[논문]미국 원전의 심층토양 제염사례 연구

이형우; 김주열; 김창락

doi:10.7733/jnfcwt.2019.17.2.213

미국 원전의 심층토양 제염사례 연구
Study on the Experiences of Subsurface Soil Remediation at Commercial Nuclear Power Plants in the United States 원문보기

Journal of nuclear fuel cycle and waste technology = 방사성폐기물학회지, v.17 no.2, 2019년, pp.213 - 226

이형우 (한국전력국제원자력대학원대학교) , 김주열 (한국전력국제원자력대학원대학교) , 김창락 (한국전력국제원자력대학원대학교)

초록
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2017년 고리 1호기 영구정지 이후 규제기관과 원전운영자는 2031년으로 예정된 부지 제염 및 복원을 수행하기 위해 사전준비 작업을 진행해오고 있다. 적절한 계획 수립 및 효과적인 규제활동을 위해서 규제지침 개발과 기술적 근거수립이 무엇보다 선행되어야 한다. 국내에선 연구용 원자로 해체경험이 있지만 상업용 원전은 없기 때문에 해외 해체 선도국의 부지복원사례연구를 통해 토양 제염과 관련한 기술사항 및 규제기준에 대한 정보를 제공한다면 고리 1호기 복원계획 및 규제기준 수립에 효과적일 것이다. 미국은 상업용 원전에 대한 다양한 해체경험을 축적해 왔으며 RESRAD 프로그램 및 MARSSIM 절차와 같은 체계를 개발 적용하여 오염된 부지의 조사, 제염, 복원 및 해제를 통합적으로 수행하고 있다. 이 논문에서는 미국의 5개 상업용 원전(해체완료 4개, 지연해체 1개)을 대상으로 심층 토양오염에 대한 부지복원 사례연구를 수행하였다. 심층토양의 경우 표층토양과 달리 미국에서도 정형화된 평가방법론이 아직 정립되어 있지 않았고, 오염평가시 지하수 영향을 고려해야 하는 특성이 있음이 확인되었다. 따라서 향후 고리 1호기 부지복원 전략수립 및 규제지침 개발에 고려할 만한 제안사항을 도출하고자 기술 및 규제 관점에서 심층토양에 대한 오염평가, 제염기준 수립, 제염작업 수행 및 결과 검증까지 단계별 주요사례를 정리하고, 미국 해체사업자가 적용한 심층토양 평가방법과 규제기관과 해체사업자 간에 논의된 주요 쟁점사항을 분석하여 시사점을 도출하였다.

Abstract ▼ AI-Helper

Regulatory agency and licensee are preparing for the site restoration of Kori unit 1, the first commercial NPP in Korea, scheduled for 2031. Developing regulatory guidelines and strategies is essential for effective restoration work. Unfortunately, Korea does not have experience of site restoration of commercial NPPs. Therefore, it is important to review cases from experienced countries to establish a strategy and regulatory standards. The U.S. has had numerous soil remediation experiences using RESRAD and MARSSIM. However, formalized evaluation methodologies for subsurface soil have not yet been established in MARSSIM. This survey focused on subsurface soil remediation by reviewing the five decommissioned NPPs under regulation of the US NRC. Overall process of remediating a contaminated subsurface soil and groundwater was reviewed to identify considerations and lessons that could be applicable in Korea. In addition, an applied methodology for evaluation of contaminated subsurface soil and related major issues between regulatory agency and licensees were reviewed in detail to support establishment of remediation strategy for Kori unit 1.

주제어

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문제 정의

has soil remediation experience with commercial NPP sites (8 sites) [1]. This survey focused on the U.S. experience in order to share the lessons learned by reviewing technical and regulatory criteria applied in their site restoration works.

제안 방법

The significant soil contaminations and leaks were found at a flange connection in the Refueling Water Storage Tank (RWST) as a result of HSA in 1988. A further sampling analysis during a site characterization survey identified three major contaminated areas : RWST, Processed Primary Water Storage Tank (PWST) and Shielded Radiological Waste Storage Area (SRWSA) in the Radiologically Restricted Area (RRA). A remediation work for a subsurface soil was performed in those areas after an analysis of contamination level and a remediation range (area, depth, etc.
It starts with the classification of contaminated areas (Class 1, 2, 3) and defining the Survey Units (SU) for a confirmation survey after remediation. Accurate technical criteria should also be established to demonstrate compliance with the site release criteria, such as the level of MDC (Minimum Detectable Concentration) and the level of site scanning and sampling requirements to be used as a basis for statistical analysis.
547×10¹² Bq) was released in one year. After a replacement of the corroded pipe (approximately 61 m long) with the excavation of adjacent subsurface soil, the monitoring wells were installed to check the groundwater characteristics and the tritium migration evaluation. The evaluation results showed that migrated contaminants did not affect the public and the environment.
Each media has different exposure pathways and is then subdivided into the release criteria referred to as Derived Concentration Guideline Levels (DCGLs) for the development of remediation plan and verification of its results. DCGLs are derived by the RESRAD (RESidual RADioactive materials) computer code using a result of an initial site assessment data and further characterization survey data for the site.
A remediation plan for radiologically contaminated media can be established by analyzing the radioactive level of each media based on the scenario and the criterion. Each media has different exposure pathways and is then subdivided into the release criteria referred to as Derived Concentration Guideline Levels (DCGLs) for the development of remediation plan and verification of its results. DCGLs are derived by the RESRAD (RESidual RADioactive materials) computer code using a result of an initial site assessment data and further characterization survey data for the site.
The current MARSSIM procedure does not provide formal guidance to a subsurface soil, unlike a surface soil due to inherent characteristics. Each plant performed a caseby-case evaluation by analyzing an exposure impact of a subsurface soil contamination to establish a technical background. According to the results of the analysis, it was confirmed that the effect of radiation exposure was limited by soil depth and the radioactive level of subsurface soil does not exceed the surface soil.
04 mSv·yr^-1 from groundwater. Exposure effect of subsurface soil was analyzed based on the resident farmer scenario in order to develop the surface soil DCGL and the subsurface soil DCGL separately. It was reasonably assumed that the resident farmer would be affected by contaminated subsurface soil due to future activities.
In response to stakeholder requirements during a review process, the plant decided the site release criteria as 0.1 mSv·yr-1 including 0.04 mSv·yr-1 from groundwater.
In addition, most of the sites considered the possibility of groundwater contamination when any subsurface soil contaminations were found. It was decided to perform longterm monitoring of groundwater without remediation at the Dresden unit 1 site with the site restoration to be performed together with units 2 and 3 later. The long-term monitoring remediation strategy can be considered in the D&D project of the Kori site (units 1 to 4).
A licensee will obtain certain DCGL levels in each media such as subsurface soil and groundwater for a radionuclide basis. The characterization survey can provide precise data about the impact zone by a systematic survey used for the RESRAD modeling, remediation planning, and FSS. Then, the survey results are compared with DCGLs to determine whether immediate remediation can be started or more survey is required.
However, it is recommended to decide DCGLs with a conservative approach for the safety of the general public and environment. The measurement procedure is to obtain site-specific data by field or laboratory measurement techniques during HSA and the characterization survey. The decision procedure provides confidence to verify compliance of data obtained from FSS by applying decision rules with statistical tests in the MARSSIM guideline.
The plant divided the site into 8 areas and classified three contaminated areas (No. 1, 2, 8) based on historical spills or leaks during operation. The HSA identified the Regenerate Hold Up Tank (RHUT), CST, Auxiliary Building (AB) and TB drains and sumps as major contaminated areas.
The plant performed a comparative analysis with respect to the development of the site-specific DCGL for surface and subsurface soil. The results showed that the exposure dose was slightly increased to 0.
The plant set a site release criterion as 0.25 mSv·yr-1 with a modified resident farmer scenario, excluded the meat and milk intake routes reflecting geographic characteristics of the site.
The result showed that each site modified the scenarios and applied the different maximum dose criteria (0.1~0.25 mSv·yr-1) to fulfill their intended purpose (greenfield or brownfield) with a conservative approach.

대상 데이터

It is necessary to verify remediation results of the site to protect the health and safety of the intended target from radiological hazards based on their purpose. The intended target is a group of people that will use the released site after remediation and will be expected to receive the maximum exposure from the residual radioactivity. This group is referred to as ‘critical group’ and has two types.

이론/모형

1402 or 1 mSv·yr-1 for restricted use that is acceptable by satisfying the ALARA (As Low As Reasonably Achievable) principle in the 10 CFR Part 20.
It was reasonably assumed that the resident farmer would be affected by contaminated subsurface soil due to future activities. The Microshield code was used for a calculation of the direct exposure dose and the RESRAD code for the groundwater contamination effect. The results showed that the subsurface soil DCGL was lower than the surface soil DCGL.
The NRC site release criteria and the stakeholder requirements should be considered before making a decision. The decided criterion is subdivided into radionuclide levels in each media through the established scenarios and models in the RESRAD code. A licensee will obtain certain DCGL levels in each media such as subsurface soil and groundwater for a radionuclide basis.
The translation process is to convert release criteria into corresponding site-specific DCGLs for each radionuclide of concern using the RESRAD modeling. Some exposure pathways in scenarios could be excluded based on a sitespecific condition and measurement data.

성능/효과

Each plant performed a caseby-case evaluation by analyzing an exposure impact of a subsurface soil contamination to establish a technical background. According to the results of the analysis, it was confirmed that the effect of radiation exposure was limited by soil depth and the radioactive level of subsurface soil does not exceed the surface soil. Therefore, the plant conservatively applied the surface soil DCGL value to the subsurface soil and performed comprehensive soil remediation work.
The core sampling results of the contaminated soil (150 m³) in TB foundation showed that residual radioactivity was limited to a 30 cm depth compared with site-specific DCGLs. However, a demolition work of TB and Compound Building (CB) foundation (30,000 m³) required an excavation (total 1,776 m²) of subsurface soil up to 4 m and substructure up to 10.
The MARSSIM process has two assumptions for subsurface soil. The first assumption is that when excavation work is performed, the mixing of radioactive materials in the subsurface soil is inevitable. In addition, the amount of soils with high level of contaminants exceeding release criteria are homogeneously mixed when it is brought to the surface.
The Microshield code was used for a calculation of the direct exposure dose and the RESRAD code for the groundwater contamination effect. The results showed that the subsurface soil DCGL was lower than the surface soil DCGL. Therefore, the surface soil DCGL value was also applied conservatively to subsurface soil [15].

후속연구

If the HSA data indicates a significant amount of residual radioactivity in the area, it should be classified as an impact zone for a detailed survey later. A licensee sets a final site release criterion based on a survey data from HSA.

참고문헌 (28)

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원문보기 상세보기
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표제어: PCR

동의어: Packet Collision Rate

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용어 설명: PCR은 세균 특이성이 있는 primer를 이용하여 적은 수의 세균이 있을지라도 쉽게 검출할 수 있는 유용한 방법이며, 이를 이용하여 구강 내 치면세균막이나 타액에서 직접 세균을 검출할 수 있게 되었다[8].

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