Numerical simulations of a creep rupture damage in Surry PWR primary circuit under TMLB accident

Abstract

In the nuclear safety field, the thermomechanical phenomena that occur during an accident in a nuclear power plant (NPP) are of particular importance. During some reactor severe accidents, the decay energy of the core is transferred via the reactor cooling system (RCS) to other parts of this circuit. The associated heatup of RCS structures can lead to pressure boundary failures; with notable vulnerabilities, i.e., the lower head vessel, the pressurizer surge line, the hot leg nozzles, etc. The potential for a primary circuit component rupture is of particular concern because fission products could be released into the environment by such a failure. The risk is higher in-vessel; the relocated debris toward the lower head vessel, following a potential melting of the core, leads to the rupture of the most important protective barrier. Primary circuit structures are subject to the effects of high pressure and high temperature throughout their service life. Such loadings generally lead to creep rupture. This risk is more likely when these structures experience transients or severe accidents. One of the most studied accidents is the station blackout (SBO). SBO without operator actions accident (TMLB' sequence) is considered as one of the most likely scenarios that may threaten the integrity of vulnerable RCS pressure boundaries. In this manuscript, we have evaluated, using the system code RELAP5/SCDAPSIM 3.4, the damage caused by rupture of some structures of the primary circuit of the Surry NPP. The structures analyzed are the surge line, the hot leg nozzles and the lower head vessel. RELAP5/SCDAPSIM 3.4 results indicate that surge line (hot leg) failures will be the first failures in the RCS pressure boundary.