| Natural background radiation is the main source of the total radiation doses received by the public,especially in high background radiation area resulted from some special geological background?e.g.,granite area,metallogenic area of radioactive element?.Radiation exposure can induce a variety of health hazards,even cause fatal cancer,distortion and heritable effects.Therefore,research on natural ambient radiation is important to the life and health of the general public,and it is significant to study the natural ambient radiation in the potential high background radiation areas.In this study,the dominant natural ambient radiation such as radionuclides in soil and gamma radiation,radon exposure from domestic water and indoor/outdoor air were studied in Shizhuyuan W-Sn-Bi-Mo polymetallic metallogenic region,which is a world-class super large ore deposit located in Chenzhou,Hunan Province,China.The main results are as follows:?1?We systematicly measured the contents of 238U,232Th and 40K in soil,and the radon concentration in drinking groundwater and indoor/outdoor air in Shizhuyuan area.In addition,we studied the annual effective dose and annual absorbed dose to different organs or tissues of human body,which is caused by ground surface gamma radiation,domestic water and inhalation of radon from indoor and outdoor air.It shows that the average contents of 238U,232Th and 40K in soil,the absorbed dose rate of ground gamma radiation and the average annual effective dose caused by surface gamma radiation are higher than the world and Chinese average.The average radon level in underground water of this area is 10.47 Bq L-1,and the annual effective dose and annual absorbed dose to different organs or tissues of human body caused by the underground water is relatively low,indicating that the groundwater is safe to be used as domestic water.Radon concentration in indoor and outdoor air is significantly higher than the world and Chinese average radon level,and results in an average annual effective dose of 14.39 mSv y-1,which is 5.2times as high as the world average and exceeds the limit recommended by ICRP.?2?Radon concentration in soil and the radon exhalation in Shizhuyuan area are relatively high.Radon concentration in soil of this area is in the range of 167-97680 Bq m3,with an average of 7509±11964 Bq m3,which is above the average radon concentration in the world and Chinese soil.The radon exhalation rate varies from 0.016 Bq m-22 s-1 to 0.729 Bq m-22 s-1,with an average of 0.072 Bq m-22 s-1,which is 4.5 times higher than the world average radon exhalation rate.?3?The influence of fractal structure of soil on radon diffusion was studied through fractal theory and field experiment.A fractal model of radon exhalation from soil was developed,and it was proved to be reliable by comparison of the radon exhalation rate calculated by this model with the measured radon exhalation rate.?4?The total annual effective dose,which results from inhalation of indoor and outdoor air,water ingestion and surface gamma external radiation received by residents in Shizhuyuan area,ranges from 11.55 mSv y-11 to 18.15mSv y-1 with an average of 15.51 mSv y-1,indicating that Shizhuyuan is a medium-high natural radiation background region.The main part of natural radiation is radon exposure from inhalation of indoor air,and the radiation dose resulted from this part accounts for 85.62%of the total dose.The second part was radon inhalation from outdoor air and surface gamma external irradiation,accounting for 7.16%and 7.09%,respectively,and the smallest part is radon exposure from water ingestion,accounting for only 0.13%.The main source of natural radiation is radon in indoor air,which is the focus of radiation protection in this area.?5?The lifetime risk probability model and the relative risk model of indoor radon induced lung cancer were used to study the lifetime risk probability of cancer and heritable effects caused by various natural radiation dose and the total radiation dose from different sources in Shizhuyuan area,and as well study the relative risk of lung cancer.Residents in this area have a high lifetime risk probability of cancer and heritable effects and high relative risk of lung cancer.The average lifetime risk probability is 6.24%,and the relative risk of lung cancer is 1.19.The main radiation source of risk is radon in indoor air,accounting for 85.62%.?6?The influence of geology and mineralization on the natural radiation environment was studied.The Mesozoic magmatic activity and magmatic-hydrothermal mineralization of W-Sn-Bi-Mo result in a relatively high content of 238U?232Th?40K in the soil and rock of this area,which lead to high radon concentration in soil and radon exhalation rate and then cause a high radon concentration in groundwater and indoor and outdoor air,forming a medium-high natural radiation background in this area.There are obvious differences in the radiation environment among different villages in this area,which can be attributed to the difference of geological characteristics and metallogenic intensity.?7?Based on the characteristics of natural radiation in this area,some measures and suggestions on radiation protection are proposed from the aspects of building material,foundation selection,engineering measures of constructions,reduction of radon content in water and indoor air. |
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