Function for Controlling Nuclide Migration of Buffer Material in the Geological Disposal for High-Level Radioactive Waste

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.

Measurements of Thermodynamic Data of Water in Ca-Bentonite by Relative Humidity Method

“…Let a H 2 O be the activity of the water, a H 2 O is defined by Equation (2). The relative partial molar Gibbs free energy of water can be obtained from Equation (3).…”

“…µ 0 (W, α): Chemical potential of α phase µ 0 (W, β): Chemical potential of β phase P ext , P 0 ext : Swelling stress when saturated with pure water and when dried [Pa] V H 2 O : Molar volume of water (=18.0686, 25 • C) [cm 3 /mol] From Equation (8), assuming that the swelling stress of bentonite under dried condition is 0 and the volume of water is constant, Equation ( 9) is derived, and the swelling stress can be calculated from Equation (9).…”

“…The properties of bentonite and montmorillonite are described below [2][3][4]. In the case of Kunigel V1 (produced by Kunimine Industries Co., Ltd., Tsukinuno, Yamagata Prefecture, Japan), which was used as a reference material in the reference case for the "Second Progress Report" [5] on the geological disposal of HLW, montmorillonite is contained in addition to chalcedony, plagioclase, calcite, dolomite, K-feldspar, and pyrite.…”

Swelling Stress of Bentonite: Thermodynamics of Interlayer Water in K-Montmorillonite in Consideration of Alteration

“…Nuclide adsorption/migration delay is the function of delaying the movement of radioactive nuclides leaked (leached) from the vitrified waste by adsorbing them onto bentonite. Montmorillonite, the main component of bentonite, has a cation exchange capacity of about 100 to 110 meq/100 g (equivalent number per 100 g of montmorillonite), which is exceptionally high compared to normal soils, rocks, and minerals [2,3]. This allows for the delayed movement of many radioactive nuclides, especially through the adsorption of cations.…”