The possibility of developing a cost competitive fusion power plant with a water-cooled blanket concept, which has much experience in nuclear power plants, was examined. The new blanket design is based on using reduced activation ferritic steel components and an advanced super-heated steam cycle which is used to realize high thermal efficiency. The high value of thermal efficiency is very effective in reducing the cost of electricity.

The allowable temperature range of the structure material, reduced activation ferritic steel, is assumed to be 350K to 900K based on expectations from the material research and development program. A mixture of lithium oxide pebbles and beryllium pebbles is installed in the breeding zone for high tritium breeding ratio and high thermal conductivity. Mixture ratio of beryllium and lithium-6 enrichment were optimized from the viewpoint of temperature distribution in the breeding zone, achievable tritium breeding ratio and its reduction due to burn up. The reference blanket system has a local tritium breeding ratio of 1.37. The arrangement of cooling channels in the breeding zones and flow rate and inlet temperature of the coolant were also optimized to keep the temperatures of structure materials, breeding materials and coolant in the allowable range. The first wall is cooled by pressurized water at about 570 K. The coolant out of the first wall is led to the breeding zone and starts to boil. The steam is super-heated up to 750 K in the blanket. This high temperature raises the thermal efficiency of the turbine to 41 %.