Corrosion of stainless steel in the nitric acid solution containing Np-237 under gamma-ray irradiation simulating the reprocessing plant environment

Irisawa, Eriko*; Kato, Chiaki   ; Yamashita, Naoki; Sano, Naruto

In the reprocessing of spent nuclear fuel, stainless steel components are exposed to highly corrosive environments, including concentrated nitric acid and gamma-ray irradiation from fission products. This study investigates the effect of gamma-ray irradiation on the corrosion behavior of ultra-low carbon stainless steel (R-SUS304ULC) in nitric acid solutions containing Neptunium-237 (Np). Immersion corrosion tests and electrochemical measurements, including polarization resistance and potentiostatic methods, were conducted under irradiation using the Co gamma-ray source. Immersion corrosion tests and electrochemical measurements, including polarization resistance and potentiostatic methods, were conducted using a Co gamma-ray source (maximum dose rate: 3.2 kGy/h). The results showed that at 55C, gamma-ray irradiation significantly reduced the corrosion rate, attributed to the radiolytic generation of nitrous acid (HNO), which reduces hexavalent neptunium (NpO), thereby lowering the oxidizing potential of the solution. At 80C, this effect was diminished due to thermal decomposition of HNO;. The polarization resistance method was validated as a reliable technique for estimating corrosion rates in Np-containing nitric acid solutions. However, under irradiation, the conversion coefficient (K) was found to be lower than expected, likely due to changes in anodic polarization behavior. Potentiostatic measurements further revealed that the increase in anodic current under irradiation was not solely due to metal dissolution. A portion of the current was attributed to other oxidation reactions, possibly involving HNO; or radiolysis-derived species from nitric acid. These findings enhance the understanding of corrosion mechanisms in nuclear fuel reprocessing environments and highlight the importance of accounting for radiation-induced chemical changes when evaluating material performance. Further investigation into the nature of these additional oxidation reactions is necessary to improve corrosion prediction models under irradiation conditions.