Estimation of conservatism of present embrittlement criteria for Zircaloy fuel cladding under LOCA

To estimate the degree of conservatism of the present embrittlement criteria, the following examinations have been conducted. Zircaloy-4 tubes were oxidized at temperatures ranging from 1173 to 1573 K, and then compressed at 373 K to determine the embrittlement due to oxygen absorption. The simulated Zircaloy-clad fuel rods were burst and oxidized in steam at temperatures within the range 1200 to 1500 K. The segment sectioned from each burst cladding was also compressed to determine the effect of hydrogen absorption on cladding embrittlement. The results obtained from these experiments have revealed that the additional embrittlement due to hydrogen absorption is found to be significant in burst cladding. The simulated Zircaloy clad fuel rods were burst and exposed to steam at isothermal temperatures ranging from 1173 to 1573 K and then bottom-flooded at the oxidation temperature or 773 K with water, after the specimen was restrained at the end of exposure time. The specimens were also bottom-flooded under a nonrestrained condition to determine the effect of restraining stress on thermal-shock-failure characteristics. The presence of restraining stress limits the permissible oxidation condition to lower temperature and shorter exposure time. Even the estimated thermal-shock-failure boundary under a restrained condition is located above the 15% oxidation line which is adopted by the present criteria