Many countries are exploring long-term disposal of used nuclear fuel in a deep geological repository using a multiple-barrier system, with a key barrier being the used fuel container (UFC). The Canadian UFC design uses a Cu-coated carbon steel (CS) vessel; the CS provides structural strength while a 3-mm thick Cu coating provides corrosion protection. Immediately after emplacement, the UFC will be exposed to a diminishing dose of gamma-radiation emitted from within the spent fuel. During the first few decades when this radiation dose is significant, trapped air or water near the UFC may undergo radiolysis, potentially producing redox active species that can alter the corrosion behaviour of the copper coating.
In order to address concerns that gamma-radiation of the environment surrounding a UFC may lead to container corrosion, a series of experiments on copper coupons have been initiated to determine the influence of gamma-radiation during the different stages of DGR conditions:
1. Copper coupons undergoing atmospheric corrosion experiments a at dose rates close to repository values, as well as much higher dose rates, to determine the earliest corrosion mechanisms that may occur;
2. Copper coupon exposures to small volumes of aqueous nitric and nitrous acids, to simulate the possible concentration of humid air radiolysis products in condensed water droplets on the container surface, during saturation of the repository; and
3. Copper coupons exposed to high dose of gamma-radiation within aqueous solutions including a range of possible groundwater anions such as SO42–, HCO3–, Cl–, to assess corrosion in an early saturating repository.
In addition, experiments have been extensively supplemented with modelling, including assessments for:
4. Production of radiolytically formed species, including to determine oxidant concentrations that may affect the copper corrosion;
5. Total corrosion of radiolytic copper corrosion in solutions containing groundwater anions such as SO42–, HCO3–, Cl– as a projection from direct measurements.
Ultimately, it is expected that a reliable model will be developed to relate radiolysis and corrosion processes with a maximum copper corrosion allowance.
Used fuel container, Corrosion, Radiation, Deep Geological Repository