An analysis of misconceptions about thorium-based fuel cycles as a technical fix to the nuclear proliferation problem. This paper analyzes several pre- rich to bomb-usable levels than natu- of the plutonium economy are taken valent misconceptions about nuclear ral uranium. Such enrichment can be as read. Nothing in this article fuel cycles that breed fissile done at a modest scale with readily should be construed to imply that: uranium-233 from thorium. Its main available centrifuges. L e the proliferation risks of conclusions are: a Several other features of mixed plutonium fuel cycles can be made a Uranium-233, despite the thorium-uranium cycles are rather tolerable, or gamma radioactivity of associated unattractive from a safeguards point 9 any commitment (even an ex- isotopes, is a rather attractive mate- of view. ploratory one) to a plutonium econ- rial for making fission bombs, and is Thus thorium cycles of any kind omy need be made for many de- a credible material for subnational as cades, perhaps even a century or two, well as national groups to use for this are not a technical fix for prolifera- tion (national or subnational) and, any improvement in existing purpose. safeguards (such as those sought by e "Pure" thorium cycles, which in though probably more safeguardable than plutonium cycles, are less so the Carter administration1 is not effect merely substitute uranium-233 worth making only because it does for plutonium, would take many than once-through uranium cycles that entail no reprocessing. not solve the whole problem, or decades and much uranium to any form of nuclear power is a While thorium cycles have some establish, and offer no significant necessary or desirable. safeguards advantage over plu- potential technical advantages, in- cluding flexibility, they cannot pro- All these fallacies are addressed tonium cycles. elsewhere [2]. While some of my ar- vide major savings in nuclear fuel re- e "Denatured" thorium-uranium guments have been previously ad- cycles, which dilute the uranium-233 sources compared to simpler ways of saving neutrons and uranium. vanced by advocates of plutonium with inert uranium-238 to a level not fuel cycles, presumably seeking to directly usable in bombs, are not an Thus while advocates of nuclear suggest that their option is accept- effective safeguard even against power may find thorium cycles able because the thorium alternative subnational bomb-making. This is worth exploring, such cycles do not to it is nearly as bad, I shall suggest because three factors-higher initial differ fundamentally from uranium that neither is acceptable. Thus any fissile content, larger mass dif- cycles in any of the respects- out-of-context quotation of this ference between isotopes, and including safeguards and fuel paper in support of plutonium cycles smaller amount of material needed resources-that are relevant to the would be perverse and misleading. for a bomb-make such "de- broader nuclear debate, and should Since proposals were widely pub- natured" uranium-233/238 fuel one not be euphorically embraced as if lished in 1976-1977, notably by or two hundred times easier to en- they did.-A.B.L. Taylor and Feiveson 131, for a "de- natured" [4] thorium cycle which If nuclear fission is to be used as an FO~~-MITREreports. Those who be- might be less proliferative (espe- energy source, whether it should use lieve plutonium is nasty and uranium cially to subnational groups) than a fuel cycles that require plutonium to may be scarce call attention to fuel- plutonium-uranium fuel cycle [5], be extracted and re-used turns on efficient, non-plutonium fuel cycles. some people [6] have jumped to the two independent questions [I]. The It is this last, perhaps most interest- conclusion that thorium cycles in first is whether plutonium is bearable ing, group that has recently recalled general and denatured thorium cy- and the second is whether a lot of attention to thorium-based fuel cy- cles in particular offer a technical fix uranium is available (geologically cles which appear to many nuclear to the proliferation problem. This is and politically) at relatively low advocates to merit study on several incorrect. It is also certainly not prices. Few if any analysts answer grounds, potentially greater resist- what such authors as Taylor, Feive- yes to both questions; and for any ance to the proliferation of nuclear son, Cochran, and von Hippel had in who do, policy choices are un- weapons being high among them. mind: they proposed denatured constrained. Those who believe It is this cluster of proposals, and thorium only as proof of the exis- plutonium is bearable and uranium some prevalent misinterpretations of tence of fuel cycles which might, on scarce build fast breeder reactors. them (especially common in Britain closer study, prove to be inherently Those who believe plutonium is and Canada), that this article briefly less proliferative than uranium/ nasty and uranium plentiful write discusses. The arguments of critics plutonium cycles. Since at least one The concern over weapons proliferation problems has lately led to serious international consideration of alternative nuclear fuel cycles. Among these. cycles involving the breeding of the uranium-233 isotope from thorium, especially with provisions for its "denaturing" with ordinary uranium, are generally believed to show great promise for a nuclear energy future without weapons proliferation. In this article the author dashes cold water on the most optimistic of such expectations.-Ed . such fuel cycle could be envisaged, 15.8 grams per cubic centimeter). ting use of a gun-type rather than an they argued, it is important not to (Corresponding figures for the alpha implosive assembly-a further op- rush ahead with a plutonium eco- phase of plutonium, at around 19.8 tion which may be easier than an im- nomy before potentially less pro- grams per cubic centimeter, are 11 plosive design. Both chemically and liferative options are carefully kilograms for nearly pure metallurgically ui'anium-233 is con- examined. plutonium-239 and 13 kilograms for siderably more tractable than The early stages of that closer "reactor-grade" plutonium pro- plutonium: it is not very pyrophoric examination have made it clear that duced at a burnup around 30 thermal and has one solid phase, not six. Its thorium fuel cycles, including de- gigawatt days per metric ton of inhalation toxicity is considered to natured ones, do not qualitatively uranium. Of course all these quan- be about 70 times lower than that of alter the prolifefation problem tities can be reduced greatly-more plutonium-239, several hundred (either national or sbbnational) and than fivefold-by reflection and im- times lower than that of reactor- in some respects offer more avenues plosion in a nuclear explosive.) grade plutonium. for proliferation than once-through Broadly speaking [7],for fast neut- The only significant disadvantage uranium cycles. The reactor-physics rons the relative neutron yield is of uranium-233 is that after produc- and safeguards considerations sup- tion, various daughters of uranium- porting this conclusion are some- 232 build up in it (peaking about 10 what elementary, but the scarcity of years after extraction), notably systematic analyses of them has and the fission cross-section thallium-208. Some uranium-232 lately generated much misplaced en- daughters emit intense and penetrat- thusiasm [6]: hence this summary, ing gamma rays, of which thallium- These conclusions are relatively in- 208 typically contributes more than Thorium and Uranium-233. There sensitive to contamination of the half the total with its 2.6- and 0.6- are two basic kinds of thorium cy- uranium-233 with traces of other megavolt emissions. Commercial cles, both of which may breed fissile uranium isotopes. handling of uranium-233, especially uranium-233 from fertile thorium- Because uranium-233 is about if it were more than a few days old 232, mainly via the intermediate three times as reactive as uranium- (whereas aging for about a year is product protactinium-233, which has 235 (whose bare-sphere critical mass desirable for full decay of the a 27-day half-life. The first kind is the at 93.5 percent enrichment is about protactinium-233 arent of the "pure" thorium cycle, a self- 50 kilograms), one might expect that uranium-2331, woul 1require heavy sustaining one relying on previously a third as much of it would be re- shielding-of order twice as much as bred uranium-233 for fission neut- quired for a bomb. In fact the re- the gamma- and neutron-emitting re- rons to breed more uranium-233 in ciprocal of this ratio, the "substitu- cycle plutonium requires-to protect fuel consisting of uranium-233 mixed tion factor," is not 3 but typically 5 workers from overexposure. But if with thorium-232. Burnup of this fuel to 10 because a physically smaller relatively small amounts of produces various isotopes, up to and core can be compressed more in a uranium-233 were being dealt with including small amounts of convergent implosion. (An in- for relatively short periods outside plutonium formed by successive compressible solid spherical shell, if the restrictions of prudent health- neutron captures. imploded, provides a pressure vary- physics practice, shielding could be From a weapons physics point of ing as the inverse fourth power of its greatly reduced or even dispensed view, uranium-233 is similar and in radius @I.) with. some respects perhaps superior to Ordinarily uranium-233 has a sub- A kilogram of uranium-233 pro- plutonium-239, the classical mate- stantial neutron background because duced at 46 thermal gigawatt days per rial. They are roughly equal in re- of (a,n) reactions with light-element metric ton of heavy metal burnup activity, as shown by the nearly impurities.