Analysis of natural circulation characteristics for pool-type fast reactor system

In the inherent safety design of the pool-type fast reactor, when the normal heat transfer routine of the primary system is lost during accident conditions, the residual heat of the reactor core is transferred to the ultimate heat sink through the natural circulation in the reactor vessel. The natural circulation phenomenon in the pool-type reactor system under accident transient is complicated. For the purpose to improve the knowledge of natural circulation characteristics of the pool-type reactor system and validate the applicability and accuracy of system analysis code for simulating natural circulation in pool systems, the RELAP5 MOD3.2 code was adopted to calculate the station blackout (SBO) test conducted on a pool-type test facility. The analysis results show that in the early stage of the transient process, the flow re-distribution phenomenon occurs in the pool-type vessel. There exists natural circulation flow path both inside core assembly simulators and outside core assembly simulators. Due to the influence of the hot fluid at the outlet of the core and the cold fluid at the outlet of the DHX, the thermal stratification phenomenon occurs in hot pool. The main thermal-hydraulic parameters calculated by the code (such as the fluid outlet temperature in the core assembly simulator, the flow rate of the primary circuit and the fluid inlet and outlet temperature of the DHX primary side) are in good agreement with the experimental results. Through the conduct of this work, the feasibility of the RELAP5 code calculation model and modeling method is verified, which can be further adopted to the natural circulation simualtion of pool-type reactors.