Corrosion of the stainless steel in the zeolite containing diluted artificial seawater under gamma-ray irradiation

As a part of consideration for long-term storage of spent zeolite adsorption vessels in the Fukushima Daiichi nuclear power station, corrosion of vessel material in the spent zeolite adsorption vessel is one of important issue. Because seawater in contaminated water, which is treated by the zeolite adsorption system for removing radioactive Cs, accelerates corrosion of stainless steel for the vessel material. However it is not well known a compounded behavior of corrosion in seawater under irradiation. We performed electrochemical tests of stainless steel (type 316L) in the zeolite containing artificial seawater under gamma-ray irradiation. Steady spontaneous potential (E_sp) and pitting potential (V_C), of type 316L was measurement. For comparison, simply immersion test without zeolite was performed. Co-60 gamma-rays source was used under irradiation. Dose rate of gamma-ray irradiation was controlled for 5kGy/h and 400Gy/h. One conditions (Cl^-: 2,000ppm) in diluted artificial seawater was used for measuring E_sp under gamma-rays irradiation. Three conditions (Cl^-: 20,000, 6,000, 2,000ppm) were used for measuring V_C under non-irradiation. In anode polarization curves, there was no clear difference under irradiation and non-irradiation. Hence it was available that V_C in anode polarization curve under irradiation is the same value under non-irradiation. The test temperature was set at 60°C for decay heat in the spent zeolite adsorption vessel. The corrosion potential of type 316L increased with increasing time after gamma-ray irradiation. This result shows that the oxidants (DO, H₂O₂) formed by radiolysis of the water promote the corrosion potential to noble. The E_sp was defined as steady corrosion potential after 500ks. The E_sp was shifted to nobler by gamma-rays irradiation, while increasing E_sp was suppressed by contacted with zeolite. The V_C contacted with zeolite decreases with increasing Cl^- ion concentration and the value is slightly smaller than the V_C in simple immersion without zeolite. (author)