that it has the bad habit of occasionally fissioning spontaneously.  This releases neutrons.  If such a stray neutron were to happen along just as the bomb material became critical, it would set off a premature chain reaction, causing the device to “fizzle.”  As a result, plutonium weapons must rely on a complicated implosion process to achieve supercriticality before the stray neutrons can do their dirty work.  Implosion weapons are much more technologically challenging to build than the gun-assembled types that can be used with HEU.  In these, one subcritical mass is simply shot into another.  However, the required mass of HEU is much larger than the amount of plutonium needed in an implosion-assembled weapon.  As it happens, the amount of U233 sufficient to build a nuclear device is about the same as the amount of plutonium, but spontaneous fission is not a problem in U233.  In other words, it combines the plutonium advantage of requiring a much smaller amount of material, and the HEU advantage of being usable in gun-assembled weapons.

    Why, then, you might ask, are we even giving Rubbia’s idea a second thought?  Because of people like Professor Egil Lillestol, who, Evans-Pritchard helpfully informs us, is “a world authority on the thorium fuel cycle at CERN.”  According to Lillestol,

    It is almost impossible to make nuclear weapons out of thorium because it is too difficult to handle.  It wouldn’t be worth trying.

    Rubbia has made similar statements, based on the same “logic.”  The rationalization for the claim that U233 is “too difficult to handle” is the supposed presence of U232, an isotope of uranium with a half-life of about 69 years, one of whose daughters (elements in its decay chain) emits a highly energetic and penetrating, and hence deadly, gamma ray.  In fact, avoiding the production of U232 in accelerator-driven reactors would be a piece of cake.  Rubbia and Lillestol must know this, making it all the more incomprehensible that they dare to foist such whoppers on unsuspecting newspaper editors.

    Only one neutron absorption is needed for the production of U233 from naturally occurring Th232.  Two are needed to produce U232.  Thus, one way to keep the level of U232 within manageable levels is to simply extract the U233 before much U232 has a chance to form.  However, there’s an even easier way.  Very energetic neutrons, with energies above a threshold of around 6 million electron volts, are necessary to produce U232.  Not many fission neutrons have that much energy, and slowing down the ones that do is simple.  Simply pass them through a “moderator” rich in hydrogen or some other light element.  Think of billiard balls.  If one of them going at a good clip hits another dead on, it stops, imparting its energy to the second ball.  Neutrons and the proton nuclei of hydrogen atoms have nearly the same mass, so the same thing can happen when they collide.  A fast neutron will typically lose a large fraction of its energy in such a collision.  In other words, the “secret” of avoiding the production of dangerous levels of U232 is as simple as passing the neutrons through a layer of hydrogen-rich material such as paraffin before allowing them to interact with the thorium.  All this should hardly come as a surprise to people like Rubbia and Lillestol.  It’s been old hat in the literature for a long time.  For a more detailed treatment, see, for example, U-232 and the Proliferation-Resistance of U-233 in Spent Fuel, a paper that appeared in the journal Science and Global Security back in 2001.

    In other words, the idea that “it is almost impossible to make nuclear weapons out of thorium” is a pipe dream.  That does not necessarily mean that thorium technology should be rejected root and branch.  It will always be necessary to exercise extreme care to insure that U233 isn’t diverted for illicit purposes.  However, managing the risk will be considerably easier in “conventional” thorium breeders, which rely on assembling a critical mass to supply the necessary source of neutrons.  Such reactors have already been built and successfully operated for years.  The U233 they produce will always be mixed with highly radioactive fission products, and can also be “denatured” by mixing it with U238, from which it cannot be separated using simple chemistry.  Such reactors would produce few of the transuranic actinides that are the main culprits in nuclear waste, potentially requiring it to be stored securely for millennia.  They could also consume the actinides produced in the current generation of reactors, so that the remaining waste could potentially become less radioactive than the original uranium ore in a few hundred years, instead of many thousands.

    If, on the other hand, the accelerators necessary to provide the neutron source for Dr. Rubbia’s subcritical facilities were to become readily available, they would be much easier to hide than conventional reactors, could be configured to produce U233 with almost no U232 contamination, and with much less radioactive fission product contamination.  In other words, they would constitute an unacceptable risk for the proliferation of nuclear weapons.  One must hope that the world will wake up in time to recognize the threat.

  • More Silly Thorium Tricks

    Posted on September 13th, 2012 Helian No comments

    You might get the idea from reading my blog that I have something against thorium.  It ain’t so!  I consider thorium a very promising candidate for supplying our future energy needs.  It’s just that there’s something about the stuff that seems to drive people off the deep end.  I actually missed the really bad thorium idea that’s the subject of this post when it turned up on the Internet about a year ago.  However, the articles are still out there, and are interesting examples of how really bad science can be promoted as perfectly plausible by people who have impressive credentials, but actually don’t know what they’re talking about.

    The idea in question was the use of thorium fueled mini-subcritical reactors to generate power for a new generation of electric cars.  It was proposed by an outfit called Laser Power Systems (LPS).  Science may not be their strong suit, but their PR people must be top drawer.  They actually convinced the people at Cadillac to embarrass themselves by designing a “concept car” around the idea.

    Steven Ashley penned an article for GE’s Txchnologist website about the idea entitled, “Thorium lasers:  the perfectly plausible idea for nuclear cars.”  Ashley, who is described as a contributing editor for Scientific American and whose work has appeared in such venues as Popular Mechanics, MIT’s Technology Review, and Physics Today, does not explain on what credentials or academic background he bases his conclusion that the idea is “perfectly plausible.”  Presumably, none, because it isn’t.  He tells us, quoting an LPS official, that they are “working on a turbine/electric generator system that is powered by ‘an accelerator-driven thorium-based laser.'”  The same individual further assured him that the new technology “would be totally emissions-free, with no need for recharging.”  Ashley adds that,

    …because a gram of thorium has the equivalent potential energy content of 7,500 gallons of gasoline, LPS calculates that using just 8 grams of thorium in the unit could power an average car for 5,000 hours, or about 300,000 miles of normal driving.

    Where, the interested reader might ask, is all this energy to come from?  Lasers, after all, are not a source of energy.  In general, they are rather inefficient energy sinks.  I found several similar articles, and none of them ever gets around to explaining this intriguing mystery.  Well, the only possible way that such a small amount of thorium could come close to producing that much energy is via fission, and even if every bit of it underwent fission, it would still produce about an order of magnitude less energy than 7,500 gallons of gasoline.  We are assured that, “only a thin layer of aluminum foil is needed to shield people from the weakly emitting metal.”  True, but the same doesn’t apply to thorium’s fission products.  They would eventually accumulate to become a potentially deadly source of radiation unless heavily shielded.  None of the articles ever gets around to explaining where, exactly, the “thorium laser” comes in, what specific atomic transitions it would rely on, how, exactly, it would be pumped, and similar seemingly obvious questions.

    What can one do but shake one’s head and congratulate the LPS people on their brilliant success in bamboozling Cadillac and a whole host of ostensibly perfectly respectable science writers into taking seriously an idea that is completely wacky on the face of it?  I’m certainly glad that I don’t fall for such pseudo-scientific nonsense.  Oh, by the way, would anyone out there like to purchase a slightly used supply of fish oil pills?

  • Nuclear Power, Thorium, and the Role of Government

    Posted on May 6th, 2012 Helian 9 comments

    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.

  • Subcritical Thorium Reactors: Dr. Rubbia’s Really Bad Idea

    Posted on September 1st, 2010 Helian 18 comments

    The Telegraph (hattip Insty) turned the hype level to max in a recent article about the potential of thorium reactors.  According to the headline, “Obama could kill fossil fuels overnight with a nuclear dash for thorium.”  Against all odds, this is to happen in three to five years with a “new Manhattan Project,” and a “silver bullet” in the form of a new generation of thorium reactors.  The author is so vague about the technologies he’s describing that it’s hard to avoid the conclusion that he simply doesn’t know what he’s talking about, and couldn’t be bothered to spend a few minutes with Google to find out.  I’ll try to translate.

    It’s claimed that thorium “eats its own waste.”  In fact, thorium is very promising as a future source of energy, but this is nonsense.  Apparently it’s based on the fact that certain types of thorium reactors actually could burn their own fuel material, as well as plutonium scavenged from conventional reactor waste and other transuranics, much more completely than alternative designs.  This is certainly an advantage, but the fission products (lighter elements left over from the splitting of uranium and plutonium) would still be highly radioactive, and would certainly qualify as waste.  Such claims are so obviously spurious that they play into the hands of opponents of nuclear power.

    It is also claimed that “all (thorium) is potentially usable as fuel, compared to just 0.7% for uranium.”  In fact, thorium is not a fissile material, meaning that, unlike uranium 235 (U235), which is the 0.7% of natural uranium the author is referring to, it cannot sustain a nuclear chain reaction on its own.  It must first be converted to a lighter isotope of uranium, U233, which is fissile.  In fact, the U238 that makes up most of the rest of the leftover 99.3% percent of natural uranium is “potentially usable as fuel” in that sense as well, by conversion to plutonium 239, also a fissile material.

    The author is vague about exactly what kind of reactors he is referring to, lumping Dr. Carlo Rubbia’s subcritical design, which depends on a proton accelerator to provide enough neutrons to keep the fission process going, and molten fluoride salt reactors, which do not necessarily require such an accelerator.  He claims that, “Thorium-fluoride reactors can operate at atmospheric temperature,” which they certainly could not if the goal were to generate electric power.  I suspect that what he means here is that, unlike plutonium breeders, which require a high energy neutron spectrum to produce more fuel than they consume, thorium breeders could potentially use “thermal” neutrons that have been slowed to the point that their average energy, when converted to a “temperature,” would be much closer to that of the other material in the reactor core. 

    In any case, the design he seems to be so excited about is Dr. Rubbia’s “energy amplifier,” which, as noted above, would be subcritical, requiring a powerful, high current proton accelerator to keep the fission process going.  It would do this via spallation, a process in which a copious source of the neutrons required to keep the reaction going would be provided via interaction of the protons with heavy nuclei such as lead, or thorium itself.  This is the process used to produce neutrons at the Oak Ridge Spallation Neutron Source.  Such reactors could easily be “turned off” by simply shutting down the source of neutrons.  However, the idea that they would be inherently “safer” is dangerously inaccurate.  In fact, they would be an ideal path to covert acquisition of nuclear weapons.  Thorium reactors work by transmuting thorium into U233, which is the isotope that fissions to produce the lion’s share of the energy.  It is also an isotope that, like U235 and Pu239, can be used to make nuclear bombs. 

    The article downplays this risk as follows:

    After the Manhattan Project, US physicists in the late 1940s were tempted by thorium for use in civil reactors. It has a higher neutron yield per neutron absorbed. It does not require isotope separation, a big cost saving. But by then America needed the plutonium residue from uranium to build bombs.

    “They were really going after the weapons,” said Professor Egil Lillestol, a world authority on the thorium fuel-cycle at CERN. “It is almost impossible make nuclear weapons out of thorium because it is too difficult to handle. It wouldn’t be worth trying.” It emits too many high (energy) gamma rays.

    What Lillestol is referring to is the fact that, in addition to U233, thorium reactors also produce a certain amount of U232, a highly radioactive isotope of uranium with a half life of 68.9 years whose decay does, indeed, release potentially deadly gamma rays.  It would be extremely difficult, if not impossible, to remove it from the U233, and, if enough of it were present, it would certainly complicate the task of building a bomb.  The key phrase here is “if enough of it were present.”  Thorium enthusiasts like Lillestol never seem to do the math.  In fact, as can be seen here, even conventional thorium breeders could be designed to produce U233 sufficiently free of U232 to allow workers to fabricate a weapon without serious danger of receiving a lethal dose of gamma rays.  However, large concentrations of highly radioactive fission products would make it very difficult to surreptitiously extract the uranium, and it would also be possible to mix the fuel material with natural or depleted uranium, reducing the isotopic concentration of U233 below that necessary to make a bomb.

    With subcritical reactors of the type proposed by Rubbia, the problem of making a bomb gets a whole lot easier.  Rogue state actors, and even terrorists groups if we “succeed” in coming up with a sufficiently inexpensive design for high energy proton accelerators, could easily modify them to produce virtually pure U233, operating small facilities that it would be next to impossible for international monitors to detect.  There are two possible pathways for the production of U232 from thorium, both of which involve a reaction in which a neutron knocks two neutrons out of a heavy nucleus of Th232 or U233.  Those reactions can’t occur unless the initial neutron is carrying a lot of energy as can be seen in figure 8 of the article linked above, the threshold is around 6 million electron volts (MeV).  That means that, in order to produce virtually pure U233, all that’s necessary is to slow the incoming spallation neutrons below that energy.  That’s easily done.  Imagine two billiard balls on a table.  If you hit one as hard as you can at the other one, what happens when they collide?  If your aim was true, the first ball stops, transferring all its energy to the second one.  The same thing can be done with neutrons.  Pass the source neutrons through a layer of material full of light atoms such as paraffin or heavy water, and they will bounce off the light nuclei, losing energy in the process, until they eventually become “thermalized,” with virtually none of them having energies above 6 MeV.  If such low energy neutrons were then passed on to a subcritical core, they would produce U233 with almost no U232 contamination. 

    It gets worse.  Unlike Pu239, U233 does not emit a lot of spontaneous neutrons.  That means it can be used to make a simple gun-type nuclear weapon with little fear that a stray neutron will cause it to fizzle before optimum criticality is reached.  And, by the way, a lot less of it would be needed than would be required for a similar weapon using U235, the fissile material in the bomb that destroyed Hiroshima. 

    We’re quite capable of blowing ourselves up without Rubbia’s subcritical reactors.  Let’s not make it any easier than it already is.  Thorium reactors have many potential advantages over other potential sources of energy, including wind and solar.  However, if we’re going to do thorium, let’s do it right.

    UPDATE:  Steven Den Beste gets it right at Hot Air.  His commenters throw out the usual red herrings about the US choosing U235 and Pu239 over U233 in the Manhattan Project (for good reasons that had nothing to do with U233’s suitability as a bomb material) and the grossly exaggerated and misunderstood problem with U232.  You don’t have to be a nuclear engineer to see through these fallacious arguments.  The relevant information is all out there on the web, it’s not classified, and it can be understood by any bright high school student who takes the time to get the facts.