Evaluation of helium cooling for fusion divertors

The divertors of future fusion reactors will have a power throughput of several hundred megawatts. The peak heat flux on the divertor surface is estimated to be 5-15MWm-2 at an average heat flux of 2MWm-2. The divertors have a requirement of both minimum temperature (100 C) and maximum temperature. The minimum temperature is dictated by the requirement to reduce the absorption of plasma, and the maximum temperature is determined by the thermomechanical properties of the plasma facing materials. Coolants that have been considered for fusion reactors are water, liquid metals and helium. Helium cooling has been shown to be very attractive from safety and other considerations. Helium is chemically and neutronically inert and is suitable for power conversion. The challenges associated with helium cooling are (1) manifold sizes, (2) pumping power, and (3) leak prevention. In this paper the first two of the above design issues are addressed. A variety of heat transfer enhancement techniques is considered to demonstrate that the manifold sizes and the pumping power can be reduced to acceptable levels. A helium-cooled divertor module was designed and fabricated by GA for a steady-state heat flux of 10MWm-2. This module was recently (August 1993) tested at Sandia National Laboratories. At the inlet pressure of 4MPa, the module was tested at a steady-state heat flux of 10MWm-2. The pumping power required was less than 1% of the power removed. These results verified the design prediction. ((orig.))