Methodology Study On Measurement Of Atmospheric Tritium In Multiforms In The Vicinity Of Nuclear Facilities

With the development of economy,the demand for energy has increased dramatically.As the situation of the environment and ecological protection is very serious,more and more attention is paied to the process of energy production.Nuclear energy with advantages of high energy density and environment friendly has been developed rapidly around the world.Whereas,radioactive nuclides would be released into the environment during the operation of nuclear power plant,and tritium is one of the main radioactive nuclides that released during the normal operation.Atmospheric tritium released from nuclear facilities are mainly in forms of tritiated water（HTO）,tritiated gas（HT）and tritiated methane（CH₃T）.As radiological hazard of atmospheric tritium is related to its chemical forms,concentrations of atmospheric tritium in multiforms should be measured simutaneously,to evaluate the influence of atmospheric tritium released from nuclear facilities and and research the translocation and transformation laws of atmospheric tritium in the environment.With the deepening research of environmental tritium,the measurement of atmospheric tritium in multiforms has become the hotspot and difficulty in current research.The main point of this research was to develop a novel atmospheric tritium sampling apparatus.And taking the Qinshan nuclear power plant（QNPP）base as an example,atmospheric tritium in forms of HTO,HT and CH₃T were sampled and measured simutaneously with the new developed novel atmospheric tritium sampling apparatus,attempting to provide technical support for the research of radiological hazards,translocation and transformation laws of atmospheric tritium.The background levels of atmospheric tritium concentrations are 10²0 mBq/m³for HTO,10³0 mBq/m³ for HT,and 2¹5 mBq/m³ for CH₃T.The measurement method for atmospheric tritium can be devided into two categories:1)On-line measurement with apparatus such as ionization chamber,proportional counter;2)Off-line measurement contains processes of accumulative sampling and liquid scintillation counter（LSC）measuring.As the minimum detection limits of ionization chamber and proportional counter are slightly higher,ionization chamber and proportional counter are usually used for the atmospheric tritium measurement in the interior of a nuclear facility.Currently,atmospheric tritium in the vicinity of nuclear facilities are monitored with the off-line measurement method.The methods used in the accumulative sampling are desiccant,bubbler,condensation,freezing et al.Currently,combined with low-level LSC measurement,the accumulative sampling apparatuses based on condensation or freezing method have a low detection limit within the background levels of atmospheric tritium concentrations,which are widely used for the atmospheric tritium sampling and measurement.The widely used atmospheric tritium sampling apparatuses have two shortcomings:1)Most of these apparatuses collect only HTO;2)Problems such as cross-contamination,blocking in the cold trap,safety problem and so on,present in the serial designed sampling apparatus.A novel atmospheric tritium sampling apparatus based on the principle of high temperature catalyzation and cold trap collection was developed in this study.And the main improvements are as follows:1)Three parallel connected sampling channels were equipped for rapidly sampling and avoiding cross contamination;2)Newly designed two-stage cold trap set with collecting efficiency above 95%was developed to solve the blocking problem;3)Hydrogen supply module was developed to replace the widely used hydrogen gas cylinder or tritium-free electrolyzer,and methane supply module was developed to replace the traditionally used methane gas cylinder;4)Optimized precious metal catalyst with catalytic efficiency above 99%for hydrogen and methane.Taking the QNPP base as an example,atmospheric tritium in chemical species of HTO,HT and CH₃T in the vicinity of QNPP base were sampled simutaneously with the novel atmospheric tritium sampling apparatus developed in this study.The results showed that:Atmospheric tritium in chemical forms of HT and CH₃T were firstly measured in the vicinity of QNPP base;The concentrations of HT and CH₃T measured during the sampling period varied within the background levels,which indicated that the sampling apparatus and measurement method used in this study were highly sensitive and could be used for sampling and measurement of atmospheric tritium within background levels.