Investigation of field calibration method for krypton-85 noble gas monitors

[Background] Radioactive noble gas monitoring is an important part of nuclear facility effuent monitoring. The absolute measurement of noble gas radioactivity concentration is based on the length-compensated internal gas proportional counting method. Most of noble gas monitors with big volumes or shieldings are required continuous operation. The absolute measurement is not suitable for on-site calibration. In addition,the value transfer system for noble gas is incomplete. [Purpose] In order to solve the problems of on-site calibration and improve the value transfer system for noble gas,CIRP(China Institute for Radiation Protection) has been established on-site calibration for noble gas monitor.[Methods] The on-site noble gas activity concentration standard device was based on gas diffusion theory. The main standard instrument was krypton-85 noble gas, tracing from National Physical Laboratory(NPL). A series of ampoules of different volumes were used for preparing the activity concentration of noble gas.Temperature,pressure,decay,volume and the other factors were considered in evaluation of uncertainty. In order to evaluate the stablity and reliablity of the devices,some on-site experiments and comparisons were finished.[Results] The device can provide krypton-85 activity concentration between 1.0×105 Bq•m-3 and 1.0×1010 Bq•m-3, the relative extend uncertainty of 6.8%-9.0% (k=2).The activity concentration of main standard instrument is 3.38×1010 Bq•m-3,and the standard uncertainty is 2.22%.The comparison error is less than 3% between HPGe measurement method and gas diffusion method. Besides, the device meets the requirements of JJF 1979-2022 calibration specification for radioactive noble gas monitors.[Conclusions] At present, there are problems with small measuring range and large nonlinear deviation of high krypton-85 activity concentration response in the use of noble gas monitors. The device is an important supplement for noble gas value transfer system in China.