Advanced helium cooled pebble bed blanket with SiCf/SiC as structural material

The Helium Cooled Pebble Bed blanket concept developed in the frame of the EPB-programme is based on the use of low activation ferritic/martensitic steel (EUROFER-97) as structural material. As the maximum allowable temperature of this steel is 550 degreesC, the coolant helium temperature can not exceed 450-500 degreesC, resulting in a relatively low thermal efficiency of the power generation system. The use of a ceramic material like SiCf/SiC with a maximum allowable temperature of 1300 degreesC allows to increase the maximum helium temperatures in the blanket, with the possibility to adopt more efficient power conversion systems. SiCf/SiC provides some other attractive features from the neutronic point of view (low neutron absorber in comparison to EUROFER) and safety (low activation). To take full advantage of the potential of this structural material, a new blanket design has been proposed. The pebble beds have been arranged in parallel to the first wall - by this configuration it was possible to reduce the required amount of beryllium, to improve the tritium breeding ratio and increasing the allowable neutron fluence. Finally, the adopted flow scheme results in a decisive reduction of the coolant pressure drop. On the basis of this design thermo-mechanic, thermo-hydraulic and neutronic calculations have been performed to optimise the design parameters (number and thickness of the beds, Li-6 enrichment, helium temperatures and pressure, etc.). An assessment of the limitation of this concept in term of maximum neutron wall, surface heating, achievable tritium breeding ratio, thermal efficiency in the power conversion system, pumping power for the blanket cooling loops has been performed. (C) 2000 Elsevier Science B.V. All rights reserved.