EP0082061B1 - Process and device for treating solutions containing tritiated water, electrode to be used in this device and its manufacturing process - Google Patents

Process and device for treating solutions containing tritiated water, electrode to be used in this device and its manufacturing process Download PDF

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Publication number
EP0082061B1
EP0082061B1 EP82402226A EP82402226A EP0082061B1 EP 0082061 B1 EP0082061 B1 EP 0082061B1 EP 82402226 A EP82402226 A EP 82402226A EP 82402226 A EP82402226 A EP 82402226A EP 0082061 B1 EP0082061 B1 EP 0082061B1
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cathode
solution
palladium
tritium
cell
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German (de)
French (fr)
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EP0082061A2 (en
EP0082061A3 (en
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Gilbert Bellanger
Pierre Giroux
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Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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Commissariat a lEnergie Atomique CEA
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    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
    • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
    • G21F9/04—Treating liquids
    • G21F9/06—Processing

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  • the subject of the present invention is a method and a device for the treatment by electrolysis of solutions containing tritiated water such as effluents from plants for reprocessing irradiated nuclear fuels, water for cooling light water or water reactors. heavy and effluents from laboratories where tritium is handled.
  • solutions containing tritiated water such as effluents from plants for reprocessing irradiated nuclear fuels, water for cooling light water or water reactors. heavy and effluents from laboratories where tritium is handled.
  • aqueous solutions containing a large quantity of tritiated water for example a content of approximately 40 Ci / m 3 , are obtained at certain stages of reprocessing. These solutions are generally obtained during the concentration by evaporation of uranium, plutonium or fission product solutions, or during the regeneration treatment of nitric acid with a view to its recycling at the dissolving stage of irradiated fuel elements. . In the latter case, these solutions are obtained during the concentration of the nitric acid which has been formed by regenerating by means of water vapor the nitrogen oxides originating from the step of destruction of the nitric acid by the formalin. It is also possible to envisage higher concentrations, either by recycling the nitric solutions, or by isotopic concentration of the effluents.
  • the present invention specifically relates to a process for treating solutions containing water formed from an isotope of hydrogen, which is suitable for the treatment of tritiated water and makes it possible to solve the problems of recovery of tritium in satisfactory conditions.
  • this process for treating a solution containing water formed from an isotope of hydrogen consists in adding to the solution an electrolyte chosen in such a way that the solution obtained can liberate by electrolysis the isotope of hydrogen in the gaseous state, to subject the solution thus obtained to electrolysis so as to obtain a release of the isotope of hydrogen, by operating in an electrolysis cell comprising a metal cathode capable of promoting the diffusion of the hydrogen isotope, said cathode forming a sealed separation wall between the solution to be electrolyzed and a compartment for receiving the hydrogen isotope, and the isotope is recovered in said compartment hydrogen desorbed from said cathode, and it is characterized in that said isotope of hydrogen is tritium and the cathode is coated with a deposit of porous palladium black on its surface in contact with the solution to be electrolyzed.
  • the cathode coated with porous palladium black thanks to the structure and the nature of the cathode coated with porous palladium black, it is possible to recover directly in the gaseous state, with good yields, the tritium released during the electrolysis, after its diffusion through the electrode wall and its desorption on the other side of the electrode.
  • the choice of a cathode of non-porous material, permeable to hydrogen and impermeable to other gases makes it possible to obtain, after release of the tritium at the cathode, an adsorption of the tritium by the cathode, then a diffusion of this in the cathode and its desorption on the other side of the cathode in the receiving compartment.
  • a slight depression is established in the receiving compartment, for example when the tritium is collected by pumping.
  • the first step constitutes the most important step because it determines the amount of tritium which could be adsorbed then diffused by the wall of the cathode in contact with the electrolyte.
  • a cathode coated on its surface is used in contact with the solution to be electrolyzed from a deposit of porous palladium black.
  • this deposit makes it possible to increase the specific surface of the cathode and to give it a higher adsorption capacity with respect to tritium.
  • a second deposit on the desorption side is also favorable but in lesser proportions.
  • tritium can also be recovered in the form of solid metal tritiide by reacting it directly in the reception compartment with a compound capable of forming a metal tritiide. Mention may be made, as compounds capable of being used, of La-Ni s compounds, Fe-Ti compounds and palladium, alloyed or not.
  • a cathode covered with porous palladium black is used on its adsorption face and, preferably, also on its desorption face. Furthermore, the presence of traces of ferric oxide on the adsorption face of the cathode is favorable and the use of annealing to restore the cathode also makes it possible to improve the results obtained.
  • the cathode is advantageously made of palladium or a palladium alloy such as a palladium-silver alloy because these metals have the property of adsorbing very large quantities of tritium.
  • a palladium alloy such as a palladium-silver alloy
  • an alloy of palladium and silver with 25% silver is used because it has a permeability substantially equal to that of pure palladium and the property of not deteriorating after repeated cycles of heating and hydrogenation.
  • metals capable of adsorbing tritium can be used, for example, pure iron, nickel, platinum and their alloys.
  • T tritium
  • the secondary reactions to avoid are: because in this case, the tritium would be discharged directly into the electrolysis cell, instead of diffusing through the wall of the electrode.
  • the adsorption of tritium by palladium is improved by subjecting the palladium or palladium alloy electrode to an activation treatment comprising a step of coating the surface of the electrode which will be in contact with the solution to be electrolyzed, a layer of finely divided and porous palladium black.
  • the porous palladium black coating is formed by electrolysis of a solution of palladium chloride in dilute hydrochloric acid. This electrolysis can be carried out at a current density of 150 mA / cm 2 for 4 min. Thus, a deposit of palladium black with a thickness of 6 I lm is obtained.
  • the annealing heat treatment makes it possible to increase the size of the meshes of the metal network of the cathode and thus to improve the diffusion of the tritium in the cathode.
  • the palladium electrodes are generally obtained by rolling and are therefore strongly hardened.
  • the grains appear little and are oriented in the direction of rolling.
  • recrystallization annealing is possible since the germs necessary for the growth of the crystals have been produced by work hardening, the most disturbed regions where the dislocation energy is concentrated playing the role of germs.
  • the metal is heated to a suitable temperature, the germs start to grow and the grain gets bigger; after a certain heating time which corresponds to the incubation period, the recrystallization actually begins.
  • time and temperature play an important role and temperature intervenes in a rather complex way.
  • the temperature is not high enough during the incubation period, the number of germs decreases and recrystallization can be suppressed, which corresponds to the phenomenon of restoration.
  • palladium electrodes good results are obtained by annealing at a temperature of around 650 ° C for one hour under vacuum.
  • the hardness decreases, the mechanical tensions are reduced and the dislocations or other imperfections of the metallic network can move towards the surface of the electrode, from where a better diffusion of the tritium in the metallic network of palladium.
  • the mechanical abrasion treatment using ferric oxide as a hydrogenation accelerator makes it possible to modify the energy required to pass the chemisorbed hydrogen into hydrogen absorbed in the interstitial sites directly below the cathode surface.
  • Iron occupies a number of sites by lending electrons to the 4d band of palladium. This model of iron adsorption which covers the cathode surface increases the permeability of hydrogen in palladium with decrease in potential and increase in current.
  • This treatment acts on the amount of tritium released over time.
  • the deposition of a thin layer of finely divided and porous palladium black on the surface of the cathode in contact with the solution to be electrolyzed makes it possible to improve the adsorption and the diffusion of tritium.
  • the existence on the surface of a very finely divided deposit of palladium black promotes and multiplies the reactions which occur at the solid-solution interface to be electrolyzed.
  • a deposit of palladium black on the desorption face improves the diffusion.
  • the electrolyte added to the solution containing the tritiated water preferably consists of an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide, which makes it possible to avoid as much as possible the formation of complex ions resulting from radiolysis phenomena and the presence of solvated electrons due to tritium.
  • an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide
  • the electrolyte concentration of this solution is advantageously from 1 ml-1 to 20 ml-1.
  • electrolysis is carried out at a temperature higher than ambient temperature, for example at a temperature of 50 to 160 ° C. because, it is thus possible to increase the current density and cell yield without bubbles forming on the cathode.
  • the operation is carried out at a temperature of 80 ° C. since this avoids the technological constraints due to the use of high temperatures as well as the appearance of unfavorable phenomena such as corrosion or secondary reactions of radiolysis.
  • the cathode when the cathode is constituted by a wall of palladium or of palladium alloy having a thickness of 50 to 250 ⁇ m, electrolysis is carried out with a current density of between 60 and 150 milliamperes / cm 2 , at a temperature 80 ° C.
  • the cathode is constituted by a hollow tube closed at one of its ends and arranged in the cell so as to be partially immersed in the electrolytic solution, the space defined at the interior of the tube constituting the tritium receiving compartment.
  • the device comprises means for extracting hydrogen and / or isotopes of hydrogen in the gaseous state, which have diffused in said receiving compartment, these means being constituted either by a suitable pump, or by a trap. based on metals and alloys such as LaNi 5 , Fe-Ti, palladium, alloyed or not, forming hydrides.
  • the device preferably comprises means for heating the solution electrolytic present in said cell.
  • the cathode is preferably made of palladium or a palladium alloy, for example a palladium and silver alloy.
  • a palladium alloy for example a palladium and silver alloy.
  • this tube is covered externally and possibly internally with porous palladium black.
  • the anode is advantageously constituted by the wall of the electrolysis cell and it is made of stainless steel.
  • the palladium-silver alloy tube which constitutes the cathode is subjected to an annealing heat treatment, then its external surface is treated by mechanical abrasion using ferric oxide before being coated with palladium black by electrolysis. .
  • the device comprises an electrolysis cell 1 made, for example, of ceramic which is not soluble in an alkaline medium, of metal or of a non-corrodable metal alloy such as steel 316L 22 CND 17-13. Preferably, it is made of passivated stainless steel.
  • the cell 1 is closed in leaktight manner at its upper part by a cover 3. Inside the cell is disposed a cathode 5 constituted by a tube closed at its lower end, and the wall of the cell constitutes the anode 7 .
  • the device comprises a condenser 15 and an electrolytic solution supply line 17 provided with a valve 18 controlled by an electrical relay associated with the probes 11 and 13, as well as a line 19 for introducing an inert gas. Furthermore, the device comprises heating means 21 of the electrolysis cell constituted by electrical resistances controlled from a thermostat which ensures thermal regulation.
  • the cathode 5 is constituted by a hollow tube 5a of circular section having a thickness of 50 to 250 ⁇ m closed at its lower end which delimits the compartment 23 for receiving tritium connected at its upper part to a device for recovery of tritium.
  • This tritium recovery device must be sealed to maintain the high purity of the diffused tritium and it can be maintained under vacuum by means of a primary vane pump.
  • this device comprises a vacuum gauge and a pressure gauge to control the vacuum, an intermediate enclosure for storing tritium, a test tube for taking gaseous samples and a trap used for storing tritium in the form of tritiide.
  • the vacuum can be obtained by a pumping group.
  • the tube constituting the cathode 5 is made of a non-porous palladium-silver alloy, permeable to hydrogen and impermeable to other gases, and it has been annealed at a temperature of 650 ° C. for one hour under a vacuum of order of 1.35 Pa to remove the orientation of the grains due to rolling.
  • the external surface of the tube intended to be in contact with the solution to be electrolyzed has undergone a mechanical abrasion treatment by means of a ferric oxide powder Fe 2 0 3 moistened, with water , for a few minutes, as an accelerator for the hydrogenation of palladium, then a layer of finely divided and porous palladium black with a thickness of 7 ⁇ m was deposited on this surface thus treated, in order to increase the active surface of palladium on contact with electrically discharged tritium.
  • This deposit of finely divided and porous palladium black was produced by electrolysis of a solution of palladium chloride containing 4 g of PdC1 2 dissolved in 20 cm3 of HCl at 12 mol / l., Then diluted to 500 cm 3 distilled water, operating at a cathode current density of 150 mA / cm 2 at a temperature of 20 ° C, for 4 min.
  • the anode 7 is formed by the wall of the cell 1 and it is connected to the positive pole of the electric current generator.
  • Such an arrangement of the anode and the cathode makes it possible to obtain a good distribution of the current over the surface of the cathode and the formation of regular equipotentials.
  • the electrolysis current is programmed using a potentiostat operating in intensiostatic mode.
  • solutions containing tritiated water can be treated in the following way: the solution to be electrolyzed which is constituted by tritiated water containing from 1 to 20 mol. 1-1 of sodium hydroxide.
  • This tritiated water was obtained by catalytic oxidation of gaseous effluents containing tritium.
  • the introduction of solution stops automatically.
  • the heating device is then switched on to bring the temperature of the solution to around 80 ° C.
  • argon is introduced via line 19 and the electrodes 5 and 7 are connected to the electric current generator to electrolyze the solution with a cathode current density of 60 mA.cm- 2 and obtain a release of gaseous tritium on the cathode 5.
  • the tritium is adsorbed by the cathode 5, then it diffuses inside the tube 5 normally under vacuum by pumping, but the process can work when the gas pressure inside the tube is much higher than the pressure of the electrolysis cell. Under these conditions, we can obtain a tritium flow rate of the order of 1 cm.min- 1 .
  • the gases released during electrolysis i.e.
  • oxygen as well as the tritium which has not diffused in the tube 5 and the water vapor, are evacuated from the cell by the current d argon to the condenser 15 in which the water vapor is condensed and then recycled inside the cell 1.
  • the gases leaving the condenser will be sent in a catalytic recombination assembly to reform tritiated water which can then be recycled inside the cell.
  • the gas evacuation pipe leaving the condenser 15 can lead to a catalytic oxidation element for residual tritium, this element consisting of palladium black fixed on alumina.
  • the tritium recombined with oxygen in the form of heavy water is then condensed in a heat exchanger and optionally recycled in cell 1. It is possible to connect a sampling bulb to the gas outlet pipe so as to analyze the gases extracted. , either at the outlet of the electrolysis cell, or after the catalytic oxidation element.
  • a device of this type has made it possible to obtain satisfactory results after operating periods of approximately six weeks without any disassembly of the cathode. At the end of this time, the latter showed no failure and the diffusion of tritium through its wall was carried out under good conditions.
  • electrolysis can be carried out at 160 ° C, at a current density of 670 mA / cm 2 , and the diffusion rate of hydrogen and tritium is under these conditions of 3.9 CM 3 . CM - 2 . M in-1.
  • the method and the device of the invention make it possible to solve the safety problems posed by the handling of tritiated water, the rejection of contaminated effluents, in particular as regards the hydrogen-tritium fraction released in the electrolysis tank. , as well as the problems of resistance of materials to tritiated water, radiolysis of tritiated water and interaction with nitrogen in the air which leads to corrosive compounds.

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Description

La présente invention a pour objet un procédé et un dispositif de traitement par électrolyse de solutions contenant de l'eau tritiée tels que les effluents provenant des usines de retraitement de combustibles nucléaires irradiés, l'eau de refroidissement des réacteurs à eau légère ou à eau lourde et les effluents provenant de laboratoires où l'on manipule du tritium.The subject of the present invention is a method and a device for the treatment by electrolysis of solutions containing tritiated water such as effluents from plants for reprocessing irradiated nuclear fuels, water for cooling light water or water reactors. heavy and effluents from laboratories where tritium is handled.

Dans les installations de retraitement de combustibles nucléaires irradiés, on obtient à certains stades du retraitement, des solutions aqueuses contenant une quantité importante d'eau tritiée, par exemple une teneur d'environ 40 Ci/m3. Ces solutions sont généralement obtenues lors de la concentration par évaporation des solutions d'uranium, de plutonium ou de produits de fission, ou encore lors du traitement de régénération de l'acide nitrique en vue de son recyclage au stade de dissolution des éléments combustibles irradiés. Dans ce dernier cas, ces solutions sont obtenues lors de la concentration de l'acide nitrique qui a été formé en régénérant au moyen de vapeur d'eau les oxydes d'azote provenant de l'étape de destruction de l'acide nitrique par le formol. On peut aussi envisager des concentrations supérieures, soit par recyclage des solutions nitriques, soit par concentration isotopique des effluents.In irradiated nuclear fuel reprocessing plants, aqueous solutions containing a large quantity of tritiated water, for example a content of approximately 40 Ci / m 3 , are obtained at certain stages of reprocessing. These solutions are generally obtained during the concentration by evaporation of uranium, plutonium or fission product solutions, or during the regeneration treatment of nitric acid with a view to its recycling at the dissolving stage of irradiated fuel elements. . In the latter case, these solutions are obtained during the concentration of the nitric acid which has been formed by regenerating by means of water vapor the nitrogen oxides originating from the step of destruction of the nitric acid by the formalin. It is also possible to envisage higher concentrations, either by recycling the nitric solutions, or by isotopic concentration of the effluents.

Pour le moment, on ne connaît pas de procédé permettant d'assurer dans des conditions satisfaisantes un traitement de l'eau tritiée en vue de récupérer le tritium qui y est contenu. En effet, les méthodes de réduction chimique de l'eau tritiée au moyen d'uranium à chaud présentent l'inconvénient de consommer de l'uranium et de conduire à des déchets d'oxyde d'uranium contaminé par du tritium. Par ailleurs, la manipulation d'eau tritiée est très difficile et pose de nombreux problèmes de contamination.For the moment, there is no known method for ensuring under satisfactory conditions a treatment of tritiated water with a view to recovering the tritium which is contained therein. Indeed, the methods of chemical reduction of tritiated water by means of hot uranium have the drawback of consuming uranium and leading to waste uranium oxide contaminated with tritium. Furthermore, handling tritiated water is very difficult and poses many contamination problems.

Toutefois, on a envisagé d'utiliser un procédé électrolytique pour traiter des effluents contenant de l'eau tritiée provenant d'un réacteur nucléaire refroidi au gaz carbonique, code cela est décrit dans le DE-A-1 965 627 déposé par Atomic Power Constructions Limited. Cependant, selon ce procédé, on ne peut récupérer le tritium dégagé à la cathode avec une grande pureté puisque celui-ci reste en présence d'impuretés gazeuses telles que la vapeur d'eau et l'oxygène. Par ailleurs, ce procédé ne permet pas d'obtenir de bons rendements en tritrium.However, it has been envisaged to use an electrolytic process to treat effluents containing tritiated water coming from a nuclear reactor cooled with carbon dioxide, code this is described in DE-A-1 965 627 filed by Atomic Power Constructions Limited. However, according to this method, it is not possible to recover the tritium released at the cathode with high purity since it remains in the presence of gaseous impurities such as water vapor and oxygen. Furthermore, this process does not make it possible to obtain good yields of tritrium.

On connaît aussi, par le brevet américain US-A-3 620 844, un système pour l'activation de l'hydrogène qui peut être utilisé pour récupérer de l'hydrogène ou du deutérium de haute pureté par électrolyse d'une solution aqueuse, en utilisant une cathode en métal capable de favoriser la diffusion de l'hydrogène qui peut être recouverte de palladium poreux sur sa face qui n'est pas en contact avec la solution aqueuse.Also known, from American patent US-A-3 620 844, a system for the activation of hydrogen which can be used to recover hydrogen or deuterium of high purity by electrolysis of an aqueous solution, using a metal cathode capable of promoting the diffusion of hydrogen which can be covered with porous palladium on its face which is not in contact with the aqueous solution.

Cependant avec un système de ce type les débits de diffusion d'hydrogène et de rendement de diffusion ne sont pas suffisammentHowever with a system of this type the hydrogen diffusion rates and diffusion efficiency are not sufficient

importants.important.

La présente invention a précisément pour objet un procédé de traitement de solutions contenant de l'eau formée à partir d'un isotope de l'hydrogène, qui est adapté au traitement d'eau tritiée et permet de résoudre les problèmes de récupération du tritium dans des conditions satisfaisantes.The present invention specifically relates to a process for treating solutions containing water formed from an isotope of hydrogen, which is suitable for the treatment of tritiated water and makes it possible to solve the problems of recovery of tritium in satisfactory conditions.

Selon l'invention, ce procedé de traitement d'une solution contenant de l'eau formée à partir d'un isotope de l'hydrogène, consiste à ajouter à la solution un électroyte choisi de façon telle que la solution obtenue puisse libérer par électrolyse l'isotope de l'hydrogène à l'état gazeux, à soumettre la solution ainsi obtenue à une électrolyse de façon à obtenir un dégagement de l'isotope de l'hydrogène, en opérant dans une cellule d'électrolyse comportant une cathode en métal capable de favoriser la diffusion de l'isotope de l'hydrogène, ladite cathode formant une paroi de séparation étanche entre la solution à électrolyser et un compartiment de réception de l'isotope de l'hydrogène, et on récupère dans ledit compartiment l'isotope de l'hydrogène désorbé de ladite cathode, et il se caractérise en ce que ledit isotope de l'hydrogène est le tritium et la cathode est revêtue d'un dépôt de noir de palladium poreux sur sa surface en contact avec la solution à électrolyser.According to the invention, this process for treating a solution containing water formed from an isotope of hydrogen, consists in adding to the solution an electrolyte chosen in such a way that the solution obtained can liberate by electrolysis the isotope of hydrogen in the gaseous state, to subject the solution thus obtained to electrolysis so as to obtain a release of the isotope of hydrogen, by operating in an electrolysis cell comprising a metal cathode capable of promoting the diffusion of the hydrogen isotope, said cathode forming a sealed separation wall between the solution to be electrolyzed and a compartment for receiving the hydrogen isotope, and the isotope is recovered in said compartment hydrogen desorbed from said cathode, and it is characterized in that said isotope of hydrogen is tritium and the cathode is coated with a deposit of porous palladium black on its surface in contact with the solution to be electrolyzed.

Selon l'invention, grâce à la structure et à la nature de la cathode revêtue de noir de palladium poreux, on peut récupérer directement à l'état gazeux, avec de bons rendements, le tritium libéré lors de l'électrolyse, après sa diffusion à travers la paroi de l'électrode et sa désorption sur l'autre face de l'électrode.According to the invention, thanks to the structure and the nature of the cathode coated with porous palladium black, it is possible to recover directly in the gaseous state, with good yields, the tritium released during the electrolysis, after its diffusion through the electrode wall and its desorption on the other side of the electrode.

En effet, le choix d'une cathode en matériau non poreux, perméable à l'hydrogène et imperméable aux autres gaz, permet d'obtenir, après dégagement du tritium à la cathode, une adsorption du tritium par la cathode, puis une diffusion de celui-ci dans la cathode et sa désorption sur l'autre face de la cathode dans le compartiment de réception. De plus, étant donné que la pression interne du tritium sur la face de la cathode en contact avec l'électrolyte est très élevée puisqu'elle varie de façon exponentielle en fonction du potentiel de la cathode, on obtient une différence très importante entre la pression interne de tritium sur la face de l'électrode en contact avec l'électrolyte et la pression interne de tritium sur l'autre face de l'électrode, c'est-à-dire dans le compartiment de réception. De ce fait, on obtient facilement la diffusion du tritium dans la paroi de la cathode, même à la température ambiante, et l'on peut ainsi récupérer le tritium dans le compartiment de réception même si la pression dans celui-ci est supérieure à 1 bar.Indeed, the choice of a cathode of non-porous material, permeable to hydrogen and impermeable to other gases, makes it possible to obtain, after release of the tritium at the cathode, an adsorption of the tritium by the cathode, then a diffusion of this in the cathode and its desorption on the other side of the cathode in the receiving compartment. In addition, since the internal pressure of tritium on the face of the cathode in contact with the electrolyte is very high since it varies exponentially as a function of the potential of the cathode, a very large difference is obtained between the pressure internal tritium on the face of the electrode in contact with the electrolyte and the internal tritium pressure on the other face of the electrode, that is to say in the receiving compartment. Therefore, it is easily obtained the diffusion of tritium in the wall of the cathode, even at room temperature, and one can thus recover the tritium in the receiving compartment even if the pressure in it is higher than 1 bar.

Toutefois dans certains cas, on établit une légère dépression dans le compartiment de réception, par exemple lorsqu'on récupère le tritium par pompage.However, in certain cases, a slight depression is established in the receiving compartment, for example when the tritium is collected by pumping.

Dans un procédé de ce type, qui comporte une première étape d'adsorption du tritium sur la paroi de la cathode, une seconde étape de diffusion du tritium à l'intérieur de la cathode et une troisième étape de désorption du tritium dans le compartiment de réception, la première étape constitue l'étape la plus importante car elle détermine la quantité de tritium qui pourra être adsorbée puis diffusée par la paroi de la cathode en contact avec l'électrolyte.In a process of this type, which comprises a first step of adsorption of tritium on the cathode wall, a second step of diffusion of the tritium inside the cathode and a third step of desorption of the tritium in the receiving compartment, the first step constitutes the most important step because it determines the amount of tritium which could be adsorbed then diffused by the wall of the cathode in contact with the electrolyte.

Selon l'invention, pour obtenir une bonne adsorption du tritium libéré par l'électrolyse, on utilise une cathode revêtue sur sa surface en contact avec la solution à électrolyser d'un dépôt de noir de palladium poreux. En effet, ce dépôt permet d'augmenter la surface spécifique de la cathode et de lui conférer une capacité d'adsorption supérieure vis-à-vis du tritium. Un deuxième dépôt sur le côté désorption est également favorable mais dans des proportions moindres.According to the invention, to obtain good adsorption of the tritium released by electrolysis, a cathode coated on its surface is used in contact with the solution to be electrolyzed from a deposit of porous palladium black. In fact, this deposit makes it possible to increase the specific surface of the cathode and to give it a higher adsorption capacity with respect to tritium. A second deposit on the desorption side is also favorable but in lesser proportions.

Selon l'invention, on peut aussi récupérer le tritium sous la forme de tritiure métallique solide en le faisant réagir directement dans le compartiment de réception avec un composé capable de former un tritiure métallique. A titre de composés susceptibles d'être utilisés, on peut citer les composés La-Nis, les composés Fe-Ti et le palladium allié ou non.According to the invention, tritium can also be recovered in the form of solid metal tritiide by reacting it directly in the reception compartment with a compound capable of forming a metal tritiide. Mention may be made, as compounds capable of being used, of La-Ni s compounds, Fe-Ti compounds and palladium, alloyed or not.

Ceci permet de stocker directement le tritium sous la forme d'un composé solide et de réaliser cette réaction dans le compartiment de réception ou dans son voisinage, ce qui évite les problèmes de contamination posés par le transfert et le stockage du tritium à l'état gazeux.This makes it possible to store the tritium directly in the form of a solid compound and to carry out this reaction in the receiving compartment or in its vicinity, which avoids the problems of contamination posed by the transfer and storage of the tritium in the state gaseous.

Pour améliorer le rendement de récupération en tritium, il est important d'agir sur les paramètres suivants:

  • - l'état de surface de la cathode, c'est-à-dire le nombre de centres actifs sur les faces
  • d'adsorption et de désorption ainsi que les accélérateurs d'hydrogénation,
  • - la structure du réseau métallique du palladium,
  • - la température, et
  • - la densité de courant qui règle la cinétique des processus électrochimiques d'adsorption, d'insertion, de diffusion et de désorption.
To improve the yield of tritium recovery, it is important to act on the following parameters:
  • - the surface condition of the cathode, i.e. the number of active centers on the faces
  • adsorption and desorption as well as the hydrogenation accelerators,
  • - the structure of the metallic network of palladium,
  • - the temperature, and
  • - the current density which regulates the kinetics of the electrochemical processes of adsorption, insertion, diffusion and desorption.

Aussi, selon l'invention, pour améliorer l'état de surface de la cathode, on utilise une cathode recouverte de noir de palladium poreux sur sa face d'adsorption et, de préférence, également sur sa face de désorption. Par ailleurs, la présence de traces d'oxyde ferrique sur la face d'adsorption de la cathode est favorable et l'utilisation d'un recuit de restauration de la cathode permet également d'améliorer les résultats obtenus.Also, according to the invention, to improve the surface condition of the cathode, a cathode covered with porous palladium black is used on its adsorption face and, preferably, also on its desorption face. Furthermore, the presence of traces of ferric oxide on the adsorption face of the cathode is favorable and the use of annealing to restore the cathode also makes it possible to improve the results obtained.

Selon l'invention, la cathode est avantageusement réalisée en palladium ou en alliage de palladium tel qu'un alliage de palladium-argent car ces métaux ont la propriété d'adsorber des quantités très importantes de tritium. On utilise, de préférence, un alliage de palladium et d'argent à 25 % d'argent car il possède une perméabilité sensiblement égale à celle du palladium pur et la propriété de ne pas se détériorer après des cycles répétés de chauffage et d'hydrogénation. Par ailleurs, on peut utiliser une cathode d'épaisseur relativement élevée, par exemple de 250 um car l'épaisseur intervient peu sur la perméabilité.According to the invention, the cathode is advantageously made of palladium or a palladium alloy such as a palladium-silver alloy because these metals have the property of adsorbing very large quantities of tritium. Preferably, an alloy of palladium and silver with 25% silver is used because it has a permeability substantially equal to that of pure palladium and the property of not deteriorating after repeated cycles of heating and hydrogenation. . Furthermore, it is possible to use a cathode of relatively high thickness, for example of 250 μm because the thickness has little effect on the permeability.

Toutefois, on peut utiliser d'autres métaux capables d'adsorber le tritium, par exemple, le fer pur, le nickel, le platine et leurs alliages.However, other metals capable of adsorbing tritium can be used, for example, pure iron, nickel, platinum and their alloys.

Le phénomène d'adsorption du tritium (T) à la cathode s'effectue suivant le mécanisme suivant:

Figure imgb0001
qui est suivi de la désorption du tritium dans le réseau métallique de l'électrode selon le mécanisme:
Figure imgb0002
 The phenomenon of tritium (T) adsorption at the cathode takes place according to the following mechanism:
Figure imgb0001
 which is followed by the desorption of tritium in the metal network of the electrode according to the mechanism:
Figure imgb0002

De ce fait, les réactions secondaire à éviter sont:

Figure imgb0003
Figure imgb0004
car dans ce cas, le tritium serait rejeté directement dans la cellule d'électrolyse, au lieu de diffuser à travers la paroi de l'électrode.Therefore, the secondary reactions to avoid are:
Figure imgb0003
Figure imgb0004
 because in this case, the tritium would be discharged directly into the electrolysis cell, instead of diffusing through the wall of the electrode.

Comme on l'a vu précédemment, on améliore l'adsorption du tritium par le palladium en soumettant l'électrode en palladium ou en alliage de palladium à un traitement d'activation comprenant une étape de revêtement de la surface de l'électrode qui sera en contact avec la solution à électrolyser, d'une couche de noir de palladium finement divisé et poreux.As previously seen, the adsorption of tritium by palladium is improved by subjecting the palladium or palladium alloy electrode to an activation treatment comprising a step of coating the surface of the electrode which will be in contact with the solution to be electrolyzed, a layer of finely divided and porous palladium black.

Ce traitement d'activation peut être réalisé de la façon suivante:

  • - on soumet tout d'abord l'électrode à un traitement thermique de recuit, puis on réalise sur la surface de l'électrode destinée à être en contact avec la solution à électrolyser un traitement d'abrasion mécanique au moyen d'oxyde ferrique humide dont les traces restant sur la cathode agissent comme accélérateur d'hydrogénation du palladium, et on revêt ensuite la surface ainsi traitée par du noir de palladium finement divisé et poreux.
This activation treatment can be carried out as follows:
  • - first of all the electrode is subjected to an annealing heat treatment, then a mechanical abrasion treatment using wet ferric oxide is carried out on the surface of the electrode intended to be in contact with the solution to be electrolyzed the traces of which remain on the cathode act as accelerators for the hydrogenation of palladium, and the surface thus treated is then coated with finely divided and porous palladium black.

De préférence, on forme le revêtement de noir de palladium poreux par électrolyse d'une solution de chlorure de palladium dans de l'acide chlorhydrique dilué. Cette électrolyse peut être réalisée sous une densité de courant de 150 mA/cm2 pendant 4 min. Ainsi, on obtient un dépôt de noir de palladium d'une épaisseur de 6 Ilm.Preferably, the porous palladium black coating is formed by electrolysis of a solution of palladium chloride in dilute hydrochloric acid. This electrolysis can be carried out at a current density of 150 mA / cm 2 for 4 min. Thus, a deposit of palladium black with a thickness of 6 I lm is obtained.

Le traitement thermique de recuit permet d'augmenter la taille des mailles du réseau métallique de la cathode et d'améliorer ainsi la diffusion du tritium dans la cathode.The annealing heat treatment makes it possible to increase the size of the meshes of the metal network of the cathode and thus to improve the diffusion of the tritium in the cathode.

En effet, les électrodes en palladium sont obtenues généralement par laminage et sont de ce fait fortement écrouies. Les grains apparaissent peu et sont orientés dans le sens du laminage. Cependant un recuit de recristallisation est possible puisque les germes nécessaires à la croissance des cristaux ont été produits par écrouissage, les régions les plus pertubées où se concentre l'énergie de dislocation jouant le rôle de germes. Lorsqu'on chauffe le métal à une température convenable, les germes commencent à croître et le grain grossit; après un certain temps de chauffage qui correspond à la période d'incubation, la recristallisation commence réellement. Ainsi, le temps et la température jouent un rôle important et la température intervient de façon assez complexe. Si la température n'est pas assez élevée au cours de la période d'incubation, le nombre de germes diminue et la recristallisation peut être supprimée, ce qui correspond au phénomène de restauration. Dans le cas des électrodes en palladium, on obtient de bons résultats en réalisant le recuit à une température d'environ 650°C pendant une heure sous vide. Ainsi, la dureté diminue, les tensions mécaniques sont réduites et les dislocations ou autres imperfections du réseau métallique peuvent se déplacer vers la surface de l'électrode, d'où une meilleure diffusion du tritium dans le réseau métallique du palladium.Indeed, the palladium electrodes are generally obtained by rolling and are therefore strongly hardened. The grains appear little and are oriented in the direction of rolling. However, recrystallization annealing is possible since the germs necessary for the growth of the crystals have been produced by work hardening, the most disturbed regions where the dislocation energy is concentrated playing the role of germs. When the metal is heated to a suitable temperature, the germs start to grow and the grain gets bigger; after a certain heating time which corresponds to the incubation period, the recrystallization actually begins. Thus, time and temperature play an important role and temperature intervenes in a rather complex way. If the temperature is not high enough during the incubation period, the number of germs decreases and recrystallization can be suppressed, which corresponds to the phenomenon of restoration. In the case of palladium electrodes, good results are obtained by annealing at a temperature of around 650 ° C for one hour under vacuum. Thus, the hardness decreases, the mechanical tensions are reduced and the dislocations or other imperfections of the metallic network can move towards the surface of the electrode, from where a better diffusion of the tritium in the metallic network of palladium.

Le traitement d'abrasion mécanique au moyen d'oxyde ferrique comme accélérateur d'hydrogénation, permet de modifier l'énergie nécessaire pour faire passer l'hydrogène chimisorbé en hydrogène absorbé dans les sites intersticiels directement sous la surface cathodique. Le fer occupe un certain nombre de sites par prêt d'électrons à la bande 4d du palladium. Ce modèle d'adsorption du fer qui recouvre la surface cathodique augmente la perméabilité de l'hydrogène dans le palladium avec diminution du potentiel et augmentation du courant.The mechanical abrasion treatment using ferric oxide as a hydrogenation accelerator makes it possible to modify the energy required to pass the chemisorbed hydrogen into hydrogen absorbed in the interstitial sites directly below the cathode surface. Iron occupies a number of sites by lending electrons to the 4d band of palladium. This model of iron adsorption which covers the cathode surface increases the permeability of hydrogen in palladium with decrease in potential and increase in current.

Ce traitement permet d'agir sur la quantité de tritium diffusée en fonction du temps.This treatment acts on the amount of tritium released over time.

Enfin le dépôt d'une couche mince de noir de palladium finement divisé et poreux sur la surface de la cathode en contact avec la solution à électrolyser permet d'améliorer l'adsorption et la diffusion du tritium. En effet, l'existence en surface d'un dépôt très finement divisé de noir de palladium favorise et multiplie les réactions qui se produisent à l'interface solide-solution à électrolyser. Bien que l'effet soit moins important un dépôt de noir de palladium sur la face de désorption améliore la diffusion.Finally, the deposition of a thin layer of finely divided and porous palladium black on the surface of the cathode in contact with the solution to be electrolyzed makes it possible to improve the adsorption and the diffusion of tritium. Indeed, the existence on the surface of a very finely divided deposit of palladium black promotes and multiplies the reactions which occur at the solid-solution interface to be electrolyzed. Although the effect is less, a deposit of palladium black on the desorption face improves the diffusion.

Selon l'invention, l'électrolyte ajouté à la solution contenant l'eau tritiée est constitué de préférence par un hydroxyde de métal alcalin tel que l'hydroxyde de sodium ou l'hydroxyde de potassium, ce qui permet d'éviter au maximum la formation d'ions complexes résultant des phénomènes de radiolyse et de la présence d'électrons solvatés dûs au tritium.According to the invention, the electrolyte added to the solution containing the tritiated water preferably consists of an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide, which makes it possible to avoid as much as possible the formation of complex ions resulting from radiolysis phenomena and the presence of solvated electrons due to tritium.

Lorsqu'on utilise l'hydroxyde de sodium, la concentration en électrolyte de cette solution est avantageusement de 1 moLl-1 à 20 moLl-1.When using sodium hydroxide, the electrolyte concentration of this solution is advantageously from 1 ml-1 to 20 ml-1.

De préférence, pour améliorer encore la diffusion du tritium dans la cathode, on réalise l'électrolyse à une température supérieure à la température ambiante, par exemple à une température de 50 à 160°C car, on peut ainsi augmenter la densité de courant et le rendement de la cellule sans qu'il y ait formation de bulles sur la cathode. De préférence, on opère à une température de 80° C car on évite ainsi les contraintes technologiques dues à l'emploi de températures élevées ainsi que l'apparition de phénomènes défavorables tels que la corrosion ou des réactions secondaires de radiolyse.Preferably, to further improve the diffusion of tritium in the cathode, electrolysis is carried out at a temperature higher than ambient temperature, for example at a temperature of 50 to 160 ° C. because, it is thus possible to increase the current density and cell yield without bubbles forming on the cathode. Preferably, the operation is carried out at a temperature of 80 ° C. since this avoids the technological constraints due to the use of high temperatures as well as the appearance of unfavorable phenomena such as corrosion or secondary reactions of radiolysis.

Avantageusement, lorsque la cathode est constituée par une paroi de palladium ou d'alliage de palladium ayant une épaisseur de 50 à 250 p.m, on réalise l'électrolyse avec une densité de courant comprise entre 60 et 150 milliampères/cm2, à une température de 80° C.Advantageously, when the cathode is constituted by a wall of palladium or of palladium alloy having a thickness of 50 to 250 μm, electrolysis is carried out with a current density of between 60 and 150 milliamperes / cm 2 , at a temperature 80 ° C.

L'invention à également pour objet un dispositif de traitement d'une solution contenant de l'eau formée à partir d'un isotope de l'hydrogène. Ce dispositif comprend:

  • - une cellule d'électrolyse (1) pour contenir une solution électrolytique capable de libérer par électrolyse l'isotope de l'hydrogène à l'état gazeux, ladite cellule comprenant une anode (7) et une cathode (5) réalisée en un métal capable d'adsorber l'isotope de l'hydrogène, ladite cathode étant telle qu'elle constitue une paroi de séparation entre la solution à électrolyser et un compartiment (23) de réception de l'isotope de l'hydrogène,
  • - des moyens pour établir une différence de potentiel entre ladite anode et ladite cathode,
  • - des moyens pour mettre en circulation dans ladite cellule ladite solution à laquelle on a jouté un électrolyte, et il se caractérise en ce que l'isotope de l'hydrogène est le ritium, ladite cathode est revêtue de noir de palladium poreux sur sa surface en contact avec la solution à électrolyser, et le dispositif comprend:
  • - des moyens pour récupérer l'oxygène dégagé dans ladite cellule, et
  • - des moyens pour condenser la vapeur d'eau formée dans ladite cellule et pour recycler la vapeur d'eau condensée dans la solution à électrolyser.
The invention also relates to a device for treating a solution containing water formed from an isotope of hydrogen. This device includes:
  • - an electrolysis cell (1) for containing an electrolytic solution capable of releasing the isotope of hydrogen in the gaseous state by electrolysis, said cell comprising an anode (7) and a cathode (5) made of a metal capable of adsorbing the hydrogen isotope, said cathode being such that it constitutes a partition wall between the solution to be electrolyzed and a compartment (23) for receiving the hydrogen isotope,
  • means for establishing a potential difference between said anode and said cathode,
  • means for circulating in said cell said solution to which an electrolyte has been added, and it is characterized in that the hydrogen isotope is ritium, said cathode is coated with porous palladium black on its surface in contact with the solution to be electrolyzed, and the device comprises:
  • means for recovering the oxygen released in said cell, and
  • - Means for condensing the water vapor formed in said cell and for recycling the condensed water vapor in the solution to be electrolyzed.

Selon un mode préférentiel de réalisation du dispositif de l'invention, la cathode est constituée par un tube creux fermé à l'une des ses extrémités et disposé dans la cellule de façon a être immergé partiellement dans la solution électrolytique, l'espace délimité à l'intérieur du tube constituant le compartiment de réception du tritium.According to a preferred embodiment of the device of the invention, the cathode is constituted by a hollow tube closed at one of its ends and arranged in the cell so as to be partially immersed in the electrolytic solution, the space defined at the interior of the tube constituting the tritium receiving compartment.

Avantageusement, le dispositif comprend des moyens pour extraire l'hydrogène et/ou les isotopes de l'hydrogène à l'état gazeux, qui ont diffusé dans ledit compartiment de réception, ces moyens étant constitués soit par une pompe adaptée, soit par un piège à base de métaux et d'alliages tels que LaNi5, Fe-Ti, palladium allié ou non formant des hydrures.Advantageously, the device comprises means for extracting hydrogen and / or isotopes of hydrogen in the gaseous state, which have diffused in said receiving compartment, these means being constituted either by a suitable pump, or by a trap. based on metals and alloys such as LaNi 5 , Fe-Ti, palladium, alloyed or not, forming hydrides.

De même, le dispositif comprend de préférence des moyens pour chauffer la solution électrolytique présente dans ladite cellule.Similarly, the device preferably comprises means for heating the solution electrolytic present in said cell.

Comme on l'a vu précédemment, la cathode est de préférence réalisée en palladium ou en alliage de palladium, par exemple en alliage de palladium et d'argent. Lorsqu' elle est sous la forme de tube creux fermé à l'une de ses extrémités, ce tube est recouvert extérieurement et éventuellement intérieurement de noir de palladium poreux. L'anode est avantageusement constituée par la paroi de la cellule d'électrolyse et elle est réalisée en acier inoxydable.As seen above, the cathode is preferably made of palladium or a palladium alloy, for example a palladium and silver alloy. When it is in the form of a hollow tube closed at one of its ends, this tube is covered externally and possibly internally with porous palladium black. The anode is advantageously constituted by the wall of the electrolysis cell and it is made of stainless steel.

De préférence, le tube en alliage de palladium-argent qui constitue la cathode est soumis à un traitement thermique de recuit, puis sa surface externe est traitée par abrasion mécanique au moyen d'oxyde ferrique avant d'être revêtue de noir de palladium par électrolyse.Preferably, the palladium-silver alloy tube which constitutes the cathode is subjected to an annealing heat treatment, then its external surface is treated by mechanical abrasion using ferric oxide before being coated with palladium black by electrolysis. .

D'autres avantages et caractéristiques de l'invention apparaîtront mieux à la lecture de la description qui suit donnée bien entendu à titre illustratif et non limitatif en référence au dessin annexé qui représente en coupe verticale un dispositif de traitement d'effluents aqueux contenant de l'eau tritiée.Other advantages and characteristics of the invention will appear better on reading the following description given of course by way of illustration and not limitation with reference to the appended drawing which represents in vertical section a device for treating aqueous effluents containing l tritiated water.

Sur cette figure, on voit que le dispositif comprend une cellule d'électrolyse 1 réalisée par exemple en céramique non soluble en milieu alcalin, en métal ou en alliage métallique non corrodable tel que l'acier 316L 22 CND 17-13. De préférence, elle est réalisée en acier inoxydable passivé. La cellule 1 est fermée de façon étanche à sa partie supérieure par un couvercle 3. A l'intérieur de la cellule est disposée une cathode 5 constituée par un tube fermé à son extrémité inférieure, et la paroi de la cellule constitue l'anode 7.In this figure, it can be seen that the device comprises an electrolysis cell 1 made, for example, of ceramic which is not soluble in an alkaline medium, of metal or of a non-corrodable metal alloy such as steel 316L 22 CND 17-13. Preferably, it is made of passivated stainless steel. The cell 1 is closed in leaktight manner at its upper part by a cover 3. Inside the cell is disposed a cathode 5 constituted by a tube closed at its lower end, and the wall of the cell constitutes the anode 7 .

Des passages de courant isolés électriquement traversent la paroi de la cellule pour alimenter respectivement la cathode 5, et deux sondes 11 et 13 qui permettent d'assurer le contrôle du niveau de solution à l'intérieur de la cellule. A sa partie supérieure, le dispositif comprend un condenseur 15 et une conduite d'alimentation en solution électrolytique 17 munie d'une vanne 18 commandée par un relais électrique associé aux sondes 11 et 13, ainsi qu'une conduite 19 d'introduction d'un gaz inerte. Par ailleurs, le dispositif comprend des moyens de chauffage 21 de la cellule d'électrolyse constitués par des résistances électriques commandées à partir d'un thermostat qui assure la régulation thermique.Electrically isolated current passages pass through the cell wall to supply the cathode 5, respectively, and two probes 11 and 13 which make it possible to monitor the level of solution inside the cell. At its upper part, the device comprises a condenser 15 and an electrolytic solution supply line 17 provided with a valve 18 controlled by an electrical relay associated with the probes 11 and 13, as well as a line 19 for introducing an inert gas. Furthermore, the device comprises heating means 21 of the electrolysis cell constituted by electrical resistances controlled from a thermostat which ensures thermal regulation.

Comme représenté sur le dessin, la cathode 5 est constituée par un tube creux 5a de section circulaire ayant une épaisseur de 50 à 250 um fermé à son extrémité inférieure qui délimite le compartiment 23 de réception du tritium raccordé à sa partie supérieure à un dispositif de récupération du tritium. Ce dispositif de récupération du tritium doit être étanche pour conserver la grande pureté du tritium diffusé et il peut être maintenu sous vide au moyen d'une pompe primaire à palettes. Généralement, ce dispositif comprend une jauge à vide et un manomètre pour contrôler le vide, une enceinte intermédiaire de stockage du tritium, une éprouvette de prise d'échantillons gazeux et un piège servant à stocker le tritium sous forme de tritiure. Le vide peut être obtenu grâce à un groupe de pompage.As shown in the drawing, the cathode 5 is constituted by a hollow tube 5a of circular section having a thickness of 50 to 250 μm closed at its lower end which delimits the compartment 23 for receiving tritium connected at its upper part to a device for recovery of tritium. This tritium recovery device must be sealed to maintain the high purity of the diffused tritium and it can be maintained under vacuum by means of a primary vane pump. Generally, this device comprises a vacuum gauge and a pressure gauge to control the vacuum, an intermediate enclosure for storing tritium, a test tube for taking gaseous samples and a trap used for storing tritium in the form of tritiide. The vacuum can be obtained by a pumping group.

Le tube constituant la cathode 5 est réalisé en alliage de palladium-argent non poreux, perméable à l'hydrogène et imperméable aux autres gaz, et il a subi un recuit à une température de 650° C pendant une heure sous un vide de l'ordre de 1,35 Pa pour faire disparaître l'orientation des grains due au laminage. Après ce traitement de recuit, la surface externe du tube destinée à être en contact avec la solution à électrolyser a subi un traitement d'abrasion mécanique au moyen d'une poudre d'oxyde ferrique Fe203 humectée, avec de l'eau, pendant quelques minutes, comme accélerateur d'hydrogénation du palladium, puis on a déposé sur cette surface ainsi traitée une couche de noir de palladium finement divisé et poreux d'une épaisseur de 7 ¡.Lm, afin d'augmenter la surface active de palladium au contact du tritium déchargé électriquement. Ce dépôt de noir de palladium finement divisé et poreux a été réalisé par électrolyse d'une solution de chlorure de palladium contenant 4 g de PdC12 dissous dans 20 cm3 d'HCI à 12 mol/l., dilué ensuite à 500 cm3 avec de l'eau distillée, en opérant sous une densité de courant cathodique de 150 mA/cm2 à une température de 20° C, pendant 4 min.The tube constituting the cathode 5 is made of a non-porous palladium-silver alloy, permeable to hydrogen and impermeable to other gases, and it has been annealed at a temperature of 650 ° C. for one hour under a vacuum of order of 1.35 Pa to remove the orientation of the grains due to rolling. After this annealing treatment, the external surface of the tube intended to be in contact with the solution to be electrolyzed has undergone a mechanical abrasion treatment by means of a ferric oxide powder Fe 2 0 3 moistened, with water , for a few minutes, as an accelerator for the hydrogenation of palladium, then a layer of finely divided and porous palladium black with a thickness of 7 μm was deposited on this surface thus treated, in order to increase the active surface of palladium on contact with electrically discharged tritium. This deposit of finely divided and porous palladium black was produced by electrolysis of a solution of palladium chloride containing 4 g of PdC1 2 dissolved in 20 cm3 of HCl at 12 mol / l., Then diluted to 500 cm 3 distilled water, operating at a cathode current density of 150 mA / cm 2 at a temperature of 20 ° C, for 4 min.

Comme on l'a vu précédemment l'anode 7 est constituée par la paroi de la cellule 1 et elle est reliée au pôle positif du générateur de courant électrique. Une telle disposition de l'anode et de la cathode permet d'obtenir une bonne répartition du courant sur la surface de la cathode et la formation d'équipotentielles régulières. Le courant d'électrolyse est programmé à l'aide d'un potentiostat fonctionnant en mode intensiostatique.As previously seen, the anode 7 is formed by the wall of the cell 1 and it is connected to the positive pole of the electric current generator. Such an arrangement of the anode and the cathode makes it possible to obtain a good distribution of the current over the surface of the cathode and the formation of regular equipotentials. The electrolysis current is programmed using a potentiostat operating in intensiostatic mode.

Dans ce dispositif, on peut traiter des solutions contenant de l'eau tritiée de la façon suivante: on met en circulation dans la cellule 1, par la conduite 17 la solution à électrolyser qui est constituée par de l'eau tritiée contenant de 1 à 20 mol.1-1 d'hydroxyde de sodium. Cette eau tritiée a été obtenue par oxydation catalytique d'effluents gazeux contenant du tritium. Dès que le niveau de solution dans la cellule détecté par les sondes 11 et 13 atteint la valeur voulue, l'introduction de solution s'arrête automatiquement. On met alors en marche le dispositif de chauffage pour amener la température de la solution à environ 80°C, on introduit de l'argon par la conduite 19 et on relie les électrodes 5 et 7 au générateur de courant électrique pour électrolyser la solution avec une densité de courant cathodique de 60 mA.cm-2 et obtenir un dégagement de tritium gazeux sur la cathode 5. Le tritium est adsorbé par la cathode 5, puis il diffuse à l'intérieur du tube 5 normalement en dépression par pompage, mais le procédé peut fonctionner lorsque la pression des gaz à l'intérieur du tube est très supérieure à la pression de la cellule d'électrolyse. Dans ces conditions, on peut obtenir un débit de tritium de l'ordre de 1 cm.min-1. Les gaz libérés lors de l'électrolyse, c'est-à-dire l'oxygène, ainsi que le tritium qui n'a pas diffusé dans le tube 5 et la vapeur d'eau, sont évacués de la cellule par le courant d'argon vers le condenseur 15 dans lequel la vapeur d'eau est condensée puis recyclée à l'intérieur de la cellule 1. Les gaz sortant du condenseur seront envoyés dans un ensemble de recombinaison catalytique pour reformer de l'eau tritiée qui pourra ensuite être recyclée à l'intérieur de la cellule.In this device, solutions containing tritiated water can be treated in the following way: the solution to be electrolyzed which is constituted by tritiated water containing from 1 to 20 mol. 1-1 of sodium hydroxide. This tritiated water was obtained by catalytic oxidation of gaseous effluents containing tritium. As soon as the level of solution in the cell detected by the probes 11 and 13 reaches the desired value, the introduction of solution stops automatically. The heating device is then switched on to bring the temperature of the solution to around 80 ° C., argon is introduced via line 19 and the electrodes 5 and 7 are connected to the electric current generator to electrolyze the solution with a cathode current density of 60 mA.cm- 2 and obtain a release of gaseous tritium on the cathode 5. The tritium is adsorbed by the cathode 5, then it diffuses inside the tube 5 normally under vacuum by pumping, but the process can work when the gas pressure inside the tube is much higher than the pressure of the electrolysis cell. Under these conditions, we can obtain a tritium flow rate of the order of 1 cm.min- 1 . The gases released during electrolysis, i.e. oxygen, as well as the tritium which has not diffused in the tube 5 and the water vapor, are evacuated from the cell by the current d argon to the condenser 15 in which the water vapor is condensed and then recycled inside the cell 1. The gases leaving the condenser will be sent in a catalytic recombination assembly to reform tritiated water which can then be recycled inside the cell.

Dans ce but, la canalisation d'évacuation des gaz sortant du condenseur 15 peut déboucher dans un élément d'oxydation catalytique du tritium résiduel, cet élément étant constitué de noir de palladium fixé sur de l'alumine. Le tritium recombiné à l'oxygène sous la forme d'eau lourde est ensuite condensé dans un échangeur thermique et éventuellement recyclé dans la cellule 1. On peut brancher une ampoule de prélèvement sur la canalisation de sortie des gaz de façon à analyser les gaz extraits, soit à la sortie de la cellule d'électrolyse, soit après l'élément d'oxydation catalytique.For this purpose, the gas evacuation pipe leaving the condenser 15 can lead to a catalytic oxidation element for residual tritium, this element consisting of palladium black fixed on alumina. The tritium recombined with oxygen in the form of heavy water is then condensed in a heat exchanger and optionally recycled in cell 1. It is possible to connect a sampling bulb to the gas outlet pipe so as to analyze the gases extracted. , either at the outlet of the electrolysis cell, or after the catalytic oxidation element.

Ainsi, on peut obtenir par le procédé de l'invention du tritium dans un état de grande pureté, exempt en particulier de vapeur d'eau.Thus, it is possible to obtain by the process of the invention tritium in a state of high purity, free in particular of steam.

Un dispositif de ce type a permis d'obtenir des résultats satisfaisants après des durées de fonctionnement d'environ six semaines sans aucun démontage de la cathode. Au bout de ce temps, celle-ci ne présentait pas de défaillance et la diffusion du tritium à travers sa paroi se faisait dans de bonnes conditions.A device of this type has made it possible to obtain satisfactory results after operating periods of approximately six weeks without any disassembly of the cathode. At the end of this time, the latter showed no failure and the diffusion of tritium through its wall was carried out under good conditions.

Ainsi on a observé un rendement de diffusion de 85 % sur une période de 330 heures, sans défaillance de la perméabilité de la cathode et sans intervention de l'opérateur.Thus, a diffusion efficiency of 85% was observed over a period of 330 hours, without failure of the permeability of the cathode and without operator intervention.

Par ailleurs, si l'on compare les résultats obtenus avec la cathode de l'invention à ceux que l'on obtient en utilisant une cathode en palladium non soumise aux traitements de recuit, d'abrasion mécanique au moyen d'oxyde ferrique et de dépôt de noir de palladium poreux, mais comportant un revêtement de noir de palladium sur sa surface interne limitant le compartiment de réception du tritium, on vérifie la supériorité de la cathode soumise au traitement de l'invention.Furthermore, if we compare the results obtained with the cathode of the invention to those obtained using a palladium cathode not subjected to annealing treatments, mechanical abrasion using ferric oxide and deposition of porous palladium black, but comprising a coating of palladium black on its internal surface limiting the tritium receiving compartment, the superiority of the cathode subjected to the treatment of the invention is verified.

En effet, pour obtenir un rendement de diffusion de 83 % avec une cathode cylindrique de 11 cm de hauteur, de 2,6 cm de diamètre et de 250 µm d'épaisseur, réalisée en alliage de palladium-argent traité conformément à l'invention, on peut réaliser l'électrolyse à 160°C, sous une densité de courant de 670 mA/cm2, et le débit de diffusion de l'hydrogène et du tritium est dans ces conditions de 3,9 CM 3.CM-2.Min-1.In fact, to obtain a diffusion efficiency of 83% with a cylindrical cathode 11 cm high, 2.6 cm in diameter and 250 μm thick, made of palladium-silver alloy treated in accordance with the invention , electrolysis can be carried out at 160 ° C, at a current density of 670 mA / cm 2 , and the diffusion rate of hydrogen and tritium is under these conditions of 3.9 CM 3 . CM - 2 . M in-1.

En revanche, si l'on utilise une cathode cylindrique de palladium-argent ayant une hauteur de 9 cm, un diamètre de 3 mm et une épaisseur de 100 lim, recouverte intérieurement de noir de palladium et n'ayant pas subi les traitements de l'invention, on peut obtenir un rendement de 83 % en opérant sous une densité de courant de 454 mA/cm-2, et dans ces conditions le débit de diffusion de l'hydrogène et du tritium est de 2,6 cm3.cm-2.min-1. Ainsi, on constate que le débit d'hydrogène et de tritium est plus important lorsqu'on utilise la cathode traitée selon l'invention.In contrast, if one uses a cylindrical cathode of palladium-silver having a height of 9 cm, a diameter of 3 mm and a thickness of 100 li m, entirely coated with palladium black and not having undergone treatments the invention, an efficiency of 83% can be obtained by operating at a current density of 454 mA / cm- 2 , and under these conditions the diffusion rate of hydrogen and tritium is 2.6 cm3.cm -2.min-1. Thus, it can be seen that the flow of hydrogen and tritium is greater when the cathode treated according to the invention is used.

De plus, grâce à la structure particulière de la cathode de l'invention, qui favorise la diffusion du tritium, on peut traiter à une température de 160°C des solutions d'eau tritiée dans la soude 20N ayant une activité de 10 à 100 Ci/I.In addition, thanks to the particular structure of the cathode of the invention, which promotes the diffusion of tritium, it is possible to treat at a temperature of 160 ° C. solutions of tritiated water in 20N sodium hydroxide having an activity of 10 to 100 Ci / I.

Enfin, le procédé et le dispositif de l'invention permettent de résoudre les problèmes de sécurité posés par la manipulation d'eau tritiée, le rejet des effluents contaminés, notamment en ce qui concerne la fraction hydrogène-tritium dégagée dans la cuve d'électrolyse, ains que les problèmes de tenue des matériaux à l'eau tritiée, de radiolyse de l'eau tritiée et d'interaction avec l'azote de l'air qui conduit à des composés corrosifs.Finally, the method and the device of the invention make it possible to solve the safety problems posed by the handling of tritiated water, the rejection of contaminated effluents, in particular as regards the hydrogen-tritium fraction released in the electrolysis tank. , as well as the problems of resistance of materials to tritiated water, radiolysis of tritiated water and interaction with nitrogen in the air which leads to corrosive compounds.

En effet, le dispositif de l'invention comprend les moyens nécessaires:

  • - pour isoler la cellule de l'atmosphère environnante,
  • - pour récupérer le tritium à l'état très pur après diffusion dans la cathode, et
  • - pour éliminer et recycler la vapeur d'eau, l'oxygène, l'hydrogène et le tritium résiduels présents dans les gaz portant de la cellule, ce qui permet d'éviter la production de nouveaux effluents radioactifs.
Indeed, the device of the invention comprises the necessary means:
  • - to isolate the cell from the surrounding atmosphere,
  • to recover the tritium in the very pure state after diffusion into the cathode, and
  • - to eliminate and recycle water vapor, oxygen, hydrogen and residual tritium present in the gases carrying the cell, which avoids the production of new radioactive effluents.

Claims (16)

1. Process for the treatment of a solution containing water formed from an isotope of hydrogen and which comprises adding to the solution an electrolyte chosen in such a way that the solution obtained can release by electrolysis the isotope of the hydrogen in the gaseous state, subjecting the thus obtained solution to electrolysis so as to obtain the giving off of the hydrogen isotope, by operating in an electrolysis cell having a cathode made from a metal able to aid the diffusion of the hydrogen isotope, said cathode forming a tight separating wall between the solution to be electrolyzed and a hydrogen isotope reception compartment and in said compartment is recovered the desorbed hydrogen isotope of said cathode, characterized in that the hydrogen isotope is tritium and the cathode is coated with a deposit of porous palladium black on its surface in contact with the solution to be electrolyzed.
2. Process according to claim 1, characterized in that the cathode is coated with a porous palladium black deposit on its desorption face.
3. Process according to one of the claims 1 or 2, characterized in that the tritium is recovered in the form of metallic tritiide in the solid state by reacting the tritium with a compound able to form metallic tritiide.
4. Process according to any one of the claims 1 to 3, characterized in that the cathode is made from palladium or a palladium alloy.
5. Process according to any one of the claims 1 to 4, characterized in that as the electrolyte is of sodium hydroxide, the electrolyte concentration of the solution to be electrolyzed is 1 mol.1-1 to 20 mol.1-1.
6. Process according to any one of the claims 1 to 5, characterized in that electrolysis is performed by maintaining the solution to be electrolyzed at a temperature of 50 to 1600 C.
7. Process according to any one of the claims 1 to 6, characterized in that the cathode is made from palladium or a palladium alloy and has a thickness of 50 to 250 um, electrolysis is carried out with a current density between 60 and 150 milliamperes/cm2 at 80°C.
8. Apparatus comprising:
- an electrolytic cell (1) for containing an electrolytic solution able to release the hydrogen isotope in the gaseous state by electrolysis, said cell having an anode (7) and a cathode (5) made from a metal able to adsorb the hydrogen isotope, said cathode being such that it constitutes a separating wall between the solution to be electrolyzed and a hydrogen isotope reception compartment (23), means for establishing a potential difference between the anode and the cathode, means for circulating the solution to which an electrolyte has been added within the cell, characterized in that the hydrogen isotope is tritium, said cathode being coated with porous palladium black on its surface in contact with the solution to be electrolyzed, the apparatus comprising means for recovering the oxygen given off in the cell, means for condensing the water vapour formed in the cell and for recycling the condensed water vapour in the solution to be electrolyzed.
9. Apparatus according to claim 8, characterized in that it comprises means for catalytically recombining the oxygen, hydrogen and tritium given off in said cell and means for recycling in the cell the thus formed tritiated water.
10. Apparatus according to claim 8, characterized in that the cathode (5) is constituted by a hollow tube, sealed at one of its ends and disposed in the cell in such a way that it is partly immersed in the electrolytic solution, the space defined within the tube constituting the tritium reception compartment (23).
11. Apparatus according to claim 10, characterized in that the hollow tube is made from palladium or palladium alloy externally coated with porous palladium black.
12. Apparatus according to claim 11, characterized in that the hollow tube is internally covered with porous palladium black.
13. Apparatus according to any one of the claims 8 to 12, characterized in that the anode is constituted by the wall of the electrolytic cell, said wall being made from stainless steel.
14. Electrode made from palladium or a palladium alloy for an apparatus for electrolyzing an aqueous solution, characterized In that it has a porous palladium black coating at least on the surface thereof which is in contact with the electrolyte.
15. Process for the preparation of an electrode according to claim 14, characterized in that it comprises subjecting a palladium or palladium alloy electrode to a thermal annealing treatment, then carrying out on the electrode surface to be in contact with the solution to be electrolyzed a mechanical abrasion treatment by means of a moist ferric oxide as the hydrogenation accelerator of the palladium and then coating the thus treated surface with finely divided, porous palladium black.
16. Process according to claim 15, characterized in that the palladium black coating is formed by the electrolysis of a solution of palladium chloride in dilute hydrochloric acid.
EP82402226A 1981-12-09 1982-12-06 Process and device for treating solutions containing tritiated water, electrode to be used in this device and its manufacturing process Expired EP0082061B1 (en)

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FR8123033 1981-12-09
FR8123033A FR2517663B1 (en) 1981-12-09 1981-12-09 METHOD AND DEVICE FOR TREATING AQUEOUS EFFLUENTS CONTAINING TRITIATED WATER, ELECTRODE FOR USE IN SUCH A DEVICE, AND METHOD FOR PREPARING THE SAME

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EP0082061A2 EP0082061A2 (en) 1983-06-22
EP0082061A3 EP0082061A3 (en) 1983-07-20
EP0082061B1 true EP0082061B1 (en) 1988-06-29

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FR2517663B1 (en) 1985-08-09
FR2517663A1 (en) 1983-06-10
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EP0082061A3 (en) 1983-07-20
JPH0129439B2 (en) 1989-06-09

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