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 st eel (EUROFER-97) as structural material. As the maximum allowable temperatu re of this steel is 550 degreesC, the coolant helium temperature can not ex ceed 450-500 degreesC, resulting in a relatively low thermal efficiency of the power generation system. The use of a ceramic material like SiCf/SiC wi th a maximum allowable temperature of 1300 degreesC allows to increase the maximum helium temperatures in the blanket, with the possibility to adopt m ore efficient power conversion systems. SiCf/SiC provides some other attrac tive features from the neutronic point of view (low neutron absorber in com parison to EUROFER) and safety (low activation). To take full advantage of the potential of this structural material, a new blanket design has been pr oposed. The pebble beds have been arranged in parallel to the first wall - by this configuration it was possible to reduce the required amount of bery llium, to improve the tritium breeding ratio and increasing the allowable n eutron fluence. Finally, the adopted flow scheme results in a decisive redu ction of the coolant pressure drop. On the basis of this design thermo-mech anic, thermo-hydraulic and neutronic calculations have been performed to op timise the design parameters (number and thickness of the beds, Li-6 enrich ment, helium temperatures and pressure, etc.). An assessment of the limitat ion of this concept in term of maximum neutron wall, surface heating, achie vable tritium breeding ratio, thermal efficiency in the power conversion sy stem, pumping power for the blanket cooling loops has been performed. (C) 2 000 Elsevier Science B.V. All rights reserved.