Design Concept of 4S-50MWe(Super Safe, Small and Simple Fast Reactor).

This report describes current status of the design of a 4S (Super Safe, Small and Simple Fast Reactor). The 4S is a fast reactor generating 50MWe electric power, which pursues simple operation system, less maintenance service, higher safety and improved economic features. To satisfy these special characteristics, a new plant concept is produced, where adoption of metallic fuel core and burnup control by annular reflector can ensure negative sodium void reactivity coefficient for the reactor core. The power is controled by the turbine system (water-steam system) alone without any control rod. Complicated fuel handling system and driving mechanism of control rod are eliminated. And the activity of operating service is also eliminated by the application of passive system to the reactor design. The 4S has a substantial safety and is a fast reactor in which fuel is confined for a long time of 10yr as the refueling interval, to realize high proliferation resistance of nuclear material.