Diffusion of Iron and Neptunium in Buffer Material

Arima, Tatsumi*; Idemitsu, Kazuya*; Xia, X.; Ishidera, Takamitsu ; Iijima, Kazuki  

For the safety assessment of high level waste disposal, it is necessary to investigate the radionuclide migration in the presence of corrosion products of carbon steel under reducing condition. In this study, the reliable apparent diffusion coefficients of neptunium (Np) in bentonite were obtained by in-diffusion method and migration of the corrosion products was investigated. Furthermore, effects of the corrosion of carbon steel on Np diffusion were discussed. The corrosion of carbon steel under reducing condition provided Feions, which were considered to migrate in the interlayer spaces of montmorillonite sheets of bentonite exchanging with two Naions. The corrosion rate of carbon steel was controlled by the diffusivity of Feions into bentonite when they were accumulated at the surface of carbon steel. The corrosion rate increased with increasing dry density of bentonite because of the increase of the diffusivity of Feions. The corrosion rate was estimated to be ~0.1 micro m/y, which was remarkable lower than the setting value, 20 micro m/y, in the second progress report. The Np profiles in the bentonite consisted of two overlapping slopes, a fast and a slow fractions, for both the experiments with and without carbon steel. The apparent diffusion coefficients of the fast and the slow fraction of Np were 10~10m/s and 10~10m/s, which were considered to be the diffusion of Np(V) and Np(IV), respectively. The corrosion of carbon steel provided strong reducing condition to keep most Np in the low oxidation state, Np(IV), which has lower solubility and mobility than Np(V). Therefore, it could be expected that the corrosion of carbon steel will restrain effectively migration of Np into the bentonite.