Co-based hardfacing alloys exposed to PWR primary coolant may be replaced with Co-free alloys to lower occupational radiation exposure. To evaluate the viability of Co-free hardfacing alloys, we conducted hot-water tests for gate valves hardfaced with NOREM™ 02 (Fe-base), Deloro™ 50 (Ni-base), and Stellite™ 6 (Co-base). Using a high flow test loop, on-off cycling tests were conducted in 280°C water. It was observed that NOREM 02 exhibited galling and excessive leak after 1000 cycle test whereas no leakage was developed with Deloro 50 after 2000 cycles. To estimate the activity reduction effect of Co-free hardfacing alloys, an existing activity transport model was modified. It is found that the main contributor of Co activity buildup is the corrosion of steam generator (SG) tubing. The Korean Next Generation Reactor (APR-1400) tubed with alloy 690 having a reduced cobalt impurity allowance is expected to have 73% lower Co activity on SG surface compared with the case of alloy 600 tubing. The complete replacement of Stellite 6 with Co-free hardfacing alloys is expected to cut additional 5% of activity which may be too small to justify the risk of galling and leakage development as revealed by the hot-water test.
Bibliographical noteFunding Information:
This work is financially supported by the Center for Advanced Reactors Development (CARD) of Korea Hydro and Nuclear Power Company. The authors would like to thank H.S. Kim and J.H. Mun of CARD, for their helpful comments. Samshin Valve Co. in Chunan, Korea kindly provided welding service for the valve installations. The National Research Laboratory (NRL) program of the Ministry of Science and Technology (MOST) provided support for meeting the publication cost for this paper.
All Science Journal Classification (ASJC) codes
- Nuclear and High Energy Physics
- Nuclear Energy and Engineering
- Materials Science(all)
- Safety, Risk, Reliability and Quality
- Waste Management and Disposal
- Mechanical Engineering