Application of modified 37 element fuel bundle in Wolsong NPP

Due to aging of the heat transport system components of CANDU reactors, Wolsong reactors has been derated since its operation for 15 years. Pressure tube(PT) diametral creep has been one of the major reactor aging mechanisms of CANDU reactors. It is caused by fast neutron irradiation, high coolant temperature and pressure. As pressure tube crept radially, coolant in fuel channel bypasses the interior sub-channels of bundles. As a result, the fuel coolability drops off while PT creep makes the hydraulic resistance reduce. To recover the aging effect, a modified design of 37-element fuel bundle (37M) has been developed by Canadian utilities since 2007. It has the very same geometry as the existing 37-element fuel bundle except center pin diameter reduced. It is looking forward to 37M fuel bundle enhancing thermal margin by increasing flow to interior sub-channels. Based on the proven technology in the Canadian nuclear industry, KHNP regarded that 37M fuel was the best option to minimize power derating with minimal design changes. KHNP has re-designed the 37M original design for CANDU6 reactor, and re-evaluated full scope reactor safety analysis using new industry standard toolset. 37M fuel demonstration irradiation was also performed at Wolsong Unit 4. Especially impact assessment was done for different fuel bundle appendage design between Wolsong and Darlington NPP, and safety analysis was also done at 37R and 37M transition core conditions. In 2019, Korea regulatory accepted full core loading of 37M fuel in Wolsong unit 2, 3, and 4. To evaluate enhancing thermal margin, regional overpower protection (ROP) trip setpoint was also evaluated and licensed for 37M full core. 37M fuel was first loaded commercially at Wolsong Unit 2 in December 2019. Since then, commercial loading has been carried out on Wolsong Units 3 and 4, and full core loading will have been completed for all Wolsong Units in August 2022. Although KHNP was not a member joining at the beginning of 37M development, 37M fuel bundle application project in Wolsong NPP has brought successful results through collaboration between COG and members since KHNP has decided to apply the new improvement. Based on the support of COG, OPG, Bruce, NBP, Kinetrics [sic], CNL, Stern. Lab during the additional CHF experiment, KHNP can actively disposition for the comments of Korea regulatory by broadening our understanding of the experimental process and results. Especially experts from OPG were invited to KHNP, and they greatly contributed to support to address regulatory issues by presenting Canadian utilities 37M fuel bundle design concept, operating experience, and applications status. By applying the 37M fuel bundle design, KHNP can enhance safety to Wolsong nuclear power plant. The experience of success in new improvement promote great confidence. It can also contribute the carbon dioxide reduction in energy grid. (author)