Friday, September 28, 2012

Uncertain promises of thorium-fueled nuclear power


Since the 1940s, thorium-fueled nuclear power has been an occasional matter of interest. The late Dr. Glenn Seaborg first isolated the fissile isotope uranium-233 from neutron-irradiated thorium in 1941. He predicted a potential for thorium as a nuclear fuel in 1946. [1] The late Dr. Alvin Weinberg, former director of Oak Ridge National Laboratory, was an early advocate for developing a thorium fuel-cycle.

Despite perennial interest, thorium has remained a research topic and has never been used to fuel commercial power reactors for sustained operation. The early days of nuclear energy were dominated by development of weapons, for which plutonium-239 produced from neutron-irradiated uranium-238 proved more versatile than uranium-233. Because of greater investment and technical knowledge, a uranium-based fuel cycle enhanced by its plutonium byproducts was adopted for military reactors and then for commercial power reactors. It has remained entrenched as the only sustained, commercial nuclear-power fuel-cycle. [2]

There is much speculation and considerable misinformation [3] available on thorium-fueled nuclear power but relatively little solid knowledge, as compared with knowledge of uranium-fueled power. Three main approaches have been actively investigated: pressurized heavy-water reactors, now active mainly in India, molten salt reactors, now active only in China, and high-temperature gas-cooled reactors, such as the former Fort St. Vrain thorium-fueled reactor in the U.S. and former THTR in Germany, now active nowhere at commercial scale. All these approaches have been found to suffer from major, unsolved problems.

Thorium has no long-lived fissile isotope as a minor constituent, comparable to uranium-235. As with production of plutonium, thorium-232 must be transmuted to fissile uranium-233 using neutron irradiation. That can be provided from a uranium-235-enriched or a plutonium fuel-cycle, in a light water-reactor, or from a natural or an enriched uranium fuel-cycle, in a heavy-water reactor. In any case, a substantial "breeding" period is needed to produce enough uranium-233 to contribute substantially to power output.

India appears committed by technology-lock-in to heavy-water reactors, which it began to develop with U.S. and Canadian help in 1956. India surreptitiously used spent fuel from its first CIRUS reactor to obtain plutonium for nuclear weapons. Efficient use of thorium in heavy-water reactors involves high burn-up, at least two to three times the current industry practice with light-water reactors, and extended fuel-rod exposure, ten years or more. Zirconium alloy cladding for fuel rods has not been validated for such conditions, and conventionally manufactured fuel rods probably would not survive. No other technology is currently proven. [4]

China has announced development of a molten-salt reactor, leveraging knowledge from the MSRE project at Oak Ridge, 1961-1969. While avoiding some problems associated with heavy-water reactors, molten-salt reactors used for power generation have other problems. One is heat-transfer piping, which must function reliably for 40 or more years in corrosive, hydrogen-bearing molten salts at very high temperatures and pressures. The MSRE project validated Hastelloy-N piping--but only at lower temperatures and pressures and only for 18 months at full power. Key, unsolved engineering problems for heat-transfer piping in molten-salt reactors are neutron and hydrogen embrittlement and corrosion-promoted stress-cracking. [5]

Of the large, high-temperature thorium-fueled reactors, Fort St. Vrain in Colorado opened first, in 1979. It suffered from leaks and component faulting. After a fire in 1987 and the discovery of pipe cracks in 1988, Public Service of Colorado shut it down. The plant reached full rated power for only two days and averaged less than 15 percent of rated capacity during nine years of commercial operation. THTR in Germany opened in 1985 and closed in 1988, with similar episodes of major problems. It averaged about 40 percent of capacity. Neither effort solved problems of fuel reprocessing, which is made especially difficult by chemically inert coatings on fuel elements of high-temperature reactors. [6]

Most thorium-fueled reactors, including all those noted, involve fuel reprocessing to extract uranium-233 and remove neutron poisons and other fission products. All reprocessing is complicated by highly penetrating gamma radiation from nuclear decay products of uranium-232, which is always being released in thorium-fueled reactors from neutron reactions with uranium-233 and its precursor, proactinium-233. Among the decay products are polonium-212 and thallium-208, which both emit high-energy gammas at about 2.6 MeV. [7]

Typical of thorium-fueled reactors, the one at Fort St. Vrain was enclosed in 15-foot-thick concrete. A smaller but more expensive alternative shield employs depleted uranium. Adequate radiation shields for bulk quantities of irradiated thorium, for its uranium-produced decay products and for radioactive waste from thorium-fueled reactors are too heavy to transport by standard rail, highway or air services. [8] Only relatively small quantities could be packaged for shipment in those ways.

Rationales for developing thorium fuel-cycles have usually focused on four claims: (1) more plentiful, less hazardous fuel at lower costs, (2) intrinsic resistance to diversion for nuclear weapons, (3) smaller amounts of nuclear waste and (4) more stable operation. [2] None of those claims stand up robustly to review. Potential thorium-fueled power reactors would present hazards and limitations similar to those of uranium-fueled power reactors, differing mainly in details.

As to claim (1), more plentiful, less hazardous fuel at lower costs: While the geologic abundance of thorium is greater than that of uranium, much of that thorium is at low concentrations that are impractical to mine. Thorium-bearing substances extracted from rare-earth mines can be more toxic than substances from uranium mines. [9] They have been associated with high incidences of severe mining-induced diseases. [10]

As to claim (2), intrinsic resistance to diversion for nuclear weapons: Uranium-233, which can be extracted from neutron-irradiated thorium by a chemical process, is a more powerful nuclear explosive than bomb-grade uranium, highly enriched in uranium-235. [11] Its claimed resistance to diversion comes from contamination at parts-per-million levels by uranium-232, with highly penetrating radiation from decay products. However, that would not necessarily deter terrorists, who often mount suicide missions. [12]

As to claim (3), smaller amounts of nuclear waste: While the fission daughter and actinide products in thorium-fuel waste are substantially less and shorter-lived than those in uranium-fuel waste, for the same yield in energy, [7] requiring isolation for perhaps ten thousand years instead of a million or more years, the shorter-term components of thorium-fuel waste, with deeply penetrating gamma radiation that remains hazardous for at least a thousand years, are much more difficult to handle safely. [13]

As to claim (4), more stable operation: While the neutron cross-section of uranium-233 is more uniform than those of other fissile isotopes, helping to stabilize performance, [7] in practice thorium-fueled reactors have been plagued with leaks, component faulting, power excursions, fires and materials failures. The practical history of thorium-fueled reactors--as contrasted with theory--recalls the long learning curves with uranium-fueled reactors. While no major disaster has occurred yet with thorium-fueled reactors, the experiences with them so far amount to only a tiny fraction of experiences with uranium-fueled reactors.

The notion of a permanently enclosed thorium-fueled reactor, operating continuously for 30 or more years, has been proposed. [14] It has not yet been proven practical from either a scientific or an engineering perspective. The gradual build-up of neutron poisons from fission would be a key challenge for long-term operation, and the unresolved engineering issues of long-term embrittlement and corrosion are at least as challenging for such an approach as for other types of thorium-fueled reactors.

Cost of nuclear fuel has long been a prominent argument for developing thorium fuel-cycles but is clearly a red herring. [15] Capital costs of reactors have dominated for more than 30 years, and fuel costs have sometimes been considered negligible. Because of requirements for extremely heavy shielding, fully robotic operations inside shielding, and fuel reprocessing, thorium-fueled reactors could be more expensive than uranium-fueled reactors of equal capacity, so costs of electricity from them might be higher. [16]

Until a shortage of uranium emerges, at least decades from now, there will not be any strong financial or environmental reason for industrially developed countries to pursue thorium fuel-cycles. Even with such a shortage developing, reprocessing spent uranium fuel to obtain plutonium fuel might be less costly and no more hazardous. The interests of power generation will be better served by long-term research investigating the unsolved engineering problems of thorium-fueled reactors.

Some 60 years ago, the late U.S. Admiral Hyman Rickover, an early leader in military applications of nuclear reactors, summarized what he already saw as jejune enthusiasms for nuclear technologies--comparing what he called "academic" reactors with practical ones. [17] Abbreviated here:

AcademicPractical
------------------------
Simple Complicated
SmallLarge
CheapExpensive
LightHeavy
Easy to buildHard to build
Not availableAvailable now

Most enthusiasts for thorium-fueled reactor technologies are academics, researchers or gadflies. Few have practical, daily working experience designing, developing, operating, maintaining, testing, regulating or certifying nuclear reactors or power systems of any kind. History has shown that such backgrounds fail to yield dependable estimates for energy technologies, so that any estimates from such sources ought to be discounted.



[1] U.S. Secretary of State, Report on the international control of atomic energy (the Acheson-Lilienthal Report), March, 1946, available at http://www.learnworld.com/ZNW/LWText.Acheson-Lilienthal.html. Unattributed, United Press, Third nuclear source bared, Tuscaloosa (AL) News, October 21, 1946, available at http://news.google.com/newspapers?id=ckxBAAAAIBAJ&pg=6357%2C2252004

[2] F. Sokolov, K. Fukuda and H.P. Nawada, Thorium fuel cycle: Potential benefits and challenges, International Atomic Energy Agency, 2005, at http://www-pub.iaea.org/mtcd/publications/pdf/te_1450_web.pdf

[3] Unattributed, quoting Prof. Reza Hashemi-Nezhad, Thorium reactor, Thorium Enthusiasts, 2011, at http://www.thorium.tv/en/thorium_reactor/thorium_reactor_1.php

[4] Paul R. Kasten, Review of the Radkowsky thorium reactor concept, Science and Global Security 7:237-269, 1998, available at http://www.princeton.edu/sgs/publications/sgs/pdf/7_3kasten.pdf

[5] Kun Chen, Thorium-fueled molten salt reactor research in Shanghai Institute of Applied Physics, Department of Nuclear Engineering, University of California at Berkeley, August 6, 2012, announcement at http://events.berkeley.edu/index.php/calendar/sn/NUC.html?event_ID=51922&date=2012-08-06

[6] Tony Kindelspire, Colorado nuclear plant at Fort St. Vrain had short, troubled life, Longmont (CO) Weekly, March 28, 2011, at http://www.longmontweekly.com/ci_17701792

[7] Michel Lung and Otto Gremm, Perspectives of the thorium fuel cycle, Nuclear Engineering and Design 180:133-146, 1998, available at http://thorium.tym.sk/TP/sizi/Perspective%20of%20the%20thorium%20fuel%20cycle.pdf

[8] L.H. Brooks, R.G. Wymer and A.L. Lotts, Progress in the thorium and urainium-233 reprocessing, Oak Ridge National Laboratory, 1974, at http://www.osti.gov/bridge/servlets/purl/4309002-8Dky0W/4309002.pdf

[9] Comparing thorium-232 with natural uranium, in Radionuclides (occupational limits), U.S. Nuclear Regulatory Commission, 2012, at http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/appb/

[10] C├ęcile Bontron, En Chine, les terres rares tuent des villages, Le Monde (France), July 19, 2012, at http://www.lemonde.fr/asie-pacifique/article/2012/07/19/en-chine-les-terres-rares-tuent-des-villages_1735857_3216.html

[11] Rene G. Sanchez, Minimum critical mass: Analytical studies, Los Alamos National Laboratory, 1993, available at http://www.fas.org/sgp/othergov/doe/lanl/lib-www/la-pubs/00215722.pdf

[12] Cameron Reed, A thorium future?, American Scientist 98(5):364, 2010, at http://www.americanscientist.org/issues/pub/2010/5/a-thorium-future

[13] Kevin Hesketh and Andrew Worrall, The thorium fuel cycle, UK National Nuclear Laboratory, 2011, at http://www.nnl.co.uk/media/27860/nnl__1314092891_thorium_cycle_position_paper.pdf

[14] Gabriele Rennie, Self-contained, portable reactor, Lawrence Livermore National Laboratory, 2004, at https://www.llnl.gov/str/JulAug04/Smith.html

[15] Robert Hargraves and Ralph Moir, Liquid-fuel nuclear reactors, Forum on Physics and Society, American Physical Society, January, 2011, at http://www.aps.org/units/fps/newsletters/201101/hargraves.cfm

[16] Arjun Makhijani and Michele Boyd, Thorium fuel: No panacea for nuclear power, Physicians for Social Responsibility, 2009, at http://ieer.org/wp/wp-content/uploads/2012/04/thorium2009factsheet.pdf

[17] Hyman G. Rickover, Letter to the Joint Committee on Atomic Energy, June 5, 1953, quoted in Nuclear and Radiation Studies Board, Internationalization of the Nuclear Fuel Cycle: Goals, Strategies and Challenges, National Academies Press, 2009, p. 60, at http://www.nap.edu/openbook.php?record_id=12477&page=60

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