Feasibility study on transmutation of long lived fission products in a fast reactor

In spent fuel there are many kinds of long-lived fission products. 99Tc and 129I are contained as fission products in HLW: they have comparatively long half lives and their toxicity remains for more than 1000 years, when the efficacy of the engineered barrier is uncertain. Fortunately, these two nuclides have large capture cross-sections in the thermal-resonance neutron energy region, so they have a good potential to be changed to stable nuclides in such a neutron flux. We tried to reduce the amount of these nuclides using a fast reactor. The neutron spectrum of a FR is too hard for effective transmutation of these nuclides, but the flux of a FR is much higher than in other commercial reactors, and the hard spectrum of a FR can be changed locally into a moderated spectrum by the addition of some moderator materials. We tried to make a good neutron flux for transmutation of these long lived fission products and to get high transmutation rates. In this survey, 60 moderated target subassemblies were modeled on the core periphery of a 600MWe fast reactor. Using a continuous energy Monte-Carlo code, some parametric surveys have been done, examining the effects on transmutation performances of: moderator fraction; pin and subassembly arrangements; moderating materials. The code system allowed us to investigate the effects of self-shielding. For 99Tc, the best transmutation rate is about 10%/y in the case of Duplex pin -the FP region is surrounded by moderator material-. With the best arrangement to maximize the amount transmuted, we can transmute 40kg 99Tc in a year. With the present case, the transmutation rate and transmuted amount of 129I are inferior to those of 99Tc. In the most effective case, the 129I transmutation rate is 5.2% and the transmuted amount is 18kg in a year. But the produced amount of 129I from a PWR is small -about 5kg from a 100MWe PWR in a year-, so the transmuted amount of 129I is equal to the output from 3PWRs. (J.P.N.)