The Qinshan Phase III (PHWR) Nuclear Power Plant has two 728 MWe CANDU6 reactors and it was found that the pressurizer steam bleed valves operated frequently during on-power refueling. The pressurizer steam bleed valves started to leak internally due to steam cutting, and the internal leakage further lead to frequent operation of degasser condenser spray valves. By investigating the response of CANDU 6 reactors during refueling operations, it was confirmed that the fueling machine flow injection caused the pressurizer level to rise during online refueling, compressing the pressurizer steam space, which is the root cause of PHT system over-pressurization. Based on the root cause analysis, a solution was proposed by adding a feed-forward control term to the PHT system inventory control logic, and the feasibility of this improvement was verified in unit online tests. After implementing the design modification, the system performances also proved the optimized logic change is effective. The research on this subject is not only beneficial to improving the safe and reliable operation of the PHT system in CANDU 6 reactors, but also provides a reference for improving the design of CANDU reactors of the next generation.

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