| ObjectivesWith the awareness of radiation to people's health increasing,significant public attention have been paid to the radiological-related safety issues and health-risk assessments.Naturally Occurring Radioactive Materials(NORM)and Technologically Enhanced Naturally Occurring Radioactive Materials(TENORM)are further enriched in utilization of uranium mines and non-uranium mines,such as coal mines and rare earth ores,resulting in radioactive concentration and higher level of natural radioactivity.There are higher risk of radioactive contamination for occupational miners.Bay an obo ores is the largest rare earth(RE)mine in the world,with higher thorium(Th)contents.RE activities may had certain impact on health when being used.At present,there are many studies on the external exposure and radon concentration of the occupational sites of the Bayan Obo RE ore,but there are insufficient research on the internal exposure of the occupational population and the publics of the surrounding areas.U,Th,226Ra and 228Ra are the common natural radionuclides and of great significance for dose contribution.It was very important to investigate the intake of U,Th,226Ra and 228Ra in the occupational population and the publics of Bayan Obo.The traditional U,Th,226Ra,and 228Ra measurement methods have the disadvantages of complex chemical operation,large amount of sample,long measurement time,and measurement of only single nuclide.Establishment of the measurement methods of U,Th,226Ra,and 228Ra for environmental and biological samples with easy operation,less sample,fast and batch-measured advantages are necessary.Finally,the morbidity/mortality risk coefficient and the external/internal exposure data are required for the traditional radiation risk assessment.Under the condition of linear-no-threshold assumption,the radiation risk is evaluated.However,the relevant data is difficult to obtain and the traditional risk assessment model does not apply to individual risk assessment.Therefore,it is of great significance to establish a risk assessment system that is easy to implement for the radiation factors of rare earth ore associated with individuals.Methods1.Combined with relevant literature reports and different experiments,the pretreatment methods of hair,urine,lamb,drinking water,aerosol and dust were determined and a method for simultaneously measure U,Th,and 226Ra and 228Ra were established by liquid scintillation counter(LSC)and Inductively Coupled Plasma Mass Spectrometry(ICP-MS).2.One of the internal irradiation research methods—biological sample monitored-was studied.Combined with the production process,the occupational population and the surrounding publics were divided into three groups:low,medium and high aerosol exposure groups.Combined the established methods,the U and Th contents in hair and urine samples of different exposed populations were measured to find the distribution of U and Th in hair and urine samples,to estimate the degree of internal exposure to Th,and to determine the biological indicator for long-term chronic Th exposure.3.Another method of internal radiation research-the intake measurement method-were studied.Combined the the questionnaire and related literature reports,the intake of mutton,drinking water,aerosol dust ware determined.The local drinking water,lamb samples,were randomly collected.Combined with the established joint method,the U,Th,226Ra and 228Ra in the mutton and drinking water investigated and estimated the corresponding dose.The Activity-Median Aerodynamic Diameter(AMAD)of U and Th were determined by multiple samples of the six-stage sampler.The aerosol dust samples for typical RE ore occupation sites and surrounding public resident areas were collected.Combined with established joint determination methods,the determination of U and Th contents in aerosol dust were determined.The internal exposure dose caused by inhalation was estimated taking into account the actual measurement AMAD for occupational groups and publics.4.Three first-level indicators related to the radiation risk of Bayan Obo RE ore were established and divided into 12 secondary indicators through expert consultation and literature research.The radiation danger score were calculated by the analytic hierarchy process(AHP)method.The exposure level score were obtained by LEC evaluation method.The product of the radiation risk score and the exposure score is the risk score of the object according to the relevant literature.The corresponding risk degree was obtained and the measure were proposed in according to risk classification principle.Results1.The minimum detection limit(MDL)of U and Th measured by ICP-MS was all 2ng/L and the extended relative uncertainty of U and Th was 0.08 and 0.16,respectively.Five mL of 0.25 M EDTA-2Na solution with distilled water medium and 16 mL Hisafe 3 were determined for the measurement of 226Ra and 228Ra by LSC.PSD were determined finally as 154 through different experiments.The immediate measurement of 228Ra with polyethylene bottle after separation and the equilibrium measurement 226Ra through their daughter 214Po with unlimited bottle material were determined by interference nuclide analysis and spectral observation.Finally,the measurement methods of 226Ra and 228Ra were evaluated by adding standard method.The measurement efficiency of 226Ra was determined to be stable,the total efficiency was 73%the relative standard deviation(RSD)was 6.7%,and the MDL and the extended relative uncertainty were 4.3 mBq/L and 0.15,respectively.However,the 228Ra measurement results were relatively poor,with a total efficiency of 11%,RSD of 16.6%,and MDL and an extended uncertainty of 41.5 mBq/L and 0.46,respectively.2.The U and Th contents in hair and urine of occupational and control populations were investigated.It has been found that the content of Th in hair were increased with the increase of exposure and there were no statistical significance of the level of Th in urine samples between three groups.There was no statistical significance of the U contents in hair and urine samples between the three groups.The U contents of hair in women was significantly higher than that of men.A linear correlation between age and Th in hair and urine were not founded.3.The contents of Th and 228Ra in drinking water and the 228Ra in mutton were all higher than the MDL.The dose contribution of natural U in drinking water was slightly higher than that of 226Ra,and the median dose values were 20.21 nSv/a and 10.54 nSv/a,respectively.In mutton,the median internal accumulated effective dose of U,Th and 226Ra was 0.52 nSv/a,1.89 nSv/a and 20.91 nSv/a,respectively,and the main dose contribution nuclide was 226Ra.The dose of inhaling Th was obviously higher than that of U and the dose of inhalation by occupational populations is higher than that of the controls.The AMAD of U and Th were 3.36?m and 3.64 ?m by multiple samples,respectively.The internal exposure doses for occupational and publics were estimated considering the factor of measured AMAD of the U and Th,mask filtration efficiency,respiratory retention of different AMAD,etc.The annual effective dose of inhaled U and Th in the occupational population were estimated to be 2.2-22.8 nSv and 5.2-24.0?Sv,respectively.The median annual effective doses of the publics inhaled U and Th were 1.5 nSv and 6.0 ?Sv,respectively.4.AHP method combined with modified LEC method were used to establish radiation risk assessment system for a typical rare earth mine.Twelve secondary indicators related to the environmental radiation of RE mines were used,combined with questionnaires and literature,the radiation risk can be evaluated for the people contacting the rare earth mines,which provides reference for RE mine management and radiation protection.Conclusion1.The method were established in this study for measuring 226Ra and 228Ra by LSC.Combined with the relatively recognized ICP-MS method for measuring U,Th,the different environmental and biology samples were digested and been measured simultaneously of U,Th,226Ra and 228Ra by LSC and ICP-MS.2.It has been found that Th in hair is more suitable as a bio-indicator for chronic Th exposure than urine and occupational groups has long-termed been exposed to Th.3.The accumulated effective dose caused by mutton and drinking water is limited.The natural U dose contribution in drinking water is slightly higher than 226Ra,while the main dose contribution nuclides in mutton is 226Ra.Compared with the average natural radiation dose received by the human body,the doses caused by inhalation of U and Th are limited.4.The radiation risk assessment of occupational population in RE ore can be carried out through the AHP method combined with the changed LEC method. |
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