For the initiation of emergency operating procedures and severe accident management of most commercial nuclear reactors worldwide, monitoring of the core temperature is required. Currently, it is not practicable to directly measure the temperature of fuel cladding surface temperature due to some technical limitations. Thus, measurement of the coolant temperature by using thermocouples at the core exit locations is widely used. However, the core exit temperature (CET) may not represent the core temperature properly because the measurement locations are somewhat distant from the heat generating part of fuel rod assembly. In this regard, it is important to assess the difference between the fuel cladding temperature and the CET. The objective of this study is to get the general insight and understanding of the boiling-induced multiphase flow inside fuel rod bundle during an abnormal operation mode following a loss-of coolant accident by comparing the calculation results of the CET deviation from the fuel cladding (or in-core) temperature for the two different cases of the present analysis model subjected to a coolant flowrate of either 100% or 50% of the nominal value. To do this, three-dimensional multi-phase computational fluid dynamics (CFD) calculations of a simplified pressurizer water reactor (PWR) core model were performed for both reactor operating modes. As a result, it was found that the calculated CETs are much lower than the maximum fuel rod cladding temperatures during both operating modes. Consequently, it is considered that the temperature deviation should be taken into account carefully to use the measured CETs for the initiation of emergency operating procedures and severe accident management of commercial nuclear reactors.

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