Reactivity feedback evaluation during the start-up of the heat pipe cooled nuclear reactors

Alkali metal heat pipe cooled reactors (HPRs) feature a compact structure, high power capacity, and longer lifetime than conventional chemical or solar power sources. Thus, HPRs could be used as energy source for long-term space missions, such as deep space or planetary surface exploration. The reactivity temperature coefficient (RTC) is a key parameter for reactor safety, and RTC for HPRs is influenced by multiple factors. The impact mechanism of the heat pipes working fluid on the reactivity temperature feedback for HPRs consists of two aspects: 1) Metal vapor status in the internal space; 2) Height variation of the liquid pool at heat pipe bottom. In this study, a HPR model is established consisting of 90 uranium nitride (UN) fuel pins and 37 heat pipes, a BeO reflector and 6 control drums with B₄C absorbers. Different types of heat pipe working fluid are simulated, including sodium and potassium. The MCNP code and the ENDF-B-VII.0 nuclear library are used for the calculation. Numerical results show that the metal vapor in the internal space of the heat pipes provides a positive feedback, while RTC provided by the liquid pool at heat pipe bottom is not monotonous. At the heat pipe operation temperature, RTC provided by the heat pipes are 0.08 pcm/K for Na HPR and 0.06 pcm/K for K HPR. Considering all the influential factors, the RTC for HPRs is negative (-0.83 pcm/K for Na and -0.91 pcm/K for K) and the HPR features strong self-stabilizing capability. This study could provide a reference for the design of HPRs.