Optimization of a portable hollow-fiber-based device for extracting radiokrypton dissolved in deep groundwater and selection of 222Rn as an indicator of Kr extraction efficiency

Groundwater dating information is important for safety assessment for geological disposal of high-level radioactive waste. Radiokrypton (Kr-81) is an excellent tracer for dating old groundwater with a residence time up to 1 Ma. We have optimized a hollow-fiber-based device for extracting Kr-81 from deep groundwater with low hydraulic conductivity for the purpose of estimating its residence time. Such a device must show a high Kr extraction efficiency and an ultra-low level of contamination from modern air and must allow for in situ evaluation of Kr extraction efficiency. The optimal devices reported herein showed minimal contamination by Kr-81 from modern air, and the efficiency of Kr extraction from high-salinity groundwater exceeded 90%. Furthermore, we confirmed that the Rn-222 was a suitable indicator of Kr extraction efficiency.