Design study of helium-solid suspension cooled blanket and divertor plate for a tokamak power reactor

Abstract

Abstract High temperature helium gas will provide a thermal efficiency greater than 40%, even using a conventional steam turbine. In addition, helium gas will not react chemically with blanket materials, and surrounding air and water. Tritium will be easily extracted from the coolant too. Thus, it is one of the most attractive coolants for a fusion power reactor, from the economical, safety and environmental points of view. However, a large volumetric flow rate is required owing to the small heat capacity when the pressure is not extremely high. This requires a larger reactor size, larger circulating power and more penetrations, which may increase radiation streaming. A relatively low heat transfer coefficient makes it difficult to apply to a component subjected to a high heat load, such as a divertor plate. We suggest in this study, a helium-solid suspension flow as coolant, to increase the heat capacity and heat transfer coefficient. A gas pressure of 5 MPa, and inlet and outlet temperatures of 400°C and 700°C were chosen. There are few candidates for the structural material which can be used at temperatures higher than 900°C. We have proposed an intermetallic compound of titanium aluminide (TiAl) as candidate structural material of the blanket. Although the database for TiA1 is incomplete, it has high strength and ductility at the operation temperature. Using TiA1, radioactive waste management will be mitigated, since its activity will decrease rapidly. The elongation at room temperature, which is only a few per cent, will be improved through research and development not only in fusion but also in other industrial fields. In this study, spherical pebbles of lithium oxide and small blocks of beryllium were chosen as the breeder and neutron multiplier. Manganese blocks were installed to enhance energy multiplication. A tritium breeding ratio of 1.38 and energy multiplication ratio of 1.35 were obtained with the blanket. The net plant efficiency exceeds 40%, including the circulating power. The peak surface heat flux on the divertor plate was decreased with gas puffing including an impurity near the striking point. In spite of this, the expected peak heat flux was still several MW m −2 from the result of numerical calculation of the edge plasma. To remove so high a heat load, an impinging helium-solid suspension jet was applied to the high heat flux region of the divertor plate. Molybdenum alloy was used as the structural material in this region to keep the surface temperature and thermal stress lower than the design limits. TiAl was used as the structural material of other regions in the divertor chamber. To prevent excessive sputtering erosion, the electron temperature in the divertor plasma must be kept lower than 20 eV.