As nuclear power plants move toward long-term operation, neutron-irradiation-induced embrittlement of reactor pressure vessels (RPVs) has emerged as a critical safety issue. During a loss-of-coolant accident (LOCA) in a pressurized water reactor (PWR), the emergency core cooling system (ECCS) injects cold water into the RPV, causing rapid cooling of the RPV wall. This rapid quenching of the RPV wall caused by ECCS injection can generate significant thermal stresses in the wall, which may be superimposed on the pressure induced membrane stresses. This phenomenon, known as pressurized thermal shock (PTS), may compromise the integrity of the RPV, particularly when pre‑existing flaws are present within RPV walls that have undergone substantial neutron‑irradiation-induced embrittlement, thereby warranting rigorous assessment.
PTS assessment methodologies have been advancing from deterministic approaches based on conservative conditions toward probabilistic fracture mechanics (PFM) approaches grounded in more realistic plant behavior. In deterministic assessments, thermal-hydraulic conditions for PTS scenarios are prescribed conservatively. In the PFM methodology, realistic input conditions are employed, and probabilistic evaluations are conducted with statistical treatment of uncertainties. In PFM assessments, it is essential to employ realistic thermal‑hydraulic conditions and to explicitly characterize the associated uncertainties.
In this study, to obtain fundamental insights that support the application of PFM methodology to PTS assessment, we investigated LOCA events (one of the principal initiators of PTS) using a four loop PWR plant model implemented in the TRACE thermal hydraulic code. A series of LOCA scenarios was analyzed to characterize system responses, with particular emphasis on the detailed thermal hydraulic behavior within the RPV downcomer during ECCS injection, a region and associated processes that are central to PTS assessment.