Magnetic fusion : Tritium breeding blankets

Tritium breeding blankets

Role of blankets
Activities of the EURATOM-CEA Association

Role of blankets

In a tokamak, the blanket is the object situated immediately behind the first wall (Cf figure opposite).

Simplified diagram of a fusion reactor.

Its main functions are as follows :

Protect the magnets and the vacuum vessel from neutron and gamma radiation.
Produce the tritium necessary for continued fusion reactions.
Convert neutron energy into heat and evacuate it to generate a cycle capable of supplying electricity.

These three functions may not be present at the same time (Cf chart below). In the first phase of ITER operation, only the 1st function is guaranteed (ie shield blanket). In a second phase of ITER operation, the 1st and 2nd functions are guaranteed (ie cold tritium breeding blanket). In the case of a reactor, functions 1, 2 and 3 are required (ie tritium breeding blanket).

	Current machines	ITER	Reactor
1 - Protection of magnets
2 - Production of Tritium		tests
3 - Converting neutron energy into heat

function required

The "Production of Tritium" equation and notably reaching auto-sufficiency is a delicate balance. Performance in terms of auto-sufficiency is characterised by the TBR (Tritium Breeding Ratio). A TBR greater than 1 indicates that the blanket is auto-sufficiency.

The production of tritium is achieved through the reaction n + 6Li --> T + He. The lithium–based material may be in liquid (liquid metal like LiPb) or solid form (ceramic like Li4SiO4 or Li2ZrO3 or Li2TiO3), i.e. liquid or solid tritium breeding blanket. For solid blanket, auto-sufficiency necessitates use of a neutron multiplier like beryllium (for liquid blanket, lead in the tritium breeding material acting as multiplier).

The fusion reaction

<-- PRODUCTION of tritium

In the case of inertial fusion, the functions of Protection, Production and Conversion are also required from the blanket. The concepts developed for magnetic fusion are thus directly transferable to inertial fusion.

Activities of the EURATOM-CEA Association

Tritium breeding blankets are the subject of a specific European-level programme. Very significant results have been obtained, such as, the demonstration that auto-sufficient production of tritium was achievable in a reactor. All the concepts developed to date have a TBR greater than 1. Two concepts are being studied at the European level :

the WCLL concept (Water Cooled Litium Lead), developed under the coordination of the CEA, uses a liquid metal (LiPb) as the tritium breeding material and water as a coolant.
The HCPB concept (Helium Cooled Pebble Bed), developed under the coordination of FZK , employs a tritium breeding material in solid form (lithiated ceramic) cooled by helium.