Development of the spherical tokamak power plant

The design of a power plant based on the spherical tokamak (ST) is being developed in order to explore the potential advantages of the ST for power production. The ST power plant operates at a low aspect ratio of typically 1.4–1.6, which allows operation at high beta. This offers stability at high elongation which permits operation at high bootstrap current fraction (>95%), leaving only a modest external current drive requirement. This high elongation also results in a relatively uniform neutron wall loading giving the first wall components a longer life before replacement is required. The ST can be readily maintained on a timescale commensurate with this material life since its simple and robust centre column can be easily removed by lowering it into a hot cell beneath the load assembly and replaced with a new or reconditioned one. This is possible because it does not require inboard breeder blankets to maintain its tritium economy, or a substantial inboard shield since the centre column consists of a water-cooled copper centre rod surrounded by a relatively thin steel neutron shield. The plasma is operated in a double null configuration, forming both an upper and lower divertor. In order to accommodate the high erosion rates and heat fluxes developed in the divertors a novel system based on the use of a cascading flow of silicon carbide pebbles has been evaluated. The layout of the main elements of the power plant has been developed with revised power flows giving the steam cycle a thermal efficiency of about 43%.