Neutron Dose Equivalent Rates Calculated from Measured Neutron Angular and Energy Distributions in Working Environments

Abstract

The estimation of non-isotropic neutron dose equivalent quantities in multidirectional fields is complicated. The estimation must be based on the neutron fluence rate as a function of both energy and angle. There are several well developed techniques for measuring neutron energy spectra. However, the directional characteristics have not usually been investigated in detail. The personal dose equivalent inside the PWR stations studied varies by a factor of 4 between AP and ISO irradiation geometries, but only by a factor of 2 between different locations for AP irradiation. This indicates the importance of knowing the directional distribution, in order to interrelate personal dose equivalent, effective dose equivalent (effective dose) and ambient dose equivalent, and to interpret dosemeter and instrument readings. A purpose built phantom has been constructed and used for the separation of the neutron fluence into 18 incident directions. Measurements with the phantom have been made at workplaces at Ringhals Nuclear Power Plant. The phantom consists of a 19 cm diameter boron doped paraffin sphere. Boron was added to decrease the transmission of neutrons through the sphere and to decrease the photon contribution from (n;gamma) reactions inside the phantom to the measurement of dose equivalent from incident photons. The phantom has proved to be capable of separating the incident neutrons into 18 directions and effective in reducing (n;gamma) reactions. Measured neutron angular distributions with coarse energy information were used with previously measured neutron spectra, to make better estimates of the neutron personal dose equivalent rates at these workplaces in Swedish nuclear facilities. The values of personal dose equivalent calculated with separation into 18 incident directions are slightly lower than those calculated for 2 or 3 directional distributions (AP, ISO and/or ROT). Results of calculations of neutron personal dose equivalent using the new (ICRP Publication 60) Q(L) function and new stopping powers, are also given, with and without resolution into 18 incident directions.