Investigation of sub-meter shields for a low aspect ratio D-T Tokamak fusion reactor

Abstract

A significant effort is being made by fusion researchers to minimize the total size of magnetic fusion devices on the path toward developing fusion energy. The spherical tokamak, which has a very low aspect ratio, is the most promising of the compact magnetic fusion reactor designs. This compactness imposes a severe material constraint on the design, as a highly compact device will have very thin inner shielding. This inner shielding, which in traditional designs is required to be around 1 meter thick, acts to protect the central solenoid and return toroidal field coil legs from material damage and nuclear heating resulting from high neutron fluxes. The use of a sub-meter inner shield creates potential for the design of a proof of principle magnetic fusion device, sacrificing the central component materials for a demonstration of temporary fusion power production. The nuclear heating of thin shields (~ 0.1 - 0.2m) of various compositions was explored using the Monte Carlo N-Particle (MCNP) transport code. The principal finding was that nuclear heating is the largest concern to the central inboard components. Nuclear heating of these sensitive materials was found to be minimized by the use of a magnesium borohydride blanket with a tungsten first wall. The resulting nuclear heating density for a 100MW, R=1m D-T tokamak employing 0.1 - 0.2m shields is shown to have the potential to threaten the ability of such a device to sustain net electricity.