A corrosion model for waste package corrosion-allowance materials

Current conceptual designs for radioactive waste disposal containers for repositories call for a thick outer barrier of corrosion-allowance material such as carbon steel and a thinner inner barrier of corrosion-resistant material. A model has been developed for calculating the contribution of the corrosion-allowance material to containment life. The model is based on the equation of Stahl for immersed aqueous corrosion of low-carbon steel. Corrosion depth was calculated as a function of time for both the expected environment (humid air) and enhanced corrosion. For the mass loadings considered, the calculated corrosion penetration after 10 (exp 5) years was never greater than 60 mm, whereas the proposed barrier thickness is 100 mm. Thus, the corrosion-allowance barrier in proposed conceptual designs will be long-lived under expected conditions. No evidence was found that high mass loadings will lead to early, catastrophic failures. Modelling of unexpected conditions produced earlier failures due to less protective oxides and larger rate constants. These effects can be mitigated by using a high mass loading to produce dry conditions