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核分裂が発見されて60年近くになり,核分裂の連鎖反応は原子炉として実用化して人類に役立っているが,励起された重い原子核が大きく変形し, 2つの核分裂片へと分離していく核分裂過程は,現在でも十分に解明されていない。これは,物理的に興味ある課題であるとともに,核燃料サイクルの基礎データとしても重要である。最近,理論と実験の両面で核分裂過程の研究が進展しつつある。本稿では,まずBrosaらが提唱したマルチチャンネル(モード)核分裂過程のモデルを簡単に紹介し,続いて核分裂過程における多くの物理量を同時計測する実験的研究について,主として筆者らの方法と結果およびBrosaらのモデルによる計算との比較を示す。

For promotion of nuclear science and engineering research, examinations were made on the possibilities and necessary measures to extend joint research at international level. The present article is a summary of the reports of investigations performed during FY 1986 through 1991 by the Special Committee of the AESJ for Feasibility Study on International Cooperation for Promotion of Nuclear Science and Engineering Research, under contract with Science and Technology Agency of Japan. Background information was collected on the present status of scientific research facilities in US, European and Asian countries on one hand, and on the expectations and prospects of Japanese scientists on the other hand. Based on the analysis of these data, some measures necessary to expand the international cooperation were proposed. It was emphasized that international joint research on a reciprocal basis would be effective in order to strengthen the technological basis of peaceful uses of nuclear energy. Problems to be solved for the new development were also discussed.

Introduced is the activities of the Joint Research Project Team on Thorium Fuel organized by mainly university researchers in Japan and supported by the Ministry of Education, Science and Culture for seven years since 1980. Four major groups were organized; (1) nuclear data, reactor physics and design, (2) nuclear fuel, (3) down stream and (4) biological effects of thorium.The first group covered measurements and analysis on nuclear data of thorium related nuclides, experiment and analysis on nuclear characteristics of thorium containing cores, basic engineering on a thorium molten salt reactor, and designs of several types of reactors. Fabrication and irradiation tests of thorium oxide fuel, and basic studies on new type thorium fuels (e.g. carbide and nitride) were studied by the second group. The third group covered the use of solutions in reprocessing of spent fuel, behavior of fission products, immobilization of high level radioactive waste, and continuous reprocessing for a molten salt reactor. The fourth group performed the trace study for patients who had been intravascularly injected with thorotrast for diagnosis of war injuries during the Second World War.