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
  • G21F9/001—Decontamination of contaminated objects, apparatus, clothes, food; Preventing contamination thereof

Definitions

• the subject of the present invention is a reducing decontaminating gel which can be used for radioactive decontamination of surfaces, in particular of metallic surfaces covered by a very adherent surface layer of metal oxides.
• the metal surfaces of these primary circuits which are usually made of stainless steel, Incoloy or Inconel, are generally covered with deposits of iron, chromium and nickel oxides which can take various forms, for example in the form nickel ferrite NiFe 2 0 4 , magnetite Fe 3 0 4 , nickel ferrite and chromium, chromium ferrite FeCr 2 0 4 , nickel chromite NiCr 2 0 4 and chromium oxide Cr 2 0 3 .
• an oxidizing decontaminating gel can be used, as described in FR-A-2 656 949.
• this oxidizing gel which makes it possible to remove the radioactive elements deposited on the part as well as the encrusted elements on its surface, does not have sufficient effectiveness with respect to the adherent metal oxide layers deposited on the surface of alloys such as austenitic steels, the Inconel 600 and the Incoloy.
• the oxide layers deposited on these metal surfaces contain radioactive elements such as 58 Co, 60 Co and 51 Cr which are emitters of gamma photons which it is therefore important to eliminate.
• the present invention specifically relates to a reducing decontaminating gel which makes it possible to eliminate these adherent metal oxide layers.
• a gel is a colloidal solution, the phases of which are difficult to define due to the molecular weight of the colloid and its large state of dispersion in solution.
• This gel structure is obtained by adding component a) to an aqueous solution, that is to say an inorganic gelling agent which does not oxidize, resists components b) and c), and has a high specific surface. , for example greater than 100m 2 / g.
• the content of gelling agent in the colloidal solution is chosen so as to obtain a viscosity sufficient to be able to maintain the gel in a layer on the surface of a part.
• the gel has a viscosity at least equal to 350mPa.s at the time of use in order to be able to apply it easily on the surface of a part, for example by spraying with a gun.
• the content of gelling agent represents 20 to 30% by weight of the solution; it depends in particular on the gelling agent used.
• the gelling agent is based on alumina A1 2 0 3 and it can be obtained by hydrolysis at high temperature.
• a gelling agent which can be used mention may be made of the product sold under the trade name Alumina C.
• the inorganic base used in the decontaminating gel of the invention can in particular be NaOH or KOH.
• NaOH is used in particular at concentrations of 7 to 10 mol / l to obtain a pH at least equal to 13.
• the reducing agent used in the gel of the invention is a reducing agent having a normal redox potential E o less than -600mV / ENH (normal hydrogen electrode) in a strong base medium (pH ⁇ 13).
• reducing agents By way of example of such reducing agents, mention may be made of borohydrides, sulfites, hydrosulfites, sulfides, hypophosphites, zinc and hydrazine.
• borohydrides sulfites, sulfides, hydrosulfites or hypophosphites are used, these are generally in the form of metal salts, for example salts of alkali metals such as sodium.
• the pH of the colloidal solution is preferably greater than or equal to 14 so that the borohydride remains stable.
• Examples of reducing decontaminating gels according to the invention include gels in which the gelling agent is based on alumina, the inorganic base is NaOH and the reducing agent is sodium borohydride, sodium sulfide, sodium hydrosulfite or hydrazine.
• the reducing decontaminating gels of the invention can be prepared in a simple manner by adding to an aqueous solution of component b), that is to say of the inorganic base, the reducing agent and the gelling agent. Generally, the reducing agent is added before the gelling agent.
• the use, in the first step, of a reducing gel at pH ⁇ 13 in accordance with the invention makes it possible to weaken and move the adherent surface metal oxide layers which are not sensitive to the action oxidizing decontaminating gels.
• oxides such as nickel ferrite NiFe 2 0 4 and magnetite Fe 3 0 4 to reduce Fe "'to Fe".
• the action of the oxidizing gel in an acid medium for example HN0 3 at a concentration of 5 to 10 mol / l, makes it possible to oxidize the chromites which are rather within the deep oxide layer.
• the simultaneous action of the oxidizing agent on the chromite and on the underlying metal facilitates decontamination.
• the decontaminating gels described in particular in FR-A-2 656 949 are used as the oxidizing gel.
• oxidizing gels By way of example of oxidizing gels, mention may be made of those in which the gelling agent is based on silica, the inorganic acid is nitric acid and the oxidizing agent is cerium sulphate.
• the action of the reducing gel in the first step alone is not decisive, but following this first step of reducing treatment, the application of the oxidizing gel in an acid medium makes it possible to achieve average decontamination factors of 2, 7 instead of 1.2 when using the oxidizing gel alone.
• the process of the invention is therefore very interesting because it allows excellent results to be obtained, when cold and with small amounts of reagent.
• the application of the various gels to the metal surface to be decontaminated can be carried out by conventional methods, for example by spraying with a spray gun, by soaking and draining, by packaging or even by means of a brush.
• the gel is applied by spraying with a spray gun, for example under a pressure (Airless compressor) at the level of the injector ranging from 50 to 100 kg / cm 2 .
• demineralized water or an aqueous solution is usually used in which the gels used can be dissolved or in which they can form a detachable film which can be entrained by water.
• the reducing gel R1 is deposited thereon by spraying with a spray gun so as to deposit 800g of gel per m 2 of surface and the gel is kept on the surface for 2 hours. .
• a rinse is then carried out with demineralized water using 51 of water per m 2 of surface.
• the oxidizing gel 01 is then applied to the thus rinsed surface by spraying with a spray gun as above and it is also maintained on the surface of the part for 2 hours using an amount of 800 g of gel per m 2 of surface.
• a rinse is then carried out with demineralized water as above.
• the activity of the surface is determined (in mrad / h).
• the decontamination factors FD have also been given which correspond to the ratio of the initial activity to the final activity.
• a contaminated part made of AISI 304 steel is treated using the decontaminating oxidizing gel 01.
• Example 1 If we compare the results of Example 1 and Comparative Example 1, we see that the preliminary step of applying a reducing gel makes it possible to significantly improve the decontamination factor.
• Example 2 the same procedure is followed as in Example 2, using the reducing gel R1 and the oxidizing gel 02 to decontaminate a part made of INCONEL 600.
• the oxidizing gel is the oxidizing gel 02 in Table 2.

Landscapes

• Engineering & Computer Science (AREA)
• Life Sciences & Earth Sciences (AREA)
• Food Science & Technology (AREA)
• Physics & Mathematics (AREA)
• General Engineering & Computer Science (AREA)
• High Energy & Nuclear Physics (AREA)
• Detergent Compositions (AREA)
• Chemically Coating (AREA)
• Adhesives Or Adhesive Processes (AREA)
• Solid-Sorbent Or Filter-Aiding Compositions (AREA)

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Citations (4)

• 1992
  • 1992-09-23 FR FR9211317A patent/FR2695839B1/fr not_active Expired - Fee Related
• 1993
  • 1993-09-21 EP EP19930402307 patent/EP0589781B1/de not_active Expired - Lifetime
  • 1993-09-21 DE DE1993624203 patent/DE69324203T2/de not_active Expired - Fee Related
  • 1993-09-21 ES ES93402307T patent/ES2132203T3/es not_active Expired - Lifetime

Patent Citations (4)

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