HLW site characterization parameters extracted from geophysical well logging data acquired in deep boreholes in Korea

Abstract

To select a suitable site for high-level radioactive waste (HLW) disposal, it is very critical to establish site assessment factors that fit the geological conditions of each country, and suitability of a site as a natural barrier of the disposal system should be evaluated and candidate sites should be narrowed down step by step based on these criteria. Site assessment factors can be largely classified into geological, rock mechanical, hydrogeological, and geochemical properties of the deep geo-environment. In Rep. of Korea, 17 items with 98 assessment factors for site assessment factors are suggested. During the period from 2020 to 2024, eight boreholes were excavated to depths of 750 meters in order to increase understanding of the deep geological environment of Korea. The locations for deep boreholes were selected in the area of Korea's four representative major bedrock types and tectonic structures. Drilling cores were recovered from all boreholes to analyze bedrock geology, and rock mechanical properties, and hydrogeological and hyrdo-geochemical pumping and injection tests were carried out to verify general features of site assessment factors of representative bed rocks found in Korea. Geophysical logging methods are a powerful tool in that one can obtain deep geo-envrionmental properties in a dense manner along the borehole walls and obtaining various site mechanical properties by correlating geophysical well logs with laboratory test results. Geophysical logs generally need recompilation and conversion to acquire suitable site assessment factors, such as dynamic elastic properties of the bedrocks, hydraulic parameters of bedrocks. In this paper, we present systematic method and relevant procedures using geophysical logging data to obtain deep geological environment assessment factors essential for the site selection and characterization for HLW disposal sites. The geophysical logging data are basically carried out through 3-Arm caliper logging; temperature, EC(electrical conductivity) of borehole water, and natural gamma logging; gamma-gamma density logging with dual spacings together with borehole diameter logs; normal resistivity logging together with SP(spontaneous potential), single point resistance, spectral gamma logging (K, U, Th), acoustic televiewer(ATV), and sonic logging with three spacing between a source and 3 receivers. The fracture information are analyzed from ATV amplitude and traveltime to be used to determine data sampling information for hydraulic tests and hydrogeochemical experiments, and p- and s-wave velocities obtained from sonic logging were used to obtain rock mechanical properties for site assessment. The characteristics of the fracture zone shows general trends which are of horizontal in mud stone boreholes and of high-angle dip in crystalline rocks, and shows a clear contrast in hydraulic experiments and hydrogeochemical experiments. All of the above results are merged into the database, D-GIVES(Deep-GeoInformation VErification System) for providing deep geo-environment information on site assessment evaluation, and this is also expected to be used as parameters which are necessary in predicting deep geological environment for long-term safety evaluation.