In case of an accident concerning a nuclear installation, two intervention phases are distinguished: an emergency phase which calls for a rapid and organized response through intervention plans, and a post-accidental phase in which postponed actions are carried out on medium and/or long-term so that the situation comes back to a state judged as acceptable by stakeholders. The PRIME project has developed a decision aiding tool for risk managers involved in an industrial accident involving radioactive substances, through the evaluation of radio-ecological sensitivity of a territory in a post-accidental phase. The proposed decision aiding tool is grounded on the integration of Multiple Criteria Decision Aid (MCDA) and a Geographical Information System (GIS). The proposed methodology relies on the concept of decision map which corresponds to a planar subdivision of the territory in which each subdivision is evaluated on the basis of several criterion maps. This results in a set of disjoint spatial units evaluated on an ordinal scale using the ELECTRE TRI method. Hence, the result is a decision map representing the radio-ecological sensitivity of the territory; such maps prove to be very useful for stakeholders to design relevant post-accidental strategies.