Abstract The pellet bed reactor (PeBR) featured with high-power density, small reactor size and full retention of fission products, has promising applications for space nuclear power system. According to the special features of the PeBR for nuclear electric propulsion (NEP), a transient analysis code is developed, where the six groups delay neutrons point kinetics model, coolant flow and heat transfer model as well as the fuel pellets heat conduction model are built and adopted. Then, the code is used to perform thermal-hydraulic analysis of PeBR, including the optimization of the frits, key parameters sensitivity analysis and the reactor transient characteristics. Results show that the optimized hot frit porosity (HFP) profile can eliminate the hot spot in the core at the steady full power operation. Conversely, changing the cold frit porosity (CFP) is inefficient on controlling the coolant flow through the PeBR core. Increasing the pellet bed porosity and decreasing the HFP from the base case have a negative effect on core flow distribution. Finally, when unprotected reactivity insertion accident (URIA) or unprotected loss of flow accident (ULOFA) occurs, the PeBR is capable of keeping the power and temperature stable and below the safety limits due to the reactor negative reactivity feedback mechanism. This study could provide suggestions and guidance for the design and safety analysis of the PeBR for NEP.

中文翻译：

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空间核电推进芯块床反应堆热工水力分析

摘要 颗粒床反应堆（PeBR）具有功率密度高、反应堆体积小、裂变产物完全截留等特点，在空间核电系统中具有广阔的应用前景。根据核电推进（NEP）PeBR的特点，开发了瞬态分析程序，建立了六组延迟中子点动力学模型、冷却剂流动和传热模型以及燃料芯块热传导模型并采用。然后，利用该代码对PeBR进行热工水力分析，包括熔块优化、关键参数灵敏度分析和反应堆暂态特性分析。结果表明，优化的热熔块孔隙率 (HFP) 分布可以在稳定的全功率操作下消除磁芯中的热点。反过来，改变冷熔料孔隙率 (CFP) 在控制通过 PeBR 芯的冷却剂流量方面效率低下。增加球团床孔隙率和降低基本情况下的 HFP 会对核心流量分布产生负面影响。最后，当发生无保护反应插入事故（URIA）或无保护失流事故（ULOFA）时，由于反应堆负反应反馈机制，PeBR能够保持功率和温度稳定并低于安全限制。本研究可为NEP用PeBR的设计和安全分析提供建议和指导。当发生无保护反应插入事故（URIA）或无保护失流事故（ULOFA）时，由于反应堆负反应反馈机制，PeBR能够保持功率和温度稳定并低于安全限制。本研究可为NEP用PeBR的设计和安全分析提供建议和指导。当发生无保护反应插入事故（URIA）或无保护失流事故（ULOFA）时，由于反应堆负反应反馈机制，PeBR能够保持功率和温度稳定并低于安全限制。本研究可为NEP用PeBR的设计和安全分析提供建议和指导。