Radionuclide Release in Geothermal Aquifers – the Role of Alpha Recoil

Abstract

Highly saline thermal waters from hydrothermal and petrothermal geothermal plants contain enhanced levels of radionuclides, mainly the Radium isotopes Ra-226, Ra-228 and Ra-224. This radioactivity content causes specific activities of precipitated scales in the order of up to some 100 Bq/g and complicates the disposal of these radioactive wastes. To establish a tool for the prediction of radionuclide concentration in fluids of planned deep geothermal plants, the origin of Radium isotopes was investigate thoroughly. A main candidate for the release mechanism at the solid/fluid interface is the recoil of nuclei as consequence of an alpha decay in the mineral. Monte Carlo simulations of alpha recoils were performed to understand the role of characteristic parameters like U/ Th content, specific surface and porosity of the aquifer rock. They allowed to reconstruct the distribution of members of the natural decay series in the near-surface region of mineral grains and to estimate the Radium concentration in the fluid as well as the expected Radium isotope ratios. The results show that alpha recoil processes satisfactorily explain the radium concentration and isotope ratios in the fluid. The model was tested on several real geothermal aquifers in Germany, namely in the North German Basin and in the Upper Rhine Valley. Typical aquifer rocks of hydrothermal and EGS systems were sampled from drilling cores and from analogue outcrops. The combination of rock and fluid analyses together with the outcomes of the Monte Carlo simulations allowed a validation of the alpha recoil model and gave some insight into the fluid transport within the aquifer systems.