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  • 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

  • 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.