Transient Analysis of the Loss of Heat Sink Accident in a New Type of Megawatt Heat Pipe Reactor

Abstract Heat pipe reactors are one of the ideal reactor types for Unmanned Underwater Vehicles (UUVs) due to high energy density, long lifecycle, modularity, and compact structure. In the reactor concept design stage, simulation and analysis of typical accidents on heat pipe reactors are key to assessing their inherent safety. The loss of heat sink accident, as one of the typical accidents in heat pipe reactors, is of great significance to the safety performance evaluation. In this study, the transient models of the passive residual heat removal system (PRHRS), including the one-dimensional flow model, the constitutive model of heat transfer, and the constitutive model of pressure drop, are developed based on the completed design of the PRHRS. Based on the models, a transient coupling analysis code for the loss of heat sink accidents is developed, which combines with the transient analysis code for heat transfer in a new type of megawatt heat pipe reactor (HTR) and the transient analysis code for the natural circulation of cooling water (SCTRAN). The results of the coupling analysis code are validated against the results of the STAR-CCM+ during the steady-state operation, with the maximum error not exceeding 8.13%. The coupling analysis code is also used to analyze the transient characteristic of the PRHRS during the loss of heat sink accident with different delayed emergency reactor shutdown times. After the loss of heat sink accident, the cooling water in the emergency cooling component (ECC) is natural flow driven by density differences without external power. Before the reactor is shut down, the heat generated in the core can only be removed through the PRHRS, and the temperature of the core remains to rise. As the heat removal capacity of the PRHRS increases and the reactor shuts down, the fuel and heat pipes gradually cool down after reaching the peak temperature, and the waste heat of the core is gradually removed. The results show that with the delayed emergency reactor shutdown time within 23s after the loss of heat sink accident, the PRHRS can remove the residual heat of the core in a timely manner, no boiling occurs in the ECC, and the temperatures of the core are lower than the temperature limit, which reflects the good safety of the new type of megawatt heat pipe reactor in the accident of the loss of heat sink.