Assessment of RELAP5/MOD3.2 for simulating density wave oscillation with a two-phase natural circulation test facility

Abstract

The slight boiling mode (SBM) of the nuclear heating reactor (NHR) designed to operating under two-phase natural circulation with low vapor quality and PWR-type reactor design is rigorously constrained by the flow instability and the extent of saturated boiling in the core. The objective of this study is to evaluate the applicability of the RELAP5 code to simulate flow instability in natural circulation systems and then to quantitatively estimate the operating parameters space of the SBM. Based on a well-scaled facility of NHR200, a RELAP5 model is developed. After a general assessment, sensitivity analysis and frequency spectrum analysis of flow resistance coefficients, numerical method, and drift velocity are further conducted for better understanding the deviations existing in simulations. It can be found that the nature of density wave oscillation (DWO) can be revealed by the RELAP5 code from the reasonable agreements between the simulation results and experimental data. Furthermore, a reasonable calibration of the drift velocity can greatly improve the accuracy of both static parameters and oscillating characteristic frequency. Ultimately, quantitative calculation of the operating parameter space is achieved with the RELAP5 code assisted by the dimensionless number Nsub/Npch. This work paves the way for studying the start-up and operating characteristics of SBM.