Cleaning and liquid contact with solids – Processes – Metal base work – acid treating
Reexamination Certificate
2002-03-20
2004-02-10
Delcotto, Gregory (Department: 1751)
Cleaning and liquid contact with solids
Processes
Metal base work, acid treating
C134S002000, C134S039000, C134S040000, C510S202000, C510S203000, C510S245000, C510S254000, C510S207000, C510S476000, C510S477000, C510S480000, C510S499000
Reexamination Certificate
active
06689226
ABSTRACT:
This invention relates to an organic gel of decontamination, usable for radioactive decontamination of surfaces, especially metallic surfaces.
According to the invention, “organic” gel is a gel in which thickening agents are essentially organic, in other words, excluding every inorganic or mineral substance.
Decontamination of parts tarnished by radioactive elements can be made either by mechanical treatments or by chemical ones.
The methods using mechanical treatments are disadvantageous in that they entail a more or less significant modification of the part's surface. Another disadvantage is that these treatments are difficult to apply on parts of complicated shape.
The methods of treatment by soaking essentially consist of carrying away radioactive elements fixed on the surface of the part by means of solution of actual appropriate agents of decontamination, especially of Ce(IV) stabilized in acid environment, highly concentrated such as nitric acid or sulfuric acid. These methods present the disadvantage of production of a large volume of effluents whose ulterior treatment, particularly by concentration, is very onerous.
Besides, the methods of treatment by soaking implementing solutions raise some problems in the treatment of pieces of large dimensions. It is very difficult to immerse and totally soak these pieces in the solution of reactors.
The solutions of decontamination, in effect, allow the treatment by soaking only for metallic disassembling pieces of limited sizes, in other words, in practice, these solutions can be practically utilized only in dismantling of radioactive installations.
On the other hand, the decontamination on the spot of radioactive installations, by spraying of watery solutions, produces large quantities of active effluents for a limited efficiency, because of a short-term contact with the pieces.
Thus, it has been suggested to transform into a viscose form the decontaminating solutions that include an active agent, which is formed by viscose/gelifying agents, in particular by solids divided in great specific areas, in small sizes of elementary particles and chemically inert.
Among the solids which comply with these requirements, supports of minerals, such as aluminum and silica, readily available in the market, present moreover a large diversity of their characteristics such as a hydrophilic or a hydrophobic character, pH, etc. and appear to be the best way to transform these solutions to viscose/gel form.
The spraying of such gels, unlike the solutions, can allow the decontamination on the spot of important metallic surfaces, which are not necessarily horizontal, but can be inclined or even vertical.
The decontaminating gels can be described as colloidal solutions including a thickening agent generally mineral, such as alumina or silica, and an active agent of decontamination, i.e. an acid, a base, an oxidizing agent, a reducing agent or a mixture thereof, which is chosen in function of the nature of the decontamination and the surface.
Thus, an alkaline gel for stainless steels and ferritic steels, will present the de-greasing properties for the elimination of the non-fixed contamination.
An oxidizing gel for stainless steels will perform the elimination of the heat or cold fixed contamination. A reducing gel will preferably be used in complement of the oxidizing gel and in an alternate way for the dissolving of oxides, heat-formed i.e. in the primary circuit of pressurized-water reactor.
Lastly, an acid gel for ferritic steels will perform the elimination of the contamination fixed in cold.
The utilization of gels for the radioactive decontamination of pieces is described particularly in document FR-A-2 380 624.
In this document, a decontaminating gel consisting of a colloidal solution of an organic or mineral compound is used in which one possibly adds a decontaminating product such as hydrochloric acid, stannous Chloride, oxine and/or sodium fluoride.
Though these gels give satisfying results, they sometime present the disadvantage of not being able to eliminate incrusted radioactivity only on a reduced depth of the part's surface, e.g. on a depth of about 1 &mgr;m.
Document FR-A-2 656 949 describes a decontaminating oxidizing gel which allows to eliminate radioactive elements deposited on the part, as well as radioactive elements incrusted on its surface.
This decontaminating gel is consists in a colloidal solution that includes:
a) from 8 to 25% by weight of a mineral-gelifying agent, preferably in silica, pyrogened silica or aluminum base;
b) from 3 to 10 mol/l of mineral base or of a mineral acid.
c) from 0.1 to 1 mol/l of an oxidizing agent such as Ce
IV
, Co
III,
or Ag
II
, that has a normal oxide-reduction potential E
0
higher than 1400 mV/EVH (standard hydrogen electrode) in a strong acid environment or in the reduced form of this oxidizing agent.
In the last case, the gel also includes from 0.1 to 1 mol/l of a compound (d), which is able to oxidize the reduced form of this oxidizing agent.
In the decontaminating gel described above, the presence of components (b) and (c) assures the elimination of radioactive accumulations, formed on the piece's surface as well as the elimination of incrusted radioactivity, by controlled erosion of the surface that has to be decontaminated.
However, this oxidizing gel does not present a sufficient effectiveness in relation to layers of adhesive metallic oxides, deposited on the surface of the alloy such as austenitic steels, Inconel 600 and Incoloy.
Document FR-A-2 695 839 describes, a reducing decontaminating gel which allows to eliminate these layers of adhesive metallic oxides, and which includes:
a) from 20 to 30% by weight of a mineral-gelifying agent, preferably of alumina base.
b) from 0.1 to 14 mol/l of a mineral base, such as NaOH or KOH, and
c) from 0.1 to 4.5 mol/l of a reducing agent, that has a potential of oxide-reduction, less than −600 mV/ENH in an environment of a strong base (pH≧13), chosen among the borohydrides, sulphites, hydrosulphites, sulfides, hypophosphites, zinc and hydrazine.
The application of gels on the surface, for example the metallic surface, is preferably accomplished by spraying, for example under a pressure that could range from 50 to 160 bars and beyond, the gel being agitated before the spraying to transform it into a homogenous gel. After a proper duration of action, the gel is rinsed by spraying water, then the effluents generated are treated by neutralization, decantation and filtration.
All the gels described above, whether they be alkaline or acid, reducing, or oxidizing, present the advantages already mentioned such as the possibility to treat the pieces of complicated shape, an easy application, a small quantity of sprayed chemical radioactive elements by specific area, i.e. a small quantity of effluents, produced by rinsing the applied gels and a perfectly controlled time of contact with the surface, hence a control of the erosion during decontamination. Moreover, the fact that it is possible to spray the gel in a distance significantly diminishes the amounts absorbed by the agents meant for the radioactive purification.
The typical gels of prior art are marketed by “F.E.V.D.I.” company, under the name of “FEVDIRAD”.
All the gels mentioned above, whether they be alkaline, reducing and especially oxidizing, present a corrosive power.
Unfortunately, they cannot stand the high shearing velocities imposed by spraying, which is the most traditional process for the application of gels.
In fact, all these gels including a mineral thickening agent, particularly silica, whether it be hydrophilic, hydrophobic, basic, or acid, have rheological properties, characterized by a thixotropic behavior; during the spraying, the viscosity diminishes under shearing, while after the cessation of shearing, the resetting process of the adhering gel on the surface starts. Such a fluid is characterized by a rheogram in hysteresis.
The control of this thixotropy is fundamental to allow a spraying and an optimal adherence of the gel onto the surface th
Bargues Stéphane
Cheung David
Favier Frédéric
Pascal Jean-Louis
Rigal Philippe
Delcotto Gregory
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
S.T.M.I. Société des Techniques en Milieu Ionisant
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