Corrosion of Copper in Gas Phase in the Final Disposal of Spent Nuclear Fuel

In Sweden and Finland the spent nuclear fuel is planned to be encapsulated in cast iron canisters that have an outer 50 mm thick shield made of copper. The copper shield shall not be penetrated by corrosion in order to prevent water from entering the canister during the first 100 000 years after deposition. The general corrosion of copper can cause a thinning of 1-2 mm during the initial phase of final disposal. The repository environment is usually described as saline groundwater. It is assumed that the gaps between the host rock, compacted bentonite, and canister and are not initially filled with water. Before the bentonite buffer has become saturated there is a thin gas phase between buffer and copper capsule. It is assumed that in the gas phase vaporisation happens at the bottom and condensation at the top.

In this work we have used Quartz Crystal Microbalance (EQCM) to measure low corrosion rates in gas phase. The EQCM measures weight change. The resolution of the EQCM was found to be good enough so that weight changes corresponding to corrosion rates in the order of μm/year can be measured. The tests have been done using electrodeposited copper layer on quartz wafer above synthetic ground water. The conditions of the synthetic Allard ground water have been varied using temperature, chloride concentration, gas flow etc.

In all tests there was a logarithmic weight increase followed by a linear period. The Δm/Δt slope of the linear period was used to calculate corrosion rates. The weight change when nitrogen is purged to the cell corresponds to thickness change -3 to +1 µm/year. In air or without any gas purging the weight change corresponds to -2 to -60 µm/year. Corrosion in oxic conditions is much faster than in anoxic conditions.