Status of EC solid breeder-blanket designs and R&D for DEMO fusion reactors

Abstract

Within the European Community Fusion Technology Programme, two solid breeder-blankets for a DEMO reactor are being developed. The two blankets have various features in common: helium as coolant and as tritium-purge gas; the martensitic steel MANET as a structural material; and beryllium as the neutron multiplier. The configurations of the two blankets, however, are different: in the breeder inside tube concept, the breeder materials are LiA1O 2 or Li 2 ZrO 3 in the form of annular pellets contained in tubes surrounded by beryllium blocks, with the coolant helium being outside the tubes; in contrast, in the breeder out of tube concept, the breeder and multiplier materials are Li 4 SiO 4 and beryllium pebbles, forming a mixed bed placed outside the tubes containing the coolant helium. The main critical issues for both blankets are the behaviour of the breeder ceramics, and of beryllium under irradiation and the tritium control. Other issues are the low temperature irradiation-induced embrittlement of the MANET; the mechanical effects caused by major plasma disruptions; and safety and reliability. The R&D work concentrates on these issues. The development of martensitic steels, including MANET, is part of a separate programme. Breeder ceramics and beryllium irradiations have been performed so far under conditions which do not cover the peak values projected for the DEMO blankets. Further irradiations in thermal reactors and in fast reactors, especially for beryllium, are required. Effective tritium control requires the development of permeation barriers and/or of methods of oxidation of the tritium in the main helium cooling system. First promising results have been obtained also in the field of mechanical effects from plasma disruptions and safety and reliability. However, further work is required in the reliability field and to validate the codes for the calculations of the plasma disruption effects.