Theoretical Research On The Direct Measurement Technology Of Tritium On The Material Surface Under ? Background Radiation

Tritium is the main fuel for fusion reactors.For example,the amount of tritium consumed by the ITER device with a fusion power of 500MW is kilogram per year.After the D-T fusion reaction,the partially unburned tritium becomes a part of the plasma facing materials(the first wall and divertor)and structural materials of the reactor.During the process of maintenance,tritium on the surface of the material can easily enter the tissues and organs through breathing and skin contact.As a kind of ? radionuclide,tritium entering human tissues can cause internal radiation hazards.Exposure to tritium contaminated surface is an important source of radiation dose in tritium operation place for tritium staffs.Therefore,from the point of view of radiation protection,it is essential to monitor the level of tritium contamination on materials' surface to ensure do no harm to staff members and environment.In addition,there exist y radiation from activated metals in tritium operation place like fusion reactor,so tritium measurement usually need to be performed under the influence of ? radiation.Then,eliminating the influence on tritium measurement caused by the ? rays is important for such measuring techs.Tritium measurement technology can't get the direct and accurate results of the level of tritium contamination at present under the ? rays surroundings.In this paper,a multi wire ionization chamber is used as a prototype of tritium ? ray detector to study surface tritium measurement in the ? background and the theoretical study method is used to determine the structure of the ionization chamber.ANSYS software has been introduced to calculate the distribution of electric field with different models and voltage.Furthermore,influence induced by the size of wire,distance between anode and cathode,and distance between wires has also been specified.Results indicate that the fluctuation of electric field in the ionization chamber can be effectively depressed by decreasing the size of wire,distance between wires and voltage applied to anode,which is helpful to obtain stable electric field.MCNP5 Monte Carlo code has been employed to simulate the tritium ? and y-rays energy deposition in ionization chamber with different geometries,and the influence of the sensitive zone height of ionization chamber on the energy deposition of ?/? ray has been studied.The results show that the energy deposition of ?/? rays in ionization chamber increases exponentially with the increase of the sensitive zone.When the sensitive area of ionization chamber is less than 0.5 cm,the change of tritium ? energy deposition will be significant.MCNP5 has been employed to simulate the ?/? rays energy deposition in ionization chamber under different operating conditions,and the effects of the ionization chamber wall material,air humidity and air pressure on ?/y rays energy deposition has also been specified.Results indicate that wall material of the ionization chamber has little effect on the energy deposition of tritium ? rays,but a great impact on the energy deposition of y in the chamber.For example,the relative proportion of energy deposition of MeV energy y rays in the aluminum alloy ionization chamber and the stainless steel ionization chamber is-85%,and the material of the chamber wall which has small atomic number can reduce the energy deposition of y rays.The calculation of air humidity on the energy deposition of ?/? ray shows that the change in air humidity will have a great effect on the energy deposition of tritium ? rays.When the air humidity is 40%,the ? rays have the minimum energy deposition.The appropriate air humidity for tritium measurement is below 20%,in which the change of air humidity have no effect on energy deposition of the ? ray.The calculation of the energy deposition of gas pressure on ?/?ray energy shows that the energy deposition of tritium ? rays reaches saturation at 1 atm and the continuous increase of pressure will not significantly change the energy deposition of ? rays in the ionization chamber,and the energy deposition of ? rays increases linearly with the pressure increasing.Based on the dry air as the detection medium,the energy deposition data of ?/? ray in a 100 cm2 stainless steel ionization chamber,the effect of ?-rays on output signal of ionization chamber produced by tritium ?-rays has been evaluated.Results indicate that the energy deposition of MeV y-rays in ionization chamber is only dozens of electron volts,which is negligible compared to the 5.05keV energy deposition of the tritium ?-rays in the same ionization chamber conditions.It is found that 1 Bq/cm2 tritium and hundreds Bq/cm2 y radionuclide produced equivalent current signals in a 100 cm2 stainless steel ionization chamber.Based on the theoretical study of the electric field configuration of the multiwire ionization chamber and the energy deposition of the ?/y ray in the ionization chamber,the feasibility of multiwire ionization chamber with 100 cm2 detection area for measuring tritium contamination on material surface under y background have been verified.A prototype of multiwire ionization chamber for measuring tritium contamination on material surface has been built,the cathode is a wire array consisting of wire spacing D of 1.0 mm and a wire radius R of 10 ?m plated tungsten wire,and the anode is aluminum alloy plate,the distance between cathode and anode is 4.0 mm.This work provides a theoretical basis for the design of ionization chamber for tritium measurement under the gamma background.