Mechanism and prediction of fuel-cladding chemical interaction in mixed-oxide fuel pins at high burnup: evidence for cladding component chemical transport

Compelling evidence is presented for the occurrence of fission product-induced cladding component transport into the fuel in irradiated stainless steel-clad oxide fuel pins. This phenomenon, morphological aspects of which are described, is most evident in fuel pins with high O:M fuels (> 1.97), cladding i.d. temperatures exceeding 6000C, and burnups of 5 at. % or greater. The proposed mechanism for this essentially chemical process (CCCT) involves dissolution of Fe, Ni, Cr, etc., by a liquid Cs, Te fission product medium and transport as dissolved species up the steep thermal gradient between the cladding and the fuel. A calculational model based on the thermodynamics and kinetics of this process is presented and used to analyze available inpile results. The CCCT process is thus shown to constitute an additional FCCI mechanism that contributes to cladding inner surface wastage at burnups exceeding ca 5 at. %. It is suggested that CCCT plays an important role in the observed axial peaking of FCCI-induced cladding inner surface wastage in long, commerical-type LMFBR fuel pins