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 N_sub/N_pch. This work paves the way for studying the start-up and operating characteristics of SBM.

设计用于在两相自然循环下运行的低蒸汽质量和 PWR 型反应堆设计的核加热反应堆 (NHR) 的微沸腾模式 (SBM) 受到流动不稳定性和堆芯饱和沸腾程度的严格限制. 本研究的目的是评估 RELAP5 代码在模拟自然循环系统中的流动不稳定性方面的适用性，然后定量估计 SBM 的运行参数空间。基于 NHR200 的大规模设施，开发了 RELAP5 模型。在综合评估后，进一步对流阻系数、数值方法和漂移速度进行灵敏度分析和频谱分析，以更好地了解模拟中存在的偏差。可以发现，从模拟结果和实验数据之间的合理一致性，RELAP5代码可以揭示密度波振荡（DWO）的性质。此外，合理校准漂移速度可以大大提高静态参数和振荡特征频率的准确性。最终通过无量纲数辅助RELAP5代码实现运行参数空间的定量计算N _sub /N _pch。这项工作为研究 SBM 的启动和运行特性铺平了道路。