A radiometric and elemental characterisation of sludge and scales found in Dutch geothermal plants

Abstract

Geothermal energy originates from the heat stored within the Earth's core. Thermal water from geological reservoirs is extracted to the surface to generate electricity or heat buildings and greenhouses. This thermal water contains various dissolved solids that can deposit, precipitate, and form scales in the installations as the water is brought to the surface. The dissolved solids contain primordial radionuclides. Workers are exposed to these radionuclides while maintaining and cleaning geothermal plants. This study complements the limited research available on naturally occurring radionuclides in geothermal installations by examining their presence in various facilities across the Netherlands. Sludge and scale samples were provided from five different surface installations. The samples were analysed using radiometric methods (alpha-particle and gamma-ray spectrometry) and a non-radiometric method (energy dispersive X-ray fluorescence, EDXRF). EDXRF identified several hazardous elements, such as Lead, Arsenic, Chromium, and Thallium. In the scales, radiometric analysis detected three main radionuclides: 228Th, 210Pb, and 210Po. Furthermore, for both scales and sludges, a notable disruption in the equilibrium was observed in parts of the natural decay chains 232Th-228Ra-228Th and 226Ra-210Pb-210Po, complicating the correct assessment of the massic activity of the radionuclides and the appropriate disposal of the materials. These findings highlight the importance of mapping Naturally Occurring Radioactive Materials (NORM) during geothermal plant maintenance and accurately assessing their massic activity.