Radiation protection aspects in conjunction with re-use of residues from the geoenergy industry

The trend towards increasing circular economy calls for re-use of all types of waste, including recycling from industrial residues such as NORM (Naturally Occurring Radioactive Materials). These secondary raw materials are currently being considered and partly used in construction materials, for critical element extraction, in medicine, and in carbon sequestration. In the geothermal, oil and gas industry mineral deposits (scales) form during deep fluid production due to changes in thermodynamic conditions and may incorporate significant amounts of natural radionuclides. Currently these residues are discarded although the scales may contain valuable metals.

Scale samples were collected from the well and aboveground geothermal facilities and analysed via XRD, XRF and ICP-MS. Their main mineralogical constituents range from Sr-rich barite, laurionite, native copper, sulphide minerals to magnetite. Some scales contain economically viable elements, such as Cu, Ba, As and Zn. Rare earth elements also occur in minor amounts (∑ ~ 50 ppm). Radionuclide activities on bulk samples were measured via gamma-spectrometry and vary from below 1 Bq/g up to 130 Bq/g, 57 Bq/g, 63 Bq/g and 40 Bq/g for Ra-226, Pb-210, Ra-228 and Th-228, respectively. Thus, for some samples the measured activities fall within the category of surveillance.

The association between mineralogy and radionuclides is investigated following partial leaching and sequential extraction. Kind and activity concentrations of radionuclides depend on the extraction method, which is determined by the mineral phase of the targeted element. The extraction process will require dose measurements followed by a calculation of the exposure to ionizing radiation. This evaluation represents a preliminary study towards the usability of the scales and demonstrates the need for further research on recycling of the residues, with due consideration given to radiation protection aspects.