Fission product transmutation effects on high-level radioactive waste forms

Abstract

The properties of potential nuclear waste forms have been under investigation for more than a decade to ensure their safe handling, packaging, transport, storage, and disposal. Two kinds of high-level radioactive waste (HLW) have received most attention: (1) that which could be produced from reprocessed commercial reactor fuel, and (2) the slightly less radioactive waste from military activities. Both kinds would be converted to solid, nondispersible forms such as glass or crystalline ceramics for disposal. After disposal, self-radiation damage, primarily radiation-induced atomic displacements, can change the properties of these waste forms and possibly increase the chances that radioactivity could be released to the environment. β decay of the fission products causes some displacements but α decay, and the associated recoil nuclei, of the actinide elements produces several orders of magnitude more displacements. Most recent studies on radiation effects have therefore concentrated on α decay1–3. A potentially more important aspect of β decay is the associated transmutation of fission products which usually results in changes of both valence and ionic size, both of which may not be readily accommodated by HLW solids. 137Cs and 90Sr are the two main sources of transmutations, but they have not been studied because of their ∼30-yr half lives. The problem has now been circumvented by preparing simulated waste forms and other solids containing 134Cs, which has only a 2-yr half life. This study will not be completed for at least another year, but we report here that results obtained after 2 yr of decay are encouraging in that negligible changes have been observed in the properties of all the materials studied.