Carbon monoxide-silicon carbide interaction in HTGR fuel particles

The corrosion of the coating-layers of silicon carbide (SiC) by carbon monoxide (CO) was observed in irradiated Triso-coated uranium dioxide particles, used in high-temperature gas-cooled reactors, by optical microscopy and electron probe micro-analysis. The mechanical failure of the coating-layer of inner dense pyrolytic carbon (IPyC) was often observed beside the area of the SiC corrosion. The grain boundaries of the SiC seemed to be selectively corroded during the early stages of corrosion. Silicon dioxide, or more stable (Si, Ce, Ba) oxide, was accumulated at the buffer-IPyC and IPyC-SiC interfaces on the cold side of the particles and the formation of (Pd, Rh, Ru, Tc, Mo) silicides was observed in the fuel kernels, which probably resulted from the vapour transport of silicon monoxide from the corroded areas. (author)