Studyon Reactor Leakage Monitoring Based On ¹³nmulti-detector Gamma Coincidence Method

The leakage of radioactive coolant within a PWR primary circuit,will not only change the operating power and affect the safe operation of the nuclear power plant,but also cause nuclear pollution to the surrounding environment and even serious safety accidents.Monitoring the leakage of the primary circuit can ensure that the leakage of the pressure boundary can be found in time,and provide help and guidance for the operators to take effective measures to prevent the expansion of the accident.In the 1990s,The method of detecting leakage by monitoring the ¹³N content in the containment was rapidly developed.This method has various advantages,such as:accurate calculation of the source term,quantitative data of the leakage rate of the primary circuit,and rapid response.At present,low-background gamma energy spectrum is used in monitoring the leakage of the primary circuit based on ¹³N.In practice,it is found that the gamma energy spectrum method has a high detection limit of the leakage rate,and after long-term operation,the stability of the spectrometer will become worse.To solve the existing problems,a?-?coincidence method to monitor the primary circuit leakage is proposed.The coincidence method can effectively suppress the background of the measurement system,but the efficiency of the coincidence detection is relatively low.For the?-?coincidence method,improving the coincidence efficiency is the key issue to reduce the detection limit of the measurement system and increase the sensitivity.In this paper,a research had be carried out by combining Monte Carlo simulation and experimental testing.The main contents are as follows:Firstly,sampling and detection devices with four Na I(TI)detector or five Na I(TI)detector were designed based on Marinelli Beaker of sampling and detection device of gamma energy spectrum system.And the annihilation behavior of positrons from ¹³N and ²²Na was simulated by using the MCNP6 program to calculate the probability of annihilation of positrons in the air in the container,on the upper and bottom surfaces of the inner wall of the container,and on the other surfaces of the inner wall of the container respectively.The result shows when the volume of the sampling container increases from 1.8L to 4.9L,the annihilation of positrons produced by ¹³N and ²²Na follows the following rules:1)The probability of annihilation in the air is proportional to the volume of the container,and gradually increases in the range of[6.57%,20.3%]and[11.76%,32.1%]respectively.2)The probability of annihilation on the upper and bottom surfaces of the inner wall of the container is proportional to the volume of the container,and gradually increases in the range of[24.66%,41.2%]and[21.33%,34.7%]respectively.3)The probability of annihilation on other surfaces of the inner wall of the container is inversely proportional to the volume of the container and gradually reduced in the range of[43.3%,64.68%]and[35.6%,59.08%]respectively.In order to improve the coincidence efficiency,we designed various sampling and measurement devices with four detectors or five detectors based on the annihilation law of positrons in the container.The Geant4 program was used to simulate and calculate the total coincidence detection efficiency(threshold E=150ke V)and Peak coincidence efficiency(E=450ke V)in different volumes and different concentrations.The law of efficiency of various structural devices changing with the capacity,the influence of different sampling times NPS and central detector efficiency on coincidence detection efficiency were discussed.The reasons for different efficiencies of different geometrical sampling and detection devices in the same capacity were analyzed.According to the coincidence efficiency and device volume,the comprehensive factor?·V values of various devices were calculated,and the law of?·V value changing with volume was analyzed.Based on the comprehensive consideration of the coincidence detection efficiency,the comprehensive analysis factor?·V value,and the practical application feasibility of the project,the geometric structure and corresponding volume of the optimal sampling measurement device were determined.The capacity of optimal device is 2.3L,and it's total coincidence efficiency and full-energy peak coincidence efficiency for gamma rays are 5.12%and 1.93%respectively.Secondly,a ¹³N standard gas source was used to test the detection efficiency of the optimal sampling and detection device(device numbered D in the paper)and other three devices with different structures.A medical cyclotron was used to generate ¹³N gas,and standard gases with different activities were prepared;an experimental system according with the efficiency calibration was built to analyze the optimal work parameters of the equipment.When the system threshold E=150ke V,the coincidence background is 2.08cps,which is about 3%of that of a single detector;when E=450ke V,the coincidence background is 0.169cps,which is 1%of that of a single detector.The total coincidence efficiency and full-energy peak coincidence efficiency of the four sampling and measurement devices were calibrated with the standard gas.Among them,the total coincidence efficiency and full energy peak coincidence efficiency of the optimal device is 5.05%and 1.89%respectively.The experimental results are in good agreement with the simulation results,and the relative error is within 5%.Finally,in order to solve the problem of the difficulty in preparation with ¹³N standard source and unsuitability for calibrating at the production site,a method of using a solid ²²Na standard source to replace ¹³N for sampling and measurement device calibration was proposed.First,the Geant4 program was used to simulate and calculate the coincidence detection efficiency of the optimal sampling and measurement container to positrons annihilation from ²²Na,the simulating results are in good agreement with the ¹³N simulation results.When the device volume is2.3L,the relative error of the total coincidence efficiency is 4.15%,and the relative error of full-energy peak coincidence efficiency is 3.17%,the simulating results prove the theoretical feasibility of the method.According to the principle of the solid source calibration,the coincidence efficiency of sampling and measurement device were obtained by applying the experimental method when the ²²Na point source replaces the ¹³N gas source.The results of point source calibration method are in good agreement with the simulating calculation results,the relative error of total coincidence efficiency is 2.34%and the full energy peak coincidence efficiency is 3.26%.Based on some parameters of the Qinshan Nuclear Power Phase II600,000 k W reactor and the coincidence detection efficiency and volume value of the optimal device in this research,the theoretical detection lower limit of the coincidence method was calculated.When sampling in the fume hood of the reactor containment drive mechanism,the detection limit of the?-?coincidence monitoring method for the leakage rate is0.756L/h.In this paper,the sampling and measurement device of the coincidence measurement system was designed by software simulation and experiment,and three different methods were used to calibrate the coincidence efficiency of the device to ensure the accuracy of the detection efficiency.The main achievements and innovations of the paper are as follows:1)A multi-probe ¹³N leakage monitoring method based on?-coincidence is proposed.2)The N-13 sampling and detection device was developed,and the detection limit was reduced by an order of magnitude.3)A ²²Na solid point source is used to replace the N-13 gas source,providing a new calibration method.The development of this research is of great significance to the diversification of reactor primary circuit leakage monitoring methods and the optimization and upgrading of the monitoring systems.