Release of materials and buildings

For many countries, clearance or the release of materials and buildings from regulatory requirements has become an indispensable part of the material management in the nuclear field. Current efforts appear to be directed largely towards refinement of processes and techniques to enhance applicability than on the development of totally new approaches. Recommended clearance values for specific radionuclides have been published during the last decade, notably by the IAEA and the European Commission and, although many countries have based heir national standards at least in part on these recommendations, unanimous international criteria have yet to be adopted. The NEA Working Party on Dismantling and Decommissioning (WPDD) has recently completed a study on the release of materials and buildings used on nuclear installations, with the aim of providing up-to-date information on current practices in participating countries. It is anticipated that the report from this study will be published during 2008. The report suggests that clearance has developed to a very mature concept allowing the swift and safe determination of compliance with clearance criteria on nearly all materials (metals, building rubble, cables, plastics etc.) and building surfaces. However, there are differences in how clearance is addressed in national regulatory frameworks and in how it is implemented at various decommissioning projects. The radiological concept for clearance is defined by the IAEA as the removal of radioactive materials or objects within authorised practices from any further regulatory control. It is generally based on the concept of triviality, e.g. with the aim of constraining individual doses resulting from cleared material to a range on the order of 10 μSv/a. There are a number of possible clearance options for unconditional clearance as well as for clearance for a specific purpose. International guidance on these issues is, for example, available from the IAEA and the European Commission, and numerous regulations exist in various countries. For key nuclides like Co 60 and Cs 137 the recommended levels in the international guidance documents for unconditional clearance fall within one order of magnitude, while the scattering of values can be much larger mainly for radionuclides of minor radiological significance. The implementation of the clearance process is generally complex and requires good planning in order to avoid high costs in the implementation phase. The variety of measurement devices suitable for the initial characterisation as well as for the measurements required to show compliance with clearance levels is large, comprising: surface contamination monitors, in situ gamma spectrometers and bulk monitors as the most commonly applied measurement techniques, as well as dose rate meters, extraction of material samples with laboratory analysis, wipe tests and other auxiliary techniques mainly used during the characterisation phase. (author)