Patent Number: 046735470
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for the separation of hydrogen and/or deuterium and tritium from an inert gas flow which is contaminated with hydrogen and/or deuterium and/or tritium, wherein the inert gas which is to be purified is conducted along the primary side of an exchange wall for hydrogen isotopes, as well as relating to an arrangement for the effectuation of the process in the cooling circuit of a gas-cooled nuclear reactor. Hydrogen (H), and its isotope deuterium (D), and tritium (T), occur as impurities, for example, in the cooling gas circuits of gas-cooled nuclear reactors in which inert gases, especially helium, are utilized as cooling gases. Thus, for example, produced in the reactor core of a high-temperature reactor (HTR) having a capacity of 500 MW.sub.th is a tritium quantity of about 10.times.10.sup.3 Ci each year. The tritium is removed, in a known manner, in a gas purifying installation which is connected to the cooling gas circuit and which is passed through by a portion of the cooling gas so that in the cooling gas circuit there is produced an equilibrium partial pressure in the cooling gas circuit which consists of about 2 .mu.bar, and for hydrogen between 10.sup.2 to 10.sup.3 .mu.bar. Due to the excess of hydrogen in comparison with water and tritium in the cooling gas circuit, due to the isotope exchange in the cooling gas flow, tritium is essentially present as hydrogen-tritium molecules. The radioactive tritium in the cooling gas circuit is conducted with the cooling gas flow to the components of nuclear reactor, whose walls it can penetrate as a result of permeation. In order to avoid the thereby occasioned environmental contaminations, it is attempted to provide for the lowest possible tritium concentration in the cooling gas circuit. 2. Discussion of the Prior Art It is known to reduce the equilibrium partial pressure for tritium, water and hydrogen through the purifying of a portion of the cooling gas flow. The branched off portion of the cooling gas flow is conveyed in a bypass conduit of the cooling gas circuit to a gas purifying installation. In the gas purifying installation, the water is quantitatively removed in the form of the H.sub.2 O, HDO or HTO, and the hydrogen in the form of H.sub.2, HD or HT. Thereby the cooling gas flow is conducted, for example, over a cooled copper oxide bed (CuO) so that hydrogen, deuterium and tritium are oxidized and condensed. In this purifying process it is disadvantageous that only a partial gas flow in the magnitude of promil and less than the total cooling gas flow for each cooling gas flow cycle can be purified, and thereby a satisfyingly low partial pressure for tritium is not achieved in the cooling gas circuit. A filter for the separation of tritium is known from U.S. Pat. No. 3,848,067, in which yttrium which evidences a high retention capacity for the hydrogen isotopes, is employed for hydrogen storage. For the separation of the hydrogen isotopes from the cooling gas circuit with yttrium, the cooling gas flow is conducted along the surface of the nickel-coated yttrium so that the hydrogen isotopes will permeate through the nickel coating and will be stored in the yttrium essentially in the form of metal hydrides. Subsequent to the enrichment of the hydrogen isotopes in the yttrium, the filter must be exchanged. It is disadvantageous that the filtering effect reduces with increasing hydrogen enrichment in the yttrium, and the filter, in accordance with partial pressure conditions of the hydrogens which are to be separated from the inert gas flow, can already become ineffective within a short time. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a process for the separation of the hydrogen and/or deuterium and tritium from an inert gas flow, which effects a continual purification of the inert gas flow even at high quantity throughputs and higher gas temperature, with a degree of purification which remains, substantially uniform during the operating period. Moreover, the process is also simple to effect when proceeding from the continual removal of the impurities, particularly the continual removal of the tritium. The foregoing object is achieved in a process of the above-mentioned type in that the inert gas which is to be purified is conducted along the primary side of an exchange wall for hydrogen isotopes, and wherein the secondary side of the exchange wall has applied thereto an agent which so chemically reacts with the permeating hydrogen isotopes that the hydrogen isotope or the hydrogen isotopes which are to be separated out of the inert gas flow which is to be purified, are bound in a reaction product which is transportable in a gas flow and which is not capable of permeating through the exchange wall, and wherein along the secondary side of the exchange wall there is conducted a carrier gas flow which conveys off the reaction product. The concentrate precipitation between the primary and secondary sides of the exchange wall which is required for the permeation of hydrogen isotopes is achieved through chemical bonding of the hydrogen isotope which is to be removed in that, on the secondary side of the exchange wall, there is added an agent which chemically reacts with the hydrogen isotopes and which is so selected that the hydrogen isotope or the isotopes which are to be removed, from the inert gas flow being purified, are bound in a reaction product which is not capable of permeating through the exchange wall, which can be taken up in a carrier gas flow conducted along the secondary side of the exchange wall and is conducted off by the carrier gas flow. Adequate for the transport of the reaction product with the carrier gas is a volumetric throughput in the magnitude of per mils relative to the volumetric throughput of the inert gas flow which is to be purified. A further advantage of the inventive process consists of in that the pressure and temperature of the carrier gas flow along the secondary side of the exchange wall are correlatable with the pressure and the temperature present in the inert gas which is to be purified on the primary side. In order to achieve a selective separation of the hydrogen isotopes, an agent is added to the carrier gas which binds the hydrogen isotope which are to separated out through isotope exchange. For the selective separation of tritium and deuterium from a hydrogen-deuterium-tritium admixture permeating through the exchange wall, suited before all is water or, relatively, steam. The agent which is added on the secondary side of the exchange wall for isotope exchange thus contains overwhelmingly hydrogen isotopes which are not to be separated so that for these, in contrast with the hydrogen isotopes which are to be bonded, there is produced a concentration equilibrium on both sides of, the exchange wall. For the isotope exchange, besides or in lieu of the preferably employed water, there can also be employed ammonia, NH.sub.3, or hydrogen sulfide, H.sub.2 S. During the addition of water for example, there is formed HTO and HDO from HT and HD pursuant to the reactions: EQU HT+H.sub.2 O.revreaction.HTO+H.sub.2, EQU HD+H.sub.2 .revreaction.HDO+H.sub.2, whereby through an increase in the partial pressure of H.sub.2 O in the carrier gas there is advanced the transition of HT into HTO and of HD into HDO. Through a change in the quantity of the agent for the isotope exchange which is passed through on the secondary side of the exchange wall or in essence, for example, through an increase in the added quantity of water, the equilibrium weight of the reaction is displaced towards the right side of the reaction equation. Through the addition of water, or in effect water vapor into the carrier gas flow, as a result of the isotope exchange, there is attained the degree of concentration between the primary and secondary side relative to the permeation-capable HT molecule, or the HT molecule even for a volumetric throughput of the carrier gas which is small relative to the.volumetric throughput on the primary side. It is sufficient to have a volumetric throughput for the carrier gas in the per mil range relative to the volumetric throughput on the primary side. H.sub.2 O is contained in the gas on the primary side as well as on the secondary side of the exchange wall at the same partial pressure. Thus, it is not removed from the inert gas flow on the primary side. The reaction products which are set forth on the right-hand side in the herein above set forth equations, are conveyed away by the carrier gas from the secondary side of the exchange wall. Separated out of the carrier gas is then subsequently that reaction product which bonds the hydrogen isotope which is to be separated out or the isotopes which are to be separated out, for example, through condensation, so that any excess of agent added for isotope exchange is concurrently removed. In an advantageous manner, in gas-cooled nuclear reactors it is possible to not only selectively clean a portion but the entire cooling gas flow for each circulation from tritium and deuterium. In order to maintain the concentration precipitate between the primary and secondary sides of the exchange wall, there can also be added to the carrier gas flow an agent which oxidizes the hydrogen isotopes. Upon the addition of such agents, during reaction with the hydrogen isotopes H.sub.2 O, D.sub.2 O, T.sub.2 O, formed is HDO as well as HTO which, in the same manner as in the previously described isotope exchange are conveyed away by the carrier gas flow and, for example, allow themselves to again be separated through condensation or rectification from the carrier gas flow. Preferably employed for the activation of the hydrogen isotopes is oxygen or, alternatively, a metal oxide, especially copper oxide or iron oxide, on the secondary side of the exchange, wall. With the utilization of metal oxide there is purposefully formed on the secondary side a metal oxide bed through which there is conducted the carrier gas flow for conveying away the formed reaction products. In accordance with the invention, as set forth in detail hereinbelow, the inert gas flow which is to be purified is conducted along the primary side of the exchange wall and the carrier gas flow along the secondary side of the exchange wall in counterflow in order to achieve a high degree of purification for the inert gas flow. Suitably, the carrier gas flow, subsequent to the separating out of the reaction products which are carried along by the carrier gas and, occasionally after renewed addition of the agent which reacts with the hydrogen isotopes, is reconveyed in a closed circuit to the secondary side of the exchange wall. Preferably, utilized as the carrier gas is a purified inert gas. The carrier gas, in this instance, may be withdrawn in advantageous manner from the purified inert gas flow flowing off from the primary side of the exchange wall. A portion of the purified inert gas is drawn off and, with the addition of an agent reacting with the hydrogen isotopes, is conveyed to the secondary side of the exchange surface. This manner there is concurrently achieved a pressure equilibrium, up to a low vacuum which essentially corresponds to the pressure loss, which is produced on the primary side, of the exchange wall during the throughflow of the inert gas through the exchange installation, and a temperature equilibrium between the primary and secondary sides of the exchange wall. In view thereof, during the reconveyance of the carrier gas flow subsequent to the separation of the reaction products, in the cleaned inert gas flow there is obviated the need for any required flow aggregates for the maintenance of the carrier gas flow. The agent which reacts with the hydrogen isotopes is introduced timely into the divided off partial flow of the inert gas, so that upon inflow of the carrier gas flow to the secondary side of the exchange wall there is present a concentration precipitate to the inert gas flow on the primary side of the exchange wall. It is advantageous that the carrier gas flow, after addition of the agent which reacts with the hydrogen isotopes and prior to flowing through the secondary side of the exchange wall, is conducted over a catalyst which accelerates the reaction between the added agent and the hydrogen isotopes. In order to produce the required concentration precipitates, the partial flow which is divided off from the purified inert gas as carrier gas, is also conducted prior to inlet on the secondary side of the exchange wall over a metal oxide bed, especially a copper oxide or iron oxide bed in which, possibly preceding inlet to the secondary side, there is yet added the agent which reacts with the permeated hydrogen isotopes. In a further embodiment of the invention, the inert gas flow is sequentially conveyed through two exchange installations, wherein there is added on the secondary side of one exchange installation an agent for the isotope exchange, and on the secondary side of the other exchange installation an agent which oxidizes the hydrogen isotopes. Through this measure, there is controllable the partial pressure relationship of the hydrogen isotopes with regard to each other through selective separation during isotope exchange, for example, during the introduction of water of water vapor through separation of deuterium and tritium, as well as the absolute partial pressure of the hydrogen through oxidation of the permeated hydrogen isotopes. Preferably the inert gas which is to be purified initially passes through an exchange installation, on the secondary side of which there is oxygen is introduced into the carrier gas flow, and thereafter an exchange installation on the secondary side of which an agent for the isotope exchange is added to the carrier gas. Hereby, the carrier gas allows itself to be conveyed through the exchange installations in a counter direction to the inert gas flow which is to be purified. When for the agent which oxidizes the hydrogen isotope there is employed a metal oxide on the secondary side of an exchange installation for example copper oxide, it is suitable to separate initially the hydrogen isotopes from the inert gas flow through isotope exchange and thereafter by oxidation by flowing through of a subsequent exchange installation. In this instance, the water which is formed on the secondary side of the second exchange installation allows itself to be subsequently employed for the isotope exchange on the secondary side of the first exchange installation. A separation of the hydrogen isotopes from the inert gas flow within two sequentially traversed exchange installations, with isotope exchange and oxidation of the hydrogen isotopes, is also of significance for the purification of the cooling gas circuit of a gas-cooled nuclear reactor since, in this manner, besides the removal of the tritium from the cooling gas circuit, there is also adjustable a desired H.sub.2 content in the cooling gas circuit.