| With the rapid development and utilization of nuclear industry in recent years, some enterprises related to nuclear will make a large number of radioactive liquid effluent with different levels in the production runtime. As soon as possible in order to find abnormal and reduce sewage emissions, many nuclear facilities are equipped with continuous monitoring system before the waste water is discharged into the storage tank. The detection efficiency of the monitoring system is one of the important indicators to measure the performance of the monitoring system. The efficiency calibration method which is often used at home and abroad is to use a liquid standard source to prepare the large volume radioactive to calibration the system. Although the method is accurate,there are also many defects: Standard radioactive solution is expensive and the examination and approval procedures are complex; After the experiment, the radioactive waste liquid must be specially treated, thus increasing the cost of the experiment; In order to cover a wider range of energy, the need to select a variety of different energy radioactive standard solution, not only increased the workload of the experiment, but also to extend the staff suffered a dose of irradiation time, etc. Theefficiency calibration method based on Monte Carlo method, using a computer simulation of the interaction between photon and substance,since the calculation is accurate, so it has been widely used in recent years.This study has established a complete set of Monte Carlo efficiency calibration method. The structure and dimensions of the HPGe detector are not detailed because of the reason that the technology is confidential.Monte Carlo method is used to simulate the point source full energy efficiency of the high purity germanium probe on the axial 25cm?30cm?40cm and the side 20 cm. By comparison with the experiment results,continuously adjusting the radius, thickness of the Ge crystal and the thickness of the copper holder, the accurate simulation size is obtained finally. The result suggests that: Use the adjusted dimensions, the fulll energy efficiency between simulate and measured results were mostly within ± 5% on the axial and ± 6% on the side of the probe.The whole monitoring unit is modeled and calculated by using the adjusted HPGe probe. For some of the common gamma radionuclides in industrial waste water, the corresponding detection efficiency is calculated and the efficiency curve is obtained. Preparation the certain ratio of 60 Co and 57 Co activity of the liquid source to calibration the system efficiency, founded the deviation between the experiment values and the simulated values is less than ±10%. It proved the feasibility andaccuracy of the calibration method with MCNP. In the case of insufficient experimental conditions, MCNP simulation can be used to replace the experimental system to complete the scale of the experiment. This study has laid the foundation for the analysis and recognition of the nuclear and the calculation of activity concentration for the latter online monitoring system. It also has an important significance for improving the existing radioactive water online monitoring technology and the construction of the radiation environmental monitoring network in China. |
PDF Full Text Request