Continuous Monitoring Of Nuclear Facilities Environmental Radiation In The High-pressure Ionization Chamber Developed A Number Of Issues

The aim of setting up a continuous monitor for environmental gamma radiation surrounding the nuclear facility is to carry out the routine environmental gamma radiation monitoring, early warning of a nuclear accident and emergency radiation monitoring. The detector with high pressure ionization chamber is taken as a continuous monitor, based on its practice application in environmental gamma radiation monitoring. It is used to investigate the promotion of the accuracy and the sensitivity of air absorbed dose-rate measurement, extention of the detection limitation in low energy and improving energy response characteristic, and expanding the measure range of the dose-rate.The method combined theory calculation with experimental verification is used, here the theory calculation adopts the Mount Carlo simulation(simple named M-C-method as follow), the experiment is carried out at the CNNC Radiation Metrology and Measurement Center.The M-C-method is adopted to calculate the response of the high pressure ionization chamber. In order to improve the characteristic of the chamber, we simulate parameters, which influence on the response of the chamber, such as the material of the wall, the thickness of the wall, the material of the gas, the pressure of the gas, the collective electrode, the volume of the chamber, the shape of the chamber, the shield material, the thickness and area of the shielding body and so on. The MCNP4B program is advantageous with simple and graphical output. Our experiment purpose at the standard radiation field is as follow: (1) The response factors are calibrated at the standard radiation field, including the Co reference gamma radiation and 137Cs reference gamma radiation , the narrow spectrum series of filtered x-radiations , and the mono-energetic 6129~7115keV gamma radiation, and the characteristic of the energy response is also studied in the standard radiation field by changing the energy range, the radiation source, and the condition. The measurement uncertainties are given.(2) To verify the M-C-method model for calculating the response of high press ionization chamber by the MCNP4B program, the three kinds of high pressure ionization chamber are simulated at high, medium and low energy, which are the spherical chamber with stainless steel wall filled with argon gas, the spherical chamber with aluminum wall filled with nitrogen gas, and the cylindrical chamber with stainless steel wall filled with argon gas respectively, the simulated results are verified with the experiment in the standard radiation field.(3) The M-C-Simulation-calculation results of improving the characteristic of the ionization chamber has been verified. The main attentions are paied to lower the detection limit in low energy and to improve the characteristic of energy response, the M-C simulative model which enwrapped the shield above the outer wall of ionization chamber was established, a variety of M-C-Simulation-calculation are carried out, the optimization results are verified by experiment.(4) To extend the measurement range of air absorbed dose-rate, the two methods are adopted currently, (i) Two chambers are adopted switching one another by the electronic systems to measure low or high dose-rate. (H) One chamber is adopted using a shifter in the electronic systems which adopts low voltage in low dose-rate and adopts high voltage in high dose-rate , and when it is non-linear between current and high dose-rate should corrected it via a Single Chip Micyoco program.The experiment method is measure the non-linear function between current and high dose-rate, our objective is to extend the air absorbed dose-rate measurement range to 109nGyh-1 for one chamber.Although a lot of work has done on environmental gamma radiation monitoring by using high pressure ionization chamber, it is needed to improve the performance when the chamber is used in continuous monitoring mainly:① To improve the stability.② To improve the sensitivity and the accuracy.③ To extend the range to meet the needs of general environmental gammaradiation monitoring and early warning of a nuclear accident and emergencyradiation monitoring.The experiment and the calculation are as following:(1) The M-C-method model for simulating the response of high pressure ionization chamber is established which can be applied to simulate the response of various chambers. The feasibility and the accuracy of the model were verified by the experimental at the standard radiation field, the results are satisfactory , for example, the results between the M-C-method and the experimental method in studying the response of the spherical steel-walled ionization chamber filled with pure argon gas with pressure of 2.53×106Pa, which the wall thickness is 2mm, the outside diameter of the chamber is 250 mm are well. The deviation between them is 1.27% in 6129~ 7115keV gamma radiation, 1.27% in Cs reference gamma radiation, and -5.5% in 60Co reference gamma radiation respectively. The deviations are increasing as the energy from in the narrow spectrum series of filtered x-radiations are decreasing, namely -9.69% at 250 keV, -4.13% at 208keV, -18.82% at 100keV, 44.72% at 48 keV . In conclusion , the results between two methods are agreement with each other except the energies of under 100keV.(2) The M-C-Simulation-calculation and the experimental validation are carried out in order to improve the characteristic of the chamber, as following :① The detection limit in low energy is lower and the energy response is higher to the guide line. The characteristic of energy response is denoted as the energy response guide line and is symbolized S,(S is the responsein certain energy, is the sensitivity in energy of 137Cs)the absorbed dose rate monitor using in environmental gamma radiation monitoring is required relatively flat response, the S is within ±30% independent for the energy range of 80keV to 3MeV and is within ±35% in the energy range of 6129~7115 keV, in the study, the energy detect limit of the spherical argon-filled steel-walledionization chamber and the spherical nitrogen-filled aluminium-walled ionization chamber has extended to 65keV, and the the energy detect limit of the spherical nitrogen-filled POM-walled ionization chamber and the spherical nitrogen-filled aluminium -walled ionization chamber has extended to 45keV by the M-C-simulation.(2) The detection characteristic in high energy of 6129—7115keV γ radiation is studied, and the detection limit of the spherical argon-filled steel-walled ionization chamber and the spherical nitrogen-filled aluminium-walled ionization chamber is extended to 6129~7115keV.③One chamber is adopted to extend the range of high dose-rate measurement to 10 9nGyh-1, the relation curue between current and dose-rate in 300v-3000v voltage was measured. Voltage 600v was selected as work, to establish the function between current and dose-rate in the range within 10% uncertainty.④The air equivalent material or the material of approximately air equivalent were adopted to improve the accuracy of air absorbed dose rate measurement.The M-C-simulation verify that the nitrogen gas is one of the approximate air equivalent, the polyoxymethylene material and the aluminium material is also the material of approximate air equivalent.To summarize our study, some features may find:(1) The response factor was calculated in the mono-energetic 6129~7115keV gamma radiation field, and the M-C-Simulation-Calculation was taken corresponding these energies, to the spherical argon-filled steel-walled ionization chamber, the two methods is agree each other within 1.27% at 137Cs standard radiation field. There is no domestic report about the study, The detection energy is usually under 3MeV in the general environmental gamma radiation monitoring, however there is high energy y radiation produced in reactor especially in nuclear power reactor and in accelerator, the work is important in continuous monitoring of high gamma radiation discharges from nuclear facilities.(2) Using the M-C-method we simulated the sensitivity factor of the sphericalargon-filled steel-walled ionization chamber and the spherical nitrogen-filled aluminium-walled ionization chamber, the cylindrical argon-filled steel-walled ionization chamber, and calibrated the sensitivity factor of the same three kinds of ionization chamber in the 137Cs standard radiation field. It shows their results agree each other between the two methods within ±1.27%. All of above conclusions has validated the M-C-method model, it shows that the M-C-Simulation-calculation model is valuable and may use in the ionization chamber investigation.(3) In order to improve the characteristic of energy response and to extend detection limit in low energy, The M-C-method model of enwrapped the shield above the outer wall of ionization chamber is established, which can be used to optimize the parameter of the shield above the outer wall of ionization chamber, the detection limit in low energy of the spherical argon-filled steel-walled ionization chamber and the spherical nitrogen-filled aluminium-walled ionization chamber are extended to 65 keV .The experiment in the standard radiation field validate the feasibility of the M-C-method. So far no domestic report about the study is found, and it can be in applied in practise.The purpose of continuous monitoring for environmental gamma radiation is to offer accurately y radiation monitoring data of radioactive discharges from nuclear facilities in routine work or in accident case, so our study is valuable. By a great deal of M-C-simulation-calculations and experiment measurements, the optimization parameters for the chamber were provided, the ionization chamber which meets the request were designed. All the methods , the parameters, and the designs are useful. Otherwise, the method combining M-C-Simulation-calculation with experimental verification is a economical and efficient technique, it will be a wide application foreground in the research of nuclear detectors.