Japan Geoscience Union Meeting 2023

Presentation information

[J] Online Poster

H (Human Geosciences ) » H-CG Complex & General

[H-CG21] Nuclear Energy and Geoscience

Fri. May 26, 2023 1:45 PM - 3:15 PM Online Poster Zoom Room (3) (Online Poster)

convener:Shinji Takeuchi(Department of Earth and Environmental Sciences, College of Humanities & Sciences, Nihon University), Takaomi Hamada(Central Research Institute of Electric Power Industry), Eiji Sasao(Tono Geoscience Center, Japan Atomic Energy Agency)

On-site poster schedule(2023/5/25 17:15-18:45)

1:45 PM - 3:15 PM

[HCG21-P05] Verification study on borehole investigation technologies (6) Quality control and assurance for laboratory tests

*Toshiyuki Matsuoka1, Hikaru Nishio1, Junichi Goto1, Stratis Vomvoris2, Armin Pechstein2, Andreas Reinicke2 (1.Nuclear Waste Management Organization of Japan, 2.National Cooperative for the Disposal of Radioactive Waste)

Keywords:Radioactive waste, Geological disposal, Borehole investigation, Laboratory test

1. Introduction
NUMO and Central Research Institute of Electric Power Industry (CRIEPI) have been conducting a collaboration research project (Verification study) in the CRIEPI Yokosuka site to optimise the applicability of existing investigation and evaluation techniques for siting a deep geological repository for high-level radioactive waste etc. Various data were obtained from deep boreholes studies (YDP-1 to YDP-3) in sedimentary rocks of the Neogene accretionary complex (e.g. Hayama Group), and it was confirmed that geological environment models could be constructed and updated. On the other hand, issues related to optimisation of drilling techniques (for reduction of core loss or borehole instabilities in the fragile and swelling Hayama group) and development of quality control and quality assurance techniques for core laboratory testing were recognised. To resolve these issues, a deep borehole study (YDP-4) was carried out in the Yokosuka site between 2019 and 2021. This report describes the results of a collaboration research with the National Cooperative for the Disposal of Radioactive Waste (Nagra) on laboratory tests on rock mechanical and hydraulic properties using core samples obtained from YDP-4 borehole study. Specifically, the laboratory tests applied by Nagra in deep borehole investigations for site selection were compared with those carried out in the verification study, and the technical knowledge was expanded to optimise the laboratory test procedures and quality control and quality assurance methods.

2. Main results
The selection criteria of core samples to be used in these laboratory tests were "selection of core samples representative of each lithology (no sampling at regular intervals without consideration of actual geological conditions)", "homogeneous in terms of grain size, minerals, sedimentary structure, and other petrological aspects" and "no fractures" – criteria used both by Nagra and NUMO. From the aspect of quality control and assurance, in order to avoid drying and release of gas that occur during the recovery of drilling cores and to maintain cores in a state close to the in-situ state, Nagra's deep borehole investigation program has developed a core sampling plan, in which the priorities for each laboratory test are clarified in advance and a system was established to enable rapid decisions according to the state of cores recovered on site. By implementing the core sampling plan, Nagra minimized the time from core recovery to core sample selection and core preparation and protection. In addition, Nagra utilizes medical X-ray CT images to evaluate the integrity of the core samples taken and to determine the location and quantity of specimens (sub-cores) that can be taken, thereby ensuring the quality of the specimens to be tested. Furthermore, the gas permeability tests performed in the verification study using core samples after drying for water content estimation indicated that an irreversible deformation of the samples might be induced due to oven drying.

3. Conclusion
In order to expand technical knowledge for optimising laboratory testing procedures and quality control and assurance methods, a collaboration research with Nagra was carried out. Laboratory tests for rock mechanical and hydraulic properties were compared between the laboratory tests applied by Nagra in the deep borehole investigations for site selection and those carried out in the verification study, using core samples obtained from YDP-4 borehole. As a result, technical knowledges were obtained on methods to shorten the time elapsed for the selection and storage of core samples, quality control of test specimens using X-ray CT images, and test procedures to ensure high data quality.