Thermal-hydraulic analysis of pellet bed reactor for space nuclear electric propulsion

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.