• Nuclear Engineering and Technology
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• 제55권10호
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• pp.3831-3843
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• 2023

The adsorption behaviour of Se(-II) onto granite and MX-80 bentonite in Ca-Na-Cl solutions has been studied utilizing adsorption experiments and surface complexation modelling. Adsorption kinetic experiments allude to steady-state adsorption periods after 7 days for granite and 14 days for MX-80 bentonite. Batch adsorption experiments were carried out to determine the influence that the physicochemical solution properties would have on Se(-II) adsorption behaviour. Adsorption of Se(-II) onto granite and MX-80 bentonite follows the trend of anionic adsorption, with a decrease in R_d values as the solution pH increased. There is also an ionic strength influence on the adsorption of Se(-II) onto granite with a decrease in the R_d value as the ionic strength increased. This effect is not found when observing Se(-II) adsorption onto MX-80 bentonite. Final experiments with a representative groundwater, determined that the adsorption of Se(-II) onto granite and MX-80 bentonite returned R_d values of (1.80 ± 0.10) m³·kg^-1 and (0.47 ± 0.38) m³·kg^-1, respectively. In support of the experiments, a surface complexation modelling approach has been employed to simulate the adsorption of Se(-II) onto granite and MX-80 bentonite, where it was determined that two different surface complexes, ≡S_Se^- and ≡SOH₂⁺_H₂ were capable of simulating Se(-II) adsorption behaviour.

• Nuclear Engineering and Technology
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• 제53권5호
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• pp.1511-1518
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• 2021

In most countries, the thermal criteria for the engineered barrier system (EBS) is set to below 100 ℃ due to the possible illitization in the buffer, which will likely be detrimental to the performance and safety of the repository. On the other hand, if the thermal criteria for the EBS increases, the disposal density and the cost-effectiveness for the high-level radioactive wastes will dramatically increase. Thus, fundamentals on the thermal behavior of the buffer under the elevated temperatures is of crucial importance. Yet, the behaviors at the elevated temperatures of the bentonite under groundwater-saturated conditions have not been reported to-date. Here, we have developed an in-situ synchrotron-based method for the thermal behavior study of the buffer under the elevated temperatures (25-250 ℃), investigated dspacings of the montmorillonite in the Korean bentonite (i.e., Ca-type) at dry and KURT (KAERI Underground Research Tunnel) groundwater-saturated conditions (KJ-ii-dry and KJ-ii-wet), and compared the behaviors with that of MX-80 (i.e., Na-type, MX-80-wet). The hydration states analyzed show tri-, bi-, and mono-hydrated at 25, 120, and 250 ℃, respectively for KJ-ii-wet, whereas tri-, mono-, and de-hydrated at 25, 150, and 250 ℃, respectively for MX-80-wet. The Korean bentonite starts losing the interlayered water at lower temperatures; however, holds them better at higher temperatures as compared with MX-80.

• Nuclear Engineering and Technology
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• 제53권3호
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• pp.894-900
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• 2021

This paper examines sorption of Pd(II) onto illite, MX-80 bentonite, and Queenston shale in Na-Ca-Cl solutions of varying ionic strength (IS) from 0.01 to 6.0 mol/L (M) and pH_c ranging from 3 to 9 under atmospheric conditions. A 2-site protolysis non-electrostatic surface complexation and cation exchange model was applied to the Pd sorption onto illite and MX-80 using PHREEQC, and the model results were compared to the experimental ones obtained in this work. Surface complexation and cation exchange constants were estimated for both illite and MX-80 through the optimization process to bring the predicted distribution coefficients from the model into alignment with the experimentally derived values. These optimized surface complexation constants were compared to existing linear free energy relationships (LFER).

• 방사성폐기물학회지
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• 제20권2호
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• pp.151-160
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• 2022

The sorption of Eu on MX-80 bentonite in Na-Ca-Cl solutions is investigated at a molal proton concentration (pH_m) range of 3 to 10 and an ionic strength (I) range of 0.1 to 6 m (mol·kgw^-1). The sorption equilibrium of Eu on MX-80 is achieved within 14 to 21 d at I = 0.1 and 6 m. The sorption distribution coefficient (K_d) values of Eu for MX-80 increase as pH_m increases from 3 to 6 for all I values, and they are independent of pH_m between 8 and 10 at I ≥ 0.5 m. Meanwhile, at I = 0.1 m, the K_d value at pH_m = 10 is slightly lower than those at pH_m = 8 and 9. The K_d values are not affected by the I values between 0.5 m and 6 m, whereas the K_d value at I = 0.1 m is greater than those at I ≥ 0.5 m, except at pH_m = 10. A two-site protolysis nonelectrostatic surface complexation and cation exchange sorption model is applied to the Eu sorption data for I ≤ 4 m, and the equilibrium constants of the sorption reactions are estimated.

• Nuclear Engineering and Technology
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• 제54권5호
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• pp.1616-1622
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• 2022

Selenium has been identified as an element of interest for the safety assessment of a deep geological repository (DGR) for used nuclear fuel. In Canada, groundwaters at DGR depths in sedimentary rocks have been observed to have a high ionic strength. This paper examines the sorption behavior of Se(-II) onto illite, MX-80 bentonite, Queenston shale, and argillaceous limestone in Na-Ca-Cl solutions of varying ionic strength (0.1-6 mol/kgw (m)) and across a pH range of 4-9. Little ionic strength dependence for Se(-II) sorption onto all solids was observed except that sorption at high ionic strength (6 m) was generally slightly lower than sorption at low ionic strength (0.1 m). Illite and MX-80 exhibited the expected results for anion sorption, while shale and limestone exhibited more constant sorption across the pH range tested. A non-electrostatic surface complexation model successfully predicted sorption of Se(-II) onto illite and MX-80 using the formation of an inner-sphere surface complex and an outer-sphere surface complex. Optimized values for the formation reactions of these surface species were proposed.

• 터널과지하공간
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• 제30권5호
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• pp.446-461
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• 2020

MX80 벤토나이트 펠렛에서의 열-수리-역학적 복합거동 특성을 파악하고자 TOUGH2-FLAC3D 시뮬레이터를 이용하여 스페인 CIEMAT에서 수행된 컬럼 시험에 대한 수치해석을 수행하였다. 수치해석에서는 실험실에서 사용된 것과 동일한 히터 파워와 물 주입압을 경계조건으로 설정하고 해석을 수행하였다. 사용된 열-수리 모델이 벤토나이트 펠렛의 복합거동 예측에 적용하기 적합한지 판단하기 위해 가열과 물 주입에 의한 벤토나이트 펠렛에서의 온도와 상대습도 변화를 시간 경과에 따라 잘 예측할 수 있는 지를 살펴보았다. 계산된 결과가 계측된 온도와 상대습도 변화 경향을 적절하게 재현 할 수 있었기 때문에 사용된 열-수리 모델은 벤토나이트 펠렛의 열-수리 복합거동을 예측하고 재현하기에 적절한 것으로 판단된다. 하지만, 물 주입 이후의 계산된 응력변화가 상대적으로 작고 느리게 변화되는 것으로 보아 사용된 탄성모델과 스웰링 모델에 한계점이 존재하는 것으로 보이며, 사용된 두 역학 모델로 완충재의 복잡한 열-수리-역학적 복합거동을 현실적으로 재현하기에 부족한 것으로 판단된다.

• 터널과지하공간
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• 제29권6호
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• pp.439-455
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• 2019

완충재는 심지층 고준위 방사성 폐기물 저장소의 주요 구성 요소이다. 벤토나이트는 높은 열전도율과 낮은 수리투과성의 특성으로 완충재의 핵심 구성 요소로 다수의 국가에서 채택되었다. 심층 처분은 지하수 유입을 일으키고 이는 완충재 및 뒷채움재의 팽윤압을 초래한다. 완충재에서 발생하는 고압의 팽윤압은 처분용기에 영향을 줄 수 있기에 정밀한 완충재의 팽윤압 예측은 안전한 처분 시스템 구축에 있어서 필수적이다. 따라서 본 연구에서는 MX-80 벤토나이트의 수리역학적 거동에 대한 팽윤압 예측 모델을 세우고, 그 결과를 토대로 민감도 분석을 시행하였다.

• Geomechanics and Engineering
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• 제16권1호
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• pp.71-83
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• 2018

Compacted bentonites were chosen as the backfill material and buffer in high level nuclear waste disposal due to its high swelling pressure, high ion adsorption capacity and low permeability. It is essential to estimate the swelling pressure in design and considering the safety of the nuclear repositories. The swelling pressure model of expansive clay colloids was developed based on Gouy-Chapman diffuse double layer theory. However, the diffuse double layer model is effective in predicting low compaction dry density (low swelling pressure) for certain bentonites, and invalidation in simulating high compaction dry density (high swelling pressure). In this paper, the new relationship between nondimensional midplane potential function, u, and nondimensional distance function, Kd, were established based on the Gouy-Chapman theory by considering the variation of void ratio. The new developed model was constructed based on the published literature data of compacted Na-bentonite (MX80) and Ca-bentonite (FoCa) for sodium and calcium bentonite respectively. The proposed models were applied to re-compute swelling pressure of other compacted Na-bentonites (Kunigel-V1, Voclay, Neokunibond and GMZ) and Ca-bentonites (FEBEX, Bavaria bentonite, Bentonite S-2, Montigel bentonite) based on the reported experimental data. Results show that the predicted swelling pressure has a good agreement with the experimental swelling pressure in all cases.

• Nuclear Engineering and Technology
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• 제56권4호
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• pp.1385-1397
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• 2024

Bentonite is a recommended material for the multiple barriers in the final disposal of low-level radioactive waste (LLW) to prevent groundwater intrusion and nuclear species migration. However, after drying-wetting cycling during the repository construction stage and ion exchange with the concrete barrier in the long-term repository, the bentonite mechanical behaviors, including swelling capacity and hydraulic conductivity, would be further influenced by the groundwater intrusion, resulting in radioactive leakage. To comprehensively examine the factors on the mechanical characteristics of bentonite, this study presented scenarios involving MX-80 and KV-1 bentonites subjected to drying-wetting cycling and accelerated ion migration. The experiments subsequently measured free swelling, swelling pressure, and hydraulic conductivity of bentonites with intrusions of seawater, high pH, and low pH solutions. The results indicated that the solutions caused a reduction in swelling volume and pressure, and an increase in hydraulic conductivity. Specifically, the swelling capability of bentonite with drying-wetting cycling in the seawater decreased significantly by 60%, while hydraulic conductivity increased by more than three times. Therefore, the study suggested minimizing drying-wetting cycling and preventing seawater intrusion, ensuring a long service life of the multiple barriers in the LLW repository.

• 방사성폐기물학회지
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• 제13권4호
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• pp.283-293
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• 2015

고준위폐기물처분장에서 완충재는 공학적방벽의 주요 구성요소 중 하나이다. 본 연구에서는 국 내외의 완충재 요구사항과 성능기준을 분석하고, 우리나라 고준위폐기물처분장에 적합한 완충재 개념 도출을 위한 접근방안을 제시하였다. 완충재의 주요 성능기준 인자항목으로, 수리전도도, 핵종 저지능, 팽윤압, 열전도도, 역학적 특성치(mechanical properties), 유기물함량(organic carbon content), 일라이트화 속도(illitization rate) 등을 고려하였다. 우리나라 고준위폐기물처분장 완충재 물질로서 국산 벤토나이트(Ca-벤토나이트)와 대안재로 MX-80 벤토나이트(Na-벤토나이트)를 제안하였다. 완충재의 기술사양은 Ca-벤토나이트 경우엔 우리나라의 성능기준을, Na-벤토나이트의 경우는 스웨덴의 성능기준을 보수적으로 만족하는 값으로 설정하였다. 완충재의 두께는 전단거동, 핵종 유출, 열전도의 측면에서 평가하여 결정하였으며, 평가결과 완충재의 두께는 0.25 ~ 0.5 m 사이가 적절하였다. 그러나 최종적인 완충재의 두께는 향후 보다 심도 있는 열-수리-역학적 평가와 경제적, 공학적 측면을 고려하여 결정하여야 할 것이다.