Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
2001-10-16
2004-03-09
Gulakowski, Randy (Department: 1746)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S028000, C134S034000, C134S037000, C134S041000, C376S308000, C376S309000, C376S310000
Reexamination Certificate
active
06702902
ABSTRACT:
BACKGROUND ART
The present invention relates to a method for the radioactive decontamination of a steel surface, and particularly to a method for the radioactive decontamination of a steel surface using pickling. The invention relates, for example, to the radioactive decontamination of an internal circuit, a metal surface, piping or item of equipment in a reprocessing plant for irradiated nuclear fuel, hereinafter referred to as the “surface”. The invention relates particularly to the radioactive decontamination of a surface made of austenitic steel, which is used to construct most surfaces of such plants.
The present invention also relates to a device for implementing the said method.
The radioactive contamination fixed on the surfaces of reprocessing plants is mainly due to surface adsorption. This contamination includes metastable radioactive contamination composed of Pu
241
, Am, U,
242, 244
Cm,
137
Cs,
90
Sr and particulate radioactive contamination with ruthenium and the following insoluble compounds: cesium phosphomolybdate, zirconium phosphate, zirconium molybdate, plutonium phosphate mixed molybdate of zirconium and plutonium, oxides of Mo, Sb, Al, Fe, colloidal plutonium oxides, etc.
It is therefore unnecessary to cause significant erosion to decontaminate the surfaces. It is generally accepted that, depending on the surface, erosion of the order of between 2 and 10 &mgr;m is sufficient.
Generally speaking, and particularly in the above example of decontamination of a reprocessing plant for irradiated nuclear fuel, radioactive decontamination may involve two major stages:
a first stage aimed at eliminating metastable contamination and the main deposits adhering to the surfaces, and
a second stage aimed at eliminating both particulate contamination fixed on the surfaces and residual deposits.
The first stage is a rinsing stage using a variety of sequences of rinsing that are non-corrosive for the surface; the second state is an erosive stage that uses reagents that are corrosive for the surface and mainly consist of oxidizing mixtures such as nitric acid/fluorhydric acid, nitric acid/cerium IV nitrate, or mixtures comprising chromic acid, nitric acid and cerium nitrate.
The nitric acid/fluorhydric acid mixture has the advantage of attacking refractory contamination deposits such as a variety of molybdates or phosphates such as Zr
4+
, MoO
2
2+
, Pu
4+
and antimony oxides.
During the corrosion stage the best decontamination factors are generally achieved when erosion is both slow, i.e. at a kinetic rate of the order of 1 &mgr;m/h or less, and regular. This is fairly difficult to achieve when the circuits to be decontaminated are complex and have zones that are more sensitive to, corrosion such as, for example, weld zones, zones of mechanical stress, etc.
Furthermore, the oxidant used for the corrosion must not be too harsh. This is why the oxidant mixture of nitric acid/fluorhydric acid cannot always be used as it is difficult to control, particularly on large extended surfaces.
Furthermore, some surfaces of the internal circuits of reprocessing plants for irradiated nuclear fuels are made of austenitic steel. It is therefore necessary on these surfaces to use an oxidant that does not cause intergranular corrosion of the steel. The oxidation-reduction couple of Ce
IV
/Ce
III
is one of the rare oxidant agents that can be used to produce a given degree of erosion on austenitic steel surfaces without causing excessive intergranular corrosion.
However, the methods of the prior art using the above oxidation-reduction couple have the particular drawback of requiring, as a precondition of effective decontamination, large quantities of cerium (of the order 0.5 mole/m
2
/&mgr;m
−1
) and hence raising the price of effluent treatment as high concentrations of cerium are not authorized in the vitreous containment matrices. The concentration of cerium required in these methods is therefore a limiting factor.
In the methods of the prior art, the concentration of Ce
IV
drops throughout the reaction, implying changing kinetics of corrosion: corrosion is too strong at the beginning and too weak at the end. This drawback is overcome by the present invention since the concentration of Ce
IV
, and therefore the kinetics, is virtually constant.
In addition, the methods of the prior art, in particular the method described in the above-mentioned document, are designed to treat oxides and not the metal surfaces of valency 0 such as austenitic steel surfaces. The nature of the alloy to be eroded or corroded which, in a reprocessing plant is exclusively austenitic steel and not INCONEL™ or INCOLOY™ or similar means that the presence of chromic acid in the decontaminating solutions of the methods described for the latter is undesirable.
For example, patent application EP-A-0 174 317 discloses a method for decontaminating chrome oxides on the surface of INCONEL™ steam generators in power plants. In this patent the Ce
IV
used as an oxidation agent is used as an agent for regenerating Cerium IV from the Cerium III formed during oxidation. The method discloses the use as a corrosion fluid of an aqueous solution of nitric acid, chromic acid and Cerium nitrate in which ozone has been dissolved. It also uses a gas-liquid contactor to put the ozone into solution.
In the decontamination of a steam generator according to the method disclosed in EP-0 174 317, a regeneration chamber for Cerium IV is coupled with the steam generator, said chamber regenerating the Cerium IV by injecting ozone to saturation point. This cannot be envisaged in a reprocessing plant due to the high &agr;, &bgr; and &ggr; radiochemical activity of the components to be decontaminated and the great complexity and variety of components to be decontaminated.
Furthermore, the method disclosed in application EP-0 174 317 reduces without eliminating the variability in corrosion speed by virtue of its direct dependence on the value of the concentration of Cerium IV which changes during the course of the reaction since Cerium IV is regenerated in batches.
DISCLOSURE OF THE INVENTION
The present invention overcomes the drawbacks described above by providing a method for the decontamination of a steel surface consisting in bringing the surface to be decontaminated into contact with a pickling solution comprising nitric acid and a first iron oxidation agent at a suitable temperature such that the face of the said surface is eroded by the oxidation of the metallic constituents such as Fe0, Cr0, Ni0, Mn0 it contains, said bringing into contact being achieved by means of the direct, continuous introduction into the pickling solution of a gas comprising a second oxidation agent such as continuously to oxidize, at least partly, said first oxidation agent reduced by the oxidation of the metallic constituents of the steel.
During the course of the method of the invention the first agent for oxidizing the metallic constituents of the steel is reduced when it oxidizes the metallic constituents of the steel surface and it is continuously regenerated by the second oxidation agent, by oxidation. The first oxidation agent is therefore selected such as to be capable of oxidizing the metallic constituents of the steel surface and the second oxidation agent is selected as being capable of oxidizing the first oxidation agent reduced by oxidation of the metallic constituents of the steel to regenerate it.
The first oxidation agent may, for example, be selected from Ce
IV
, Ag2+, etc.
The gas constituting the second oxidation agent may be selected from gases comprising ozone.
Advantageously, according to one embodiment of the method of the present invention, the first oxidizing agent may be cerium IV, for example as cerium IV nitrate, and the second oxidizing agent may be a gas including ozone.
According to this embodiment, the present invention makes it possible in particular to eliminate the above drawbacks relating in particular to the methods of the prior art using cerium.
The present invention proposes means for continuousl
Dalard Francis
Delagrange Jacques
Fulconis Jean-Michel
Commissariat a l'Energie Atomique
Gulakowski Randy
Kornakov M.
Pearne & Gordon LLP
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