CA1217412A

CA1217412A - Potassium ferrate decontamination process

Info

Publication number: CA1217412A
Application number: CA000389872A
Authority: CA
Inventors: Paul H.C. Spekkens
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-11-12
Filing date: 1981-11-12
Publication date: 1987-02-03

Abstract

ABSTRACT
In the method of decontaminating a corroded metallic surface in a water-containing system of a nuclear reactor by contacting the surface with an aqueous decontaminating solution, for example a citrox reagent in solution, for dissolving and removing metallic oxides therefrom, the surface is first treated by reacting the corrosion layer with an aqueous solution of potassium ferrate which oxidizes insoluble oxides in the layer to a soluble, higher oxidation state to facilitate their removal by the decontaminating solution.

Description

~2~74i2 This invention relates to a method of decontaminating metallic surfaces, more particularly metallic surfaces which have become corroded in a water-containing system of a nuclear reactor.
Most metallic surfaces which are exposed to hot water become corroded and develop a layer of oxide on them. If this oxide is formed ir. water containing radioactive species, or is otherwise exposed to water containing radioactive species, the radioactive species can become trapped in the corrosion layer.
The accumulation of radioactive material on the internal surfaces of the piping, tubing and components of the water cooling system of a nuclear reactor can produce undesirable radiation fields which present a serious hazard to maintenance workers. The usual method of removing this hazard, which has been known for many years, is to employ an aqueous decontamina-ting solution, i.e. a chemical solution which will dissolve and/or loosen the contaminated oxide layer, and flush the dissolved or loosened materials from the system. This process is commonly referred to as chemical decontamination.
The type of oxide formed by the corrosion process depends upon the particular alloy of the metal component and the chemical conditions in the water. When austenitic alloys are in contact with high temperature water under reducing conditions, oxidation of the alloy produces a layer containing a chromium-rich oxide, i.e. iron chromite (FeCr2O4). The conditions under which this oxide is formed exist in the cooling systems of certain types of nuclear reactors, more particularly pressurized water reactors, and in certain components of CANDU reactors.
The oxide is extremely resistant to direct dissolution by the decontaminating agents generally used in decontamination applications. However, it is common in the nuclear industry to employ an oxidizing chemical reagent to react with the * Trade Mark -- 1 -- ,~.

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chromlum in the iron chromite so as to oxidize it from -the +III
oxidation state to the +VI state which is soluble in water and so can be flushed away. This leaves behind a porous iron oxide matrix which can be readily dissolved by any of a number of aqueous decontaminating solutions, which are usually based on mixtures of oxalic and citric acids or their salts and are referred to as citrox reagents.
The oxidizing chemical reagent most frequently used for this purpose is alkaline potassium permanganate (K~lnO4).
This reagent is commonly used, but it has certain serious disadvantages, namely:
1) it is extremely corrosive to certain alloys, particularly chromium-plated surfaces and *

Stellite surfaces;

2) unless the pH of the solution is kept very high, the product of the reaction is the insoluble manganese dioxide which will be deposited on the metallic component to be cleaned and will be difficult to remove.
The deposit will trap contaminated material and, if not removed before the component is returned to service in the nuclear system, will produce a radiation hazard by activation of the natural manganese to its radioactive isotope;

3) waste solutions containing potassium permanganate are incompatible with some known solidification techniques used to render the wastes suitable for storage.
These solutions require additional chemical treatment before they can be solidified, * "Stellite" is a trade mark denoting a family of hard-facing alloys supplied by Haynes Stellite Co.

~2~79~:~L2 thus adding -to the cost of waste dlsposal.
The object of the present invention is to provide an alternative method of pre-treating the corroded surface without using alkaline potassium permanganate and so avoiding the disadvantages referred to above. This is achieved by using an aqueous alkaline solution of potassium ferrate (K2FeO4) as the oxidizing chemical reagent. The solu-tion must be alkaline for the ferrate to have sufficient stability for practical use and a pH in the range 14-15 has been found suitable.
Thus, the method of the present invention for decontam-inating a corroded metallic surface in a water containing system of a nuclear reactor by contacting the surface with an oxidizing chemical reagent in aqueous solution preparatory to contacting said surface with an aqueous decontaminating solution for dissolving and removing residual metallic oxide therefrom is characterized in that the oxidizing chemical reagent is potassium ferrate.
The invention is especially applicable to cases in which the metallic component is of a chromium-containing alloy, for example a chromium-iron alloy such as an austenitic steel or a chromium-nickel alloy such as Inconel , the corrosion layer on the surface of the component comprising a chromium-rich metallic oxide. In this connection the term "metallic oxide"
means an oxidation product of the alloy and will usually be a complex compound containing, for example, iron chromite, magnetite, and/or nickel ferrite.
The method is also applicable to the decontamination of surfaces of other alloys having a component such as copper which may be oxidized to a water-insoluble form but which may be further oxidized to a higher oxidation s-tate in which it is readily soluble. Thus, the method is especially applicable -to the decontamination of surfaces of nickel-copper alloy * "Inconel" is a trade mark ~Z~7~

components used in the nuclear reac-tors.
One major advan-tage of this method is that it introduces no metals other than iron to the system. The decomposi-tion of potassium ferrate or its reaction wi-th the corroded surface produces insoluble ferric oxide. This oxide is often already present in many of the systems to be cleaned and can readily be removed in the second oxide-dissolution step using the aqueous decontaminating solution. Leaving small amounts of ferric oxide behind in the system will have only minimal adverse effects.
Another important advantage of the method is that the potassium ferrate solution spontaneously decomposes according to the equation

4 4H2O >2Fe2O3 + 80H +32 Thus, a potassium ferrate solution, when allowed to stand after having been used, will spontaneously lose its oxidizing power, making it compatible with standard solidification techniques.
The ferric oxide which is formed is a precipitate and can be removed from the mixture by filtration. This precipitate will adsorb a significant amount of the dissolved activity present in the solution, making the filtrate easier to dispose of The method has been tested on metal test specimens of different alloys, namely "Inconel 600", AM 355, 17-4 PH and 304 SS under similar conditions. In each case the specimen contaminated in high temperature water was treated using an aqueous solution containing 0.06 M potassium ferrate in 3 M
sodium hydroxide. The corroded surface of the specimen was contacted with the solution for one hour at 70C. The specimen was rinsed and then treated for two hours at 85C using an aqueous decontaminating solution comprising a citrox reagent, namely a solution containing 10 g/L ethylenediaminetetraacetic acid, 50 g/L ammonium citrate, 25 g/L oxalic acid, 1 mL/L

~2~ 7~L2 "Triton X-100", 0.5 mL/L "Chevron NI-W", 0.5 mL/L "Rodine 31A"
and adjusted -to pH 3.5 with ammonia, -to dissolve and remove the residual oxides from the surface.
"Triton X-100" is a trade mark denoting a non-ionic surfactant, containing ethoxylated alkyl phenols, supplied by Rohm and Haas.
"Chevron NI-W" is a trade mark denoting a non-ionic surfactant, containing ethoxylated alkyl phenols, supplied by Chevron Chemical Co~
"Rodine 31A" is a trade mark denoting a corrosion inhibitor containing pyridine derivatives, thiourea derivatives, sulphuric acid and a surfactant, supplied by Amchem Products Inc.
The level of radioactivity on the specimens before and after the treatment was measured using a Geiger tube, and the average decontamination factors (ratio of activity before and after treatment) are given in the following table, along with those obtained when only the aqueous citrox solution was used without the potassium ferrate pretreatment.

Decontamination Factor Without Ferrate AlloyUsing Ferrate Treatment Treatment "Inconel 600" 7 3

Claims

THE EMBODIMENTS OF THE INVENTION IN WIIICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of decontaminating a corroded metallic surface in a water-containing system of a nuclear reactor, the corrosion layer on said surface comprising a copper-rich metallic oxide, which method comprises contacting said layer with an aqueous alkaline solution of potassium ferrate thereby to convert the oxide to a water-soluble form preparatory to contacting the layer with an aqueous decontaminating solution for dissolving and removing residual metallic oxide from the surface.