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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

The Bottom Line

Even with a whole-assembly seed-and-blanket core, where each type of fuel assembly is of homogenous construction, it is clear that the manufacture of the fuel and its management within the reactor would be more complicated than usual. In a typical power reactor, the fuel assemblies are shuffled at intervals so that each will be exposed, on average, to the same conditions of heat and radiation. In a seed-and-blanket core, the seeds must sustain power levels that are significantly above average, while the blanket assemblies experience far less stressful conditions. Thus the fuel in the seed rods reaches higher temperatures, releases more of the gaseous fission products into the limited space allowed for them within the fuel rods and requires more cooling than does the fuel used in the blanket regions.

Figure 7. Nuclear power reactorsClick to Enlarge Image

These demands can be accommodated in various ways—for example, by allowing more coolant to flow through the seeds and by making the fuel materials less resistant to the flow of heat. In the Radkowsky-Kurchatov approach, the seed rods are made from a metallic uranium alloy (following designs that have been tested in Russian submarines), which improves their thermal conductivity. In the MIT-Brookhaven scheme, the uranium oxide pellets within the seed rods are hollow, which lowers their temperature. Although the blanket rods are less problematic in this regard, they too must be carefully engineered so that the exterior cladding holds up well, the working lifetime of these rods being in some designs as long as 13 or 14 years.

In addition to examining these various engineering concerns, investigators at CANES have also quantified the advantages of the seed-and-blanket designs in terms of their contribution to averting the proliferation of bomb- making materials, and we have also tried to evaluate their economics. We found that the seed-and-blanket arrangements produce less plutonium than competing schemes in which uranium and thorium are mixed at finer scales. But our results are not quite as optimistic as Radkowsky's earlier work had indicated: We calculate a reduction of only 60 percent (for the whole-assembly system) or 70 percent (if both seed and blanket rods are used within each assembly), compared with Radkowsky's estimate of an 80-percent reduction for the latter.

Our calculations of plutonium production do, however, support Radkowsky's assertions that the spent fuel would contain appreciable amounts of plutonium-238, a highly radioactive isotope, which thus produces a lot of heat. Indeed, the plutonium-238 content would be three to four times higher than with conventional uranium fuels. As Radkowsky pointed out, the heat given off by this isotope would make it quite difficult if not impossible to fabricate and maintain a nuclear weapon.

The production of such large amounts of plutonium-238 comes about because more of the fuel is consumed (or "burned up," in the lingo of nuclear engineers) than is the case in conventional uranium-fueled reactors. An equivalent amount of plutonium-238 could be created using an all- uranium fuel, but this would require a higher initial amount of fissile uranium (235U) than is typical in today's practice, and the economic projections for that are discouraging.

Thus our recent work amply confirms that the various engineering concerns can be met and that running reactors on thorium could indeed forestall clandestine efforts to use the spent fuel for making bombs. But the results of our investigation into the economics of thorium are less clear-cut. We estimate that thorium-based fuels could cost anywhere from 10 percent less to about 10 percent more than conventional nuclear fuels. The wide range stems from fundamental uncertainties about the cost of the seed uranium (which must be four times more enriched in uranium-235 than is the case with typical nuclear fuels), the cost of fabricating the fuel assemblies and the savings that might accrue in the future as a result of the reduction in the amount of spent fuel in need of disposal.

Although it seems unlikely that economics alone could drive the adoption of thorium fuels, there are no technical "show-stoppers" here. Modifications to the existing commercial infrastructure would clearly be needed, but no fundamentally new technology is required. And the fact that the relevant materials (thorium and enriched uranium) have a long record of experimental use in reactors lends credibility to the notion that this scheme could one day find widespread application, should policymakers push the nuclear industry in that direction.

Bibliography

  • Garwin, R. L., and G. Charpak. 2002. Megawatts and Megatons. Chicago: University of Chicago Press.
  • Mark, C. 1992. Explosive properties of reactor-grade plutonium. Science and Global Security 3:1–13.
  • Radkowsky, A., and A. Galperin. 1998. The nonproliferative light water reactor: A new approach to light water reactor core technology. Nuclear Technology 124:215–222.
  • Shwageraus, S. , X. Zhao, M. Driscoll, P. Hejzlar, M. S. Kazimi and J. S. Herring. In press. Micro-heterogeneous thoria-urania fuels for pressurized water reactors. Nuclear Technology.







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