실내 건축자재의 방사능농도지수 측정에 관한 연구

Abstract

인체에 영향을 미칠 수 있는 생활주변 환경방사선에 대한 불필요한 노출에 국민적 관심과 우려가 제기되고, 특히 실내 건축마감재로 널리 사용되고 있는 화강석으로부터 방출되는 주요 자연방사성핵종인 라돈에 대해 농도 관리의 필요성이 제기되었다. 이에 우리 정부는 IAEA 권고지침과 EU회원국에서 적용하는 방법 중 방사능농도지수를 이용한 평가 및 관리방안으로 국내 실정에 맞는 ‘건축자재 라돈 저감ㆍ관리 지침서(2020.06.01. 시행)’를 발간하였다. 본 연구에서는 ‘방사능농도지수’ 측정 및 평가방법의 유효성과 최적화 방안을 도출하기 위해서 실내마감재로 국내에서 유통 중인 27종의 실내마감재 화강석 시료 표면에서의 라돈 방출 준위에 따라 4개의 Group으로 분류하여 대표 9종의 시료를 선정하였으며, 감마핵종분석을 위해 시료 전처리(절단, 건조, 분쇄, 충전) 과정을 거쳐 3주간(약 20일 이상) 방사평형 후에 고순도게르마늄(HPGe) 핵종분석기로 측정하였다. 방사능농도지수 측정ㆍ평가는 시료에 포함된 Ra-226, Th-232, K-40의 감마선에너지 스펙트럼을 측정하여 방사능 농도를 평가하였는데, 연속붕괴 계열을 이루는 우라늄계열과 토륨계열의 Ra-226과 Th-232은 각각 모핵종과 딸핵종과의 방사평형을 이용하여 그 농도를 평가하였다. 측정결과 9개 시료의 방사능농도지수는 S1(1.01), S2(1.74), S3(0.91), S4(1.55), S5(0.51), S6(1.19), S7(0.25), S8(0.32), S9(0.75)으로 4종의 화강석이 기준치를 초과하였으며 Ra-226, Th-232, K-40의 방사능농도지수의 비율은 7.773 % ~27.59 %, 31.99 % ~ 51.61%, 22.61 % ~ 60.23%로 대부분 Th-232와 K-40에 의해 방사능농도지수가 평가되었다. 또한 자연 우라늄(U-238) 존재비로 토대로 186.10 keV 혼합피크에서의 226Ra에 대한 계수치 비율(0.5762)로 보정한 Ra-226은 Ra-226과 방사평형 된 딸핵종 Bi-214(609.3 keV)의 방사능농도와 비슷한 값으로 Ra-226의 직접측정 방법으로 타당 하였으며, 이를 적용 시 3주간 방사평형절차 없이 신속한 평가와 222Rn의 가스 누출로부터 시료의 건전성이 확보될 수 있다. 또한 K-40(1460.75 keV, 10.67 %)은 Ac-228(1459.2 keV, 0.9 %)과의 간섭으로 인해 K-40의 농도가 과대평가 되어 보정이 필요하였으며, 이에 Ac-228 간섭에 의한 피크 계수치(net area counts) 기여분 정도를 평가한 결과 K-40(1460.75 keV) 방사능농도에 Ac-228의 방사능농도의 11.24 %를 차감할 경우 정확한 평가가 가능하였다. 본 연구에서 이를 종합하여 방사능농도지수에 대한 보정식을 추가하면서 향후 실내 건축마감재에 대한 방사능농도지수 평가에 있어 보다 신속하고 간편하며 정성적, 정량적인 측정을 위한 유용한 데이터로 활용되기를 기대한다.

The general public has raised high interest and concerns due to unnecessary exposure to environmental radiation from around life. Recently, the necessity of national management has been raised as radon radiation is released from granite, which is widely used as a finishing material for indoor buildings. Therefore, for effective management of radon, the government introduced an evaluation and management method using the IAEA guidelines and the “Activity Concentration Index” applied in EU countries as a selection tool. In addition, the “Guidelines for Radon Reduction and Management in Building Materials (Enforced on January 1, 2020)” was published. In this research, 'activity concentration index' was measured for 27 granite samples of indoor building finishing materials distributed in Korea, and an optimization method was derived for the evaluation and effectiveness of radioactivity in the sample. First, in order to measure the ‘activity concentration index’, the radon radioactivity emitted from the surface of 27 granite samples was measured from each sample. In addition, 9 samples with relatively high radon radioactivity were selected for gamma ray spectroscopy analysis, and were classified into 4 groups according to the radioactivity concentration level and evaluated. The second selected 9 granite samples were radioactive equilibrium for about 3 weeks after pretreatment(crushing, drying, grinding, filling) for gamma ray spectroscopy analysis, and measured using a high purity germanium(HPGe) detector. The measurement and evaluation of the ‘activity concentration index’ was evaluated by measuring the gamma ray energy emitted from the Ra-226, Th-232, and K-40 nuclides contained in the sample. Ra-226 and Th-232 of the uranium and thorium series can be measured indirectly using the radioactive equilibrium between the parent and daughter nuclides included in the radioactive decay series. As a result, the ‘activity concentration index’ values from 9 samples were S1(1.01), S2(1.74), S3(0.91), S4(1.55), S5(0.51), S6(1.19), S7(0.25), S8(0.32), S9(0.75) was measured, exceeding the reference value in 4 granite samples. In addition, the ratio of the ‘activity concentration index’ of Ra-226, Th-232, and K-40 was evaluated as 7.773 %~27.59 %, 31.99 %~51.61 %, and 22.61 %~60.23 %, and Th-232 and K-40 nuclides accounted for most. In this paper, in order to measure and evaluate Ra-226 radioactivity more quickly and accurately, the gamma ray energy emitted from Ra-226 was directly measured and compared with the result value using the radioactive equilibrium relationship. The gamma ray energy 186.10 keV directly emitted from the Ra-226 nuclide is measured as a mixed peak because the energy difference is small from the 185.70 keV emitted from U-235, and there is a great difficulty in evaluating the radioactivity of Ra-226. However, in the case of using the natural abundance ratio of uranium (U-238), it is possible to quickly and directly measure the radioactivity concentration of Ra-226 without waiting for 3 weeks or more by correcting by applying the counts ratio (0.5762) at the 186.10 keV peak. The direct radioactivity measurement result of Ra-226(186.10 keV) using the natural abundance ratio of uranium was compared with the indirect radioactivity concentration between Bi-214(609.30 keV) in radioactive equilibrium relationship, and the result was well matched in 8 samples. Therefore, it was confirmed that the direct measurement method of the gamma ray energy of Ra-226 is a reliable level, and it is a valid method to evaluate the ’activity concentration index’ more quickly of the sample. In addition, K-40(1460.75 keV, 10.67 %) requires correction because the radioactivity concentration of K-40 is overestimated due to interference between Ac-228(1459.2 keV, 0.9 %). The effect of the radioactivity concentration of Ac-228 on the radioactivity concentration of K-40(1460.75 keV) was evaluated as a maximum of 11.24 %, and it was confirmed that the change was large depending on the radioactivity concentration of Th-232. Through this research, a more reliable radioactivity analysis method was presented by adding a correction equation from the general equation of the ‘Activity concentration index’. In the future, since the measurement and evaluation of the ‘activity concentration index’ for indoor building finishing materials can be measured more quickly and accurately, it is expected to contribute to the work of reducing the exposure from natural radiation in the living area.