Classifications

• • 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
  • G21F9/28—Treating solids
  • G21F9/34—Disposal of solid waste

Definitions

• the invention is included within the field of nuclear power, since graphite is used in nuclear reactors as a moderator and neutron reflector inside the nucleus.
• the graphite During its stay inside the nucleus, the graphite is subject to an intense neutron flow which activates its structural carbon and the impurities it contains. Furthermore, during the operation of the reactor, it suffers a radioactive pollution process which adds to the activation process. As a consequence, the graphite becomes radioactive graphite and when it is extracted from the nucleus it must be conditioned to allow its storage.
• radioactive graphite comes basically from four kinds of power reactors the nuclei of which is formed by a stack of prismatic graphite rods with several cavities where combustible elements are housed.
• the graphite is a solid, quite inert element, with good mechanical properties.
• the graphite used in nuclear reactors is amorphous and formed by microcrystals with a low content of impurities.
• radioactive graphite The kind and concentration of radionuclides present in radioactive graphite depend on the content of impurities, the neutronic flow to which it has been subject and the pollution suffered, either at the nucleus or subsequently.
• radionucleids present in graphite are H-3, C-14 and Co-60.
• fixation and lixiviation capabilities depend on the aforementioned properties.
• H-3 is mainly generated from fuel, and it comes off as a gas. It is introduced in the graphite by fixing itself to active points of its structure, by means of chemical bonds.
• the H-3 found at the outer layers interchanges isotopically with the hydrogen of hydrocarbonated compounds and water absorbed from the atmosphere.
• the graphite escapes when the former come off. Therefore, the outer layers are less tritium rich than the adjacent inner ones.
• the C-14 comes from the activation of the carbon found in the graphite structure and therefore, it is very stable, and it only comes off by contact due to the unctuousness of the surface.
• the Co-60 comes from the external activation and pollution. That coming from the activation can be found throughout the graphite, and that from the pollution is found on the surface and it is much more lixiviable.
• the conditioning techniques used until present consist of the graphite coating with epoxy resins, bitumen and hydraulic binder.
• the ultimate aim of the graphite conditioning by means of metallic coating is to seal the radioactive graphite obtained in nuclear plants, to minimise the lixiviation of the radionucleids that it contains and eliminate the unctuousness of its surface, to prevent any pollution during its handling.
• the coating application system is based on the placement of two overlapped metallic layers with a thickness of a few microns, the first one made of copper and the second one of nickel, by means of the method known as "electroless”, or electrodeless catalytic reduction.
• the ions present at the metal salt are sequestered, releasing them as needed.
• Free copper ions are converted into metallic copper, preferably on the catalysed surface, where the activating energy of the reaction is smaller, thanks to the action of the reducer.
• the problem has been solved by applying an initial semi-permeable copper layer which serves as a backbone to smooth the surface irregularities and where an amorphous nickel layer is deposited to make the previous layer water tight.
• Copper has been used for the first layer since it is a metal easy to deposit and that serves to adjust the size of the metallic granules.
• the second layer is made of nickel since it is amorphous, very compact and resistant to corrosion.
• an intermediate granule size has been chosen, that enables to cover the surface cavities and pores, leaving small intergranular spaces that can be sealed by means of a nickel coating.
• the size of the granule has been achieved through an adequate catalysation process and the incorporation of an additive to the solution. The higher the number of active points is, the more nucleation points will be obtained and more copper crystals will be formed during the metallization, thus decreasing its size with the number.
• the concentration of nickel cations present in the solution has been limited to reduce the deposition speed and to prevent an excessively quick growth which might cause cracks.
• This kind of coatings is used to modify the properties of the surfaces, to make it more corrosion-resistant, to incorporate new properties or to improve their aesthetic aspect.
• it competes with electroplating, advantageously in some cases due to the possibility of modifying the characteristics of the deposit with additives.
• the possibility of depositing metals on non-conducting surfaces enables the technique to obtain metal-coated plastics.
• the polymeric surfaces are coated with a thin metal coat to make them more attractive, to protect them against corrosion or to modify its electric or magnetic properties.
• the aim is to change the aspect of the surface, to modify its electric or magnetic properties, or to protect it against external agents, but it has never been used as a barrier to prevent leakages of the products contained inside it, and never in the case of radioactive products.
• the interest has been focused on the composition of the deposited layer and on the characteristics of the metallic granules, but not on the room left between granules and the water tight coating which is the basis of this patent.
• the coating application system is based on the depositing of a metallic layer through the method known as "electroless” or electrodeless self-catalytic reaction.
• the process for the complete conditioning consists of four basic stages: catalysation, metallisation with copper, activation and metallisation with nickel.
• the parts are introduced and extracted from one bath to another, either draining them or not, as indicated below.
• the operations are performed at a room temperature between 18 and 25 °C.
• the bath tanks do not have necessarily to comply with any special requisite except, obviously, that they should not react with the solution.
• the object of the first stage consists of depositing on the active points of the surface a series of small particles of a catalyst metal that will serve as focuses for the copper deposition.
• the piece is subject to three operations:
• the graphite piece is preferably submersed into a acetone bath for a period exceeding one minute -depending on the graphite- although sometimes the period should be of ten minutes, which is the recommended time.
• the piece is taken out from the bath and dried at open air during five minutes, to eliminate the acetone.
• the piece After the regeneration, the piece is introduced inside a sensitization bath (Table 1, formula 2) of cobalt salts or nickel during 10 minutes.
• the piece is removed from the previous bath and drained for 1 minute to avoid that the remnants of the solution are reduced, forming metal particles suspended in the solution.
• the dried piece is introduced inside a reducer bath of potassic borohydride (Table 1 formula 3) during 30 minutes where the chemically absorbed cations are reduced to metal.
• a layer of crystalline compact metallic copper is deposited in two stages.
• the piece is submersed into an aqueous solution bath formed by Co(CH3COO)2 or CoCl2 (table I, formula 4) for 5 minutes.
• the piece After it has been taken out from the rinsing solution, the piece is submersed into the plating bath for a period ranging between 0.5 and 10 hours, and the pH value is set to 12 throughout the whole process.
• the plating solution contains a copper salt, a salt sequester, a reducer and additives (Table I, formula 5).
• the piece is taken to the nickel plating bath, where the sequester is sodium citrate and the reducer, hypophosphite (Table I, formula 6) for a period ranging between 5 and 10 hours. Finally, the piece is rinsed under abundant water.
• the pH should be kept between 8.7 and 9, by means of NaOH.
• the catalyst is fixed on the graphite surface, it is necessary to regenerate the active points which are saturated with molecules and atoms adhered by chemical absorption. This is done by eliminating the molecules or atoms by chemical attack or dissolution.
• the catalyst solution can be Ni, Pd, Co and, in general, any transitional metal.
• the metal cations are adsorbed through chemical adsorption at the active surface points.
• the only task of the rinsing bath is to carry the borohydride remnants.
• the plating concentrations and times defined in the invention are optimised for a room temperature ranging between 18 and 25 °C. At higher temperatures, the invention can also work provided that the concentrations and times are adjusted accordingly. However, this would entail the additional evaporation problems that the patent intends to avoid.
• the solution must be prepared in accordance with the sequence of operations defined in Table 2, in order to achieve a good dissolution of the products.
• the baths used throughout the whole plating process should be replaced or regenerated after several uses, since they get damaged or consumed.
• the rinsing bath used before submersing the piece inside the copper plating bath gets corrupted by exhaustion of CO2+, which is reduced to metallic Cobalt dispersed within the solution. It is regenerated by adding Co(CH3-COO)2 or CoCl2 and filtering it.
• the copper plating bath gets exhausted as the plating progresses.
• a regeneration process must be implemented. It is done by filtering, by adding the consumed copper salt and the necessary amount of Na2EDTA to dissolve all the copper, and by adding the consumed HCHO and setting the pH to 12 by means of NaOH.
• the nickel plating bath undergoes a process similar to the one of the copper bath. therefore, it must also be regenerated by filtering it and adding the nickel salt and the necessary NaH2PO2 and NH3 to restore the consumed products.
• the piece was dried inside a lab drier with silica gel and was subsequently weighted. It was appreciated that the weight had increased some 8 ⁇ m.
• the content of Co-60 present at the graphite dust collected from cutting area was analyzed.
• piece 1 - which had been treated with acetone - showed a metal coating while piece 2 - which has not been subject to this treatment - had no coating.
• piece 1 seemed to have a nickel coating and piece 2 only a copper one.

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• 1994
  • 1994-09-23 ES ES9402010A patent/ES2107936B1/es not_active Expired - Fee Related
  • 1994-11-03 EP EP94500171A patent/EP0703587A3/fr not_active Withdrawn
  • 1994-12-28 RU RU94045153A patent/RU2123213C1/ru active

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