Divertor engineering studies for a double null demonstration reactor based on the net configuration

The increase in fusion power on extrapolating from a double-null version of the Next European Torus (NET-DN, 600 MW) to a demonstration reactor (DEMO-DN, 2500 MW) raises the temperature and flux of particles in the scrape-off plasma, which means for the poloidal divertors used for impurity control (i) an increase in the peak heat flux from 5 MW m/sup -2/ to 10 or 15 MW m/sup -2/, and (ii) a large increase in the sputtering erosion rate of the plate surface. Two possible types of divertor design have been considered for DEMO-DN: (i) a solid tungsten alloy plate containing poloidally-disposed cooling tubes separated by deep stress-relieving grooves, and (ii) a porous plate made of sintered tungsten alloy and protected against undue sputtering erosion by sacrificial liquid metal. Cooling both by supercritical water (at 3500C, 350 bar) and by helium (at 3500C, 400 bar) has been considered. With the thickness of overlying plate material (which should be as large as possible to maximize the erosion allowance) and the coolant temperature rise as parameters, the cooling tube diameter has been optimized against various limitations, viz. surface temperature, primary and secondary stress, erosion-corrosion, pumping power and pressure drop. Twisted-tape heat transfer enhancement has been assumed in all cases. Viable designs are found for a solid plate cooled by water or by helium and for a water-cooled porous plate. The latter is necessarily thinner, and the tubes narrower (-- 2 mm diameter), which also means that a helium-cooled porous plate design is ruled out by an excessive pumping power requirement. Mechanical life estimates based on two-dimensional finite-element analyses give acceptable results for all three 'viable' designs provided that the sputtering erosion rate is sufficiently low