Chemical processes in defective LWR fuel rods

The results of several experimental studies aimed at improving understanding of the chemical processes that cause severe degradation of defective light-water reactor fuel cladding are reported. The competition between oxidation and hydriding of zirconium and zircaloy exposed to steam-hydrogen mixtures at 70 bar and 350-400 C was studied by thermogravimetry. A critical H2/H2O ratio of the gas separates regimes of these two types of reaction. For sponge-Zr, the critical ratios at 350 and 400 C are ∼5000 and ∼200, respectively. Radiolysis of steam by alpha particles was studied mass spectrometrically. The yield of the hydrogen radiolysis product in saturated steam at 290 C was found to be ∼8 molecules per 100 eV of deposited energy. An in-reactor experiment demonstrated that fission-fragment-irradiated steam can oxidize UO2 to UO2+x. (orig.)