Geochemical influences on the design, construction and operation of a nuclear waste vault

Abstract

The design, construction and operation of a vault for used nuclear fuel in crystalline rock may be influenced by a number of geochemical factors. During the period when a site is being selected, information is needed regarding the rock type, heterogeneities in its composition and the mineralogy of permeable zones, because these will cause variations in thermal conductivity, strength and radionuclide sorptive properties of the rock. These factors may affect decisions regarding depth of vault construction, tunnel dimensions and spacing of panels and waste containers. During site characterization, borehole drilling from the surface and subsequent hydraulic testing will introduce both chemical and microbiological contaminants that may further influence this decision. During vault construction, the geochemistry of the rock may cause changes to the design and construction methods used. High salinity fluids in the rock matrix could limit the ability of radar surveys to detect fractures in the surrounding rock and may also cause unacceptably high total dissolved solids loadings in water discharged from the facility. The presence of toxic, corrosive or radioactive constituents in inflowing groundwater may require grouting or development of treatment facilities both above and below ground. The use of explosives will cause high organic and nitrate loadings in service water as well as the possible impregnation of these chemicals in the disturbed wall-rock surrounding an excavation. These chemicals may remain despite cleaning efforts and act as nutrients to promote microbial activity in the post-closure phase. In the operational phase, inflow of increasingly saline groundwater may affect predicted container corrosion-rates and buffer and grout stabilities following vault resaturation and increase in temperature. This, in turn, may affect designs for container materials and spacing. Salt accumulations and microbial growths on vault walls may extend into the rock mass and require skimming of wall-rock before closure. This paper describes geochemical influences that might be anticipated in the disposal of nuclear waste with examples from the construction and operation of Canada's Underground Research Laboratory, Manitoba, in a granitic batholith on the Canadian Shield.