高レベル放射性廃棄物地層処分における緩衝材の核種移行抑制機能

Abstract

Among the properties for controlling radionuclide migration of bentonite, used as a buffer material composing engineered barriers for the geological disposal of high-level radioactive waste, studies on the water penetration, swelling, chemical buffering and nuclide migration retardation properties were reviewed. For water penetration properties, the effects of bentonite dry density, silica sand mixture, temperature and salinity are approximately clear, and a model, applicable for various dry densities, silica sand contents and montmorillonite contents also has been developed. For swelling properties, the effects of bentonite dry density, silica sand mixture, salinity, interlayer cation, montmorillonite content and saturation degree are approximately clear, and a thermodynamic model, applicable for various dry densities, silica sand contents, salinities and montmorillonite contents has been developed. For chemical buffering properties, although data acquisition by a batch method and studies to directly measure pH and chemical component of the porewater of compacted bentonite have been conducted, data acquisition in compacted system and geochemical analysis have just started. Finding accumulation and data acquisition for various conditions such as high pH and saline groundwater are still needed. For nuclide migration retardation properties, the effects of bentonite dry density, salinity, silica sand mixture, montmorillonite content, particle size, interlayer cation, the charge of diffusion species and temperature on diffusion have been comprehensively studied. Data on the effect of interlayer cation on diffusion coefficient and activation energy and data on diffusion coefficient and activation energy for effective diffusion coefficient are however still insufficient. Inconsistency between batch sorption and retardation in diffusion should be additionally considered together with data setting method for safety assessment analysis.After the year 2000, studies for saline groundwater conditions have been powerfully conducted, and it is understood that saline groundwater conditions affect all properties described above. Continuing these studies, studies on the long-term behaviour considered the effect of alteration are necessary.