Thermal hydraulic analysis of loss of flow accident in the JRR-3M research reactor under the flow blockage transient

The transient thermal-hydraulic behavior of loss of flow accident (LOFA) in the JRR-3M 20MW pool-type research reactor under flow blockage is investigated using RELAP5/MOD3.4 code. The core is divided into 7 hot channels, 1 average channel and 1 bypass channel to take into account the interaction between the blocked channel and adjacent channels. Blockage ratios considered in the work includes 0%, 40%, 50%, 60%, 70%, 80% and 100%. MDNBR and fuel central temperature are investigated to estimate the integrity of the fuel plate. Compared with the accident consequence of flow blockage, the results indicate that it is more likely for LOFA under flow blockage to occur departure from nucleate boiling (DNB) in the same blockage ratio. Even when the flow channel is totally blocked, the fuel temperature is still within the safety limit, which is set as 400 °C. And the emergency shutdown of the main loop low-flow signal can effectively mitigate the accident consequences. The reactor core is ultimately in a safe quasi-steady state when the natural circulation is completely established.