Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/14—Cleaning or pickling metallic material with solutions or molten salts with alkaline solutions
• • 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
  • G21F9/001—Decontamination of contaminated objects, apparatus, clothes, food; Preventing contamination thereof
  • G21F9/002—Decontamination of the surface of objects with chemical or electrochemical processes
  • G21F9/004—Decontamination of the surface of objects with chemical or electrochemical processes of metallic surfaces

Definitions

• the present invention relates to a method for dissolving the solids formed in a nuclear installation.
• solids which have formed on the walls of the apparatus and pipes, or which have accumulated at the bottom of the apparatuses of a nuclear fuel processing plant, or the tanks for storing liquid effluents originating in particular from reprocessing.
• These solids are formed on the walls of devices, tanks, containers, pipes and piping, in the form of scaling layers, or accumulate at the bottom of devices, tanks and other containers in the form of solid deposits.
• These solids consist essentially of the following crystalline forms:
• the solubility of a zirconium molybdate compound is less than 0.2 g / l in 4N nitric acid.
• One of the methods of the prior art dissolves part of these solids by two successive operations: namely an attack in basic medium by soda followed by a recovery of solids by nitric acid.
• the attack with sodium hydroxide allows the solubilization of ions with strong oxidation like molybdenum, but precipitates the other ions, of which the most troublesome are zirconium and plutonium, by formation of hydroxides with macromolecular structure [4].
• the penetration of the basic attack into the scaling layers is very limited by the re-precipitation of these compounds.
• soda is also penalizing for the operator because the possible presence of plutonium in the deposits requires at all times to guarantee the safety-criticality of the rinsing process by ensuring a non-accumulation of plutonium in hydroxylated form and requires rapid reacidification of alkaline solutions to avoid irreversible formation of hydrated plutonium oxide [4].
• the object of the invention is to provide a process for dissolving the solids formed in the apparatus and piping of a nuclear installation which meets, among other things, the needs indicated below and which satisfies some of the criteria and requirements. mentioned above, in particular, with regard to the dissolution medium.
• the object of the invention is also to provide a method for implementing the dissolution of the solids formed in the apparatus and piping of a nuclear installation which does not have the drawbacks, defects, limitations and disadvantages of the methods of the prior art and which provides a solution to the problems of the processes of the prior art.
• This object, and others still, are achieved, in accordance with the invention by a process for dissolving the solids formed in the apparatus and pipes of a nuclear installation in which said solids are brought into contact with an aqueous solution of dissolution chosen.
• aqueous solutions of carbonate ions with a concentration greater than or equal to 0.3M from aqueous solutions of carbonate ions with a concentration greater than or equal to 0.3M, aqueous solutions of bicarbonate ions, and aqueous solutions of a mixture of nitric acid and a polycarboxylic acid chosen from oxalic acid and triacids.
• the process of the invention uses aqueous solutions, the use of which for dissolving solids formed in the apparatus and piping of a nuclear installation, has never been mentioned or suggested in the prior art.
• the method of the invention meets all of the needs indicated above; in particular, the dissolution medium chosen from the aqueous solutions listed above satisfies all the criteria and all the requirements for such a dissolution medium.
• the contacting is generally carried out at moderate temperature, for example from 20 to 60 or 80 ° C., preferably at room temperature, for example 20-25 ° C.
• the contacting is relatively short, even to result in total dissolution of the solids. It will, for example, last from 1 to 24 hours depending on the physical form and the quantity of the compounds to be dissolved.
• the process of the invention also relates to a process for dissolving the solids formed in the apparatus and pipes of a nuclear installation.
• solid formed is meant the solid formed which is not the result of a normal process carried out in these installations, that is to say that it is undesirable, undesirable, parasitic solids which form in installations due in particular to the side reactions (undesired) which take place there or the fluids which circulate there.
• devices we mean all types of devices that can count the facilities mentioned above: it could be for example separation devices, dissolution devices, desorption, concentration, denitration, clarification, solution transfer, bubbling rods, measuring rods or nozzles.
• apparatus also includes tanks, reservoirs, vats, basins, enclosures for storing reagents, or liquid effluents, for example liquid effluents from reprocessing.
• piping is meant all piping and piping for fluid transfer which may be encountered in the installations described above.
• the solids which it is sought to eliminate, to dissolve, in the process of the invention are normally insoluble precipitates which are generally formed on the walls of the apparatus and pipes in the form of scaling layers or accumulated at the bottom of the apparatus. in the form of solid deposits.
• a solution can be circulated continuously over the deposits and / or layers to be eliminated, by rinsing the walls of the apparatus and pipes with the solution.
• these devices can be filled with the solution and left to act for the time necessary for the dissolution of the solids.
• the nature of the solids is variable, the compounds or crystalline forms which may be included in the composition of these solids are chosen, for example, from:
• the process according to the invention is just as effective, whatever the main constituent of the solids.
• the aqueous solution used in the process of the invention can be chosen from solutions of carbonate ions with a concentration greater than or equal to 0.3 M.
• the carbonate ion at these concentrations acts by the majority formation of charged ions soluble zirconium tetra-carbonate and plutonium following, for example, the reaction below for zirconium molybdate:
• the concentration of carbonate ions in the aqueous solution will preferably be 0.4M at the limit of solubility in water of the carbonate salt (from which the ion is derived). This limit varies, depending on the carbonate used and the temperature, it is generally from 2 M at 20 ° C to 3.4 M at 30 ° C, for example for sodium carbonate, for example it is d '' about 3M at 25 ° C for sodium carbonate.
• the solubility of the elements of the solid to be dissolved varies linearly with the initial concentration of carbonate ions up to the maximum concentration of carbonate ions (approximately 3 mol / 1 for sodium carbonate in water at 25 ° VS) .
• the solubility of zirconium molybdate is 315 g / 1 at 25 ° C for a carbonate concentration of 3 mol / 1 and the initial carbonate / dissolved Zr molar ratio is generally from 4 to 5 for example.
• the volume of dissolving solution used to dissolve the solids varies according to the concentration of the solution used but it is generally from 3 ml to 100 ml per gram of solids, for example for a 1 M carbonate solution is 10 to 30 ml per gram.
• the plutonium from the dissolved solids is stable for periods which exceed one week in the solution for dissolving carbonate ions, in the presence of the other dissolved elements. Its concentration is for example around 8 g / l in 1M carbonate medium. As with zirconium, the charged carbonate complexes are responsible for this stability.
• the salt from which the carbonate ions are derived is generally chosen from alkali metal ions such as sodium and potassium, alkaline earth metal ions and ammonium ions.
• alkali metal ions such as sodium and potassium, alkaline earth metal ions and ammonium ions.
• Sodium carbonate is preferred, but the use of different salts such as potassium or ammonium carbonates can give identical results while limiting the possibilities of coprecipitation of zirconium when hot (60 ° C.).
• the solubility of radio-contaminants other than plutonium can be increased by an appropriate choice of the counterion.
• the potassium counterion makes it possible to dissolve the basic forms of the antimony.
• the advantages of the carbonate ion as a dissolving reagent are numerous.
• an acid solution is added to the aqueous dissolution solution containing the carbonate ions; preferably a nitric acid solution.
• the destruction of the carbonate ion is complete after such acidification of the dissolution solution, for example with nitric acid.
• the aqueous dissolution solution can also be chosen from aqueous solutions of bicarbonate ions, or hydrogen carbonate, the concentration of these solutions is generally from 0 to 2 M in bicarbonate ions.
• the aqueous dissolution solution can finally be chosen from aqueous solutions comprising a mixture of nitric acid and a polycarboxylic acid chosen from oxalic acid and triacids.
• concentration of nitric acid in this solution is generally 0.05 to 1 M
• concentration of polycarboxylic acid in this solution is generally 0.3 to 1 M.
• the polycarboxylic acid which is used is therefore, according to the invention, generally chosen from oxalic acid and triacids such as citric acid. Oxalic acid is preferred.
• the mixture of oxalic and nitric acids acts by formation, when the oxalate concentration is sufficiently high (greater than 0.5 M), soluble charged oxalate complexes of zirconium and plutonium
• the dissolution of solids by the mixture of oxalic and nitric acids is at least as effective as sodium hydroxide and does not lead, under certain conditions, to the formation of solid species of zirconium and plutonium, for example when the concentration of oxalate ions is large enough (greater than or equal to about 0.5 M).
• the solubility of zirconium molybdate by this medium can be attributed by analogy with plutonium to the formation of complexes charged with zirconium oxalate, Zr (C 2 0) 3 ⁇ or Zr (C 2 0 4 ) 4 ⁇ preventing its condensation.
• the concentration of oxalate ions should preferably be sufficiently high (greater than or equal to about 0.5 M) and the concentration of nitric acid sufficiently low (less than or equal to 1 M) to limit the formation of neutral complexes capable of precipitating .
• the dissolution is carried out at a temperature of 20 to 80 ° C, for example 60 ° C and the solution resulting from the dissolution is stable at 25 ° C.
• the contacting step can be advantageously followed by a step of destroying the acids in the dissolution solution by oxidation, for example under the following conditions: nitric acidity of 3 N in the presence of Mn 2+ at 0.01 M at 100 ° C.
• the initial mass divided by the added volume is 96 + ⁇ g / 1: this is a value which increases the solubility in grams per liter.
• a lower value is obtained by analysis of an identical solution saturated with solid. To this end, 1.5 grams of zirconium molybdate crystals are placed in a flask containing 10 ml of 1 M sodium carbonate at a temperature of 20 ° C. The whole is stirred by a magnetic bar. After 10 hours, the solution is filtered with a filter of porosity 0.3 ⁇ m. The filtrate is dried for 6 days at 40 ° C until stabilization of the mass (the mass varies from less than 2% on a day of drying).
• the difference in mass before and after contact divided by the volume of the solution therefore 94 + 2 g / 1 in this example, is a lowering of the solubility.
• the solubility of zirconium molybdate in 1 M sodium carbonate at 20 ° C is therefore estimated to be between 92 and 97 g / L.

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• 2000
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