Analysis of IPWR neutronic-thermohydraulic operation characteristics under low power condition

In nuclear industry, using coupling of different best estimate 3-D neutron kinetic (N-K) and thermal hydraulic (T-H) codes to analyze neutronic-thermohydraulic features of reactor core is one of the most concerned research areas, which could help improve simulation fidelity and optimize nuclear design. In this paper, to further understand low power operation features of Integral PWR-200 (IP200), the coupled code combining RELAP5 and 3-D two-group neutron diffusion code had been developed. This paper reported the detailed processing of synchronizing different time steps explicitly and spatial mapping between T-H and N-K codes. To verify and validate the coupled code, the benchmark test results showed good agreement with the existing Qinshan nuclear power plant (NPP) operation data. IP200's entire system and reactor core were modeled using the coupled code. The simulation tasks gave descriptions of different scenarios of operation strategies, including rated power, natural circulation (NC) and once-through steam generators' (OTSGs) group operation under low power conditions. For forced circulation (FC) operation, reactor power, coolant flow and temperature features were mainly influenced by fuel assembly enrichment, control rods configuration and pumps' thrust. Under 25%FP low power operation, NC showed different coupled effects with that of FC, whose above core features were mainly influenced by loss of pumps' driven force. Besides, 25%FP OTSGs group operation transient conquered secondary-side flow instability, but also characterized by a strongly asymmetric behavior of the primary system which was caused by non-uniform coolant temperature distribution at the core inlet. The coupled code improved simulation precision of different low power operation strategies under the premise that it could fully generalize characteristics and performance of the whole system. Furthermore, the obtained knowledge of coupled code provided a better understanding of integral pressurized water reactor (IPWR) operation features with strong neutronic-thermohydraulic coupling effects, which would be beneficial to improve coupling other codes in future work. (C) 2019 Elsevier Ltd. All rights reserved.