Fission Product Source Term Chemical Form and Release of Fission Products at Elevated Temperature

摘要

From experimental release data at temperatures above 1500/sup 0/C, a theory is derived for the fission products release. The release is described in terms of intragranular diffusion, driven by small gradients of concentration and temperature. About one half of the fission product atoms diffuse interstitially with a relatively low activation energy; as they are larger than the interstitial holes they diffuse with a positive excess entropy, which is a strong function of their size. The other half diffuses substitutionally with a relatively high activation energy. Other transport processes, like diffusion along grain boundaries and cracks, are faster than this intragranular diffusion and thus do not limit the release. It is speculated that all fission products are released as single atoms, except the lanthanides, the ytttrium, the zirconium and the niobium. The use of such a theory might be, e.g. that some deliberately introduced disorder in the UO/sub 2/-matrix could reduce the excess entropy of the diffusion and thereby possibly reduce the release of large interstitial atoms. 25 refs. (ERA citation 12:040929)