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Removal of the More Noble Nonvolatile Fission Products from Uranium-Bismuth Fuels

Orrington E. Dwyer, Herbert E. Howe, Edward R. Avrutik

Nuclear Science and Engineering

Volume 12 / Number 1 / January 1962 / Pages 15-22


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The liquid-metal-fuel reactor concept, which has received much attention in recent years, in its commonest version is a thermal breeder and employs as a fuel a dilute solution of U in molten Bi. About 28% of the nonvolatile fission products are less reactive chemically than U. This group, customarily referred to as the FPN group, is further divided into three subgroups according to the proposed methods of removal. The FPN-I's would be removed by oxidizing them to chlorides with fused salts, the FPN-II's by precipitating them directly from the liquid fuel, and the FPN-III's by reaction with Zn to form low-density intermetallic compounds which are insoluble in Bi. The FPN-II's, representing about 90% of the FPN's, would be removed continuously, while the others, because of their low yields or relatively low thermal cross sections, would be allowed to build up in the fuel for several years without causing any particular concern. Eventually, however, they would have to be removed. The FPN-I's would be removed by the same continuous process proposed for removing those nonvolatile fission products which are more reactive than U, while the FPN-III's would be removed in a batch process similar to that currently used in the refining of Bi. The following paper includes information on the rates of build-up of the several important FPN elements in the fuel, steady-state concentrations of the FPN-II elements, reactor poisoning level of the FPN's, and experimental results in support of the proposed methods of removal.

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