Heat transfer analysis of a first wall radiation shield for a fusion reactor

The analysis is based on a graphite liner interposed between the plasma and the first wall (vacuum wall). A radiation boundary condition is assumed at the rear face or the shield, with no other mode of heat transfer at that boundary being considered. The first wall is assumed to be composed of stainless steel and is held at a constant temperature of 5400C. Neutronics calculations, using ANISN, were performed for the base case of 1 MW/m2 surface flux. Volumetric heat generation rates were then scaled as a linear function of wall loading. A transient finite difference scheme was used which accounts for heat generation, heat loss by convection, conduction and/or radiation at each node, and physical property variations with temperature. Simplified one-dimensional geometry was assumed throughout. The interposition of the liner significantly altered both the heat transfer environment and neutron flux at the first wall. The reduction in neutron density requires the addition of beryllium as a neutron multiplier to attain a tritium breeding ratio of one. Since the addition of the liner alone significantly altered the neutronics, the incorporation of small amounts of internal cooling will not further aggravate the problem. The particular design investigated allowed molten tin (melting point 2300C, boiling point 22700C) to flow parallel to the toroidal circumference. As an alternative, LiF--BeF2 (flibe) was used as a coolant. The addition of coolants does not significantly alter the neutronic performance of the liner. However, the internally cooled concept allows neutron fluxes as high as 20 MW/m2, at the front face of the first wall liner, to be accommodated without compromising the structural integrity of the vacuum wall. In addition the use of low Z graphite reduces the interaction of the wall materials and the plasma. (U.S.)