Status report of the Finnish spent fuel geologic repository programme and ongoing corrosion studies

Abstract

Posiva Oy is preparing to submit an operating licence application for a spent nuclear fuel geologic repository located at Olkiluoto, Finland. The construction licence for the underground disposal facility and encapsulation plant was granted in 2015 and construction of the disposal facility began in December 2016. Safe disposal is achieved by the KBS‐3 concept, developed in collaboration with Posiva's Swedish counterpart, the Swedish Nuclear Fuel and Waste Management Co, (SKB). The KBS‐3 concept consists of disposal canisters (a cast iron insert and a copper overpack), surrounded by swelling bentonite and disposed in crystalline bedrock. The copper overpack ensures containment of radioactive materials and its performance against corrosion is a key driver for the design of the engineered barrier system and the repository overall. This paper describes the current status of the Posiva programme, recent learnings from the construction of the repository, and the implications for the long‐term performance of the canister.

Treatment of canister corrosion in Posiva's safety case for the operating licence application

“…An example of this potential disconnect between laboratory and repository is the difference between the flux of sulphide ions to corrosion specimens in small‐scale experiments and that for the canister surrounded by compacted buffer material. [ 3,8 ] The flux of sulphide ions to the corroding copper surface is important because for fluxes below a threshold value, of between 3 × 10 −14 and 2 × 10 −12 mol cm −2 s −1 , the rate of interfacial consumption is faster than the rate of supply and, as a result, the interfacial [HS − ] is essentially zero. [ 8 ] Therefore, localised corrosion and SCC mechanisms that require the presence of HS − ions within the pit or at the tip of the crack are not operative under transport‐limited conditions.…”

“…[ 3,8 ] The flux of sulphide ions to the corroding copper surface is important because for fluxes below a threshold value, of between 3 × 10 −14 and 2 × 10 −12 mol cm −2 s −1 , the rate of interfacial consumption is faster than the rate of supply and, as a result, the interfacial [HS − ] is essentially zero. [ 8 ] Therefore, localised corrosion and SCC mechanisms that require the presence of HS − ions within the pit or at the tip of the crack are not operative under transport‐limited conditions. [ 3,8 ] It is interesting to note that the laboratory conditions under which crack‐like features have been reported in SCC tests in sulphide environments are characterised either by a threshold sulphide flux of approximately 10 −10 mol cm −2 s −1 or the addition of a phosphate buffer which is believed to slow the rate of the interfacial reactions so that the overall corrosion reaction is interfacially limited, rather than being transport controlled.…”

“…[ 8 ] Therefore, localised corrosion and SCC mechanisms that require the presence of HS − ions within the pit or at the tip of the crack are not operative under transport‐limited conditions. [ 3,8 ] It is interesting to note that the laboratory conditions under which crack‐like features have been reported in SCC tests in sulphide environments are characterised either by a threshold sulphide flux of approximately 10 −10 mol cm −2 s −1 or the addition of a phosphate buffer which is believed to slow the rate of the interfacial reactions so that the overall corrosion reaction is interfacially limited, rather than being transport controlled. [ 3,8,9 ] The sulphide flux, or more likely the resulting interfacial [HS − ], also determines the properties of the precipitated Cu 2 S film, with porous, nonprotective films observed at fluxes below the transport‐limited threshold value and compact, protective films formed at higher fluxes.…”

Status and Implications of Hydrogeochemical Characterization of Deep Groundwater for Deep Geological Disposal of High-Level Radioactive Wastes in Developed Countries

“…2018년 6월부 터는 실제 처분용기를 처분하고 벤토나이트로 채운 후 모니터링하는 현장 규모의 실험(FISST, Full Scale In Situ System Test)이 시작되었다. FISST에는 용존 황화물 유 입에 의한 구리 캐니스터의 부식 및 기계적 성능의 변화 평가 연구가 포함되어 있다 (Salonen et al, 2020). 고준위 방사성 폐기물 지질처분을 위한 해외 선진국의 심부 지하수 환경 연구 동향 분석 및 시사점 도출 749…”

Measuring Key Parameters Governing Anion Transport Through Mx-80 Bentonite

“…The compacted bentonite serves many purposes, one of which is to retard the migration of corrosive species to the UFC. Microbially influenced corrosion is one of the key corrosion mechanisms that is currently under investigation by NWMO and other nuclear agencies (Salonen et al 2021). That is, sulfate-reducing bacteria at the interface of the bentonite and host rock may produce bisulfide (HS -) if a sufficient supply of sulfate is present (Briggs et al 2017b, Hall et al 2021).…”