TEPC Performance in the CANDU Workplace

Abstract

Tissue-equivalent proportional counters (TEPCs) have a number of features that make them an attractive option for neutron monitoring around power reactors. These features are principally the ability of the TEPC to operate in a mixed field environment, the direct determination of the dose equivalent from first principles and a reasonably well-understood response to the wide range of neutron energy encountered. A spherical TEPC from Far West Technology (5" diameter) and a commercial, TEPC-based, neutron monitor (REM 500) have been used to map the neutron fields at different locations in a CANDU 6 (Canadian Deuterium Uranium) power reactor operated by New Brunswick Power. Neutron ambient dose equivalent rates ranged between 8 µSv.h-1 and 500 µSv.h-1 as measured with a multisphere spectrometer and photon dose equivalent rates from 40 µSv.h-1 to 1.2 mSv.h-1 as determined with a TEPC. It is shown that, for the CANDU workplace, the energy response of TEPCs is still not known accurately enough to enable field correction factors to be derived from neutron fluence measurements or to avoid the need for more appropriate calibration procedures based on simulated 'workplace' neutron fields. The sensitivity of TEPCs is sufficient for workplace monitoring, but needs consideration when counters of 2" diameter or smaller are proposed for use. For mixed-field work, the use of a TEPC for photon dosimetry as opposed to photon discrimination requires attention to count rates and instrument dead time.