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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.

  • The Bomb and the Nuclear Posture Review

    Posted on April 22nd, 2017 Helian No comments

    A Nuclear Posture Review (NPR) is a legislatively mandated review, typically conducted every five to ten years.  It assesses such things as the role, safety and reliability of the weapons in the U.S. nuclear stockpile, the status of facilities in the nuclear weapons complex, and nuclear weapons policy in areas such as nonproliferation and arms control.  The last one was conducted in 2010.  The Trump Administration directed that another one be conducted this year, and the review is already in its initial stages.  It should be finished by the end of the year.  There is reason for concern about what the final product might look like.

    Trump has made statements to the effect that the U.S. should “expand its nuclear capability,” and that, “We have nuclear arsenals that are in very terrible shape.  They don’t even know if they work.”  Such statements have typically been qualified by his aides.  It’s hard to tell whether they reflect serious policy commitments, or just vague impressions based on a few minutes of conversation with some Pentagon wonk.  In fact, there are deep differences of opinion about these matters within the nuclear establishment.  That’s why the eventual content of the NPR might be problematic.  There have always been people within the nuclear establishment, whether at the National Nuclear Security Administration (NNSA), the agency within the Department of Energy responsible for maintaining the stockpile, or in the military, who are champing at the bit to resume nuclear testing.  Occasionally they will bluntly question the reliability of the weapons in our stockpile, even though by that very act they diminish the credibility of our nuclear deterrent.  If Trump’s comments are to be taken seriously, the next NPR may reflect the fact that they have gained the upper hand.  That would be unfortunate.

    Is it really true that the weapons in our arsenal are “in very terrible shape,” and we “don’t even know if they work?”  I doubt it.  In the first place, the law requires that both the Department of Energy and the Department of Defense sign off on an annual assessment that certifies the safety and reliability of the stockpile.  They have never failed to submit that certification.  Beyond that, the weapons in our stockpile are the final product of more than 1000 nuclear tests.  They are both safe and robust.  Any credible challenge to their safety and reliability must cite some plausible reason why they might fail.  I know of no such reason.

    For the sake of argument, let’s consider what might go wrong.  Modern weapons typically consist of a primary and a secondary.  The primary consists of a hollow “pit” of highly enriched uranium or plutonium surrounded by high explosive.  Often it is filled with a “boost” gas consisting of a mixture of deuterium and tritium, two heavy isotopes of hydrogen.  When the weapon is used, the high explosive implodes the pit, causing it to form a dense mass that is highly supercritical.  At the same time, nuclear fusion takes place in the boost gas, producing highly energetic neutrons that enhance the yield of the primary.  At the right moment an “initiator” sends a burst of neutrons into the imploded pit, setting off a chain reaction that results in a nuclear explosion.  Some of the tremendous energy released in this explosion in the form of x-rays then implodes the secondary, causing it, too, to explode, adding to the yield of the weapon.

    What could go wrong?  Of course, explosives are volatile.  Those used to implode the primary might deteriorate over time.  However, these explosives are carefully monitored to detect any such deterioration.  Other than that, the tritium in the boost gas is radioactive, and has a half life of only a little over 12 years.  It will gradually decay into helium, reducing the effectiveness of boosting.  This, too, however is a well understood process, and one which is carefully monitored and compensated for by timely replacement of the tritium.  Corrosion of key parts might occur, but this too, is carefully checked, and the potential sources are well understood.  All these potential sources of uncertainty affect the primary.  However, much of the uncertainty about their effects can be eliminated experimentally.  Of course, the experiments can’t include actual nuclear explosions, but surrogate materials can be substituted for the uranium and plutonium in the pit with similar properties.  The implosion process can then be observed using powerful x-ray or proton beams.  Unfortunately, our experimental capabilities in this area are limited.  We cannot observe the implosion process all the way from the initial explosion to the point at which maximum density is achieved in three dimensions taking “snapshots” at optimally short intervals.  To do that, we would need what has been referred to as an Advanced Hydrodynamic Facility, or AHF.

    We currently have an unmatched suite of above ground experimental facilities for studying the effects of aging on the weapons in our stockpile, including the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, the Z Machine at Sandia National Laboratories, and the Dual-Axis Radiographic Hydrodynamic Test facility (DARHT) at Los Alamos.  These give us a very significant leg up on the international competition when it comes to maintaining our stockpile.  That is a major reason why it would be foolish for us to resume nuclear testing.  We would be throwing away this advantage.  Unfortunately, while we once seriously considered building an AHF, basically an extremely powerful accelerator, we never got around to doing so.  It was a serious mistake.  If we had such a facility, it would effectively pull the rug out from under the feet of those who want to resume testing.  It would render all arguments to the effect that “we don’t even know if they work” moot.  We could demonstrate with a very high level of confidence that they will indeed work.

    But that’s water under the bridge.  We must hope that cooler heads prevail, and the NPR doesn’t turn out to be a polemic challenging the credibility of the stockpile and advising a resumption of testing.  We’re likely to find out one way or the other before the end of the year.  Keep your fingers crossed.

  • 3+2: More Tinkering with the Nuclear Arsenal

    Posted on October 10th, 2016 Helian No comments

    It really seems as if the weapon designers at the nation’s three nuclear weapons laboratories, Los Alamos, Livermore, and Sandia, never really believed that nuclear testing would ever end.  If so, they were singularly blind to the consequences.  Instead of taking the approach apparently adopted by the Russians of designing and testing robust warheads that could simply be scrapped and replaced with newly manufactured ones at the end of their service life, they decided to depend on a constant process of refurbishing old warheads, eliminating the ability to make new ones in the process.  When our weapons got too old, they would be repeatedly patched up in so-called Life Extension Programs, or LEPs.  Apparently it began to occur to an increasing number of people in the weapons community that maintaining the safety and reliability of the stockpile indefinitely using that approach might be a bit problematic.

    The first “solution” to the problem proposed by the National Nuclear Security Administration (NNSA), the semi-autonomous agency within the Department of Energy (DOE) responsible for maintaining the nuclear stockpile, was the Reliable Replacement Warhead (RRW).  It was to be robust, easy to manufacture, and easy to maintain.  It was also a new, untested design.  As such, it would have violated the spirit, if not the letter, of Article VI of the Non-proliferation Treaty (NPT).  If it had been built, it would also very likely have forced violation of the Comprehensive Nuclear Test Ban Treaty (CTBT), which the U.S. has signed, but never ratified.  It was claimed that the RRW could be built and certified without testing.  This was very probably nonsense.  There have always been more or less influential voices within NNSA, the Department of Defense (DoD), and the weapons labs, in favor of a return to nuclear testing.  That would not have been a good thing then, and I doubt that it will be a good thing at any foreseeable time in the future.  In general, I think we should do our best to keep the nuclear genie bottled up as long as possible.  Fortunately, Congress agreed and killed the RRW Program.

    That didn’t stop the weaponeers.  They just tried a new gambit.  It’s called the “3+2 Strategy.”  There are currently four types of ballistic missile warheads, two bombs, and a cruise missile warhead in the U.S. arsenal.  The basic idea of 3+2 would be to reduce this to three “interoperable” ballistic missile warheads and two air delivered weapons (a bomb and a cruise missile), explaining the “3+2.”  In the process, the conventional chemical explosives that drive the implosion of the “atomic bomb” stage of the weapons would be replaced by insensitive high explosives (IHE).  The result would supposedly be a safer, more secure stockpile that would be easier to maintain.  The price tag, in round numbers, would be $60 billion.

    I can only hope Congress will be as quick to deep six 3+2 as it was with the RRW.  The 3+2 will require tinkering not only with the bits surrounding the nuclear explosive package (NEP), but with the NEP itself.  In other words, its just as much a violation of the spirit of Article VI of the NPT as was the RRW.  The predictable result of any such changes will be the “sudden realization” by the weapons labs somewhere down the line that they can’t certify the new designs without a return to nuclear testing.  There’s a better and, in the long run, probably cheaper way to maintain the stockpile.

    In the first place, we need to stop relying on LEPs, and return to manufacturing replacement weapons.  The common argument against this is that we have lost the ability to manufacture critical parts of our weapons since the end of testing, and in some cases the facilities and companies that supplied the parts no longer exist.  Nonsense!  The idea that a country responsible for a quarter of the entire world’s GDP has lost the ability to reproduce the weapons it was once able to design, build and test in a few years is ridiculous.  We are told that subtle changes in materials might somehow severely degrade the performance of remanufactured weapons.  I doubt it.  Regardless, DOE has always known there was a solution to that problem.  It’s called the Advanced Hydrodynamic Facility, or AHF.

    Basically, the AHF would be a giant accelerator facility capable of producing beams that would be able to image an imploding nuclear weapon pit in three dimensions and at several times during the implosion.  Serious studies of such a facility were done as long ago as the mid-90’s, and there is no doubt that it is feasible.  In actual experiments, of course, highly enriched uranium and plutonium would be replaced by surrogate materials such as tungsten, but they would still determine with a high degree of confidence whether a given remanufactured primary would work or not.  The primary, or “atomic bomb” part of a weapon supplies the energy that sets off the secondary, or thermonuclear part.  If the primary, of a weapon works, then there can be little doubt that the secondary will work as well.  The AHF would be expensive, which is probably the reason it still hasn’t been built.  Given the $60 billion cost of 3+2, that decision may well prove to be penny-wise and pound-foolish.

    The whole point of having a nuclear arsenal is its ability to deter enemies from attacking us.  Every time people who are supposed to be the experts about such things question the reliability of our stockpile, they detract from its ability to deter.  I think a remanufacturing capability along with the AHF is the best way to shut them up, preventing a very bad decision to resume nuclear testing in the process.  I suggest we get on with it.

  • 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

  • Do We Really Need New Nukes?

    Posted on December 2nd, 2014 Helian No comments

    If an article that just appeared in the LA Times is any indication, the agitation for jump-starting the nuclear weapons program at the Department of Energy (DOE) and the three nuclear weapons laboratories (Lawrence Livermore, Los Alamos, and Sandia National Laboratories) continues unabated. Entitled “New nuclear weapons needed, many experts say, pointing to aged arsenal,” it cites all the usual talking points of the weaponeers. For example,

    Warheads in the nation’s stockpile are an average of 27 years old, which raises serious concerns about their reliability, they say. Provocative nuclear threats by Russian President Vladimir Putin have added to the pressure to not only design new weapons but conduct underground tests for the first time since 1992.

    “It seems like common sense to me if you’re trying to keep an aging machine alive that’s well past its design life, then you’re treading on thin ice,” said Rep. Mac Thornberry (R-Texas), chairman-elect of the House Armed Services Committee. “Not to mention, we’re spending more and more to keep these things going.”

    Thornbury also offered support for renewed testing, saying, “You don’t know how a car performs unless you turn the key over. Why would we accept anything less from a weapon that provides the foundation for which all our national security is based on?”

    Such comments are entirely typical. They would make a lot of sense if the U.S. nuclear weapons program existed in a vacuum. However, it doesn’t. It exists in a world with several other major nuclear powers, and they all have the same problems. Under the circumstances, the fact that such problems exist and are shared by all the nuclear powers is less significant than the question of which nuclear power is best equipped to deal with them. The question of who will benefit by the building of new weapons and a resumption of nuclear testing depends on the answer to that question. If one country has a significant advantage over its rivals in dealing with a common problem as long as the status quo is maintained, then it would be very ill-advised to initiate a change to the status quo that would allow them to catch up.  At the moment, the United States is the country with an advantage. As noted in the article,

    The U.S. has by far the greatest archive of test data, having conducted 1,032 nuclear tests. Russia conducted 715 and China only 45.

    Beyond that, we have the ability to conduct tests with conventional explosives that mimic what goes on in the initial stages of a nuclear explosion, and superb diagnostics to extract a maximum of data from those tests. Perhaps more importantly, we have an unrivaled above ground experimental, or AGEX, capability. I refer to machines like Z at Sandia National Laboratories, or the NIF at Livermore, which are far more capable and powerful than similar facilities anywhere else in the world. Those who say they can’t access physical conditions relevant to those that occur in exploding nuclear weapons, or that they are useless for weapon effects or weapon physics experiments, either don’t know what they’re talking about or are attempting to deceive.

    As far as the NIF is concerned, it is quite true that it has so far failed to achieve its fusion ignition milestone, but that by no means rules out the possibility that it ever will. More importantly, the NIF will remain a highly useful AGEX facility whether it achieves ignition or not. Indeed, before it was built, many of the weapons designers showed little interest in ignition. It would merely “muddy the waters,” making it more difficult for the diagnostics to precisely record the results of an experiment. The NIF could access weapons-relevant conditions without it. In fact, in spite of its failure to achieve ignition to date, the NIF has been a spectacular success as far as achieving its specifications are concerned. It is more than an order of magnitude more powerful than any previously existing laser system, its 192 laser beams are highly accurate, and its diagnostic suite is superb.

    Another problem with the resumption of testing is that it will lead to the development of weapons that are much more likely to be used. Once the nuclear genie is out of the bottle, it will likely prove very difficult to put it back in. For example, again quoting the article,

    John S. Foster Jr., former director of Lawrence Livermore National Laboratory and chief of Pentagon research during the Cold War, said the labs should design, develop and build prototype weapons that may be needed by the military in the future, including a very low-yield nuclear weapon that could be used with precision delivery systems, an electromagnetic pulse weapon that could destroy an enemy’s communications systems and a penetrating weapon to destroy deeply buried targets.

    The commonly heard narrative at DOE goes something like this: “We need to develop small, precise, penetrating nuclear weapons because they will be a much better deterrent than the existing ones. Potential enemies are unlikely to believe that we would ever use one of the high yield weapons that are all that remain in the current arsenal. They would be far more likely to believe that we might use a small bunker buster that would minimize the possibility of significant collateral damage.” The problem with that narrative is that it’s true. We would be far more likely to use such a weapon than the ones in the current arsenal, and there would be no lack of voices within DOE and DoD calling for its use if an appropriate opportunity ever arose.

    I can understand the agitation for a resumption of testing. It’s a lot sexier to make things that go boom than to serve as custodians for an aging pile of existing nukes. Unfortunately, the latter course is the wiser one. By resuming nuclear testing we would really be unilaterally surrendering a huge advantage, playing into the hands of our enemies and destabilizing the nuclear landscape at the same time.

  • Comments on Some Comments on the National Ignition Facility

    Posted on September 23rd, 2014 Helian No comments

    We live in a dauntingly complex world.  Progress in the world of science is relevant to all of us, yet it is extremely difficult, although certainly not impossible, for the intelligent layperson to gain a useful understanding of what is actually going on.  I say “not impossible” because I believe it’s possible for non-experts to gain enough knowledge to usefully contribute to the conversation about the technological and social relevance of a given scientific specialty, if not of its abstruse details, assuming they are willing to put in the effort.  Indeed, when it comes to social relevance it’s not out of the question for them to become more knowledgeable than the scientists themselves, narrowly focused as they often are on a particular specialty.

    To illustrate my point, I invite my readers to take a look at a post that recently appeared on the blog LLNL – The True Story.  LLNL, or Lawrence Livermore National Laboratory, is one of the nation’s three major nuclear weapons research laboratories.  It is also home of the National Ignition Facility, which, as its name implies, was designed to achieve fusion “ignition” by focusing a giant assembly of 192 powerful laser beams on tiny targets containing a mixture of deuterium and tritium fuel.  The process itself is called inertial confinement fusion, or ICF.  Ignition is variously defined, but as far as the NIF is concerned LLNL officially accepted the definition as fusion energy out equal to total laser energy in, in the presence of members of a National Academy of Sciences oversight committee.  This is a definition that puts it on a level playing field with the competing magnetic confinement approach to fusion.

    According to the blurb that appears on the home page of LLNL – The True Story, its purpose is “for LLNL present and past employees, friends of LLNL and anyone impacted by the privatization of the Lab to express their opinions and expose the waste, wrongdoing and any kind of injustice against employees and taxpayers by LLNS/DOE/NNSA.”  The post in question is entitled ICF Program is now Officially Owned by WCI (Weapons and Concepts Integration).  It’s certainly harmless enough as it stands, consisting only of the line,

    ICF program is now officially owned by WCI.  A step forward or an attempt to bury it out of sight?

    This is followed by an apparently broken link to the story referred to.  This gist can probably be found here.  Presumably the author suspects LLNL might want to “bury it out of sight” because the first attempt to achieve ignition, known as the National Ignition Campaign, or NIC, failed to achieve its goal.  What’s really of interest is not the post itself, but the comments following it.  The commenters are all listed as “anonymous,” but given the nature of the blog we can probably assume that most of them are scientists of one tribe or another.  Let’s take a look at what they have to say.  According to the first “anonymous,”

    If (takeover of NIF by WCI) is an attempt to keep funding flowing by switching milestones from energy independence to weapons research.  “Contingency Plan B”.

    Another “anonymous” writes in a similar vein:

    Reading between the lines it is clear that the new energy source mission of the NIF is over and now it’s time to justify the unjustifiable costs by claiming it’s a great too for weapons research.

    Perhaps the second commenter would have done better to read the lines as they stand rather than between them.  In that case he would have noticed that energy independence was never an official NIF milestone, not to mention its “mission.”  NIF was funded for the purpose of weapons research from the start.  This fact was never in any way a deep, dark secret, and has long been obvious to anyone willing to take the trouble to consult the relevant publicly accessible documents.  The Inertial Confinement Fusion Advisory Committee, a Federal Advisory Committee that met intermittently in the early to mid-90’s, and whose member included a bevy of heavyweights in plasma physics and related specialties, was certainly aware of the fact, and recommended funding of the facility with the single dissenting vote of Tim Coffey, then Director of the Naval Research Laboratory, based on that awareness.

    Be that as it may, the claim that the technology could also end our dependence on fossil fuel, often made by the NIF’s defenders, is credible.  By “credible” I mean that many highly capable scientists have long held and continue to hold that opinion.  As it happens, I don’t.  Assuming we find a way to achieve ignition and high gain in the laboratory, it will certainly become scientifically feasible to generate energy with ICF power plants.  However, IMHO it will never be economically feasible, for reasons I have outline in earlier posts.  Regardless, from a public relations standpoint, it was obviously preferable to evoke the potential of the NIF as a clean source of energy rather than a weapons project designed to maintain the safety and reliability of our nuclear arsenal, as essential as that capability may actually be.  In spite of my own personal opinion on the subject, these claims were neither disingenuous nor mere “hype.”

    Another “anonymous” writes,

    What’s this user facility bullshit about?  Only Livermore uses the facility.  Cost recovery demands that a university would have to pay $1 million for a shot.  How can it be a user facility if it’s run by the weapons program?  This isn’t exactly SLAC we’re talking about.

    Here, again, the commenter is simply wrong.  Livermore is not the only user of NIF, and it is, in fact, a user facility.  Users to date include a team from MIT headed by Prof. Richard Petrasso.  I’m not sure how the users are currently funded, but in the past funds for experiments on similar facilities were allocated through a proposal process, similar to that used to fund other government-funded academic research.  The commenter continues,

    By the way, let’s assume NIF wants to be a “user facility” for stockpile stewardship.  Since ignition is impossible, the EOS (Equation of State, relevant to the physics of nuclear weapons, ed.) work is garbage, and the temperatures are not relevant to anything that goes bang, what use is this machine?

    NIF does not “want to be a user facility for stockpile stewardship.”  Stress has always been on high energy density physics (HEDP), which has many other potential applications besides stockpile stewardship.  I was not surprised that NIF did not achieve ignition immediately.  In fact I predicted as much in a post on this blog two years before the facility became operational.  However, many highly competent scientists disagreed with me, and for credible scientific reasons.  The idea that ignition is “impossible” just because it wasn’t achieved in the first ignition campaign using the indirect drive approach is nonsense.  Several other credible approaches have not yet even been tried, including polar direct drive, fast ignitor, and hitting the targets with green (frequency doubled) rather than blue (frequency tripled) light.  The latter approach would enable a substantial increase in the available laser energy on target.  The EOS work is not garbage, as any competent weapons designer will confirm as long as they are not determined to force the resumption of nuclear testing by hook or by crook, and some of the best scientists at Livermore confirmed long ago that the temperatures  achievable on the NIF are indeed relevant to things that go bang, whether it achieves ignition or not.  In fact, the facility allows us to access physical conditions that can be approached in the laboratory nowhere else on earth, giving us a significant leg up over the international competition in maintaining a safe and reliable arsenal, as long as testing is not resumed.

    Anonymous number 4 chimes in,

    I love this quote (apparently from the linked article, ed.):

    “the demonstration of laboratory ignition and its use to support the Stockpile Stewardship Program (SSP) is a major goal for this program”

    Hey guys, this has already failed.  Why are we still spending money on this?  A lot of other laboratories could use the $$.  You’re done.

    The quote this “anonymous” loves is a simple statement of fact.  For the reasons already cited, the idea that ignition on the NIF is hopeless is nonsense.  The (very good) reason we’re still spending money on the project is that NIF is and will continue into the foreseeable future to be one of the most capable and effective above ground experimental (AGEX) facilities in the world.  It can access physical conditions relevant to nuclear weapons regardless of whether it achieves ignition or not.  For that reason it is an invaluable tool for maintaining our arsenal unless one’s agenda happens to be the resumption of nuclear testing.  Hint:  The idea that no one in DOE, NNSA, or the national weapons laboratories wants to resume testing belongs in the realm of fantasy.  Consider, for example, what the next “anonymous” is actually suggesting:

    Attempting to get funding for NIF and computations’s big machines was made easier by claiming dual purposes but I always felt that the real down and dirty main purpose was weapons research.  If you want to get support from the anti-weapon Feinstein/Boxer/Pelosi contingent you need to put the “energy” lipstick on the pig.  Or we could go back to testing.  Our cessation of testing doesn’t seem to have deterred North Korea and Iran that much.

    Yes, Virginia, even scientists occasionally do have agendas of their own.  What can I say?  To begin, I suppose, that one should never be intimidated by the pontifications of scientists.  The specimens on display here clearly don’t have a clue what they’re talking about.  Any non-technical observer of middling intelligence could become more knowledgeable than they are on the topics they’re discussing by devoting a few hours to researching them on the web.  As to how the non-technical observer is to acquire enough knowledge to actually know that he knows more than the scientific specialists, I can offer no advice, other than to head to your local university and acquire a Ph.D.  I am, BTW, neither employed by nor connected in any other way with LLNL.

     

  • The United States’ Nuclear Future

    Posted on February 23rd, 2014 Helian No comments

    There are lots of great ideas out there for improving the way we do nuclear power.  For instance, Transatomic Power recently proposed a novel type of molten salt reactor (MSR).  The Next Generation Nuclear Plant (NGNP) Industry Alliance, with support from the U.S. Department of Energy, has chosen a high temperature gas reactor (HTGR) as its reactor of the future candidate.  Small modular reactors (SMRs) are all the rage, and a plethora of designs have been proposed.  Unlike the others, Terrapower’s traveling wave reactor (TWR), which is backed by Bill Gates, actually has a fighting chance to be built in the foreseeable future – in China.  With the possible exception of SMR’s, which have strong military support, the chances of any of them being built in the United States in the foreseeable future are slim.  Government, the courts, and a nightmarish regulatory process stand in the way as an almost insuperable barrier.

    It wasn’t always this way.  A lot of today’s “novel” concepts are based on ideas that were proposed many decades ago.  We know they work, because demonstration reactors were built to try them out.  More than a dozen were built at Oak Ridge National Laboratory in Tennessee.  No less than 53 were built at Idaho National Laboratory!  Virtually all of them were completed more than half a century ago.  There are few historical precedents that can match the sudden collapse from the vitality of those early years to the lethargy and malaise prevailing in the nuclear industry today.  It’s sad, really, because the nuclear plants that actually are on line and/or under construction are artifacts of a grossly wasteful, potentially dangerous, and obsolete technology.

    The light water reactors (LWRs) currently producing energy in this country use only a tiny fraction of the energy available in their uranium fuel, producing dangerous transuranic actinides that can remain highly radioactive for millennia in the process.  Many of the new designs are capable of extracting dozens of times more energy from a given quantity of fuel than LWRs.  Molten salt reactors would operate far more efficiently, could not melt down, and would consume dangerous actinides in the process, leaving such a small quantity of waste after several decades of operation that it would be less radioactive than the original ore used to fuel the reactor after a few hundred years rather than many millennia.  Besides also being immune to meltdown, HTGRs, because of their much higher operating temperatures, could enable such things as highly efficient electrolysis of water to produce hydrogen fuel and greatly improved extraction techniques for oil and natural gas from shale and sand.  Why, then, aren’t we building these improved designs?

    It’s highly unlikely that the necessary initiative will come from industry.  Why would they care?  They’re in the business to make a profit, and LWRs can be built and operated more cheaply than the alternatives.  Why should they worry about efficiency?  There’s plenty of cheap uranium around, and it’s unlikely there will be major shortages for decades to come.  Ask any industry spokesman, and he’ll assure you that transuranic radioactive waste and the potential proliferation issues due to the plutonium content of spent LWR fuel are mere red herrings.  I’m not so sure.

    In other words, strong government leadership would be needed to turn things around.  Unfortunately, that commodity is in short supply.  The current reality is that government is a highly effective deterrent to new reactor technology.  Take the Nuclear Regulatory Commission (NRC) for example.  Read Kafka’s The Trial and you’ll have a pretty good idea of how it operates.  So you want to license a new reactor design, do you?  Well, most of the current regulations apply specifically to LWRs, so you’ll have to give them time to come up with new ones.  Then you’ll need to spend at least a decade and millions of dollars explaining your new technology to the NRC bureaucrats.  Then you can expect an endless stream of requests for additional information, analysis of all the threat and failure scenarios they can dream up, etc., which will likely take a good number of additional years.  After all, they have to justify their existence, don’t they?  If you ever manage to get past the NRC, the court system will take things up where they left  off.

    What to do?  I don’t know.  It really doesn’t upset me when reactors built with legacy technology are pulled off line, and replaced with fossil fueled plants.  They just waste most of their fuel, throwing away energy that future generations might sorely miss once they’ve finally burned through all the coal and oil on the planet.  Maybe the best thing to do would be to just buy up all the available uranium around and wait.  We might also stop the incredibly block-headed practice of converting all of our “depleted” uranium into ammunition.  The Lone Ranger’s silver bullets were cheap by comparison.  Future generations are likely to wonder what on earth we were thinking.

    Things were a lot better in the “apathetic” 50’s, but the novelist Thomas Wolfe had it right.  You can’t go home again.

     

  • New Reactors in the UK and the Future of Nuclear Power

    Posted on October 22nd, 2013 Helian 2 comments

    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.

  • 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.

  • Fusion Update: The NIF Inches Closer to Ignition

    Posted on August 30th, 2013 Helian No comments

    In a recent press release, Lawrence Livermore National Laboratory (LLNL) announced that it had achieved a yield of 3 x 1015 neutrons in the latest round of experiments at its National Ignition Facility, a giant, 192-beam laser facility designed, as its name implies, to achieve fusion ignition.  That’s nowhere near “ignition,” but still encouraging as it’s three times better than results achieved in earlier experiments.

    The easiest way to achieve fusion is with two heavy isotopes of hydrogen; deuterium, with a nucleus containing one proton and one neutron, and tritium, with a nucleus containing one proton and two neutrons.  Deuterium is not radioactive, and occurs naturally as about one atom to every 6400 atoms of “normal” hydrogen, with a nucleus containing only a single proton.  Tritium is radioactive, and occurs naturally only in tiny trace amounts.  It has a half-life (the time it takes for half of a given amount to undergo radioactive decay) of 12.3 years, and must be produced artificially.  When tritium and deuterium fuse, they release a neutron, a helium nucleus, or alpha particle, and lots of energy (17.6 million electron volts).

    Fortunately (because otherwise it would be too easy to blow up the planet), or unfortunately (if you want to convert the energy into electricity), fusion is hard.  The two atoms don’t like to get too close, because their positively charged nuclei repel each other.  Somehow, a way must be found to make the heavy hydrogen fuel material very hot, causing the thermal motion of the atoms to become very large.  Once they start moving fast enough, they can smash into each other with enough momentum to overcome the repulsion of the positive nuclei, allowing them to fuse.  However, the amount of energy needed per atom is huge, and when atoms get that hot, the last thing they want to do is stay close to each other (think of what happens in the detonation of high explosive.)  There are two mainstream approaches to solving this problem; magnetic fusion, in which the atoms are held in place by powerful magnetic fields while they are heated (the approach being pursued at ITER, the International Thermonuclear Experimental Reactor, currently under construction in France), and inertial confinement fusion (ICF), where the idea is to dump energy into the fuel material so fast that its own inertia holds it in place long enough for fusion to occur.  The NIF is an ICF facility.

    There are various definitions of ICF “ignition,” but, in order to avoid comparisons of apples and oranges between ICF and magnetic fusion experiments, LLNL has explicitly accepted the point at which the fusion energy out equals the laser energy in as the definition of ignition.  In the experiment referred to above, the total fusion energy release was about 10,000 joules, give or take.  Since the laser energy in was around 1.7 million joules, that’s only a little over one half of one percent of what’s needed for ignition.  Paltry, you say?  Not really.  To understand why, you have to know a little about how ICF experiments work.

    Recall that the idea is to heat the fuel material up so fast that its own inertia holds it in place long enough for fusion to occur.  The “obvious” way to do that would be to simply dump in enough laser energy to heat all the fuel material to fusion temperatures at once.  Unfortunately, this “volumetric heating” approach wouldn’t work.  The energy required would be orders of magnitude more than what’s available on the NIF.  What to do?   Apply lots and lots of finesse.  It turns out that if a very small volume or “hot spot” in the fuel material can be brought to fusion conditions, the alpha particles released in the fusion reactions might carry enough energy to heat up the nearby fuel to fusion conditions as well.  Ideally, the result would be an alpha “burn wave,” moving out through the fuel, and consuming it all.  But wait, it ain’t that easy!  An efficient burn wave will occur only if the alphas are slammed to a stop and forced to dump their energy after traveling only a very short distance in the cold fuel material around the hot spot.  Their range is too large unless the fuel is first compressed to a tiny fraction of its original volume, causing its density to increase by orders of magnitude.

    In other words, to get the fuel to fuse, we need to make it very hot, but we also need to compress it to very high density, which can be done much more easily and efficiently if the material is cold!  Somehow, we need to keep the fuel “cold” during the compression process, and then, just at the right moment, suddenly heat up a small volume to fusion conditions.  It turns out that shocks are the answer to the problem.  If a train of four shocks can be set off in the fuel material as it is being compressed, or “imploded,” by the lasers, precisely timed so that they will all converge at just the right moment, it should be possible, in theory at least, to generate a hot spot.  If the nice, spherical symmetry of the fuel target could be maintained during the implosion process, everything should work just fine.  The NIF would have more than enough energy to achieve ignition.  But there’s the rub. Maintaining the necessary symmetry has turned out to be inordinately hard.  Tiny imperfections in the target surface finish, small asymmetries in the laser beams, etc., lead to big deviations from perfect symmetry in the dense, imploded fuel.  These asymmetries have been the main reason the NIF has not been able to achieve its ignition goal to date.

    And that’s why the results of the latest round of experiments haven’t been as “paltry” as they seem.  As noted in the LLNL press release,

    Early calculations show that fusion reactions in the hot plasma started to self-heat the burning core and enhanced the yield by nearly 50 percent, pushing close to the margins of alpha burn, where the fusion reactions dominate the process.

    “The yield was significantly greater than the energy deposited in the hot spot by the implosion,” said Ed Moses, principle associate director for NIF and Photon Science. “This represents an important advance in establishing a self-sustaining burning target, the next critical step on the path to fusion ignition on NIF.”

    That’s not just hype.  If the self-heating can be increased in future experiments, it may be possible to reach a threshold at which the alpha heating sets off a burn wave through the rest of the cold fuel, as described above.  In other words, ignition is hardly a given, but the guys at LLNL still have a fighting chance.  Their main challenge may be to stem the gradual evaporation of political support for NIF while the experiments are underway.  Their own Senator, Diane Feinstein, is anything but an avid supporter.  She recently turned down appeals to halt NIF budget cuts, and says the project needs to be “reassessed” in light of the failure to achieve ignition.

    Such a “reassessment” would be a big mistake.  The NIF was never funded as an energy project.  Its support comes from the National Nuclear Security Administration (NNSA), a semi-autonomous arm of the Department of Energy charged with maintaining the safety and reliability of the nation’s nuclear arsenal.  As a tool for achieving that end, the NIF is without peer in any other country.  It has delivered on all of its performance design goals, including laser energy, illumination symmetry, shot rate, the precision and accuracy of its diagnostic instrumentation, etc.  The facility is of exceptional value to the weapons program even if ignition is never achieved.  It can still generate experimental conditions approaching those present in an exploding nuclear device, and, along with the rest of our suite of “above-ground experimental facilities,” or AGEX, it gives us a major leg up over the competition in maintaining our arsenal and avoiding technological surprise in the post-testing era.

    Why is that important?  Because the alternative is a return to nuclear testing.  Do you think no one at NNSA wants to return to testing, and that the weapon designers at the National Weapons Laboratories wouldn’t jump at the chance?  If so, you’re dreaming.  It seems to me we should be doing our best to keep the nuclear genie in the bottle, not let it out.  Mothballing the NIF would be an excellent start at pulling the cork!

    I understand why the guys at LLNL are hyping the NIF’s potential as a source of energy.  It’s a lot easier to generate political support for lots of electricity with very little radioactive waste and no greenhouse gases than for maintaining our aging arsenal of nuclear weapons.  However, IMHO, ICF is hopeless as a source of electricity, at least for the next few hundred years.  I know many excellent scientists will disagree, but many excellent scientists are also prone to extreme wishful thinking when it comes to rationalizing a technology they’ve devoted their careers to.  Regardless, energy hype isn’t needed to justify the NIF.  It and facilities like it will insure our technological superiority over potential nuclear rivals for years to come, and at the same time provide a potent argument against the resumption of nuclear testing.