Swedish Nuclear Fuel and Waste Management Co, SKB, applied for a permit to build a KBS-3 type final repository for spent nuclear fuel at the Forsmark site in 2011. The application has been tried by the Swedish Radiation Safety Authority (SSM) under the Act on Nuclear Activities and by a Swedish Land and Environmental Court under the Environmental Code. On January 23, 2018, the Land and Environmental Court approved in its statement to the Government the parts relating to the choice of Forsmark as the site for the repository, post-closure aspects related to the rock and the buffer and the environmental impact assessment. It also considered that supplementary information regarding five issues related to the long-term integrity of the copper canisters be presented and evaluated. The five issues are a) corrosion due to reaction in oxygen-free water, b) pitting due to reaction with sulfide, c) stress corrosion cracking due to reaction with sulfide, d) hydrogen embrittlement, and e) the effect of radiation on pitting, stress corrosion cracking and hydrogen embrittlement. This submission is a summary of the findings regarding these issues, as reported by SKB to the Government in April 2019.
Regarding copper corrosion in pure, oxygen-free water, it is concluded that there is no reason to assume that this corrosion mode occurs to an extent that exceeds that predicted by established thermodynamic data, based on a thorough evaluation of available scientific evidence on the matter.
New studies were performed with copper exposed to i) aqueous sulfide solutions at room temperature, ii) hydrogen sulfide gas at elevated temperatures, and iii) nutritious growth media containing sulfate reducing bacteria. The copper surfaces were investigated for signs of localised corrosion damage. It is concluded that pitting corrosion due to sulfide exposure can be excluded since pitting requires the formation of a passive copper sulfide film on the copper surface, and such film formation has never been observed on copper in sulfide solutions under naturally corroding conditions. There are some laboratory observations that have been interpreted as micro-galvanic corrosion of copper in sulfide solutions. When these laboratory conditions are translated to repository conditions, it cannot be conclusively ruled out that micro-galvanic corrosion could occur for the highest sulfide fluxes that could occur in the repository in the case of eroded bentonite buffer. The occurrence of localised corrosion under a biofilm of sulfate-reducing bacteria (SRB) is also discussed and ruled out, in particular based on the fact the groundwater at repository depth is not sufficiently rich in nutrients to support the formation of a biofilm and/or high metabolic activity of SRB.
Further studies were performed with slow strain rate testing of copper in sulfide solutions, and compared to earlier studies. It is concluded that simultaneous tensile stresses and sulfide exposure are not expected to jeopardise the canister integrity in the repository environment, even though tensile stresses cannot be ruled out in parts of the canister during certain periods. In laboratory studies, microscopic fractures have been observed in the surfaces of copper specimens exposed to tensile stresses in combination with high concentrations of sulfide. However, a mechanistic description has been formulated and suggests that the observed phenomenon is not an example of “traditional” stress corrosion cracking, SCC, but rather a kind of intergranular corrosion. When evaluating the conditions under which these observations were made, it is concluded that the sulfide concentrations and fluxes required for the phenomenon to be observed, are such that they will not occur at the canister surface in the repository.
It is concluded that hydrogen embrittlement will not jeopardise canister integrity in a KBS-3 repository. The conclusion is based on assessments of possible influxes into the material of atomic hydrogen liberated in corrosion processes, and on an evaluation of experiments and model calculations of how such influxes would affect the metal. Exposure conditions leading to damaging effects in thin surface layers in laboratory experiments are much more aggressive than in the repository environment.
Based on revised calculations of radiation damage and to some degree also demonstrated by new experiments it is concluded that irradiation will cause insignificant levels of radiation damage in the canister material. In further experimental studies of radiolytic corrosion of copper it was found that catalytic decomposition of hydrogen peroxide on copper oxide surfaces produces molecular oxygen, which explains both earlier laboratory results and modelling calculations.
Since localised corrosion in the form of micro-galvanic sulfide corrosion, could not be conclusively ruled out for the highest sulfide fluxes that could be expected in the repository environment, an updated dose consequence calculation based on a pessimistic interpretation of available data on micro-galvanic pit formation was carried out. It shows that the impact of micro-galvanic corrosion on peak doses is marginal. The overall result of the work presented to the Swedish Government is that the main conclusion in the safety assessment SR-Site is unaltered: A KBS-3 repository built at the Forsmark site will fulfil regulatory requirements on post closure safety.
