The Stress Corrosion Cracking of Copper Containers for the Disposal of High-Level Nuclear Waste

Stress corrosion cracking (SCC) is a possible failure mode for Cu containers in an underground nuclear waste disposal vault. It is difficult to guarantee that SCC will never initiate on Cu containers based only on the results of relatively short-term experiments. Therefore, the extent of SCC is being predicted based on the argument that the rate of crack propagation will be limited. Several environmental factors will limit the rate of cracking, including: the general absence of aggressive SCC agents in the vault, the limited time of rapid strain of the container shell and the limited supply rate of oxidants (principally dissolved O₂) to the container surface through the compacted clay-based material in which the containers will be placed. In the first part of this study, the effect of oxidant flux on the crack velocity is being determined.

The SCC behaviour of two high-copper alloys has been determined in nitrite-containing environments over a range of oxidant fluxes. In NO₂ solutions, transgranular SCC is observed. There is evidence for discontinuous crack advance, including crack arrest markings on fracture surfaces and correlated noise events in the electrochemical potential and applied load signals. Crack velocities of 4-8 nm·s⁻¹ are observed in constant extension rate tests in 0.1 mol·dm⁻³ sodium nitrite (NaNO₂) with applied current densities of 0.1 - 1.0 μA·cm⁻².

The maximum crack length for a Cu container has been estimated based on the observed dependence of the crack velocity on the oxidant flux and the predicted time dependence of the oxidant flux to the containers in a disposal vault.