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For the last 10 years different research programs have been carried out in France by the Institute for Radioprotection and Nuclear Safety (IRSN) in order to develop a methodology for predicting areas with a strong potential for radon exhalation at the soil surface. In parallel, several local authorities initiated regional studies based on the geological radon potential in order to define priority areas for radon risk management. The review of these regional initiatives has highlighted the large variability in the selected criteria, the mapping scale, and the methods to present the results on a map.
Currently the French regulation for radon risk management defines 31 priority areas that correspond to administrative subdivisions, where the radon detection is obligatory for several types of public buildings. These areas have been selected on the basis of the radon monitoring campaign in dwellings. The implementation on the field showed that this zoning raises some practical difficulties for local radon risk management.
In order to improve regulatory tools the French Nuclear Safety Authority (ASN) thus asked the IRSN to propose a harmonised methodology to derive a single French map of the geogenic radon potential.
The proposed approach consists in determining the capacity of the geological units to produce radon and to facilitate its transfer to the atmosphere. This method is based on the interpretation of the existing national geological data and their cartographic analysis using a Geographic Information System.
We present the results obtained through the test application of this method to an area located in the region Bourgogne (French Massif Central). This region was selected because of the significant variations of the radon potential displayed over its territory. The results are discussed by comparing the geogenic map with the results of radon measurements in dwellings.
This work firstly allows to propose a classification of the geological formations as a function of their radon source, on the basis of criteria such as lithology, geochemistry (uranium content), petrography. The application of this classification can then lead to a radon source potential map of the rocks.
This initial map is then optimized by taking into account the main additional parameters that control the radon transport through rocks (faults, natural or anthropological cavities etc) and that can increase the radon exhalation potential at the soil surface.
The results are in general in good agreement with the radon measurements in buildings.
The geogenic map obtained leads to a zoning more precise than the scale of the existing priority areas.
The implementation of this methodology over the whole French territory is planned for 2009 in the national plan for the radon risk management approved by the government. This map shall help updating the existing geographical limits of the priority areas, with a precision superior to that currently based on the sole administrative boundaries.
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