Design And Verification Of Key Indicators For Extremely Low-level Radioactive Waste Measurement System

The research on the extremely low level radioactive waste measurement technology and the successful development of the system are the important basis for the realization of the minimization of radioactive waste、the expansion of the dynamic range of the classification check of existing home-made equipment and the safe disposal of radioactive waste.In order to meet the demand for the correct and fast classification of radioactive waste and free release detection,this paper designs a very low level radioactive waste measurement system based on the combination of Monte Carlo simulation and experimental verification.The design of the system mainly focuses on improving the efficiency of the system,reducing the background of the system by designing the active shield and the layout of detectors and so on.300Bq/kg is considered as an important constraint for the design of a very low level radioactive waste measurement system,calculating and analyzing the gamma ray emission rate of the waste package and the technical problems between improving the efficiency of the system and reducing the background of the environment,and preliminarily constructing the overall layout of the system.In order to realize the extremely low level radioactive waste package measurement by high reliability,the system should have high detection efficiency and meet the spatial uniformity requirement of radial and axial efficiency,and the optimal layout、thickness and space uniformity of detection efficiency of NaI detector are obtained by theoretical simulation,In the case of certain detection efficiency,in order to reduce the background level to the minimum,the combination of lead and copper shielding scheme is adopted to obtain the relationship between the thickness of the lead layer and the background count by experimental measurement and theoretical simulation,and accomplishing the design of lead and copper layer thickness of the active shield to reduce the background level of the system.At the same time,the energy spectrum,type and intensity of environmental gamma ray which affect the background level of the system are analyzed,and the technology shielding method of controlling the counting energy area is used to further deduct the influence of 1.4MeV,2.614 MeV and other high-energy gamma rays in the environment.By using the existing NaI detector,the relationship between the base counting rate under the condition of 10mm lead shielding and the unshielded window area of the detector is established to calculate the base counting rate of the designed system.The relationship between a nuclide gamma ray emission rate,the system count rate and the detection limit(MDA)is simulated with the characteristics of a nuclide energy spectrum which is very close to the energy spectrum of the 152Eu radiation source.Simulation research and experimental verification show that the efficiency of the design system is better than 30%,and the background count rate is less than 10500(cps).When the measurement time is 1000s,the weight of waste bag is 15kg,the MDA of system is better than 300Bq/kg.The results of the study are helpful to the successful development of the extremely low level waste measurement system and provide a useful attempt and exploration for the safety disposal scheme of radioactive waste.