[en] A design study for medical reactor for Boron Neutron Capture Therapy (BNCT) has been carried out since 1988, with the cooperation of Kobe Univ. and Mitsubishi Heavy Industries Ltd. The intention of this study is to investigate reactor concepts suitable for BNCT and to decide the major specifications of the reactor which is to be used exclusively for the treatment of cancer and for biomedical research. At present, clinical treatment with BNCT is being carried out only in Japan and good results have been achieved by single radiation regimen using currently available research reactor such as the Musashi Reactor, KUR and JRR-2. However these research reactors are not convenient for the treatment because of the difficulty in scheduling due to other researcher uses. On the other hand, in Europe, USA and Australia, BNCT by epithermal neutrons is being studied because this method makes it possible to treat various deep-seated cancers without surgery. In this case, it is important and necessary to sufficiently suppress the fast neutron contamination in the therapeutic neutron beam. Considering these points, the basic design requirements for the medical reactor have been set as follows in this study. (1) The reactor is to be used exclusively for medical purpose. (2) The reactor is to be capable of treating or diagnosing 50,000 patients during its design life without refueling. (3) The thermal power is to be less than 2MW. (4) The core is to be cooled by natural convection. (5) Separate thermal and epithermal neutron beam holes are to be provided. (6) No heavy water is to be used in the core. (7) Proven technologies are to be utilized. The design concept of the reactor facilities has been defined successfully using semitight hexagonal lattice with low enrichment UO₂ fuel rods of the type used incurrent PWRs and a pool type design. (author)