Econ. Environ. Geol. 2023; 56(4): 421-433

Published online August 30, 2023

https://doi.org/10.9719/EEG.2023.56.4.421

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

A Review of the Influence of Sulfate and Sulfide on the Deep Geological Disposal of High-level Radioactive Waste

Jin-Seok Kim*, Seung Yeop Lee, Sang-Ho Lee, Jang-Soon Kwon

Disposal Performance Demonstration R&D Division, Korea Atomic Energy Research Institute, Daejeon 34057, Republic of Korea

Correspondence to : *jskim84@kaeri.re.kr

Received: April 17, 2023; Revised: August 10, 2023; Accepted: August 23, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

The final disposal of spent nuclear fuel(SNF) from nuclear power plants takes place in a deep geological repository. The metal canister encasing the SNF is made of cast iron and copper, and is engineered to effectively isolate radioactive isotopes for a long period of time. The SNF is further shielded by a multi-barrier disposal system comprising both engineering and natural barriers. The deep disposal environment gradually changes to an anaerobic reducing environment. In this environment, sulfide is one of the most probable substances to induce corrosion of copper canister. Stress-corrosion cracking(SCC) triggered by sulfide can carry substantial implications for the integrity of the copper canister, potentially posing a significant threat to the long-term safety of the deep disposal repository. Sulfate can exist in various forms within the deep disposal environment or be introduced from the geosphere. Sulfate has the potential to be transformed into sulfide by sulfate-reducing bacteria(SRB), and this converted sulfide can contribute to the corrosion of the copper canister. Bentonite, which is considered as a potential material for buffering and backfilling, contains oxidized sulfate minerals such as gypsum(CaSO4). If there is sufficient space for microorganisms to thrive in the deep disposal environment and if electron donors such as organic carbon are adequately supplied, sulfate can be converted to sulfide through microbial activity. However, the majority of the sulfides generated in the deep disposal system or introduced from the geosphere will be intercepted by the buffer, with only a small amount reaching the metal canister. Pyrite, one of the potential sulfide minerals present in the deep disposal environment, can generate sulfates during the dissolution process, thereby contributing to the corrosion of the copper canister. However, the quantity of oxidation byproducts from pyrite is anticipated to be minimal due to its extremely low solubility. Moreover, the migration of these oxidized byproducts to the metal canister will be restricted by the low hydraulic conductivity of saturated bentonite. We have comprehensively analyzed and summarized key research cases related to the presence of sulfates, reduction processes, and the formation and behavior characteristics of sulfides and pyrite in the deep disposal environment. Our objective was to gain an understanding of the impact of sulfates and sulfides on the long-term safety of high-level radioactive waste disposal repository.

Keywords deep geological repository, sulfate and sufide, sulfide induced stress corrosion cracking, bentonite, sulfate reducing bacteria

고준위방사성폐기물 심층처분에 미치는 황산염과 황화물의 영향에 대한 고찰

김진석* · 이승엽 · 이상호 · 권장순

한국원자력연구원 저장처분성능검증부

요 약

원자력발전소의 사용후핵연료(Spent Nuclear Fuel: SNF)에 대한 최종처분은 지하 심부의 지질학적 저장소에서 이루어진다. 사용후핵연료를 감싸는 금속처분용기는 주철과 구리 등으로 제작되어 방사성핵종을 장기간 격리할 예정이며, 공학적방벽과 천연방벽으로 구성된 다중방벽처분시스템에 의해 보호를 받도록 설계된다. 지하 심부의 환경(심층처분환경)은 점차 무산소의 환원환경으로 바뀌게 되며, 이러한 환경에서 구리처분용기의 부식을 일으킬 수 있는 유력한 물질 중 하나는 황화물이다. 황화물에 의한 응력균열부식은 구리처분용기의 안정성을 크게 저하시켜 처분장의 장기안전성에 큰 영향을 미칠 수 있다. 심층처분환경에는 황산염이 다양한 형태로 존재 또는 유입될 수 있으며, 황산염환원미생물에 의해 황화물로 전환되어 구리처분용기의 부식에 기여할 수 있다. 완충재와 뒤채움재의 유력한 후보물질인 벤토나이트에는 주로 석고(CaSO4)와 같은 산화형태의 황산염 광물이 포함되어 있다. 심층처분환경 내에 미생물이 생장할 만한 공간이 있고 유기 탄소 등 전자공여체가 충분히 공급된다면 미생물 활동에 의해 황산염이 황화물로 환원될 수 있다. 하지만 근계영역에서 생성된 황화물과 지권으로부터 유입되는 황화물 중 대부분은 완충재에 의해 차단되어 극히 일부만이 처분용기에 도달할 것이다. 처분환경에서 존재가능한 황화철 광물 중 하나인 황철석은 용해과정에서 황산염을 발생시켜 구리처분용기의 부식에 기여할 수 있다. 하지만 황철석의 극히 낮은 용해도로 인해 산화 생성물의 양은 매우 적을 것이고 포화된 벤토나이트의 낮은 수리전도도로 인해 처분용기로 산화 생성물의 이동은 제한될 것이다. 우리는 심층처분환경에서 황산염의 존재와 환원 그리고 황화물과 황철석의 형성 및 거동 특성 등에 관한 주요 연구 사례 등을 종합적으로 분석, 정리하였고, 고준위방사성폐기물 처분장의 장기안전성에 대한 황산염과 황화물의 영향을 이해하고자 하였다.

주요어 심층처분장, 황산염 및 황화물, 황화물 응력균열부식, 벤토나이트, 황산염환원박테리아

Article

Review

Econ. Environ. Geol. 2023; 56(4): 421-433

Published online August 30, 2023 https://doi.org/10.9719/EEG.2023.56.4.421

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

A Review of the Influence of Sulfate and Sulfide on the Deep Geological Disposal of High-level Radioactive Waste

Jin-Seok Kim*, Seung Yeop Lee, Sang-Ho Lee, Jang-Soon Kwon

Disposal Performance Demonstration R&D Division, Korea Atomic Energy Research Institute, Daejeon 34057, Republic of Korea

Correspondence to:*jskim84@kaeri.re.kr

Received: April 17, 2023; Revised: August 10, 2023; Accepted: August 23, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

The final disposal of spent nuclear fuel(SNF) from nuclear power plants takes place in a deep geological repository. The metal canister encasing the SNF is made of cast iron and copper, and is engineered to effectively isolate radioactive isotopes for a long period of time. The SNF is further shielded by a multi-barrier disposal system comprising both engineering and natural barriers. The deep disposal environment gradually changes to an anaerobic reducing environment. In this environment, sulfide is one of the most probable substances to induce corrosion of copper canister. Stress-corrosion cracking(SCC) triggered by sulfide can carry substantial implications for the integrity of the copper canister, potentially posing a significant threat to the long-term safety of the deep disposal repository. Sulfate can exist in various forms within the deep disposal environment or be introduced from the geosphere. Sulfate has the potential to be transformed into sulfide by sulfate-reducing bacteria(SRB), and this converted sulfide can contribute to the corrosion of the copper canister. Bentonite, which is considered as a potential material for buffering and backfilling, contains oxidized sulfate minerals such as gypsum(CaSO4). If there is sufficient space for microorganisms to thrive in the deep disposal environment and if electron donors such as organic carbon are adequately supplied, sulfate can be converted to sulfide through microbial activity. However, the majority of the sulfides generated in the deep disposal system or introduced from the geosphere will be intercepted by the buffer, with only a small amount reaching the metal canister. Pyrite, one of the potential sulfide minerals present in the deep disposal environment, can generate sulfates during the dissolution process, thereby contributing to the corrosion of the copper canister. However, the quantity of oxidation byproducts from pyrite is anticipated to be minimal due to its extremely low solubility. Moreover, the migration of these oxidized byproducts to the metal canister will be restricted by the low hydraulic conductivity of saturated bentonite. We have comprehensively analyzed and summarized key research cases related to the presence of sulfates, reduction processes, and the formation and behavior characteristics of sulfides and pyrite in the deep disposal environment. Our objective was to gain an understanding of the impact of sulfates and sulfides on the long-term safety of high-level radioactive waste disposal repository.

Keywords deep geological repository, sulfate and sufide, sulfide induced stress corrosion cracking, bentonite, sulfate reducing bacteria

고준위방사성폐기물 심층처분에 미치는 황산염과 황화물의 영향에 대한 고찰

김진석* · 이승엽 · 이상호 · 권장순

한국원자력연구원 저장처분성능검증부

Received: April 17, 2023; Revised: August 10, 2023; Accepted: August 23, 2023

요 약

원자력발전소의 사용후핵연료(Spent Nuclear Fuel: SNF)에 대한 최종처분은 지하 심부의 지질학적 저장소에서 이루어진다. 사용후핵연료를 감싸는 금속처분용기는 주철과 구리 등으로 제작되어 방사성핵종을 장기간 격리할 예정이며, 공학적방벽과 천연방벽으로 구성된 다중방벽처분시스템에 의해 보호를 받도록 설계된다. 지하 심부의 환경(심층처분환경)은 점차 무산소의 환원환경으로 바뀌게 되며, 이러한 환경에서 구리처분용기의 부식을 일으킬 수 있는 유력한 물질 중 하나는 황화물이다. 황화물에 의한 응력균열부식은 구리처분용기의 안정성을 크게 저하시켜 처분장의 장기안전성에 큰 영향을 미칠 수 있다. 심층처분환경에는 황산염이 다양한 형태로 존재 또는 유입될 수 있으며, 황산염환원미생물에 의해 황화물로 전환되어 구리처분용기의 부식에 기여할 수 있다. 완충재와 뒤채움재의 유력한 후보물질인 벤토나이트에는 주로 석고(CaSO4)와 같은 산화형태의 황산염 광물이 포함되어 있다. 심층처분환경 내에 미생물이 생장할 만한 공간이 있고 유기 탄소 등 전자공여체가 충분히 공급된다면 미생물 활동에 의해 황산염이 황화물로 환원될 수 있다. 하지만 근계영역에서 생성된 황화물과 지권으로부터 유입되는 황화물 중 대부분은 완충재에 의해 차단되어 극히 일부만이 처분용기에 도달할 것이다. 처분환경에서 존재가능한 황화철 광물 중 하나인 황철석은 용해과정에서 황산염을 발생시켜 구리처분용기의 부식에 기여할 수 있다. 하지만 황철석의 극히 낮은 용해도로 인해 산화 생성물의 양은 매우 적을 것이고 포화된 벤토나이트의 낮은 수리전도도로 인해 처분용기로 산화 생성물의 이동은 제한될 것이다. 우리는 심층처분환경에서 황산염의 존재와 환원 그리고 황화물과 황철석의 형성 및 거동 특성 등에 관한 주요 연구 사례 등을 종합적으로 분석, 정리하였고, 고준위방사성폐기물 처분장의 장기안전성에 대한 황산염과 황화물의 영향을 이해하고자 하였다.

주요어 심층처분장, 황산염 및 황화물, 황화물 응력균열부식, 벤토나이트, 황산염환원박테리아

    Fig 1.

    Figure 1.Schematic diagram of multi-barrier disposal system and disposal site(KBS-3 model)(from Goo et al., 2022).
    Economic and Environmental Geology 2023; 56: 421-433https://doi.org/10.9719/EEG.2023.56.4.421

    Fig 2.

    Figure 2.Schematic diagram of canister-buffer-backfill unit surrounded host rock showing the main potential sources of sulfide in the different compartments(modified from Wersin et al., 2014).
    Economic and Environmental Geology 2023; 56: 421-433https://doi.org/10.9719/EEG.2023.56.4.421

    Fig 3.

    Figure 3.Schematic diagram of “ingredients” involved in sulfate reduction and sulfide generation(modified from Wersin et al., 2014).
    Economic and Environmental Geology 2023; 56: 421-433https://doi.org/10.9719/EEG.2023.56.4.421

    Fig 4.

    Figure 4.Graphic representation of the mean copper sulfide production rates on copper plates that were exposed(ex) and embedded(em) at different saturated bentonite densities(1500, 1800 and 2000 kg/m3). The treatments were unfiltered(uf) or filtered(f) in experiment G and exposed to 25℃(25) and 120℃(120) for 15 h(from Masurat et al., 2010b).
    Economic and Environmental Geology 2023; 56: 421-433https://doi.org/10.9719/EEG.2023.56.4.421

    Fig 5.

    Figure 5.Schematic diagram of hydrogeochemical site model of baseline groundwater conditions with the main water-rock interactions at Olkiluoto(from Posiva, 2012a).
    Economic and Environmental Geology 2023; 56: 421-433https://doi.org/10.9719/EEG.2023.56.4.421

    List of performed experiments with various bentonite for investigating microbial sulfide producing activity(Bengtsson et al., 2017a; 2017b).


    YearTested bentonite(s)Planned wet densities (kg m-3)Planned dry densities (kg m-3)Chemical composition of tested bentonite (wt%)
    S - totalSiAlFeNaCa
    2012-2013MX-801750, 20001171, 15620.04267.4021.204.142.251.46
    2013-2014Asha, MX-801850, 1900, 1950, 2000, 19001300, 1406, 1453, 1529, 14060.04267.4021.204.142.251.46
    0.1055.7021.6014.802.320.87
    2014-2015Calcigel1850, 1900, 19501333, 1411, 14900.0354.7017.505.050.472.94
    2015-2016Asha1500, 1750, 1850765, 1147, 13000.1055.7021.6014.802.320.87
    2016Asha1600, 1700917, 10700.1055.7021.6014.802.320.87
    2016Rokle1750, 1850, 19501112, 1260, 1408046.6012.9013.000.201.75
    2016-2017GMZ1750, 1850, 19501160, 1315, 1469067.4314.202.405.81.13
    2019Calcigel1750, 19001153, 13830.0354.7017.505.050.472.94


    Summary of solid iron sulfides and their properties(King, 2013).


    MineralCompositionStructureProperties
    MackinawiteFeSmTetragonalMetastable, principal species formed by precipitation of Fe(II) in aqueous solution
    Cubic FeSFeScCubicUnstable, formed prior to FeSm
    TroliteFeStHexagonalStoichiometric end-member of the pyrrhotite group
    PyrrhotiteFe1-xSVariousStable, non-stoichiometric iron sulphide with x < 0.2
    GreigiteFe3S4gInverse spinelMetastable mixed Fe(II)/Fe(III) inverse thiospinel
    PyriteFeS2pCubicStable Fe(II) disulphide
    MarcasiteFeS2mOrthorhombicMetastable Fe(II) disulphide

    KSEEG
    Dec 29, 2023 Vol.56 No.6, pp. 629~909

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