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  • Nuclear Fusion Update

    Posted on October 17th, 2016 Helian No comments

    At the moment the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is in a class by itself when it comes to inertial confinement fusion (ICF) facilities.  That may change before too long.  A paper by a group of Chinese authors describing a novel 3-axis cylindrical hohlraum design recently appeared in the prestigious journal Nature.  In ICF jargon, a “hohlraum” is a container, typically cylindrical in form.  Powerful laser beams are aimed through two or more entrance holes to illuminate the inner wall of the hohlraum, producing a burst of x-rays.  These strike a target mounted inside the hohlraum containing fusion fuel, typically consisting of heavy isotopes of hydrogen, causing it to implode.  At maximum compression, a series of shocks driven into the target are supposed to converge in the center, heating a small “hot spot” to fusion conditions.  Unfortunately, such “indirect drive” experiments haven’t worked so far on the NIF.  The 1.8 megajoules delivered by NIF’s 192 laser beams haven’t been enough to achieve fusion with current target designs, even though the beams are very clean and uniform, and the facility itself is working as designed.  Perhaps the most interesting thing about the Chinese paper is not their novel three axis hohlraum design, but the fact that they are still interested in ICF at all in spite of the failure of the NIF to achieve ignition to date.  To the best of my knowledge, they are still planning to build SG-IV, a 1.5 megajoule facility, with ignition experiments slated for the early 2020’s.

    Why would the Chinese want to continue building a 1.5 megajoule facility in spite of the fact that U.S. scientists have failed to achieve ignition with the 1.8 megajoule NIF?  For the answer, one need only look at who paid for the NIF, and why.  The project was paid for by the people at the Department of Energy (DOE) responsible for maintaining the nuclear stockpile.  Many of our weapons designers were ambivalent about the value of achieving ignition before the facility was built, and were more interested in the facility’s ability to access physical conditions relevant to those in exploding nuclear weapons for studying key aspects of nuclear weapon physics such as equation of state (EOS) and opacity of materials under extreme conditions.  I suspect that’s why the Chinese are pressing ahead as well.  Meanwhile, the Russians have also announced a super-laser project of their own that they claim will deliver energies of 2.8 megajoules.

    Meanwhile, in the wake of the failed indirect drive experiments on the NIF, scientists in favor of the direct drive approach have been pleading their case.  In direct drive experiments the laser beams are shot directly at the fusion target instead of at the inner walls of a hohlraum.  The default approach for the NIF has always been indirect drive, but the alternative approach may be possible using an approach called “polar direct drive.”  In recent experiments at the OMEGA laser facility at the University of Rochester’s Laboratory for Laser Energetics, the nation’s premier direct drive facility, scientists claim to have achieved results that, if scaled up to energies available on the NIF would produce five times more fusion energy output than has been achieved with indirect drive to date.

    Meanwhile, construction continues on ITER, a fusion facility designed purely for energy applications.  ITER will rely on magnetic plasma confinement, the other “mainstream” approach to harnessing fusion energy.  The project is a white elephant that continues to devour ever increasing amounts of scarce scientific funding in spite of the fact that the chances that magnetic fusion will ever be a viable source of electric power are virtually nil.  That fact should be obvious by now, and yet the project staggers forward, seemingly with a life of its own.  Watching its progress is something like watching the Titanic’s progress towards the iceberg.  Within the last decade the projected cost of ITER has metastasized from the original 6 billion euros to 15 billion euros in 2010, and finally to the latest estimate of 20 billion euros.  There are no plans to even fuel the facility for full power fusion until 2035!  It boggles the mind.

    Magnetic fusion of the type envisioned for ITER will never come close to being an economically competitive source of power.  It would already be a stretch if it were merely a question of controlling an unruly plasma and figuring out a viable way to extract the fusion energy.  Unfortunately, there’s another problem.  Remember all those yarns you’ve been told about how an unlimited supply of fuel is supposed to be on hand in the form of sea water?  In fact, reactors like ITER won’t work without a heavy isotope of hydrogen known as tritium.  A tritium nucleus contains a proton and two neutrons, and, for all practical purposes, the isotope doesn’t occur in nature, in sea water or anywhere else.  It is highly radioactive, with a very short half-life of a bit over 12 years, and the only way to get it is to breed it.  We are told that fast neutrons from the fusion reactions will breed sufficient tritium in lithium blankets surrounding the reaction chamber.  That may work on paper, but breeding enough of the isotope and then somehow extracting it will be an engineering nightmare.  There is virtually no chance that such reactors will ever be economically competitive with renewable power sources combined with baseline power supplied by proven fission breeder reactor technologies.  Such reactors can consume most of the long-lived transuranic waste they produce.

    In short, ITER should be stopped dead in its tracks and abandoned.  It won’t be, because too many reputations and too much money are on the line.  It’s too bad.  Scientific projects that are far worthier of funding will go begging as a result.  At best my descendants will be able to say, “See, my grandpa told you so!”

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