USE AND PERFORMANCE OF GRAPHITE AND METAL ISSECS IN TOKAMAK FUSION REACTORS

Carbon and the metallic elements molybdenum, niobium, vanadium and tungsten have been considered for use as an ISSEC (Internal Spectral Shifter and Energy Converter) in tokamak fusion reactors. All five materials have been shown to reduce the radiation damage in the 316 SS structural first wall and thus increase the first wall lifetime. On a per unit thickness basis, a tungsten ISSEC is most effective in this regard followed by Mo, Nb, V and carbon ISSECs in decreasing order. If the ISSEC is restricted to transfer its heat to the first structural wall by thermal radiation only, the maximum allowable thickness a carbon ISSEC can have is limited to 9.5 cm, Mo ISSEC to 7.5 cm, Nb to 8.5 cm, V to 4.5 cm and a W ISSEC to 6 cm for a 1 MW/m2 neutronic and 4 W/cm2 surface heat loading. If the ISSEC is cooled by radiation plus conduction, the maximum allowable thickness goes up to 13 cm for C, 10 cm for Mo, 8 cm for V and stays the same for Nb and W. The only ISSEC material to result in an overall reduction in total blanket radioactivity at shutdown is carbon while all of the metallic ISSECs increase the total activity. On the other hand, the long term activity (at 1000 years after shutdown) is increased for Mo, Nb and V ISSECs while it is reduced by C and W. The carbon and V ISSECs reduce the energy production per fusion while Nb and W increase it slightly and Mo results in a 15-17% energy production increase. On a relative cost basis, metallic ISSECs cost 30-55 times more than a carbon ISSEC when used at the maximum thicknesses given above. Among the four metals studied, Mo is considered to be the best material for use as an ISSEC. A definitive choice between a graphite and Mo ISSEC is difficult at this time as both materials have strong positive features; carbon being superior from radioactivity, afterheat, cost and fabricability standpoint, but molybdenum being more effective in reducing the radiation damage in the first structural wall and increasing the energy multiplication in the blanket. © 1978.