Evolution of the Natural Radioactivity on the Soultz-sous-Forêt EGS Power Plant and Implication for Radiation Protection

Abstract

The Soultz-sous-Forêts EGS power plant exploits a reservoir hosted in fractured granitic rocks, which naturally contain radionuclides. The circulation of geothermal fluid can leach some radionuclides and bring them to the surface, where radioactivity may happen: this implies to set up radiation protection procedures in accordance to the nuclear safety regulation. As asked by ASN (French national agency for nuclear safety), several monitoring surveys have been carried out since 2009 to follow the evolution of natural radioactivity on the geothermal installation and to better understand the chemical and physical mechanisms leading to the presence of radioactivity. Hence around 400 contact and ambient dose rate measurements were regularly sampled on both GPK1 and GPK2 platforms, allowing a precise monitoring and mapping of the evolution of radioactivity. For all surveys, results show a general increase of the dose rate values as a function of circulated volume. Moreover, the highest values are mostly found on the reinjection line: here the lower fluid temperature (~70°C) tends to induce the precipitation of sulfates (solid solutions between barite, BaSO4, and Celestine, SrSO4) and sulfides (Galena, PbS), which are able to trap radionuclides (mainly 226Ra for sulfates and 210Pb for Galena) during their formation. The presence of radioactivity on surface installation requires setting up radiation protection procedure for workers and public, as asked by the French nuclear safety regulation. A protected zone and a controlled zone have been defined around the installation, with specific access authorization. Adapted protection equipments must be worn by workers especially for operation where they may be in direct contact with radioactive material (heat exchanger and filter cleaning, pipe dismantling). Finally a procedure has also been established for the disposal and removal of radioactive waste (mainly residues from filters) in agreement with the French regulation.