Abstract
A composite container for the retention and disposal of used CANDU™ nuclear fuel was structurally analyzed. The design employs an inner steel vessel for support against external pressure and a close-fitting outer copper vessel for corrosion resistance. The container is deposited in an underground disposal vault in granitic rock and is surrounded by a clay-based buffer that swells when contacted by groundwater. It must withstand the swelling pressure of the buffer plus the hydrostatic pressure of the groundwater at a minimum 500-m depth. The case was analyzed in which a small manufacturing defect penetrates the copper shell, allowing groundwater to seep between the vessels and gradually convert the steel of the inner vessel to magnetite. Because the specific volume of magnetite is approximately twice that of steel, the outer surface of the steel vessel progressively swells while the remaining (uncorroded) shell thins. Outward deformation of the copper vessel is resisted by the buffer and surrounding rock, therefore pressure on the steel vessel gradually increases as it corrodes.
The finite element code, ANSYS, was used to predict the minimum corrosion that would produce plastic collapse of the steel vessel. Knowledge of the rate of conversion of steel to magnetite under disposal-vault conditions could then allow an estimate of the time-to-collapse following the initial ingress of groundwater.
