Experimental study of the fracture kinetics of a tubular 16MnNiMo5 steel specimen under biaxial loading at 900 and 1000 °C. Application to the rupture of a vessel bottom head during a core meltdown accident in a pressurized water reactor

This study proposes an experimental analysis of the creep, crack initiation and crack propagation phases in a 16MnNiMo5 steel subjected to thermomechanical loading representing a core meltdown accident in a pressurized water reactor involving the transfer of a molten corium bath to the bottom head. The experimental setup enabled a biaxial mechanical loading (internal pressure + tension) to be applied to a tubular specimen at 900 and 1000 °C. In addition to the usual temperature, load, displacement and pressure measurements, the specimen was observed by two high-speed numerical cameras and an infrared camera. The crackʹs initiation and propagation conditions and the depressurization law were inferred from these measurements. At such temperatures, creep induces very large strains prior to the occurrence of the cracks which, in the worst-case scenario, can propagate at velocities as high as several meters per second. The design of the experiment enabled us to study the influence of the temperature (magnitude and hoop distribution), of the toughness of the steel (two grades were studied) and of the volume of pressurized gas. The results show that creep and crack propagation are highly dependent on temperature, and also that crack initiation and propagation are highly dependent on the degree of heterogeneity which is responsible for the localized initiation of the crack.