The Breeder Blanket system surrounds the plasma and has two critical functions: heat extraction and tritium fuel breeding. It is perhaps the most complex and challenging system in the whole fusion 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 page 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 the first reaction emits energy while the second absorbs it. Use of the isotope 6LI is favoured and so enrichment facilities are being developed.
Beryllium is another essential component of breeder designs as neutron to give a higher tritium gain. One neutron interacting with beryllium gives two neutrons out, via the reaction:
Lead is also used as neutron multiplier in some blanket concepts.
The illustration below shows the full series of reactions from fusion, via neutron multiplication to tritium breeding.
The big question, is how many tritium nuclei are produced, for one fusion reaction, on average at the end of the reaction series. The value of this is the Tritium Breeder Ratio (TBR) and in practice it is likely to have a maximum value of 1.15. The figure below from, Sawan & Abdou (2006), summarizes the inherent breeding capacity for thick breeder blankets with no structural material and no "external" neutron multiplier. "Internal" neutron multiplier options are included. The plot show TBS as a function of the energy gain in the reaction series.
Tritium Breeding Ratio (TBR) for candidate breeding materials with the corresponding energy multiplication factors.
The addition of structural material in practical breeder blankets reduces the TBR by factors of up to 30%. Further reductions in practical TBR, result from gaps in breeder blanket coverage caused by the entry ports into the vacuum vessel. With all factors, estimates are that the overall TBR will be in the range 1.05-1.15.
After 40 years of R&D on breeder blanket technologies, many different concepts have been developed with different materials and principles of operation. The major technologies being currently pursued are: liquid metal concepts; ceramic breeder concepts and molten salt concepts. Abdou et al. (2015) provides a detailed review of breeder technologies.
Main Breeder Blanket concepts studied, showing flow circuits. Circulating fluids: helium (purple); hydrogen (green); liquid lead-lithium (orange); water (blue). Primary Cooling System (PCS), Tritium Extraction System (TES), Coolant Purification System (CPS).
Decisions have been taken to focus on two breeder blanket designs for both ITER and EU-DEMO. These technologies are:
The Lithium-Lead blanket systems have the possibility for higher tritium breeding ratios but the liquid metal has the possibility of serious corrosion. The Pebble-Bed blanket is a lower safety risk choice and is shown in the figure below.
HCPB Breeder Blanket in EU-DEMO
The coolant pipes transport the fluids to the tritium recuperation system, where the tritium must be rapidly processed for re-injection into the tokamak, as well as passing through the heat exchangers to generate steam to drive the electricity turbines.
The final choice of the breeder blanket technology involves many considerations:
This choice is completely critical for a fusion reactor and it seems very necessary to have dedicated test facilities, beyond ITER, to enable the right decisions.