Radiation Effects on KBS-3 Barriers: SKB’s work so far

Abstract: In the Swedish concept for final disposal of spent nuclear fuel, referred to as KBS-3, a three-layered protection system is used. The system consists of a copper canister holding the spent nuclear fuel deposited 500 meters in a repository built in groundwater saturated granitic rock. The copper canisters are placed in deposition holes, buffered and backfilled by bentonite clay. One of the challenges associated with this system is the long-term exposure of the engineered barriers i.e. the canister including the spent fuel and the cast iron insert as well as the bentonite buffer to ionizing radiation. The possible effects of radiation on the materials in the engineered barriers have been studied not only by the Swedish Nuclear Fuel and Waste Co (SKB), but also by academia nationally and internationally. In this work, literature studies have been carried out to investigate whether all possible/potential effects of ionizing radiation from the spent nuclear fuel on the engineered barriers have been considered by SKB. Apart from a general summary of the relevant issues in chapter one, regions within the KBS-3 design where enhanced radiation may occur have been identified and the related radiation induced processes have been summarized in chapter two. These issues include effects of water-radiolysis on the spent fuel, the cast-iron insert and other ferrous materials, the copper shell and the bentonite buffer. Three types of possible damages have been identified: the microstructural defects in the ferrous materials by direct radiation, the radiation-induced microstructural alterations of the spent fuel and the bentonite and radiation-induced oxidation i.e. dissolution of the fuel and corrosion of the ferrous metals and the copper. The relevant SKB documents have been identified (in chapter 3) to be the SR-Site main report, the underlying Process reports and reports of FUD-programs. Apart from these documents, other SKB technical reports and open literature are used as basis for assessing whether all the issues mentioned in chapter two of this study have been considered by SKB.  According to the assessment, most of the significant issues mentioned in chapter two have been mentioned and discussed in the identified SKB documents. In the main safety assessment, i.e. the SR-Site project, the significance of most of the radiation processes has been based on dose levels expected at the different regions within the KBS-3 design. Irrespective of the assessments in SR-Site though, the current understanding of the relevant issues and plans for future research efforts summarized in FUD-programs reflect the research developments. However, the recent developments in understanding of radiation-induced effects on montmorillonite and the corrosion of copper in argon atmosphere need to be given due importance in coming FUD-programs. A separate investigation of radiation effects on FSW-joints may contribute to removal of the remaining uncertainties. In addition, mechanism involved in radiation-induced fuel dissolution and copper corrosion may require more research efforts than those planned.