Modelling of intergranular corrosion using cellular automata-I Characteristics and corrosion rates of stainless steels in modified nuclear reprocessing solution

Nuclear fuel reprocessing process is important on the viewpoint of effective usage for uranium resources. Concentrated and hot nitric acid solution is used in PUREX process which is adopted in Japanese reprocessing plant. Some troubles occurred at the beginning of the development. Then, ultra-low carbon type austenitic stainless steels were utilized to ensure highly corrosion resistance in such an environment. Even though, these improved stainless steels have been corroded in some typical conditions. A lot of experiments were conducted to clarify the mechanism of these corrosion problems. Modified solutions were used to simulate the actual reprocessing solution. Mainly boiling nitric acid solution containing some cations were used. Vanadium and Rutenium are typical cations used for reproducing intergranular corrosion of these stainless steels. The data indicated that corrosion rates increased with time at the initial stage, and these stayed at constant value with time duration. It was because that intergranular corrosion propagated at grain boundary in the initial stage and intergranular corrosion attacked whole grain boundary causing drop out of grains. Corrosion rates (weight loss) of steady state were sum of intergranular corrosion amounts and weight losses of dropped grains. Surface appearances and cross sections of corroded samples were also analyzed in detail. The results indicated that the initial stage of intergranular corrosion was characterized by the ratio of corrosion rates between grain boundary and matrix. These ratios differed from individual grain boundaries. Total corrosion rates were affected by the distribution of these ratios. These data were based on the numerical modelling of intergranular corrosion using cellular automata. And also, calculated results were compared with these analytical data.