SSM perspective
Background
The concept that the Swedish nuclear power industry plans to utilise for the final disposal spent nuclear fuel is called KBS-3 method, which is based on three different barriers to prevent spreading of radioactive substances: copper canisters, bentonite buffers and the surrounding Swedish bedrock. In the current KBS-3 design, the spent nuclear fuel will be placed in cast iron inserts which will be protected by a 50 mm thick copper shell. The cast iron insert provides mechanical strength and radiation shielding, while the copper constitutes the corrosion barrier of the canister. The canisters should then deposited in a repository at the Forsmark site at a depth of about 500 m, where each canister is enclosed by a bentonite buffer, primarily to limit transport of groundwater to and from the canister’s surface. When evaluating The KBS-3 system’s long-term protective capability, the understanding of long-term processes that can affect the canister containment function, for example, local corrosion processes such as stress corrosion cracking and pitting corrosion under reducing chemical conditions, is of great importance. Research studies can help elucidate conditions and expand understanding of these types of fundamental processes and mechanisms, which therefore constitute important evidence in this context. To determine the meaning of individual experimental results for the assessment of a long-term KBS-3 repository performance however, the radiation safety analysis requires consideration of the current environmental conditions in the repository environment, and the long-term development of those conditions, which in this context are represented by the most important parameters for the processes. For pitting corrosion during reducing conditions, those are primarily concentrations and fluxes of hydrogensulphide ions, as well as the chloride content, which need to be based on understanding and analysis of both groundwater chemical conditions and transport processes in the vicinity of a canister. For stress corrosion cracking, which additionally requires occurrence of tensile stresses in the shell in addition to the parameters controlling the conditions for pitting corrosion in the repository environment, such parameters need to be based on an understanding of the loading conditions that may occur in the repository as well as the development of stresses in the canister shell.
Results
Stress corrosion of OFP copper in a reducing sulphide environment has been studied with the SSRT method, in which the copper samples are subjected to tensile stresses under slow loading conditions, at 90ºC and at the same time exposed for different sulphide concentrations (0.001 M and 0.00001 M, respectively). At the higher sulphide concentration level, intercrystalline defects in the copper material were observed, which was not the case for the samples exposed at the lower sulphide concentration level. Unloaded samples from copper base material as well as from welded copper material were also exposed in the same experimental environment, in order to determine the effect of mechanical loading on the extent of hydrogen charging of the copper material. The highest hydrogen charging was observed in the unloaded welded copper samples. For the base material it was found that mechanically loaded copper samples exhibit a slightly higher hydrogen content after exposure as compared with the unloaded samples. For the unloaded samples exposed at the lower the sulphide concentration level, it was noted that hydrogen content was lower after exposure compared to before exposure. It was observed that the sulphidation of oxide films formed on the copper samples before exposure were very effective. A more detailed summary of the results can be found in “Summary” section in the beginning of the report.
Relevance
This research project is a continuation of an SSM-funded study (SSM 2017: 02) aimed at elucidating chemical and mechanical conditions required for stress corrosion of oxygen-free, phosphorus-doped copper, so-called OFP copper, in a sulphide environment under chemically reducing conditions by slow loading of copper samples in solutions with varied sulphide concentration (SSRT; Slow Strain Rate Testing). This continuation study focuses on confirming and more accurately establishing the limiting values for sulphide concentration and tensile stress, below which stress corrosion could not be observed in the previous project, and also to study the extent of hydrogen charging as a result of sulphide corrosion in both loaded and unloaded copper samples.
Need for further research
Stress corrosion is a form of local corrosion that can be regarded as complex as it involves an interaction between both chemical and mechanical conditions. The present study has contributed to estimating both chemical, mainly sulphide concentration, and mechanical limiting values below which the process cannot be observed. Considering the complexity of stress corrosion as a process, SSM considers it to be relevant to continue these studies under chemical reducing conditions, along with the mechanical conditions related to the process, as well as also covering mechanisms for the hydrogen loading of copper. A direction of these studies, based on a KBS-3 final repository relevance, could be experimental studies aimed at higher sensitivity and detection capability which simulate hydrogen charging of copper corresponding to the final repository environment, as well as the effect of hydrogen charging in the perspective of stress corrosion.