THE CAUSE ANALYSIS AND TREATMENT OF THE POSITIVE DEVIATION FOR RPVL/RPVDT MEASURED TEMPERATURES IN AN EPR NUCLEAR POWER PLANT

Abstract

The Reactor Pressure Vessel Level (RPVL) system and Reactor Pressure Vessel Dome Temperature (RPVDT) system are all made of some thermocouples of type K in the European Pressurized Reactor (EPR) nuclear power plant. However, positive deviations appeared on them when comparing measured temperatures (T_M) with real temperatures (T_R) during commissioning in an EPR plant in China, and some of these positive deviations reached 6.2℃ maximally when the reactor temperature was around 300℃, which exceeded the uncertainty requirement of ±5℃for RPVDT system. According to the structure, manufacturing process and operating principle of RPVL/RPVDT probe in the EPR nuclear power plant, this paper draws a conclusion that the positive deviation of measured RPVL/RPVDT in-core temperatures is caused by the change of the Seebeck coefficient of the thermocouple material, and the influences and strategies in such situation are indicated.