The thermal conductivity of filler materials and permeability of a cement sealant for deep borehole repositories for high level nuclear waste

Abstract

The Department of Energy is contractually obligated to begin the removal of spent nuclear fuel from reactor sites by the year 2020 at the risk of increased liabilities. The Blue Ribbon Commission on America's Nuclear Future proposed in 2012 that further development is necessary for geological repositories for spent nuclear fuel (SNF) and high level waste (HLW). They also noted that deep boreholes drilled into granite bedrock may be a viable option. Among the major concerns regarding this type of repository are the retention of radionuclides and the tolerance of heat from the canisters in situ. As a barrier against buoyancy-induced flows of groundwater through the borehole, a special cement formulation has been proposed as a sealant for the waste emplacement zone. Such a sealant must be expansive to prohibit flow through lateral gaps and should have a permeability that is less than or equal to that of the surrounding bedrock. Tests of the cement cured under pressure were conducted using a pressure decay method and water as a pore fluid. Data indicate that bulk permeability of this ideal sample is on the order of 0.1[mu[D, which is sufficient to inhibit flow through the bulk material for the immense expanse of time needed for long-lived radioactive species (e.g. 1-129) to decay. However, a less homogenous variation cured under atmospheric conditions indicated a two order-of-magnitude increase in permeability when subjected to increasing temperature and pressure. Furthermore, the harsh aquatic environment is likely to induce chemical changes that may impact longevity and durability. The decay heat of the waste canisters is conceptually able to induce water flows through air gaps, cracks and voids in the borehole and can lead to enhanced degradation of the canisters themselves. Therefore, thermal conductivity tests have been performed in an apparatus simulating the annular gap between waste canisters and the borehole wall liner on materials that can function as fillers for the gap and canisters themselves. These include mixtures of bentonite and crushed granite, bentonite mud, salt, and dehydrated borax. The effects of air and helium as fillers for the void space of porous materials was also analyzed, along with convection effects in vertical and horizontal orientations. The procedure involved controlling the linear power (13, 50, and 190 W/m) of a rod-shaped electrical heat source surrounded by an annulus of material in an insulated steel pipe and measuring the average temperature change across the gap at steady-state. These data have promoted a 3:7 mixture of bentonite and granite as an optimal gap filler with a value of thermal conductivity at 0.30 W/m-K and adequate absorptive characteristics when in contact with water. Furthermore, helium enhances the thermal conductivity of anhydrous borax, a candidate for a canister filler, by a factor of 1.9, and water increases that for bentonite (in the form of clay) by a lesser degree. Data overall indicate that the horizontal orientation of a canister is optimal at increasing the thermal conductivity of the filler due to enhanced convective heat transfer, which favors slanted borehole designs. The findings of this thesis can be used in future studies involving computational fluid dynamics of the system as a whole, and future work is suggested in the analysis of compacted materials, corrosion inhibitors, and variations on the cement formulation that optimize swelling/voiding at high pressures and temperatures. The latter would involve varying curing temperatures and pressures and employing X-ray crystallography to analyze the phases that are present.