Achievements in workplace neutron dosimetry in the last decade: lessons learned from the EVIDOS project.

The availability of active neutron personal dosemeters has made real time monitoring of neutron doses possible. This has obvious benefits, but is only of any real assistance if the dose assessments made are of sufficient accuracy and reliability. Preliminary assessments of the performance of active neutron dosemeters can be made in calibration facilities, but these can never replicate the conditions under which the dosemeter is used in the workplace. Consequently, it is necessary to assess their performance in the workplace, which requires the field in the workplace to be fully characterised in terms of the energy and direction dependence of the fluence. This paper presents an overview of developments in workplace neutron dosimetry but concentrates on the outcomes of the EVIDOS project, which has made significant advances in the characterisation of workplace fields and the analysis of dosemeter responses in those fields.

Neutron area survey instrument measurements in the EVIDOS project.

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.

Characterisation of mixed neutron photon workplace fields at nuclear facilities by spectrometry (energy and direction) within the EVIDOS project.

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.

Application of workplace correction factors to dosemeter results for the assessment of personal doses at nuclear facilities.

Ratios of H(p)(10) and H*(10) were determined with reference instruments in a number of workplace fields within the nuclear industry and used to derive workplace-specific correction factors. When commercial survey meter results together with these factors were applied to the results of the locally used personal dosemeters their results improved and became within 0.7 and 1.7 of the reference values or better depending on the response of the survey meter. A similar result was obtained when a correction was determined with a prototype reference instrument for H(p)(10) after adjustment of its response. Commercially available survey instruments both for photon and neutron H*(10) measurements agreed with the reference instruments in most cases to within 0.5-1.5. Those conclusions are derived from results reported within the EC supported EVIDOS contract.

Evaluation of individual dosimetry in mixed neutron and photon radiation fields (EVIDOS). Part I: Scope and methods of the project.

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.

Summary of personal neutron dosemeter results obtained within the EVIDOS project.

Within the EC project EVIDOS ('Evaluation of Individual Dosimetry in Mixed Neutron and Photon Radiation Fields'), different types of active neutron personal dosemeters (and some passive ones) were tested in workplace fields at nuclear installations in Europe. The results of the measurements which have been performed up to now are summarised and compared to our currently best estimates of the personal dose equivalent Hp(10). Under- and over-readings by more than a factor of two for the same dosemeter in different workplace fields indicate that in most cases the use of field-specific correction factors is required.

Direction distributions of neutrons and reference values of the personal dose equivalent in workplace fields.

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 Krümmel (boiling water reactor and transport cask), at Mol (Venus research reactor and fuel facility Belgonucléaire) and at Ringhals (pressurised reactor and transport cask) are presented.

Evaluation of individual dosimetry in mixed neutron and photon radiation fields (EVIDOS). Part II: Conclusions and recommendations.

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.

Overview of active personal dosemeters for individual monitoring in the European Union.

In 2001, the European Radiation Dosimetry Group, EURADOS, started a working group on 'Harmonisation of Individual Monitoring in Europe and the Dissemination of Information on New Techniques in this Field'. Within this group, one of the projects consisted of analysing the status of active personal dosemeters (APDs) in Europe. This paper reviews the regulatory requirements for APDs in 15 EU member states and summarises the main characteristics of commercial and new developments in this field. In particular, it focuses on the comparison of APD performance and standard passive dosimetry systems. Based on this information, an evaluation is initiated to establish why several countries are reluctant to accept APDs for dose records.

Evaluation of individual monitoring in mixed neutron/photon fields: mid-term results from the EVIDOS project.

EVIDOS is an EC sponsored project that aims at an evaluation and improvement of radiation protection dosimetry in mixed neutron/photon fields. This is performed through spectrometric and dosimetric investigations during different measurement campaigns in representative workplaces of the nuclear industry. The performance of routine and, in particular, novel personal dosemeters and survey instruments is tested in selected workplace fields. Reference values for the dose equivalent quantities, H(*)(10) and H(p)(10) and the effective dose E, are determined using different spectrometers that provide the energy distribution of the neutron fluence and using newly developed devices that determine the energy and directional distribution of the neutron fluence. The EVIDOS project has passed the mid-term, and three measurement campaigns have been performed. This paper will give an overview and some new results from the third campaign that was held in Mol (Belgium), around the research reactor VENUS and in the MOX producing plant of Belgonucléaire.

Aspects of harmonisation of individual monitoring for external radiation in Europe: conclusions of a EURADOS action.

Following the publication of the EU Council Directive 96/29, EURADOS coordinated two working groups (WGs) for promoting the process of harmonisation on individual monitoring of occupationally exposed persons in Europe. An overview of the major findings of the second WG is presented. Information on the technical and quality standards and on the accreditation and approval procedures has been compiled. The catalogue of dosimetric services has been updated and extended. An overview of national regulations and standards for protection from radon and other natural sources in workplaces has been made, attempting to combine the results from individual monitoring for external, internal and workplace monitoring. A first status description of the active personal dosemeters, including legislative and technical information, and their implementation has been made. The importance of practical factors on the uncertainty in the dose measurement has been estimated. Even if a big progress has been made towards harmonisation, there is still work to be done.

Active personal dosemeters for individual monitoring and other new developments.

In this paper, we will present a first (but not complete) status description of active personal dosemeters (APDs) and their implementation in European countries. In modern radiation protection practices, APDs are becoming absolutely necessary operational tools for satisfying the ALARA principle. Despite their success, they are relatively new for individual monitoring of workers. Regulation, legal requirements and calibration procedures are different in European member states. A catalogue of commercially available and prototype devices is presented. Improvement on devices and in implementation of calibration method are expected in the forthcoming years. End-user feedback experience and requirements are reported.

Numerical and experimental results of the operational neutron dosemeter 'Saphydose-N'.

Since 1993, the Institute for Radiological Protection and Nuclear Safety (IRSN) has lead, in association with Electricité de France (EDF), a R&D study of a neutron personal electronic dosemeter. This dosemeter, called 'Saphydose-N', is manufactured by the SAPHYMO company. This paper presents first the optimisation of some detector components using Monte Carlo calculations, and second the test of the manufactured Saphydose-N under radiation following the IEC 1323 standard's recommendations for active personal neutron dosemeters. The measurements with the manufactured dosemeter were performed on the one hand at PTB (Physikalisch-Technische Bundesanstalt) in mono-energetic neutron fields and, on the other hand at IRSN in neutron fields generated by a thermal facility (SIGMA), radionuclide ISO sources and a realistic spectrum (CANEL/T400). The manufactured dosemeter Saphydose-N was also tested during measurement campaigns of the European programme EVIDOS ('Evaluation of Individual Dosimetry in Mixed Neutron and Photon Radiation Fields') at different nuclear workplaces. The study showed that Saphydose-N complies with the recommendations of standard IEC 1323 and can be used at any workplace with no previous knowledge of the neutron field characteristics.

Individual neutron monitoring in workplaces with mixed neutron/photon radiation.

EVIDOS ('evaluation of individual dosimetry in mixed neutron and photon radiation fields') is an European Commission (EC)-sponsored project that aims at a significant improvement of radiation protection dosimetry in mixed neutron/photon fields via spectrometric and dosimetric investigations in representative workplaces of the nuclear industry. In particular, new spectrometry methods are developed that provide the energy and direction distribution of the neutron fluence from which the reference dosimetric quantities are derived and compared to the readings of dosemeters. The final results of the project will be a comprehensive set of spectrometric and dosimetric data for the workplaces and an analysis of the performance of dosemeters, including novel electronic dosemeters. This paper gives an overview of the project and focuses on the results from measurements performed in calibration fields with broad energy distributions (simulated workplace fields) and on the first results from workplaces in the nuclear industry, inside a boiling water reactor and around a spent fuel transport cask.

The first operational dosemeter for neutrons which complies with IEC standard 1323.

Individual neutron dosimetry represents one of the current difficulties in the field of radiological protection of workers. Since March 1999, the regulatory requirements in France for active (i.e. operational) dosimetry have been those of ICRP Publication 60, applicable from May 2000, necessitating the introduction of a new generation of neutron dosemeters. Over the last few years, the Institute for Nuclear Safety and Protection has been studying an individual electronic dosemeter for neutrons based on a semiconducting detector, capable of meeting the specifications laid down by a neutron dosimetry work group, including members from all the main players in the French nuclear industry. In 1998, the IPSN began transferring technology to the Saphymo company which, by the end of 2001, will be marketing Saphydose-n, the first individual dosemeter for neutrons which complies with IEC Standard 1323. This dosemeter is of compact design and can assess the individual dose equivalent Hp(10) in mixed neutron and gamma radiation fields. It wil be usable in any nuclear facility without prior knowledge of the average neutron spectrum or of the neutron-gamma ratio. It will be possible to connect the Saphydose-n dosemeter to any of the existing gamma deserter terminals to read the dose data and recharge the batteries.

Study of the photon radiation performance of electronic personal dosemeters.

Following the specifications and test methods given by international standards IEC-61526 and ISO 4037, the dosimetry department of the IPSN studied the photon radiation performance of seven recent electronic personal dosemeters: The personal dosimetric performance of each piece of equipment was tested with X and gamma radiation between 12 keV and 1.25 MeV.

An active personal neutron dosemeter based on microdosimetric principles: CIME.

Over the last few years IPSN has been developing a small, tissue equivalent proportional counter (TEPC) with multielement geometry for personal radiation protection monitoring. This paper presents the last prototype, which is insensitive to microphony, and the experimental results. Numerical modelling results using CERN codes are partly presented and allow an understanding of the nuclear and electrostatic physics involved in a TEPC.