Computer simulation is playing a critical role in designing accelerator-driven transmutation of waste (ATW) systems that may help solve the problem of disposing of nuclear waste. ATW systems are designed to destroy long-lived fission products, reducing the time required for the waste to decay naturally from 10,000 to less than 1,000 years. One of the key components in the ATW concept is a target that is bombarded with protons from a linear accelerator, which in turn produces neutrons that sustain the transmutation of the waste to a stable or less radioactive material. A huge amount of heat is generated during this process, called spallation, which can be removed with a liquid lead-bismuth eutectic (LBE). One of the greatest design challenges in developing a working ATW system is designing the target and its operating conditions to maintain the proper temperature. Researchers at Los Alamos National Laboratory are using finite element analysis based computational fluid dynamics software to analyze target designs produced by engineers at the Institute of Physics and Power Engineering (IPPE) and the Experiment and Design Organization-Gidropress (EDO-GP) in Russia. The results have been used to significantly improve succeeding design iterations. Nuclear waste from commercial power plants contains large quantities of plutonium, other fissionable actinides and long-lived fission products that are potential proliferation concerns and create challenges for long-term storage. The current United States policy is to store unprocessed spent fuel in a geological repository. Long-term uncertainties are hampering the acceptability and eventual licensing of such a repositow and driving up its cost. The greatest concerns are the potential for radiation release and exposure from the spent fuel for tens of thousands of years and the possible diversion and use of the actinides contained in the waste for weapons construction. In the ATW concept, spent fuel would be shipped to a site where the plutonium, transuranics, and selected long-lived fission products would be destroyed by fission or transmutation using an accelerator-driven subcritical burner cooled potentially by liquid LBE and limited pyrotechnical treatment of the spent fuel and residual waste. This approach contrasts with the presentday reprocessing practices in Europe and Japan in which high-purity plutonium is produced and used in the fabrication of fresh mixed oxide fuel that is shipped off-site for use in light water reactors.
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