The event described here can be limited to specific settings and might not be encountered in every geothermal projects. More details about this topic are presented in a report that can be found on GEOENVI website.
In the infrequent case of abundant circulation and drilling in natural radioactive rocks, like granite for example, there is a potential for radioactive contamination at surface. The natural radioactivity of geothermal fluid is usually very low and can’t be detected above the ambient radioactivity, but cuttings from well drilling and deposits (scale) that develop within casing and surface equipment can be significantly more radioactive (Scheiber et al., 2012). In some, restricted sites, where the radioactivity level appears above the natural level due to specific geological conditions and to the accumulation of NORM (Naturally Occurring Radioactive Material) in the surface installations, mitigation measures are required to protect population and environment.
The geothermal fluid is in equilibrium with the reservoir and may be enriched in radionuclides by water-rocks interaction, in the case of natural radioactive rocks (Eggeling et al, 2013). Scales present at surface are formed by deposition of minerals present in the brine due to physico-chemical changes at surface (such as temperature or pressure) in comparison to reservoir conditions. These scales may trap radioactive elements present in the brine. Thus, radioactivity levels in geothermal facilities depend on both the geological and industrial contexts. The presence of scales not only affects the industrial capacity, inducing external additional costs, but also represents a potential hazard for the surrounding environment, workers and local population. Consequently, radioactivity deserves to be correctly monitored, but most importantly avoided thank to the application of specific treatment.
As for any planned, existing or emergency exposure situation, which involves a risk from exposure to ionizing radiation, geothermal operations dealing with radioactive materials must be compliant to the European regulation and its national applications, which establish thresholds for dynamic and cumulative radioactive doses, and safety standards. These are enforced by the environmental impact assessment procedure for permission to operate a geothermal plant, which prescribes provisions for management of radioactivity, usually based on the worst-case scenario.
Monitoring and prevention measures are applied only in those few cases involving radioactive material. Radioactivity is an impact from geothermal operations, but due to the type of involved radionuclides, types of emissions, and generally low level of radiation, the inherent risk can be easily minimized using appropriate working procedures.
The table here below provides an overview of this event in term of risk and impact assessment i.e. its causes, consequences, the phases concerned, the influencing context or the principals monitoring and mitigation measures that can be adopted.
