Determination of Neutron and Photon Dose Equivalent at Workplaces in Nuclear Facilities in Sweden (A joint SSI-EURADOS Comparison Exercise)

Abstract

The accuracy of the dosimetry (personal and area monitoring) has been investigated inside the containment buildings of two pressurised water reactors and in the environment of a transport cask with spent fuel elements. The dosimetric quantities of main interest were ambient dose equivalent and personal dose equivalent, as operational quantities, and effective dose equivalent, as limit quantity. They were either directly determined by means of instruments and dosemeters or calculated from the experimentally determined directional spectral neutron fluences. Several groups employing different techniques carried out the investigations. The comprehensive comparison exercise has shown that a well specified Bonner sphere spectrometer and a set of proton recoil detectors are well-suited to determine the neutron field for reference purposes. The neutron fields were rather soft with up to 70% of the neutron dose equivalent contributed by neutrons of energies less than 100 keV. On account of their energy dependence, rem counters overestimate, and TEPC systems underestimate, the neutron dose equivalent, even if calibrated in the field of a D2O-moderated 252Cf source. Only a recently developed active dosemeter, based on superheated drop detectors (only used outside the containment), measured the ambient dose equivalent in good agreement with the results obtained with Bonner spheres). Measurements with several personal dosemeters irradiated on a phantom were used to estimate the directional properties of the neutron field. This is required for the calculation of personal and effective dose equivalent values. Some personal dosemeters gave dose equivalent results in satisfactory agreement (±20%) with reference values. In any case the measured dose equivalent values were conservative estimates of the corresponding effective dose equivalent values. The application of the new recommendations of the International Commission on Radiological Protection (ICRP) will result in about 50% higher values of neutron ambient dose equivalent. In the determination of the photon ambient dose equivalent, which amounts to about 30% of the total dose equivalent, differences up to 50% were observed between the readings of GM counters and TEPC's, chiefly caused by high energy photons present in the containment building.