A Method For Control Of Key Parameters In Thoron Chambers

More attention has been paid to radiation exposure due to inhalation of thoron progeny.The risk of inhalation of thoron cannot be neglected in China since its thorium-rich soil.More important,thorium resource and rare earth were quickly developed under the help of national policy,but effective dose should be limited for workers working in the process in mining,milling,abstracting,utilizing and disposing of thorium resource.Indoor thoron equilibrium factor does not follow a homogenous distribution for its short half-life?55.6s?.Exposure assessment for thoron may not be accurate by calculation of thoron and thoron equilibrium factor.Because of this,estimation of dose and associated risk from thoron and its progeny in the room relies on conducting accurate measurements of thoron progeny.However,thoron progeny measurement is inevitable interfered by radon progeny,which lead to measurement error.Thus,it is necessary to establish a standard thoron chamber to ensure measurement quality by calibration of thoron progeny measurement devices.But there is several thoron chambers and most of them cannot sustain the stability of thoron progeny concentration when simultaneously conduct calibration of some active monitors since the long half-life of ²¹²Pb lead to long time to reach a stable activity concentration.In order to obtain reliable measurement data of thoron progeny concentration at measurement field,a novel method and related theoretical model are developed to establish stable and homogenous thoron progeny concentration in thoron chamber in this study,which will meet the needs of for calibrating thoron progeny monitors and then improve the level of thoron progeny measurement.Control of thoron progeny is based on thoron source.Several various activity of thoron sources with high and stable emanation coefficient were used and an aging chamber with total volume of 443 L.The injection amount of thoron can be controlled by adjusting flow rate and volume of tube in the thoron injection system.The existing thoron chamber was reconstructed and simulation sampling loop was designed.The thoron chamber increases the function of a thoron progeny chamber after reconstructed.Control method and a theoretical model were also developed for controlling key parameters in thoron chamber.Thoron progeny in the calibration chamber was continuously replenished by the aging chamber with high thoron progeny concentration and then increase rate of thoron progeny would reach a dynamic equilibrium with the loss caused by sampling,decay and ventilation in calibration.Theoretical relationship between key parameters and control variable was described in the model for verification of the effectiveness of the method.The theoretical model can be as a guide to control key parameters in thoron chamber.A method for determination of deposition velocity was also developed for calculation of deposition.The calculation results were compared with the value determined by fitting the thoron progeny concentration as a function of timeThe test of thoron progeny chamber operation in static state shows that stability and homogeneity of thoron progeny concentration below10%and are not affected by various temperature and relative humidity.The experimental value of stable thoron progeny concentration is close to the theoretical value.The aging chamber as source of thoron progeny in the calibration chamber improves the product rate of thoron progeny.Various but stable activity ratio of ThC/ThB ranging from 0.33 to 0.83and thoron progeny concentration ranging from 100 to 6000 Bq·m^-3 are available by altering the flow rate of the simulation loop.Not only time for thoron progeny concentration to reach stable was shortened,but also fluctuation of thoron progeny concentration caused by sampling of active monitors with the operation of the simulation loop.The test of thoron chamber operation in air flow field is shown that stability and homogeneity of thoron concentration below 5%and those of thoron progeny below 10%.There is an acceptable error between estimation value and the experimental value for the stable thoron progeny concentration.Thoron progeny concentration in the range of 800 Bq·m^-3is available.Thoron equilibrium factor can be controlled in the range of environmental air.This study established the two different sets of thoron chamber running in static and dynamic model and both of them can control thoron progeny concentration in a stable state,which take the lead in solving the difficulty in stable control thoron progeny concentration when several continuous measurement instruments sample air simultaneously or one-time sampling with large flow rate.The theoretical model can be used to estimate concentration of thoron progeny in similar thoron chamber.The results can meet the need for metrology of thoron progeny,evaluation of environmental thoron and the national development of thorium-based nuclear energy.