The Breeder Blanket system surrounds the plasma just outside the first wall. It has two critical functions: heat extraction and tritium fuel breeding. It is perhaps the most complex and challenging system in the reactor because of the many requirements imposed by its two functions, as well as requiring survival in the enormous neutron flux.
Neutrons from the fusion reactions in the plasma are stopped in the material of the blanket producing heat which is extracted by a circulation fluid. The requirements for tritium breeding are described in the section Tritium Fuel Supply .
The figure below illustrates the dual purpose of the system with a cooling circuit toward a turbine to generate electricity and a circuit to retrieve and purify tritium to re-inject it in the machine.
Lithium, which has two stable isotopes 6Li (7.5%) and 7LI(92.5%) is an efficient breeder of tritium via the reactions:
where first reaction emits energy while the second absorbs it. Also 6LI gives a higher Tritium Breeding Ratio and so is favoured leading to efforts to make lithium enrichment plants: Development of a viable route for lithium-6 supply of DEMO and future fusion power plants .
Beryllium is another component of breeder designs where it multiplies neutrons via the reaction:
In different designs, these elements appear in various compounds. Many blanket concepts have been proposed worldwide over the past 40 years using different combinations of materials and principles of operation. These can be classified as: liquid metal concepts, ceramic breeder concepts, and other concepts such as molten salts. Four samples designs are give below.
Details of these designs with discussions of their relative merits can be found in: Blanket/first wall challenges and required R&D on the pathway to DEMO
The number and diversity of the breeder blanket design must reflect the importance as well as the difficulty of the issue. ITER will test some prototype modules but may not be able to find a perfect, or even acceptable, solution for a power reactor.