Tritium breeding blanket
In a tokamak, the breeding blanket is located immediately behind the first wall. It has several purposes:
1) Protecting the magnets and the vacuum chamber from neutron and gamma radiation.
2) Recovering the energy that the neutrons give up heating the equipment. A heat transfer fluid circulates in the structure taking away this heat to create a cycle that can supply electricity.
3) In the case of deuterium-tritium fusion, producing the tritium required for the fusion reaction (using breeding blankets).
These three functions cannot take place at the same time. In the physical study stage of ITER only function 1 is provided (with the so-called blanket module). In the technology study stage of ITER only functions 1 and 3 are provided (referred to as cold breeding blanket). For a reactor, functions 1, 2 and 3 are required (breeding blankets). Source: http://www-fusion-magnetique.cea.fr/cea/next/couvertures/blk.htm