GEOCHEMICAL INFLUENCES ON THE DESIGN, CONSTRUCTION AND OPERATION OF ANUCLEAR WASTE VAULT

The design, construction and operation of a vault for used nuclear fue l in crystalline rock may be influenced by a number of geochemical fac tors. 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 variation s in thermal conductivity, Strength and radionuclide sorptive properti es 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 s urface and subsequent hydraulic testing will introduce both chemical a nd microbiological contaminants that may further influence this decisi on. During vault construction, the geochemistry of the the rock may ca use 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 unaccepta bly 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 treat ment facilities both above and below ground. The use of explosives wil l cause high organic and nitrate loadings in service water as well as the possible impregnation of these chemicals in the disturbed wall-roc k surrounding an excavation. These chemicals may remain despite cleani ng efforts and act as nutrients to promote microbial activity in the p ost-closure phase. In the operational phase, inflow of increasingly sa line groundwater may affect predicted container corrosion-rates and bu ffer and grout stabilities following vault resaturation and increase i n temperature. This, in turn, may affect designs for container materia ls and spacing. Salt accumulations and microbial growths on vault wall s may extend into the rock mass and require skimming of wall-rock befo re closure. This paper describes geochemical influences that might be anticipated in the disposal of nuclear waste with examples from the co nstruction and operation of Canada's Underground Research Laboratory, Manitoba, in a granitic batholith on the Canadian Shield.