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Cesium Sorption to Granite in An Anoxic Environment

무산소 환경에서의 화강암에 대한 세슘 수착 특성 연구

  • Cho, Subin (Groundwater Environment Research Center, Korea Institute of Geoscience and Mineral Resources) ;
  • Kwon, Kideok D. (Department of Geology, Kangwon National University) ;
  • Hyun, Sung Pil (Groundwater Environment Research Center, Korea Institute of Geoscience and Mineral Resources)
  • 조수빈 (한국지질자원연구원 지하수환경연구센터) ;
  • 권기덕 (강원대학교 자연과학대학 지질학과) ;
  • 현성필 (한국지질자원연구원 지하수환경연구센터)
  • Received : 2022.05.31
  • Accepted : 2022.06.21
  • Published : 2022.06.30

Abstract

The mobility and transport of radioactive cesium are crucial factors to consider for the safety assessment of high-level radioactive waste disposal sites in granite. The retardation of radionuclides in the fractured crystalline rock is mainly controlled by the hydrochemical condition of groundwater and surface reactions with minerals present in the fractures. This paper reports the experimental results of cesium sorption to the Wonju Granite, a typical Mesozoic granite in Korea, performed in an anaerobic chamber that mimics the anoxic environment of a deep disposal site. We measured the rates and amounts of cesium (133Cs) removed by crushed granite samples in different electrolyte (NaCl, KCl, and CaCl2) solutions and a synthetic groundwater solution, with variations in the initial cesium concentration (10-5, 5×10-6, 10-6, 5×10-7 M). The cesium sorption kinetic and isotherm data were successfully simulated by the pseudo-second-order kinetic model (r2= 0.99) and the Freundlich isotherm model (r2= 0.99), respectively. The sorption distribution coefficient of granite increased almost linearly with increasing biotite content in granite samples, indicating that biotite is an effective cesium scavenger. The cesium removal was minimal in KCl solution compared to that in NaCl or CaCl2 solution, regardless of the ionic strength and initial cesium concentration that we examined, showing that K+ is the most competitive ion against cesium in sorption to granite. Because it is the main source mineral of K+ in fracture fluids, biotite may also hinder the sorption of cesium, which warrants further research.

고준위방사성폐기물 내에 핵분열 생성물로 존재하는 방사성 세슘의 이동은 화강암을 기반으로 하는 고준위방사성폐기물 처분장의 안전성 평가를 위한 중요한 고려 항목이다. 지하수의 수리화학적 특성과 모암을 이루는 광물과의 반응 특성은 처분장에서 방출된 세슘의 이동 속도를 결정하는데 있어서 중요한 요소이다. 알칼리 금속인 세슘은 지하수 내의 주요 양이온들과 수착 자리를 놓고 경쟁하는 것으로 알려져 왔고, 흑운모는 화강암의 핵종 수착 특성에 중요한 기여를 하는 구성광물로 알려져 왔다. 이 논문은 심부 처분장의 무산소 환경을 모사하기 위해 혐기성 챔버에서 전형적인 중생대 원주화강암에 대한 세슘 수착특성을 연구한 결과를 보고한다. 분쇄한 화강암에 대하여 전해질(NaCl, KCl, CaCl2) 용액과 합성지하수를 사용하여 세슘 초기농도(10-5, 5×10-6, 10-6, 5.0×10-7 M)를 달리하여 세슘 수착속도와 수착량을 측정하였다. 세슘 수착 실험 결과는 유사 2차 속도 모델(r2 = 0.99)과 프로인들리히(Freundlich) 등온선 모델(r2 = 0.99)로 잘 모사되었다. 특히 염화포타슘(KCl)은 모든 이온강도 및 세슘 초기농도에서 다른 전해질에 비해 가장 강력하게 세슘 수착을 제한하였다. 이는 포타슘 이온(K+)이 세슘의 가장 효과적인 경쟁 이온임을 지시하는 것으로 판단된다. 흑운모 함량에 따른 세슘 수착 실험 결과 흑운모의 세슘 수착 분배계수는 화강암 자체의 값보다 약 2배 이상 높았으며 선형관계를 나타냈다. 포타슘 이온은 주로 흑운모에 의해 공급되기 때문에, 이러한 결과는 처분심도 ~500 m 깊이에서 화강암체 내에 존재하는 흑운모가 세슘의 효율적인 수착재로 작용하지만 또한 지하수의 수리화학 조건에 따라 세슘의 수착을 저해하는 역할을 할 수 있음을 지시하며, 향후 이에 대한 상세한 후속 연구가 필요함을 보여준다.

Keywords

Acknowledgement

이 연구는 한국지질자원연구원 "HLW 심층처분을 위한 지체구조별 암종 심부 특성 연구" 과제의 재정적 지원으로 수행되었다. 실험에 도움을 준 충남대학교 이임정 학생에게 감사드린다.

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