The Atomic Energy Commission of Japan formulated the 'Long-term Program for Research, Development and Utilization of Nuclear Energy' ¹⁾ (henceforth 'Long-term Program') in June, 1994, which laid down the basic policy stipulating that 'scientific researches on deep geological environment as the basis for research and development of geological disposal should be forwarded'. The Japan Nuclear Cycle Development Institute (henceforth 'JNC') has been promoting the scientific researches on deep geological environment under the name of 'geoscientific researches'. The Long-term Program gave the following positions to the underground research facility to emphasize the importance.

Deep underground research facilities play an important role in research relating to geological disposal. They allow us to understand the characteristics of the geological environment and to improve the reliability of the models used for performance assessment of disposal system. They also provide opportunities for comprehensive research that will contribute to overall understanding of Japan's deep geological environment.

It is recommended that more than one facility should be constructed, considering the range of characteristics and features of Japan's geology and other relevant factors.

It is important to plan underground research facilities on the basis of results obtained from research and development work already carried out, particularly the results of scientific studies of the deep geological environment. Such a plan for underground research facilitied should be clearly separated from the development of an actual repository.

Based on the basic policy, the Mizunami Underground Research Laboratory (henceforth 'MIU') Project commenced. This report is to summarize the results of geoscientific researches carried out from 1996 FY to 1999 FY (henceforth 'Phase I-a', See Chapters 2 and 3.2) of the MIU Project. The researches are carried out following the 'Master Plan of Mizunami Underground Research Laboratory' (henceforth 'Master Plan')²⁾ and the annual plan of geoscientific researches^{3), 4), 5), 6), 7)} of the MIU Project.

The Long-term Program, which was revised in November, 2000, stipulated as follows.
Based on the results of past researches, JNC should steadily carry on researches and development activities to verify the reliability of geological disposal technologies and to establish a safety assessment method, using research facilities for deep geological environment. The research facility for deep geological environment will serve not only as a place for scientific investigation, but also as a place for deepening public understanding of researches and development activities related to the geological disposal of nuclear waste.