Pressurized Thermal Shock (PTS) Transient Scenarios Screening Analysis With TRACE

Abstract

A number of postulated accidents that can lead a nuclear power plant (NPP) to a rapid cooling condition known as Pressurized Thermal Shock (PTS) are analysed. Such PTS, which induce high thermal stresses in the RPV wall, can occur at high or low internal pressure conditions and thus can challenge the integrity of the reactor pressure vessel. It is getting more crucial during operating of a NPP due to radiation embrittlement of the reactor pressure vessels (RPV). In this work, an overview of different transients that could contribute to the risk of a vessel failure for a pressurized water reactor (PWR) is presented. In total, 25 different scenarios were analyzed including large, intermediate and small break Loss-of-Coolant Accidents (LOCA), steam generator tube rupture (SGTR), main steam line break and stuck-open pressurizer (PZR) relief valve scenarios, with different assumptions of emergency core cooling system (ECCS) availability, temperature and operator response time. Screening of all these transients is done using a TRACE model of the reference NPP. Analysis of all transients is performed by calculating key parameters that are characterizing the cooling rate of the internal reactor vessel surface and downcomer (DC), i.e. the minimum fluid temperature in the vessel, primary system pressure and heat flux on the inside of the vessel wall. Comparisons between one-dimensional (1D) and three-dimensional (3D) cylindrical spatial representations of the downcomer and lower plenum are considered. Results of the screening analysis showed that the 3D vessel model is especially crucial during an asymmetric injection of the emergency cooling water by the safety system, for instance, in case of LOCA in a cold leg (CL). The main outcome of the LOCA screening analysis is that a large break and some intermediate break LOCAs in either the CL or hot leg (HL) lead to the most severe PTS conditions. Other analyzed transients, e.g. small break LOCA, stuck-open PZR relief valve (RV) and SGTR, are not severe enough to contribute significantly to the PTS risk, with the possible exception of a stuck-open PZR-RV valve since this has a higher probability of occurrence.