Performance of core exit thermocouple for PWR accident management action in vessel top break LOCA simulation experiment at OECD/NEA ROSA Project

Suzuki, Mitsuhiro; Takeda, Takeshi ; Nakamura, Hideo  

Presented are experiment results of the LSTF with a focus on core exit thermocouple (CET) performance to detect core overheat during a vessel top break LOCA simulation experiment. The break size is equivalent to 1.9% cold leg break. The accident management (AM) action to rapidly open the SG relief valves was initiated when CET temperature rose up to 623 K. The core overheat, however, was detected with a time delay of about 230 s and a large temperature discrepancy was observed between the CETs and the hottest core region. This paper clarified the reasons of time delay and temperature discrepancy between the CETs and heated core including three-dimensional steam flows in the core and core exit. The paper discusses applicability of the LSTF CET performance to PWR conditions and a possibility of alternative indicators for earlier AM action is studied by using symptom-based plant parameters such as a reactor vessel water level detection.