Reliability of core exit thermocouple for accident management action during SBLOCA and abnormal transient tests at ROSA/LSTF

Presented are experimental results on the general performances of core exit thermocouples (CETs) to detect core overheat for accident management (AM) action by using the Large-Scale Test Facility (LSTF) of the ROSA Program of the Japan Atomic Energy Agency. The LSTF is a full-height, full-pressure, and 1/48-volumetric-scaled model of a 4-loop pressurized water reactor (PWR). This study was motivated by a significant delay in the time and temperature rise of the CETs from core heat-up during a vessel top head small break loss-of-coolant accident (SBLOCA) test. A certain delay in time and temperature rise of the CETs was also observed in various SBLOCA and abnormal transient tests. Such CET performances are derived from thirteen LSTF tests as follows: (1) general CET performances are obtained in the form of equations including cases under limited influences of water fall-back from hot legs, (2) the major reason for the delay is the interaction of three-dimensional steam flows with low-temperature structures in and around the core and core exit, (3) break location was insignificant except for the PV top and bottom break cases, and (4) CET superheat is suitable for AM instead of the temperature value for significantly high or low pressure transients. The applicability of the results to PWR is discussed further. (author)