Futher enhancement of nuclear reactor safty and safe and steady management of the radioactive wastes are most important issues for sustainable progress in nuclear energy. We are studying on fission product behavior in severe accident of light water reactor for improvement of source term evaluation technology. We are studying on nuclear fuels for transmutation of long-lived actinides (so called "minor actinides") included in spent nuclear fuels in the aspect of fabrication technologies, material properties, irradiation behavior, and resource recycling. Our research activities cover wide range of issues concerning high temperature science on fuel and fission product materials, represented by chracteristics of "fuel debris" for decommissioning of Fukushima Daiichi Nuclear Power Station.