Safety evaluation of the modified auxiliary feedwater system for the Chinese improved PWR

• The plant model of the Chinese improved PWR is built.
• Safety evaluation of modified ASG under of LOFW FWLB sequences is studied.
• Minimum feedwater flow rate and volume are obtained.
• Mitigation capacity analysis of the modified ASG with 900 m3 is investigated.

Chinese National Nuclear Safety Administration (NNSA) has issued the general technique requirements of the modification actions for the Nuclear Power Plants (NPPs). The enlargement of the tank volume of the auxiliary feedwater system (ASG) is one of the modification actions to enhance the safety margin for the Chinese improved PWR NPPs. Therefore, it is necessary to evaluate the tank volume of ASG can supply enough cooling water to the secondary side of the steam generators under the transient accidents, to maintain the reactor under safety situation. Typical accident sequences, such as the category II accidents of the loss of feedwater (LOFW) with no loss of offsite power (no-LOOP) and the category IV accidents of the feedwater line break (FWLB) with no-LOOP and the FWLB combined with LOOP, are selected to analyze the minimum flow rate and volume for ASG, based on the RCC-P requirements, based on the model of the typical improved generation II 1000 MWe nuclear power plant, built by using the mechanical safety analytical code. The auxiliary feedwater flow rates of 5 m3/h, 10 m3/h, 15 m3/h and 25 m3/h to each steam generator (SG) for the LOFW with no-LOOP are investigated by analyzing the transient progress, and the results show that the minimum flow rate is 25 m3/h. FWLB transients with different flow rates of 30 m3/h, 35 m3/h and 45 m3/h are also analyzed to show that 45 m3/h is the minimum flow rate considering a safety margin. The minimum volume of the auxiliary feedwater tank is 713 m3 for the LOFW, 723 m3 for the FWLB with no-LOOP and 799 m3 for the FWLB combined with LOOP. Therefore, considering the safety margin, the minimum auxiliary feedwater flow rate necessary to prevent bulk boiling in the reactor coolant system is 45 m3/h for each unbroken SG, and a 900-m3 volume of auxiliary feedwater tank can satisfy the RCC-P requirements for the improved PWR NPPs, to give more time to take mitigation measurements.