Patent Number: 056334230
Section: summary

TECHNICAL FIELD OF THE INVENTION The present invention relates to the processing of effluents containing small amounts of radionucleides (below approximately 5.10.sup.3 kBq/liter), such as e.g. strontium 90 (.sup.90 Sr), antimony 125 (.sup.125 Sb), cesium 137 .sup.137 Cs), and ruthenium 106 (.sup.106 Ru). More specifically, the invention relates to the decontamination of such effluents by the electrodissolution of an anode permitting the concentration in solid form and in a small volume allowing easy storage of the contamination present in the effluent. PRIOR ART A process for the decontamination of a solution containing ruthenium 106 by the electrodissolution of an iron anode is known from the article by F. KEPAK et al, published in "International Journal of Applied Radiation and Isotopes", vol. 19, pp 485-487, 1968. According to the procedure described in this document, the ruthenium is trapped by iron hydroxide particles. These particles are obtained by precipitation or dissolution by electrolysis of an iron anode. This procedure is applied to a solution containing a high ruthenium radioactivity (42.times.10.sup.6 kBq/liter), i.e. a high concentration. Tests relating to the decontamination by electrodissolution of an iron anode on effluents, whose radioelement concentrations are approximately 10.sup.4 times lower have made it possible to obtain max 20% decontamination factors for ruthenium. DESCRIPTION OF THE INVENTION The invention therefore relates to a consumable anode, a process for the electrodissolution of said anode and an apparatus for the performance thereof making it possible to attain high decontamination rates, at least for effluents with low radioelement concentrations. More specifically, a consumable anode according to the invention has between 20 and 70% by weight iron, between 20 and 40% by weight cobalt and between 5 and 30% by weight aluminium, the sum total of the weight percentages of these three elements being equal to or below 100. The electrodissolution of such an anode makes it possible to form, in the solution in which the anode is immersed, mixed hydroxides formed from iron, cobalt and aluminium. These hydroxides have an entraining effect on the radioelements present in the solution. The effectiveness of this entraining effect is linked with the simultaneous presence of the three elements. According to variants of the anode according to the invention, it can also contain nickel (less than 20 wt. %) and/or titanium (less than 10 wt. %) and/or copper (less than 5 wt. %) and/or niobium (less than 5 wt. %). The presence of these additional elements makes it possible to improve the precipitation and extraction of certain specific elements present in the solution. Thus, the presence in the anode of niobium and/or titanium makes it possible to improve antimony extraction. The presence in the anode of nickel and/or copper makes it possible to improve ruthenium extraction. The composition of the anode alloy determines the type and proportions of the insoluble hydroxide mixture, which are absorbing supports which, by coprecipitation, dissolve the radioelements. Thus, the weight of the metallic hydroxides formed is a function of the dissolved anode weight. The invention also relates to a process for the decontamination of radioactive liquid elements comprising the following stages: placing an anode like that described hereinbefore, as well as a cathode, in the liquid effluent, PA1 bringing the pH of the effluent to a value equal to or higher than 1, PA1 producing a direct current between the anode and the cathode, so that the potential of the anode is above 2 V/NHE (Normal Hydrogen Electrode). With such a process, the anode is dissolved to form insoluble metallic hydroxides, which entrain by coprecipitation the radioelements of the effluent. With this process it has proved possible to attain decontamination rates above 90% for certain elements, e.g. antimony and strontium. According to an embodiment of the process, the current is circulated until the electricity quantity consumed is equal to at least 8 coulombs/milliliter. According to another embodiment of the invention, part of the mud or sludge obtained after precipitation and entrainment of the radioelements can be recycled into the reactor, the other part undergoing a separation stage by decanting or settling, filtration or centrifuging in order to separate the insoluble precipitate of metallic hydroxides from the effluent. According to another aspect of the invention, an apparatus for performing the decontamination process comprises a reactor for receiving the effluent to be decontaminated, an anode like that described hereinbefore, a cathode, a direct current supply to which are connected the anode and the cathode and a means for raising the pH of the solution to a value equal to or greater than 1. According to an embodiment of the apparatus, it can also comprise a reference electrode. It can also comprise a temperature control thermocouple and a heat exchanger permitting the maintaining of the effluent at ambient temperature. It can finally incorporate means for recycling part of the sludge passing out of the reactor and solid/liquid separating means for treating another part of the extracted sludge.