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Thorium Fuel for Nuclear Energy

An unconventional tactic might one day ease concerns that spent fuel could be used to make a bomb

Mujid Kazimi

How might a determined terrorist group get hold of the uranium or plutonium needed to make an atom bomb? That question has been weighing heavily on many people's minds. The easiest way is probably to buy it, perhaps from North Korea, which, according to intelligence reports, may have the means to produce a modest stockpile. Although the nuclear aspirations of Pyongyang have been much in the news this year, experts also worry about other "nations of concern" obtaining these terrifying weapons. The North Korean example is, however, rather clear-cut, and the details illuminate a longstanding problem of international security, one that nuclear engineers like myself would dearly like to help solve.

Almost a decade ago the world breathed a sigh of relief when diplomatic efforts, including those of former President Jimmy Carter, defused what then threatened to become a violent conflict: At the time, North Korea was interfering with the monitoring work of the International Atomic Energy Agency, in clear breach of that country's obligations as a signatory to the nuclear nonproliferation treaty. In particular, the North Koreans were asserting that they had produced just a tiny amount of plutonium from the spent fuel taken from their nuclear reactors—far too little to make even one bomb—but they refused to allow inspectors to verify this claim.

Figure 1. Uranium fuels mostClick to Enlarge Image

Why was North Korea reprocessing nuclear fuels in the first place? After all, the peaceful pursuit of civilian nuclear power does not require any reprocessing, as the United States and several other countries have demonstrated. (At U.S. power plants, the spent fuel materials are simply stored in dry casks or in cooling ponds, in preparation for their eventual disposal at the Yucca Mountain Repository in Nevada, which should begin operation around 2010.) Was not North Korea's decision to reprocess spent nuclear fuels prima facie evidence that it intended to extract the plutonium generated within its reactors and use it to fabricate nuclear bombs?

Not exactly. North Korea's nuclear power reactors are quite different in design from the ones now operating in the United States, which use water both as the coolant and the moderator, the substance that slows the neutrons released during nuclear fission, allowing them to initiate further fission reactions. The North Korean "magnox" reactors (a name derived from the magnesium oxide alloy that encloses the uranium fuel) use gas as the coolant and graphite as the moderator, having a design similar to one long in use in the United Kingdom. It turns out that the spent fuel from magnox reactors cannot safely be stored: It must be reprocessed to a form that is less susceptible to oxidation in air or water. So the fact that the North Koreans were reprocessing their spent fuels could not in itself be taken as evidence of ill intent. Their interference with international inspectors was, however, quite troubling.

The accommodation that Carter helped to work out alleviated many worries: In return for mothballing their graphite-based reactors, Pyongyang received assistance from Washington in obtaining nuclear power plants of the type used in the United States, along with a generous aid package. The solution was, at least in part, a technical fix, offering the North Koreans a way to develop a peaceful program of nuclear energy. They could then continue to operate nuclear power plants without creating so much concern abroad that in the course of reprocessing spent fuel they might extract plutonium for bombs.

Of course, without adequate oversight the North Koreans could conceivably use their newer reactors for breeding plutonium, by reprocessing the spent fuels at some secret site. Indeed, their penchant to work clandestinely to obtain bomb-making materials became obvious last year, when it was reported that Pakistan had sent North Korea high-speed centrifuges—equipment for making weapons-grade uranium—in return for missile technology. Such apparatus is growing increasingly easy to obtain, and thus efforts to transform ordinary uranium into the highly enriched form suitable for bombs are becoming harder and harder to police. So the world will probably always face that threat. But what of the problem of spent nuclear fuels being used for bomb making?

One of the important barriers to such a diversion of spent fuel is that it remains highly radioactive for centuries after discharge, thus requiring remote handling and facilities with adequate shielding for extracting the plutonium. Might there be effective technical solutions to further limit the problem of spent nuclear fuels being exploited for military ends? That is a question that the designers of nuclear fuels have asked themselves over and over. Here, I would like to explore one possible answer that has garnered much recent interest: thorium.

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