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 LSTF Test 6-1 results, which is the first test of the OECD/NEA ROSA Project focusing on core exit thermocouple (CET) performance to detect core overheat during a PWR vessel top break loss-of-coolant accident (LOCA) simulation experiment. The break size is equivalent to 1.9% cold leg break. The detected CET temperature at 623K is used to start accident management (AM) action to quickly depressurize steam generator secondary-sides. The core overheat, however, was detected with a time delay of about 230s and a large temperature discrepancy between the CETs and the hottest core. This paper clarifies the reasons of these findings 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 by using symptom-based plant parameters such as a reactor vessel water level detection.