A major concern for the design of the fuel cycle for deuterium and tritium fusion plants is the efficient and economic recovery of tritium from the breeding eutectic Pb- 17Li.

An article"Tritium Extraction from Pb-17Li by Bubble Columns"by C. Malara, published in FUSION TECHNOLOGY Vol. 28, Oct. 1995, p. 693 to 699 describes a column in which the breeder eutectic flows from the top to the bottom and a bubbler injects pressurized helium and produces a counter current of bubbles rising from the bottom to the top of the column. A flux of tritium atoms from the eutectic to the ascending bubbles increases progressively the axial molar fraction at the gas phase and extracts the tritium. At present, it is commonly accepted that the actually projected fusion reactor extractor columns are large and of low effi- ciency, and series of extractor columns are needed.

The invention aims in improving the tritium extrac- tion rate thus allowing to reduce the size of the extractor and thereby the tritium inventory in the extractor. This aim is achieved by the method as claimed 1 and by the extractor implementing this method as defined in claim 2. As far as preferred embodiments of the extractor are concerned, refer- ence is made to the secondary claims.

The invention will now be described in more detail by means of the accompanying drawings.

Figure 1 shows schematically an extractor tube according to the invention.

Figure 2 is a diagram showing the permeation rate of hydrogen as a function of temperature through an extractor according to the invention and, for comparison reasons, through a steel tube.

Now referring to figure 1, a tube 1 is made from a ceramic composite material mainly composed of silicon car- bide fibres in a silicon carbide matrix. Such materials are known, for example from EP-A-0 489 636. They are commonly called SiCf/SiC composite ceramics.

The fibres are compressed at high temperature and the matrix is formed by infiltration from the gaseous or liquid phase. The final tube can be characterized as follows : fibre content : 40% Porosity : 10% Density : above 2.4 g/cm3 This material has been proposed until now as struc- tural material in fusion experiments because of its stabil- ity against thermal and radiation stresses.

While the gas permeation through this material is of minor importance for structural elements of the reactor and have even not been evaluated, it has been found that the permeation rate for hydrogen isotopes through the wall of a tube made of this material subjected to liquid Pb-17Li eutectic is enormously large and renders this material particularly apt to constitute a selective gas permeation wall for the extraction of hydrogen isotopes from a liquid Pb-17Li eutectic.

Figure 2 shows the permeation rate for hydrogen through a membrane A as a function of temperature compared to that of a structural steel wall B of equal thickness (lmm) the pressure difference between inside and outside being in both cases 1 kPa. This diagram shows that the permeation rates for steel are immeasurable for temperatures below 500°C whereas the permeation rate for the SiCf/SiC material are several magnitudes higher even for temperatures only slightly above room temperature.

Returning to figure 1, the tube 1 made from this material is fed with the liquid eutectic from the upper side (see arrow 2) and the liquid is withdrawn from the lower side (arrow 3). The gas permeation rate through the tube 1 allows hydrogen isotopes to pass through the wall according to arrows 4, and these gases are conveyed away by a helium flux sweeping the outside of the tube (arrows 5).

This flux according to arrows 5 can in a variant embodiment be inversely directed or be perpendicular to the tube axis if desired. The extractor can further be conceived like a heat exchanger with a multiplicity of tubes in paral- lel between two manifolds.

The extractor according to the invention can finally be combined to a classical hydrogen extraction system accor- ding to the bubble principle mentioned above. In this case the lower end of the tube is connected to a breeder eutectic withdrawal line and a source of inert gas, whereas the upper end of the tube is connected to a breeder eutectic supply line and a gas extraction line for extracting gases having bubbled through the liquid eutectic and having been released from the liquid surface. Such a combination adds the extrac- tion rate of the bubble system to that of the permeation system.

Besides the high permeation rate which ensures a high gas extraction rate, a major advantage of the present inven- tion is the low temperature at which such high permeation rates are achieved (see figure 2). It is therefore not necessary to heat the liquid leaving the reactor expressly for enhancing the permeation rate.

The extractor according to the invention is simple because no complex geometry is needed to increase the resi- dence time of the bubbles in the tube. It allows the use of relatively small extraction columns thus reducing the dynamic fuel inventory in the extractor and improving the safety of the fusion fuel cycle.

Because of the high efficiency at low temperature, that means below the temperature threshold for the corrosion of SiCf/SiC by Li, no major aging effect is observed in the ceramic material. This results in long operation times.