Diffusion of Ni in Compacted Sodium Bentonite

抄録

Diffusion coefficients of key nuclides in bentonite are one of the important parameters required for performance assessment of geological disposal of high-level radioactive waste (HLW). Diffusion is generally classified into two states: non-steady state and steady state. Nonsteady state diffusion represents the diffusion coefficient in a transient state, which is called apparent diffusion coefficient (Da). In performance assessment, Da is accounted as an important parameter for evaluating nuclide retardation in bentonite. Nickel-59 is produced by activation of the corrosion products in a power reactor and is one of the important radionuclides for performance assessment because of its long half-life of 7.5×10⁴ yr. Christiansen and Torstenfelt have reported Da of Ni in bentonitew⁽¹⁾. However, Da for Ni in Kunigel V1^R bentonite (Kunimine Industries Co., Ltd.), which is produced in Japan, has not been measured yet. In underground, such 1, 000m deep, bentonite temperature is higher than that of ground surface because of temperature gradient⁽²⁾. This temperature increase may affect diffusion behaviors of nuclides in bentonite. For example, it has been reported that the apparent diffusion coefficients for Cs, Na, HTO and Sr in bentonite become larger with increasing temperature^(3)-(7). However, studies for temperature dependence on diffusion of nuclides in compacted bentonite are quite limited.
In this study, the apparent diffusion coefficients for Ni in Kunigel V1 were measured as functions of density of the bentonite and temperature, then the activation energy for diffusion was calculated from the temperature dependence of the Da.