Search

Within the EC project EVIDOS, 17 different mixed neutron-photon workplace fields at nuclear facilities (boiling water reactor, pressurised water reactor, research reactor, fuel processing, storage of spent fuel) were characterised using conventional Bonner sphere spectrometry and newly developed direction spectrometers. The results of the analysis, using Bayesian parameter estimation methods and different unfolding codes, some of them especially adapted to simultaneously unfold energy and direction distributions of the neutron fluence, showed that neutron spectra differed strongly at the different places, both in energy and direction distribution. The implication of the results for the determination of reference values for radiation protection quantities (ambient dose equivalent, personal dose equivalent and effective dose) and the related uncertainties are discussed.

In the framework of the EVIDOS (Evaluation of Individual Dosimetry in Mixed Neutron and Photon Radiation Fields) project, funded by the EC, measurements with PADC personal neutron dosemeters were carried out at several workplace fields of the nuclear industry and at simulated workplace fields. The measured personal neutron dose equivalents of the PADC personal neutron dosemeter are compared with values that were assessed within the EVIDOS project by other partners. The detection limits for different spectra types are given. In cases were the neutron dose was too low to be measured by the PADC personal neutron dosemeter, the response is estimated by convoluting the responses to monoenergetic neutrons with the dose energy distribution measured within EVIDOS. The advantages and limitations of the PADC personal neutron dosemeter are discussed.

In this paper the present status of the Direct Ion Storage Neutron (DIS-N) prototype dosemeter (RADOS) is described. The separation of neutron from photon dose equivalent has been improved by adding tin shieldings. The neutron energy response has been changed by additional plastic covers containing 40% B4C in order to reduce the over-response to thermal neutrons. The responses of the dosemeters were determined for standard photon and neutron fields (monoenergetic neutrons, neutron sources and simulated workplace fields). Irradiations in real workplaces were also performed. The dependence of the neutron response on the angle of incidence was measured for different neutron sources.

A new personal neutron dosimetry system at the Paul Scherrer Institute (PSI) is based on chemically etched CR-39 detectors (PN3) with different radiators, and an automatic readout system, the Autoscan 60. The most relevant neutron fields at the PSI accelerator have an energy range up to 600 MeV with peak energies around 1-5 and 50-100 MeV due to evaporation and cascade processes in the concrete shieldings, and partly thermalised spectra that occur at the entrance to the labyrinths. The main radiators used for PN3 are polyethylene (PE) for fast neutrons and PE with lithium for thermal and fast neutrons. For extremely high track densities (corresponding to a dose up to 5 Sv) a transmission spectrometer is used for evaluation, since automatic counting by Autoscan 60 saturates at doses above 100 mSv. The system has been tested for linearity, energy dependence up to 300 MeV neutron energy, angular dependence, and radon background response. It was then calibrated for realistic neutron fields at PSI. The detection limit for these high energy fields is about 0.3 mSv. In addition, calculations have been performed to determine the response of the dosemeter to typical spectra in nuclear power plants.

The physical basis for the DIS (Direct Ion Storage) dosemeter is reviewed along with some of the issues associated with the expected long-term stability and reliability of the MOSFET structure under various operating conditions. Recent data from tests performed to assess the dosimetric accuracy, stability and reliability of the first commercially produced units for photon and beta radiation is presented. Special detectors have been developed for neutron dosimetry. Their design may be optimised either for a flat energy response or for a very high sensitivity at thermal neutron energy. Results of test irradiations in neutron fields from thermal up to 45 MeV neutron energy are presented with indication of dependence of the detection limit for neutrons on the photon dose for various dosemeter designs.

Neutron survey instruments have been exposed at all the measurement locations used in the EVIDOS project. These results have an important impact in the interpretation of the results from the project, since operationally the survey instrument will be used for an initial assessment of and routine monitoring of the ambient dose equivalent dose rate. Additionally, since the response of these instruments is in some cases very well characterised, their systematic deviations from the reference quantities provide an important verification of the determination of those quantities.

Supported by the European Commission, the EVIDOS project started in November 2001 with the broad goal of evaluating state of the art dosimetry techniques in representative workplaces of the nuclear industry. Seven European institutes joined efforts with end users at nuclear power plants, at fuel processing and reprocessing plants, and at transport and storage facilities. A comprehensive programme was devised to evaluate capabilities and limitations of standard and innovative personal dosemeters in relation to the mixed neutron-photon fields of concern to the nuclear industry. This paper describes the criteria behind the selection of dosimetry techniques and workplaces that were analysed, as well as the organisation of the measurement campaigns. Particular emphasis was placed on the evaluation of a variety of electronic personal dosemeters, either commercially available or previously developed by the partners. The estimates provided by these personal dosemeters were compared to reference values of dose equivalent quantities derived from spectrometry and fluence-to-dose equivalent conversion coefficients. Spectrometry was performed both with conventional multisphere and with some original instrumentation providing energy and direction resolution, based on silicon detectors and superheated drop detectors mounted on or in spherical moderators. The results were collected in a large, searchable database and are intended to be used in the harmonisation of dosimetric procedures for mixed radiation fields and for the approval of dosimetry services in Europe.

Within the EC project EVIDOS, double-differential (energy and direction) fluence spectra were determined by means of novel direction spectrometers. By folding the spectra with fluence-to-dose equivalent conversion coefficients, contributions to H*(10) for 14 directions, and values of the personal dose equivalent Hp(10) and the effective dose E for 6 directions of a person's orientation in the field were determined. The results of the measurements and calculations obtained within the EVIDOS project in workplace fields in nuclear installations in Europe, i.e., at Krummel (boiling water reactor and transport cask), at Mol (Venus research reactor and fuel facility Belgonucleaire) and at Ringhals (pressurised reactor and transport cask) are presented.

The paper presents the main conclusions and recommendations derived from the EVIDOS project, which is supported by the European Commission within the 5th Framework Programme. EVIDOS aims at evaluating state of the art neutron dosimetry techniques in representative workplaces of the nuclear industry with complex mixed neutron-photon radiation fields. This analysis complements a series of individual papers which present detailed results and it summarises the main findings from a practical point of view. Conclusions and recommendations are given concerning characterisation of radiation fields, methods to derive radiation protection quantities and dosemeter results.

In the framework of the EVIDOS project, funded by the EC, measurements were carried out using dosemeters, based on ionisation chambers with direct ion storage (DIS-N), at several workplace fields, namely, at a fuel processing plant, a boiling and a pressurised water reactor, and near transport and storage casks. The measurements and results obtained with the DIS-N in these workplaces, which are representative for the nuclear industry, are described in this study. Different dosemeter configurations of converter and shielding materials were considered. The results are compared with values for personal dose equivalent which were assessed within the EVIDOS project by other partners. The advantages and limitations of the DIS-N dosemeter are discussed.

The novel DIS-1 dosemeter developed by RADOS is based on ionisation chambers with so-called Direct Ion Storage (DIS). The dosemeter can measure H p (10) and H p (0.07) of photon and H p (0.07) of beta irradiation. The characteristics of the commercially available DIS-1 dosemeter were studied at the Paul Scherrer Institute, particularly in respect to the requirements laid down in the Swiss Dosimetry Ordinance. Detailed tests were carried out in terms of linearity, photon and beta responses, angle dependence, long-term stability of the signal, reproducibility and environmental conditions. The DIS-1 dosemeter has been qualified by the authority to conform to the requirements of the Swiss Dosimetry Ordinance for personal photon and beta dosemeters. It is now used as a legally approved personal dosemeter system at PSI.

The European Commission recently sponsored an international research and development project, in which three 'active' personal neutron dosemeters were developed. Comparative irradiation experiments showed good results with respect to sensitivity and energy dependence of the response.

Ionisation chambers are sensitive to both neutrons and photons. In order to produce a neutron dosemeter based on an ion chamber a double-chamber system which allows for differential readings has to be built. The system consists of one chamber with high neutron sensitivity (e.g. A-150 or polyethylene with 10 B or 6 Li compounds) and one chamber with low neutron sensitivity (e.g. graphite or Teflon). Different combined dosemeter prototypes were produced and their responses for standard photon and neutron radiation fields, as well as various field spectra, were determined. The feasibility of neutron dosimetry with ion chambers and direct ion storage (DIS) electronics has been proved. The results obtained with prototype dosemeters indicate the system's promising potential for legal approval in the future. Apart from dosimetric properties, the advantages of the system are its small size and weight, easy readout and relatively low production cost.