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Sustainable nuclear energy breakthrough.

Discussion in 'Real Life Discussion' started by funflash, Feb 13, 2014.

  1. funflash

    funflash First Year

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  2. Perspicacity

    Perspicacity High Score: 3,994 Prestige DLP Supporter

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    TL;DR: It's headline-grabbing hype and oversells what is an otherwise very nice scientific result. But that's really all it is: a bit of good science, not a landmark achievement or major milestone.

    The somewhat longer version:

    In inertial confinement fusion (ICF), you take a big bank of lasers, whose amplifiers take up the space of about three football fields, and you focus the laser energy (an amount needed to bench press a brachiosaurus five times) into the ends of a cylinder about a centimeter long and made out of a mixture of gold and uranium called a hohlraum.

    The 192 laser beams hit the inner walls of the hohlraum and heat it up. Hot things glow. The hotter a thing is, the higher the energy of the radiation it gives off. The universe glows, a bit of leftover from the Big Bang, but in the microwaves. People glow in the infrared, which is how night-vision goggles work. The surface of the sun, at 5800 K temperature, glows in the visible range. The inner walls of the hohlraum wall glow after the laser hits them, but as x-rays. So you can think of the hohlraum as a fancy converter of radiation from visible light (the NIF lasers are blue, at about 351 nm wavelength) to x-rays, acting as a very hot oven. This conversion loses quite a lot of energy, however, 80% or more of the incident laser energy.

    The x-rays from this x-ray oven are absorbed in a plastic or diamond capsule and they compress it very symmetrically (though not quite symmetric enough, as we'll see in a moment). Here's a picture of a capsule:

    [​IMG]

    The laser energy is absorbed in the capsule and causes the outer part to boil off. Momentum is conserved, so to make stuff boil off the outside of the capsule, the thing has to be pushed inward, rather like a rocket engine, though think of rocket engines strapped all around the capsule each firing simultaneously and causing thing to compress. This squishing happens until the deuterium-tritium "fuel" in the core goes from cryogenic conditions cold enough to freeze hydrogen to roughly 20 times the density of lead and 100 million Kelvin, at which point fusion reactions should start to go. This requires shrinking the radius of the capsule by a factor of around 35.

    If you get everything to work just so, and if our models for how thermonuclear burn works are accurate, then you create the conditions for thermonuclear fusion to happen in the "hot spot" at the center of the capsule. If you get the hot spot fuel burning fast enough, the alpha particles (helium nuclei) that form as a bi-product of the deuterium + tritium -> alpha (3.5 MeV) + neutron (14.1 MeV) reaction can heat the surrounding fuel, bringing it up to fusion burning temperatures as well, "igniting" it in the manner of kindling in your fireplace igniting the nearby logs. And ignition is the real milestone, which hasn't happened (not even close).

    So why did NIF fail to ignite after all these years? For a long time, we didn't know. That's what this campaign was to find out. It turns out that squishing things in radius by a factor of 35 is incredibly hard to do. Try this experiment yourself sometime: take a balloon and try to squeeze the balloon to half its volume--you have enough strength in your body to do so, but you'll find the balloon bulging out between your fingers before you can. NIF tries to do the same thing, but with more of a factor-of-10000 compression in volume. When squishing stuff, whether a balloon with your hands or a NIF capsule, you tend to find "low mode asymmetries," a fancy way of saying that stuff doesn't like being squeezed and it squirts out the places that aren't squeezing as much. To really squish stuff well, you need exquisite symmetry.

    In the NIF point design (variants of which folks have been trying to get to ignite for about three years now), they do this squeezing with four exquisitely timed shock waves that arrive at the center of the fuel at the same instant. Even the slightest errors in timing or symmetry get amplified during compression, making the final assembly anything but a nice, symmetric hot spot.

    The "high foot" campaign being reported in the article on was designed to figure out why NIF failed to ignite and to test the hypothesis that it was these low mode asymmetries that were the cause. The idea was to make compressed fuel with fewer shocks--not four but three (or perhaps even two) shock waves, each of which is stronger than the four shocks in the point design, but leading to less overall compression and an implosion that is far less error prone.

    This campaign, designed to test a reason for failure, succeeded in getting a much more symmetric hot spot, releasing quite a lot more fusion energy than before, and even showing evidence of sufficient symmetry of fuel compression that the alpha particles from the burnt DT plasma in the hot spot indeed coupled into the surrounding fuel. It's a noteworthy landmark, proving that the original design principles were right for the most part, but didn't take into account the sources of asymmetry properly (whose origins we still don't understand). It was conceived along the lines of if you try and fail, it's far worse not to know why you failed than to know. The latter lets you go back and try to fix things. Since a single shot on the NIF costs hundreds of thousands of dollars, you can't afford to "shoot in the dark" to find out why failure happened.

    So why do I say this is oversold? Because the high foot path is not one that realistically leads to fusion ignition. The fuel doesn't get compressed enough for that. The only way anyone knows to get the kind of compression is to back off on the very things that made the high-foot shots more symmetric. ("High foot," incidentally, is a descriptive term referring to the size of the initial laser pulse sent into the hohlraum--the "point design" used a low foot, meaning a very weak first shock wave, but also unpredictable behavior in the plasma, probably seeding some bad asymmetries.) Omar himself, who led this study, will admit that this design is probably not on the path to ignition.

    You can't look at a huge, generational project and not factor in the people element. Those at LLNL, the Lab who has the laser, are getting beat up for overselling how easy they thought fusion ignition would be and for largely shutting out the rest of the scientific community. They need a feel-good story and they need it yesterday to stave off impending layoffs of scientists and engineers. And Nature, the journal where they published, is notable in that they are both the cachet journal for big, splashy results, and a commercial outlet that coordinates press blitzes and hypes the hell out of results. This is part of the journal's business model which is a bit cringeworthy as a scientist, though something that LLNL is happy to capitalize on in this era of depressing and devastating budget cuts.

    A technical reason it's oversold is that yes, the energy put into the hot spot fuel is overcome by fusion energy produced. But this artificial metric is so far from practical fusion power as to be laughable. You're not counting the energy put into the rest of fuel and capsule ablator, which is greater by about a factor of at least 10. You're not counting the additional factor of 10 or so energy loss between energy put into the hohlraum walls and re-radiated as x-rays absorbed in the fuel. You're not counting that the lasers themselves are only about 10% efficient. That's a lot of factors of ten for something that's unlikely to ever ignite. While ignition leads to a big multiplier, even ignition wouldn't be at engineering break-even. Honestly, to imagine that this is some kind of major milestone on the path to fusion energy is to be optimistic to the point of self-delusion.

    Don't get me wrong: it's great that we now have a better sense of low mode asymmetry being the culprit for failure. But what can we actually do about it? The only sure-fire way of getting to ignition that anyone can think of would be to build a bigger laser so one can recover margin in the design elsewhere. (If you can compress more fuel, you need to compress it by less.) This just ain't going to happen--it's a political impossibility in the best of budgetary times and it certainly won't happen in anything like the present climate, where Congress hasn't seen any science and engineering R&D that they didn't think, "Hey, let's cut that."

    Lost in this ocean of recent hype is that NIF's main mission has never been and will never be fusion power production; that was all window dressing. NIF's mission is validating our understanding of the conditions inside nuclear weapons. The NNSA (the part of the Dept. of Energy that manages our nuclear weapons) has zero interest in advancing fusion energy. It's not their job. The part of DOE and the only part of the government that does care about fusion energy (the Fusion Energy Science Program of the Office of Science) funds primarily magnetic fusion, not inertial fusion, and their budget cuts have been so horrendous over the last couple of decades that the U.S. fusion energy program is depleted beyond any sort of sustainable unit and is merely withering on the vine as the last researchers in the field retire.

    If fusion energy ever does happens, it won't be the U.S. leading the charge.

    In short, I see no way that this result impacts fusion energy in any meaningful sense beyond maybe stirring up a bit of public enthusiasm. But you can only play that game for so long before the public starts asking, "Hey, where's our flying cars?"
     
    Last edited: Feb 13, 2014
  3. funflash

    funflash First Year

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    Thanks for replying with an incredible post.

    Since the cost of testing seems so expensive which countries or international teams are still experimenting? I remember you stating in another post about nuclear weapons that the physicists that worked on those projects are now retiring and few if any are taking their places (I assume its the same in the other areas of nuclear research).

    Where does the new generation of nuclear physicists come from? With funding in decline it seems like a career path with limited options for a person with a 'high IQ' that has to specialise in nuclear technology.

    I'm from NZ where we have had very anti nuclear laws for many years and other European countries that are phasing out nuclear power , how does this stigma affect international research?
     
  4. Darth Disaster

    Darth Disaster The Waking Sith Prestige DLP Supporter

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    Reason number #4 I hang out on DLP.

    There's generally SOMEONE who can speak with authority about matters of importance, and explain them in ways that make it understandable. Just reading that post set my mind abuzz with ideas and enthusiasm for science.

    Thank you, Pers, for a brilliant and informative post.
     
  5. Perspicacity

    Perspicacity High Score: 3,994 Prestige DLP Supporter

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    I can't comment on which countries may or may not be testing, though the U.S. is not and hasn't since the 1980s. Instead of testing, we work to improve our "predictive capability" for how such devices work and we have facilities like the NIF that allow experiments in the right regimes to test models in our computer codes. Also, we do sub-critical tests as permitted by international treaty.

    Downsizing is happening across the physics disciplines to a greater or lesser extent as the post-WWII expansion in higher education has reversed and universities are downsizing or eliminating physics departments, not up-sizing. Grad school incoming classes are a small fraction of what they were when I started and comprise mostly foreign nationals coming to the U.S. for their degrees. Also, a smaller percentage of the U.S. citizens who stay on to complete their Ph.D. Why would someone bright enough and assiduous enough to get a Ph.D. in physics invest six or seven years when the job prospects are so bad and nothing suggests it's going to get any easier with time?

    I can assure you that the ephemeral novelty of putting "Dr." in front of your name doesn't make up for it.

    In the nuclear weapons labs, new staff come from top engineering programs (dominantly nuclear, but also mechanical and electrical) as well as top physics and astrophysics programs. There's a lot of pipelining--"mafias" of alum from one place tend to draw from that same place through their connections. MIT, UCLA, Michigan, Princeton, UIUC, and Texas A&M seem to be the "go-to" universities in my area of work (the last because of proximity more than anything). We tend to pay exceptionally well (our postdocs receive the highest stipends of any postdocs in the country; our senior scientists make about as much as full professors in research universities) which gives us "pick of the litters."

    It's understood that you have to retrain whoever you hire because they're unlikely to have the skills and knowledge to contribute right away. This typically takes five to ten years before a staff member can function competently and independently. And it's true that only a few scientists remain at the Labs with any experience in nuclear testing. Within a decade, that number will likely be zero.

    Now is a poor time to go into science in the U.S. no matter how often the President talks about the need for STEM graduates in his public speeches. The reality is quite depressing for anyone wishing to make a career out of basic R&D. Scientists in plasma and fusion science have it particularly bad in the U.S. as the double-hit of a rapidly downsizing energy program and the failure of NIF to ignite has turned funding agencies and Congress against further investment in these areas. The U.S. is in the process of closing one of its premier fusion centers (at MIT) and no graduate program is producing very many scientists in these areas anymore.

    Nuclear fusion will never be "clean" in the sense that it is normally sold to the public (for technical reasons I could go into, aneutronic fusion like p + B-11 is just too difficult to take seriously) and even the "simple" problem of DT fusion has proved to be far harder than anyone thought initially. The envisioned fusion facilities have slipping time scales and ballooning costs (NIF had a cost overrun of about 4x; ITER will dwarf that, costing around $40B by the time it's complete, an enormous sum for such a "test reactor") that will make people realize that fusion is probably not the right path for affordable power anytime soon.

    In the end it boils down to an economics game*: given the precipitous drop in alternative, clean sources of energy (solar, wind), fusion, even if it were technically feasible today would face an enormous economic barrier to entry.

    * The one place where fusion has a distinct advantage is for spacecraft power generation, though we'd have to develop some ultracompact reactor and nobody has any (credible) ideas for how to accomplish this.
     
  6. Zenzao

    Zenzao 500 Club King Prestige

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    Thank you for the detailed and, more to the point, understandable explanations here, Pers. First time I've read this kind of thing and not felt like a lack wit.
     
  7. World

    World Oberstgruppenführer Moderator DLP Supporter

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    Aww, you totally ruined all the good cheer for the future that the news generated.

    How would you describe the chances of ITER of achieving, well, anything of note in our lifetimes?
     
  8. Perspicacity

    Perspicacity High Score: 3,994 Prestige DLP Supporter

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    I never seem to post on science stuff without being a wet blanket.

    Honestly, the biggest risk I see with ITER is that it is never completed as the schedule and cost keep slipping off into oblivion. (In the U.S., its effect on the fusion energy program is such that it's been nicknamed EatER and other countries are also experiencing sticker shock.) I'd estimate (total WAG) its chances of premature cancellation are around 30% and it's possible that some of the partners back out of their treaty obligations.

    If it's built, I think they'll get plasma and results in our lifetimes, though I expect that I'll be retired or close to it by the time they get first plasma (which should be around 2025-2028 the way things are trending). I also think the operating costs are going to lead to shutting the machine down early because they haven't solved the disruption problem, which I could go into if you're interested.
     
  9. Giovanni

    Giovanni God of Scotch

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    Yes please. Pers. Yes, please.
     
  10. Solfege

    Solfege High Inquisitor DLP Supporter

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    Considering "Bollocks" and "How Science Goes Wrong", that is, the general decline in the quality of science over the past 60 years, you're well justified in keeping the rest of us mortals grounded.

    Please, enlighten us further.
     
    Last edited: Feb 13, 2014
  11. Perspicacity

    Perspicacity High Score: 3,994 Prestige DLP Supporter

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    Tokamaks like ITER have nested tori (doughnut-shaped surfaces) of magnetic field lines that wrap around one another and confine the plasma (electrons and ions travel, for the most part, in spiraling orbits along the magnetic field lines). Because the tori are closed surfaces, they are able to keep the hundred million degree plasma away from the walls, which tend to be made of some solid like graphite or tungsten.

    The problem is that certain configurations of hot and cold plasma (ones with large "gradients" or changes of temperature over small distances) inside the vessel are susceptible to having the confining magnetic field "disrupt," meaning that the pressure in the core plasma causes the field lines to bulge out like an aneurism on a blood vessel, making the hot plasma blob slam into the wall and wreak all sorts of havoc.

    These disruptions can be triggered by impurities in the plasma, which almost certainly will be present since the diverters (think of magnetic field exhaust pipes used to extract the alpha particle "ash" from the confined plasma) will be made of tungsten, which gets sucked back into the plasma and radiates away the thermal energy to the walls as x-rays, cooling the plasma locally.

    On small research tokamaks that don't contain all that much energy, this is not a huge deal, an annoyance more than anything. ITER, however, is of such a large size that dumping the stored energy into the wall is a very big deal. To give you a sense of scale, ITER will contain around of order half a billion Joules of energy when operating. A disruption would dump the bulk of this energy into a section of the wall of the confining vessel.

    Half a billion Joules is about as much energy as is needed to melt a ton of copper. It's the amount contained in 125 sticks of dynamite. It's also the amount of kinetic energy that four Blue Angels aircraft have at takeoff, all slamming into the wall.

    [​IMG]

    Needless to say, after every major disruption (and every large tokamak to date has suffered major disruptions, typically every several tens of shots), massive repair work would be required with long periods (months? years?) of downtime. And after all these decades of research, nobody understands completely why tokamaks disrupt, nor has an especially good solution for how to prevent these disruptions. Even if they did, systems, particularly mechanical systems of the type that have been proposed (launching pellets into the plasma to quench the core) tend to be fallible. Would you trust an automated mechanical control system to oversee the integrity of a nearly 1000 cubic meter vacuum vessel filled with hundred-million-degree tritium plasma?

    And here folks thought that Three Mile Island was a bit of a big deal. Such a disruption on ITER with tritium in the chamber would be akin, in terms of radiological catastrophe, to a fission core meltdown.

    Until the disruption thing is solved once and for all, ITER will be the last tokamak every built.
     
  12. Russano

    Russano Disappeared

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    Pers always makes me feel inadequate in so many different ways.
     
  13. Oz

    Oz Heir to Hogwarts Moderator DLP Supporter

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    I'd say he's packing one serious weapon.