Patent Number: 042773636
Section: summary

This invention relates to a method for processing a mixture of air and rare gases, especially xenon and krypton, and more especially for processing gaseous effluents arising from the reprocessing of irradiated nuclear fuels, a further object of this processing operation being to recover the xenon contained in these mixtures. The gaseous effluents from irradiated fuel reprocessing plants essentially consist of air having a concentration of oxygen which can be lower than the natural concentration, and rare gases such as xenon and krypton; such effluents usually contain water vapor, carbon dioxide gas, traces of hydrocarbons and also of nitrogen oxides, the presence of which is due to the dissolution of fuels in a nitric acid medium. The known methods for processing said gaseous effluents essentially comprise a purification step for delivering a mixture of air and rare gases which is free from other impurities, then a step consisting of preconcentration of the rare gases in one of the two constituents of air, namely oxygen or nitrogen. In the process which consists in preconcentrating the rare gases in nitrogen, said preconcentration step is carried out after catalytic reduction of the entire quantity of oxygen contained in the mixture with hydrogen and consists in concentration of the rare gases in a fraction of the nitrogen by cryogenic distillation. The mixture of krypton and xenon in the nitrogen is then subjected to a further cryogenic distillation in order to separate the krypton and nitrogen from the xenon. This method suffers from a disadvantage by reason of the low solubility of xenon in liquid nitrogen, thus giving rise to clogging of distillation columns as a result of crystallization of the xenon. In the other method which consists in preconcentrating the rare gases in oxygen, this preconcentration is carried out by removing the entire quantity of nitrogen contained in the mixture as well as part of the oxygen by cryogenic distillation. The krypton and xenon mixture concentrated in the oxygen which is then obtained is first subjected to a step involving catalytic reduction of the oxygen with hydrogen, then to a cryogenic distillation step in order to separate the krypton from the xenon. Compared with the method of preconcentration in nitrogen, the advantage of this method lies in the fact that xenon has much greater solubility in liquid oxygen than in liquid nitrogen. By reason of the radioactivity of the medium, however, ozone is formed in the boiler of the distillation column; this presence of ozone gives rise to explosion hazards. These explosion hazards could quite conceivably be forestalled by continuous destruction "in situ" of the ozone which is formed; the design development of an ozone removal system of this type, however, gives rise to a certain number of problems and has never been carried into practical effect. The present invention is precisely directed to a method of processing, especially of gaseous effluents arising from the reprocessing of irradiated fuels, which overcomes the disadvantages recalled in the foregoing. In fact, the method under consideration does not give rise to the formation of ozone and there is consequently no attendant danger of explosion. In regard to crystallization of xenon, such problems can readily be circumvented. The method according to the invention essentially comprises a step involving concentration of rare gases in solution in liquid argon by cryogenic distillation of light gases and mainly nitrogen from the liquefied mixture. Preferably, said method further comprises, after said concentration step, a step involving recovery of the xenon by cryogenic distillation of the other gas or gases from the concentrated and liquefied mixture. In fact, the present Applicant has studied the behavior of xenon from the point of view of solubility in three cryogenic solvents, namely oxygen, nitrogen and argon. In the studies made by the Applicant, it has been sought to determine the maximum pressure at which a solid-liquid-vapor xenon-solvent equilibrium can exist. In order to prevent any danger of crystallization of the xenon, it was therefore important to carry out the preconcentration step at the notable pressure aforesaid. The researches conducted by the present Applicant have led to the discovery that, in the case of an oxygen-xenon system, said notable pressure has a maximum value of 17 bar absolute, that said notable pressure has a maximum value of 18.5 bar absolute in the case of an argon-xenon system, and that said notable pressure would be appreciably higher and of the order of 35 bar absolute, for example, in the case of a nitrogen-xenon system. For the foregoing reason, the fact of carrying out a step involving preconcentration of xenon and of krypton in liquid argon in the method according to the invention has the advantage over the method of preconcentration of xenon and krypton in nitrogen of preventing crystallization of the xenon by carrying out cryogenic distillation at a distinctly lower pressure. A further advantage over the method of preconcentration of xenon and krypton in oxygen lies in the fact that ozone formation is avoided. The invention is also directed to a method of processing of effluent gases arising from the reprocessing of irradiated nuclear fuels constituted by a mixture containing at least radioactive krypton and xenon in air. The method essentially and successively comprises a first step involving removal of any impurities such as hydrocarbons, nitrogen oxides, carbon dioxide gas, water vapor, a second step involving concentration of xenon and krypton in solution in liquid argon by distillation of the light gases and mainly nitrogen from the liquefied mixture, a third step involving removal of argon by cryogenic distillation of the separated mixture of argon, xenon and krypton which has previously been liquefied, and a fourth step involving separation of xenon and krypton by cryogenic distillation of the separated mixture of xenon and krypton which has previously been liquefied. In the method under consideration, the argon required for the second concentration step can be introduced into the gas mixture to be processed either before the first step involving removal of various impurities or before the second concentration step. According to an advantageous feature of the method of the invention, part of the krypton which has been separated from the xenon and obtained during the fourth step is introduced into the mixture obtained at the end of the first step before proceeding to the second step of concentration of the xenon and krypton in solution in liquid argon. This re-introduction of a certain quantity of the krypton obtained at the end of the process makes it possible to reduce the pressure at which the second step of concentration of xenon and krypton in liquid argon is carried out. It is readily apparent that the invention also extends to suitable installations for the practical application of the method. Installations of this type comprise in particular catalytic reactors and cryogenic distillation columns with all their ancillary equipment units which are known per se, arranged and connected together by means of pipes for the circulation of different products, thus making it possible to perform the successive operations of the method defined in the foregoing.