Patent Number: 048759457
Section: description

DETAILED DESCRIPTION OF THE INVENTION The exhaust gas from the final cycle of a fusion reactor has approximately the following composition: 80 to 85 Mol% He, Ar PA0 15 to 20 Mol% NQ.sub.3, CQ.sub.4, Q.sub.2 O, Q.sub.2, CO, CO.sub.2, N.sub.2 and O.sub.2. (Q=H, D, T). PA0 R (P.sub.CO =2 mbar)=(8.26.+-.1.6).times.10.sup.-7 (mol cm/min cm.sup.2) PA0 R (P.sub.CO =20 mbar)=(9.10.+-.0.8).times.10.sup.-7 (mol cm/min cm.sup.2) PA0 R (P.sub.CO =200 mbar)=(7.84.+-.1.4).times.10.sup.-7 (mol cm/min cm.sup.2). Referring to the drawing, in the present invention, the exhaust gas 2 is brought into a palladium/silver permeator 1 operating at a temperature of 450.degree. C. or below. The small amounts of ammonia which are usually present and expected in the exhaust gas are converted into its elements (Q.sub.2 and N.sub.2, Q=H, D, T) by a catalytic reaction on the palladium/silver surface of permeator 1. The reaction product hydrogen Q.sub.2 from the decomposition of the ammonia and the elemental hydrogen Q.sub.2 which is originally present in the exhaust gas selectively penetrate or permeate through permeator 1 where they emerge as the main hydrogen gas process stream 3. Process stream 3 contains a major fraction of the elemental heavy hydrogen originally present in the exhaust gas and of the elemental hydrogen formed by any ammonia decomposition. A major fraction of the light hydrogen isotope H, if present, will also appear in stream 3. The permeator 1 is operated preferably at temperatures between 400.degree. and 450.degree. C. because under these conditions carbon monoxide does not lower the hydrogen permeation rate. The other gaseous components in the exhaust gas and any nitrogen formed as a result of ammonia decomposition do not permeate through permeator 1 and emerge from permeator 1 as a residual gas stream 4 containing impurities which include water (Q.sub.2 O), hydrocarbon, CO.sub.2 and CO. After emerging from permeator 1, residual gas stream 4 is brought into contact with a catalyst bed 5 so that the hydrogen isotopes bound in water (Q.sub.2 O) contained in the residual gas stream 4 are released by reaction with CO in excess according to the reaction equation EQU CO+Q.sub.2 O.fwdarw.CO.sub.2 +Q.sub.2 Catalyst bed 5 is comprised of a CuO/Cr.sub.2 O.sub.3 /ZnO catalyst and is at temperatures between 150.degree. and 200.degree. C. The CuO/Cr.sub.2 O.sub.3 /ZnO catalyst can have a Cu content, as CuO, of 39 wt.%, a Cr content, as Cr.sub.2 O.sub.3, of 18 wt.%, and a Zn content, as ZnO, of 38 wt.%. In order to achieve 100% reaction of Q.sub.2 O, a CO/Q.sub.2 O ratio of at least 1.5 is required. If the amount of CO in excess is lower than 1.5, CO gas has to be introduced into the gas stream 4 through a CO supply line 6 before gas stream 4 enters catalyst 5 to bring the ratio up to at least 1.5. At the outlet of the CuO/Cr.sub.2 O.sub.3 /ZnO catalyst bed 5, the resulting gas stream consists preferably of helium with small amounts of hydrocarbon (methane, ethane and higher aliphatic and aromatic hydrocarbons), nitrogen, carbon monoxide, carbon dioxide, and Q.sub.2. In order to release the hydrogen isotopes D and T bound in the hydrocarbons, the gas mixture is passed through a palladium/silver permeator 7 i.e., consisting of a tube array of 2.5. wt.% Ag and 75 wt.% Pd, the tubes containing a nickel/aluminum oxide bulk catalyst at a temperature of 300.degree.-450.degree. C. (Alternatively, the gas mixture can be passed into a nickel catalyst bed followed by a palladium/silver permeator). Here the hydrocarbons are decomposed into their elements by splitting, and the elemental hydrogen Q.sub.2 released by the splitting and the hydrogen Q.sub.2 that was formed by the reaction of the water vapor in the previous step are separated simultaneously from the remaining gaseous components by selectively permeating through permeator 7. This combination of the splitting reaction and hydrogen permeation allows a high detritiation of the process gas to be achieved. The hydrogen which is separated in permeator 7 emerges as a hydrogen stream 9 and is combined with the main fraction of hydrogen in stream 3, and the resulting combined stream is transferred to the hydrogen isotope separation system by means of a pump 10. The heavy hydrogen from this separation system can then be fed back into the fuel cycle of the fusion reactor. The remaining gaseous components in permeator 7 which do not permeate through permeator 7 emerge as a decontaminated residual gas stream 11 which does not contain any hydrogen. Thus, gas stream 11 contains tritium-free and deuterium-free impurities, and can be withdrawn either directly, that is, it can be discharged directly into the atmosphere, or it can be recycled in case cleanup is inadequate. As shown in the drawing, stream 11 can be recycled through a line 12 into the CuO/Cr.sub.2 O.sub.3 /ZnO catalyst. In order to counteract a loss in efficiency of the catalyst in permeator 7 due to carbon deposits, the catalyst has to be regenerated periodically using hydrogen or deuterium from a separate source which is fed to permeator 7 from a hydrogen supply 8. The CuO/Cr.sub.2 O.sub.3 /ZnO catalyst described in step (c) is known from H. Yoshida et al., Nucl. Technology/Fusion 5, 178 (1984). However, the bound hydrogen in the Yoshida et al process is oxidized to water by addition of oxygen and has to be reduced into water again in another process step so that a complicated process consisting of a number of individual steps results in cleanup of the exhaust gas. The process according to the present invention, by contrast, comprises only a few process steps. Decontamination is achieved in the process of the present invention solely by physical or catalytic processes so that a plant working on the process according to the present invention allows the operation to be optimum in terms of availability, waste arisings and safety, especially because the bound hydrogen does not oxidize to become water and the addition of oxygen, be it by direct supply or via an oxygen releasing fixed bed, is avoided. The present invention is explained in more detail by the following examples. EXAMPLE 1 In order to study the regeneration capacity of the Ni-catalyst an He/CH.sub.4 -mixture (330 mbar CH.sub.4 and 705 mbar He) was first passed over a catalyst heated to 460.degree. C. in a closed loop (Volume=8.8 liters). The hydrogen released by methane splitting was separated continuously with the help of a Pd/Ag permeator connected to the circuit. Separation of hydrogen causes the equilibrium to shift up to methane concentrations below the detection limit of gas chromatography. Due to the fact that the methane splitting reaction is a reversible one, the carbon deposited on the catalyst can be recovered quantitatively as methane into the gas phase after treatment with hydrogen at temperatures between 450.degree. and 550.degree. C. Without noticeable loss in catalyst efficiency a total of 18 splitting/regeneration cycles were possible on 10 g catalyst. The amount of methane reacted was 2.2 mol (about 0.12 mol/cycle). Under optimized conditions the reaction half-time was about 4.2 minutes. EXAMPLE 2 In a closed loop (Volume=8.8 liters) H.sub.2 /He mixtures (P.sub.H2 =200 mbar, total pressure 1 bar) with different carbon monoxide partial pressures (P.sub.CO =2, 20 and 200 mbar, respectively) were passed through a palladium/silver permeator, heated to 450.degree. C., with an active surface of 289 cm.sup.2. The hydrogen permeation constants R were evaluated and were as follows: These results show that within the error band carbon monoxide concentrations up to 20 mol% have no significant effect on the permeability of the metallic membrane to hydrogen. EXAMPLE 3 In tests carried out in a closed loop in which a gaseous mixture consisting typically of 2 kPa H.sub.2 O and 6 kPa CO diluted in He up to approx. 100 kPa were passed over 5 g of a zinc stabilized copper chromite catalyst (CuO-ZnO-Cr.sub.2 O.sub.3 -Al.sub.2 O.sub.3) it was shown that at temperatures between 420 and 520 K. thermodynamic equilibrium can be attained at a sufficiently fast rate. The addition of up to 10 kPa methane to this gaseous mixture influences neither the reaction rate nor the chemical equilibrium. It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.