Materials issues in fusion reactors

Abstract

The world scientific community is presently engaged in one of the toughest technological tasks of the current century, namely, exploitation of nuclear fusion in a controlled manner for the benefit of mankind. Scientific feasibility of controlled fusion of the light elements in plasma under magnetic confinement has already been proven. International efforts in a coordinated and co-operative manner are presently being made to build ITER – the International Thermonuclear Experimental Reactor – to test, in this first step, the concept of 'Tokamak' for net fusion energy production. To exploit this new developing option of making energy available through the route of fusion, India too embarked on a robust fusion programme under which we now have a working tokamak – the Aditya and a steady state tokamak (SST-1), which is on the verge of functioning. The programme envisages further development in terms of making SST-2 followed by a DEMO and finally the fusion power reactor. Further, with the participation of India in the ITER program in 2005, and recent allocation of half – a – port in ITER for placing our Lead – Lithium Ceramic Breeder (LLCB) based Test Blanket Module (TBM), meant basically for breeding tritium and extracting high grade heat, the need to understand and address issues related to materials for these complex systems has become all the more necessary. Also, it is obvious that with increasing power from the SST stages to DEMO and further to PROTOTYPE, the increasing demands on performance of materials would necessitate discovery and development of new materials.Because of the 14.1 MeV neutrons that are generated in the D+T reaction exploited in a tokamak, the materials, especially those employed for the construction of the first wall, the diverter and the blanket segments, suffer crippling damage due to the high He/dpa ratios that result due to the high energy of the neutrons. To meet this challenge, the materials that need to be developed for the tokamaks are steels for the first wall and other structurals, copper alloys for the heat sink, and beryllium for facing the plasma. For the TBMs, the materials that need to be developed include beryllium and/or beryllium-titanium alloys for neutron multiplication, lithium-bearing compounds for tritium generation, and the liquid metal coolants like lead-lithium eutectic in which lead acts as a neutron multiplier and lithium as a tritium breeder. The other materials that need attention of the materials scientists include superconductors made of NbTi, Nb3Sn and Nb3Al for the tokamaks, coatings or ceramic inserts to offset the effect of corrosion and the MHD in liquid metal cooled TBMs, and a host of other materials like nano-structured materials, special adhesives and numerous other alloys and compounds. Apart from this, the construction of the tokamaks would necessitate development of methodologies of joining the selected materials.This presentation would deal with the issues related to the development, characterization and qualification of both the structural as well as the functional materials required to carry forward the challenging task of harnessing fusion energy for use of mankind in engineered systems.