Blanket and first wall conceptual design issues of the spherical tokamak power plant

Abstract

This paper presents aspects of the conceptual design of the vessel and in-vessel components of the spherical tokamak power plant. Some of the critical parameters for power plant operation and design of the first wall, blanket and shield, are the sufficient production of tritium needed for sustaining the fusion reaction, the neutron and photon heating rates of the various components and the displacement damage of the structural material. The concept under consideration is of the double null configuration, operating at 4.0 GW of fusion power, with an aspect ratio of 1.4, and a wall loading at the midplane of 5 MW m −2. The structural material is assumed to be 9% Cr–martensitic stainless steel, and the coolant helium gas, with the exception of the copper alloy central column which is pressurised water cooled and uses vanadium alloy as the structural material. The neutron multiplier and tritium generating materials are varied to optimise tritium production, the preferred option being a combination of beryllium and lithium oxide. Taking into consideration the relatively severe thermal and neutron induced loading in the spherical tokamak, the main features of this design are the first wall structure which is independent of the blanket, and the radial ribs that occupy all the space under the toroidal field coils. Detailed neutronics calculations have been carried out, in both one (radial, infinite cylinders) and two (radial-poloidal, toroidally axisymmetric) dimensions, to obtain the neutron spectrum, heating rates, tritium generation and displacement damage at various locations around the machine. In one-dimensional calculations the tritium generation ratio values are in the range of 1–1.5 depending on several factors, such as the first wall thickness and multiplier to tritium generating materials ratio in the blanket. The two-dimensional calculations, with a mixture ratio of beryllium to lithium oxide 4 : 1, result in a tritium generation ratio of 1.34, which is within the design target. Of importance are the materials used as a shield for the central column. Various shields have been analysed and one concept that gave good results was the combination of tungsten–carbide, and vanadium alloy with helium cooling. The results of neutronic calculations for the displacement damage give a maximum dose at the first wall of 55 dpa year −1 for the martensitic steel and 2.1 dpa year −1 for the outer layer of the copper alloy central column at the midplane. These dose values give in-service lifetimes within the minimum design target of 2 years for the structural material and 6 years for the copper alloy in the central column.