Why map the radon potential?
Radon is present everywhere in the territory and its concentration in buildings is highly variable: from a few becquerels per cubic meter (Bq.m-3) to several thousand becquerels per cubic meter.
Among the factors influencing the concentration levels measured in buildings, geology, in particular the uranium content of the underlying ground, is one of the most determining. It determines the radon potential of geological formations: in a given geographical area, the greater the potential, the greater the probability of the presence of radon at high levels in buildings. In certain sectors, the existence of particular characteristics of the subsoil (faults, mining works, hydrothermal sources) can constitute an aggravating factor by facilitating the conditions for the transfer of radon to the surface and thus lead to a local modification of the potential.
Knowledge of the characteristics of the geological formations on the territory thus makes it possible to draw up a map of the zones in which the presence of radon at high concentrations in buildings is most probable. This work was carried out by IRSN at the request of the Nuclear Safety Authority and made it possible to establish a map of the radon potential of geological formations in mainland France and overseas.
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The mapping of the radon potential of geological formations established by IRSN leads to the classification of municipalities into 3 categories:
Category 1 (yellow)
Municipalities with category 1 radon potential are those located in geological formations with the lowest uranium content. These formations correspond in particular to the limestone, sandy and clayey formations that make up the large sedimentary basins (Paris basin, Aquitaine basin) and basaltic volcanic formations (Massif Central, French Polynesia, West Indies, etc.).
On these formations, a large majority of buildings have low radon concentrations. The results of the national measurement campaign in metropolitan France thus show that only 20% of buildings exceed 100 Bq.m-3 and less than 2% exceed 300 Bq.m-3.
Category 2 (yellow/orange)
Municipalities with category 2 radon potential are those located on geological formations with low uranium contents but on which specific geological factors can facilitate the transfer of radon to buildings.
The municipalities concerned are in particular those intersected by major faults or whose subsoil shelters underground mining works… These particular geological conditions can locally facilitate the transport of radon from the rock to the surface of the ground and thus increase the probability of high concentrations in buildings.
Category 3 (orange)
Municipalities with category 3 radon potential are those which, over at least part of their area, have geological formations whose uranium content is estimated to be higher compared to other formations. The formations concerned are in particular those constituting granite massifs (Armorican Massif, Central Massif, French Guiana, etc.), certain volcanic formations (Central Massif, French Polynesia, Mayotte, etc.) but also certain sandstones and black shales.
On these formations richer in uranium, the proportion of buildings with high radon concentrations is greater than in the rest of the territory. The results of the national measurement campaign in metropolitan France thus show that more than 40% of the buildings located on these sites exceed 100 Bq.m-3 and more than 10% exceed 300 Bq.m-3.
Note: in the case of municipalities with a large surface area – as is the case in particular for certain Overseas Territories -, the formations concerned sometimes only occupy a limited proportion of the municipal territory. In this case, the mapping by municipality does not represent the actual area of a territory affected by a radon potential but, in a way, the probability that there is a source of exposure to radon on the territory of a municipality. high, even very localized. In order to visualize different zones within the municipal territory and to better appreciate the real radon potential on this territory, it is advisable to refer to the cartography represented according to the contours of the geological formations.
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