Candidate iodides for LLFP transmutation in FR core

Tachi, Yoshiaki  ; Wakabayashi, Toshio*

It is very attractive technique to transmute long-lived fission products such as iodine-129 included in spent fuel by using Fast Reactors (FRs) to minimize environmental burden and toxic risk due to high level waste disposal. Iodine has low melting point (386K) and low boiling points (457K) compared with FR core temperature. Furthermore, it is corrosive against iron based materials. Then, it is the most important issue to search suitable chemical forms for FR core conditions, having high stability at elevated temperature and good compatibility with cladding material. From the viewpoint of melting point, nuclides generation by neutron irradiation, reactivity with stainless steel, easy fabrication method and recyclability, 5 kinds of iodides of CuI, MgI, YI, RbI and YI were selected as the candidate chemical form of iodine for transmutation in FR. In order to evaluate stability of the candidate iodides at elevated temperature, TG-DTA of CuI, RbI, BaI and YI were performed. According to TG-DTA results, no significant mass change showed in BaI beyond those temperatures. Mass of CuI, RbI and YI started to be lost severely at the lower temperature than their melting points. Compatibility test between iodides and cladding materials were conducted. Cladding materials in contact with iodides were heated in a capsule filled with pure Ar gas at 873K for 5000h maximum. Results of cross-section observation showed that MgI and YI made pit corrosion on SUS316 and ODS. It appeared that the upper part of the inner surface test capsules with RbI, YI were degraded severely. Based on the experimental results, the most applicable iodide for transmutation by FR is BaI from the viewpoints of stability at elevated temperature and compatibility with cladding material.