Research On The Key Technologies Of Online Gamma-energy Spectrum Measurement Of Water

With the expansion of nuclear energy scale and the extensive application of non-power nuclear technology in China,the discharge pressure of low-level radioactive effluent from nuclear facilities is increasing day by day,and the radioactivity level of inland receiving water body has become an important object of radiation environmental monitoring around nuclear facilities.The measurement technology of online gamma-energy spectrum for the receiving water body is an important monitoring technology to ensure the public radiation safety.Therefore,it is of great significance and practical value to study the online monitoring method of gamma-energy spectrum.Guided by the requirement of online monitoring of radioactive pollution in water,this paper completed the design and implementation of online gamma-energy spectrum measurement system for water body,and carried out the research and application of key technologies of online gamma-energy spectrum measurement for water body.The main research contents and achievements are as follows:?1?Design and implementation of online gamma-energy spectrum detection device for water.Based on the functional requirements and the minimum detectable limits of the online gamma-energy spectrum measurement system of water,the N-type coaxial electric refrigeration high-purity germanium detector with better energy resolution was selected as the gamma ray detector whose energy resolution was not greater than 2.3 ke V@1332 ke V peak(⁶⁰Co).Through theoretical calculation of effective detection distance and taking into account the equipment cost and load-bearing,we designed a cylindrical lead chamber with an outer diameter of?720mm×875 mm and the inner diameter of?500 mm×600 mm which lined with 2 mm thick oxygen-free copper with the measured indoor background of lead being 2.69 cps?below 3Me V?.An automatic calibration unit is designed on the outside of the lead,which consisted of a low-activity ¹³⁷Cs source,a shield and a shield valve,thus realizing the automatic calibration of the spectral drift and detection efficiency of the online gamma-energy spectrum measurement system of water.Experiments showed that the minimum detectable limit of the water online gamma-energy spectrum measurement system was 0.006 Bq/L(¹³⁷Cs,24h).Based on the physical structure design of the water online gamma-energy spectrum detection device,we used the Monte Carlo numerical simulation method to complete the calculation of the passive detection efficiency of the online gamma-energy spectrum measurement of water,and constructed the calibration curve and mathematical model of the passive detection efficiency for different energy rays.In order to verify the accuracy of the mathematical model of passive efficiency calibration,we prepared ²²⁶Ra,¹³⁷Cs and ⁴⁰K calibration solutions of different concentrations and measured the relative deviation between the detection efficiency of the gamma-energy spectrum measurement system and the detection efficiency obtained by Monte Carlo numerical simulation,which turned out to be less than 8%.?2?The design of online automatic sampling device of water and the research on the distribution uniformity of radionuclides in water sample.In terms of the representativeness of water sample collection,we designed a porous water sampler with self-adjusting sampling depth and dual channel water sampling redundant circuit,and then we designed sedimentation tank and a screen in the front of the measurement room to pretreat the water sample by filtration and self-precipitation.The measurement room adopts the design of the upper water inlet on one side and the lower water outlet at the bottom of the middle.Based on this,we established the mathematical relationship between the gamma ray full-energy peak area of the radionuclide characteristic of the water sample in the measurement room and both the sampling flow rate and sampling time of the water sampling circuit under ideal uniform conditions.In terms of the distribution uniformity of radionuclides in water sample,based on research of static and dynamic diffusion models,we used the Standard k-?model of Fluent simulation software to simulate the distribution of radionuclide in the measurement room,established a convection-diffusion distribution model.On that basis,we used Monte Carlo to simulate the detection efficiency in different distribution states,and constructed a functional model of the characteristic peak net area count rates with time to realize the prediction of the early warning time of abnormal concentration of radionuclide activity in water.At the same time,we constructed a prediction function model of nuclide activity concentration,which provides a theoretical basis for the rapid prediction of nuclide activity concentration in water.?3?Research on online gamma-energy spectrum data processing and quantitative analysis of radioactivity of water.Aiming at the natural water bodies characteristics,such as the high scattering background of gamma-energy spectrum instruments,the high interference of gamma-energy spectrum and the weak characteristic peaks,as well as the objective needs of fast processing of spectral data,we proposed an instrumental spectral analysis method based on spectral line denoising by wavelet filtering,SNIP?Sensitive nonlinear iterative peak?and FFT?Fast fourier transform?background subtraction and total peak area method to calculate the characteristic peak areas.We used ²²⁶Ra,¹³⁷Cs and ⁴⁰K calibration solutions with different activity concentrations,selected the 186 ke V,661 ke V and 1460 ke V gamma ray full-energy peaks on the instrument spectrum,and carried out the radioactive nuclide activity quantitative calibration of the online gamma-energy spectrum measurement system with the activity concentration fits of ²²⁶Ra,¹³⁷Cs and ⁴⁰K nuclides being 0.9998,1.0 and 0.999,respectively.We investigated the effects of non-radioactive and radioactive suspended particles?turbidity?on the measurement results of on-line gamma spectroscopy by using Monte Carlo numerical simulation and physical experiments.When the turbidity was less than 5000,the non-radioactive suspended particles of the water affected the detection efficiency of characteristic gamma rays from ²²⁶Ra,¹³⁷Cs and ⁴⁰K little,and the relative error was less than 0.4%,the activity concentration of radionuclides in water increases linearly with the increase of the turbidity of radioactive suspended particles.The self-precipitation method of water can effectively reduce the impact of radioactive suspended particles on the measurement results.Based on the analysis of gamma-energy spectrum characteristics emitted by?and?^-radiators in natural and artificial radionuclides,a mathematical equation to estimate the total?/total?radioactivity of water was established on account of the gamma ray fluence rate of radionuclides,which provide a technical method for estimating the total?/total?radioactivity in water by online gamma-energy spectrometry.?4?Preliminary application and effect of online gamma-energy spectrum measurement system for water.The online gamma-energy spectrum measurement system developed in this paper was installed in Pinglesi Water Quality monitoring Station in Pidu District of Chengdu.From March 2018 to August 2018,the measurement system has carried out pilot production measurement on water of Pinglesi Water Quality monitoring Station,and has continuously collected 114 gamma-energy spectrum data.The results showed that the arithmetic mean of total?activity concentration was 0.0214±0.0152 Bq/L in the range of 0.0002?0.0748 Bq/L,the range of total?activity concentration was 0.0073?0.1267 Bq/L,and the arithmetic mean value was 0.0506±0.0248 Bq/L.In March 2018,the monthly mean values of total?and total?activity concentrations measured by the online gamma ray spectrometry system were 0.0408 Bq/L and 0.0846 Bq/L,respectively,with relative deviations of 3.03%and 15.89%compared with the laboratory analysis results.The total alpha activity concentration and total beta activity concentration measured in the drinking water source were lower than 0.5 Bq/L and 1.0 Bq/L respectively in the drinking water quality standard?GB5749-2006?.The total alpha/total beta radioactivity reported by the measurement system has become the daily monitoring basis for the radioactive environmental monitoring of drinking water sources in Chengdu by Sichuan Radiation Environmental Management Center Station.