[en] This report summarizes the research and development activities in the Department of Nuclear Energy System during the fiscal year of 1998 (April 1, 1998 - March 31, 1999). The Department has been organized from April 1998. The main research activity is aimed to build the basis of the development of future nuclear energy system. The research activities of the fiscal year cover basic nuclear and atomic and molecular data evaluation, conceptual design of reduced-moderation water reactor, development of reactor analysis code, reactor physics study on fast neutron system, control and sensing technology development for nuclear reactor, experiment and analysis of thermal-hydrodynamics, development of advanced material for reactor, lifetime reliability assessment on structural material for advanced reactor, development of advanced nuclear fuel, design of marine reactor and the research for nuclear ship system. The maintenance and operation of reactor engineering facilities belonging to the Department are undertaken. The activities of the research committee to which the Department takes a role of secretariat are also summarized in this report. The 98 papers are indexed individually. (J.P.N.)