Stress-Strain State evaluation for the WWER-1000 RPV in case of Pressurized Thermal Shock

Since about a decade an important issue appeared in nuclear technology and in nuclear reactor safety also due to reactors aging: notwithstanding the taken countermeasures it became clear that neutron flux may damage Reactor Pressure Vessels (RPV). The damage depends upon the fluence and is largely affected by impurities present in the carbon steel material. The key problem is expected in the welding present in the RPV. In a number of cases the composition of welding is unknown with main reference to the Russian type WWER (Water-Cooled, Water-Moderated, Energy Reactor). The Pressurized Thermal Shock (PTS) constitutes a key consequence of the radiation damage: in the course of an accident, cold water may be injected by ECCS (Emergency Core Cooling Systems) and enter in contact with possibly embrittled (by the radiation) RPV walls. This generates local stresses which may overcome the limit stress for the propagation of cracks (possibly present in the material). Thus, it is essential to calculate the fluid temperature close to the vessel walls. This temperature depends upon the overall transient scenario and may need, in order to be calculated with some accuracy, the application of Computational Fluid Dynamic (CFD) codes. The present document, issued in a cooperation between Universities of Pisa and Genoa, deals with fundamental analysis to characterize the stress –strain relation in a WWER-1000, following a PTS type of event..