Reduced deposition of 110m Ag colloids by improved oxidation operation process during shutdown of a PWR

For some pressurized water reactors, 110m Ag colloids deposite on the piping surface and auxiliary systems of the primary loop during the oxidation operation process, becoming one of the most important radionuclides. The decreasing concentration of hydrogen peroxide (H 2 O 2 ) during the oxidation operation process may be the cause of the deposition of 110m Ag colloids. In this work, Ag colloids with particle size of 20–60 nm were first synthesized. Then the effects of reaction temperature, concentration of H 2 O 2 and reaction time on the oxidation and removal of Ag colloids were studied. The results showed that at least 5 ppm H 2 O 2 was needed to achieve significant oxidation and removal of Ag colloids at 80 °C. The results of this work were then applied during the shutdown of a PWR. The oxidation operation process was improved to maintain the concentration of H 2 O 2 at 4–8 ppm during the whole process. The deposition of 110m Ag colloids was then effectively mitigated, and the RCV radiation index was reduced by 28.2%. This work demonstrates the importance of H 2 O 2 in reducing the deposition of Ag colloids during the oxidation operation process, providing a new idea and methodology to reduce shutdown dose rate of PWRs.