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Publication numberUS3496017 A
Publication typeGrant
Publication dateFeb 17, 1970
Filing dateApr 28, 1966
Priority dateApr 28, 1966
Also published asDE1621670A1
Publication numberUS 3496017 A, US 3496017A, US-A-3496017, US3496017 A, US3496017A
InventorsWeed Ronald D
Original AssigneeAtomic Energy Commission
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method and composition for decontamination of stainless steel surfaces
US 3496017 A
Abstract  available in
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Claims  available in
Description  (OCR text may contain errors)

States ABSTRACT OF THE DISCLOSURE The tenacious magnetite film containing included radioactive contaminants which forms on stainless steel heat exchange surfaces of nuclear reactors is removed by a two step treatment. The first step utilizes a hot, alkaline, aqueous solution of potassium permanganate. The second utilizes a hot aqueous solution of oxalic acid, dibasic ammonium citrate, ferric sulfate or nitrate and diethyl thiourea.

CONTRACTUAL ORIGIN OF THE INVENTION This invention was made in the course of or under a contract with the United States Atomic Energy Commission.

Introduction The invention relates to a method of and a composition for the removal of radioactive contamination from metal surfaces. It is effective for removing the extremely tenacious radioactive film which forms on stainless steel surfaces on long exposure to hot Water containing radioisotopes. It is also effective for removing the less tenacious radioactive films formed on carbon steel, zircaloy, brass, bronze and other metals. At the same time there is no objectionable corrosion to any of these metals.

The principal object of the invention is the decontamination of nuclear reactor cooling systems composed primarily of stainless steel. The radioactive corrosion films characteristic of recirculating water-cooled nuclear reactors must be periodically removed to permit contact maintenance and continuity of operation. Before these systems can be cleaned, procedures and processes must be developed which are suitable for use in the system. These processes must meet certain criteria which are discussed in the following paragraphs.

A decontamination reagent must dissolve the radioactive corrosion film and remove it from the system. Large nuclear plants are no exception to other types of plants; they are built with inherent low velocity areas and other traps where particulate material will settle out. The particulates in these areas must be dissolved for successful removal. Therefore, a successful decontamination reagent must dissolve as well as remove the radioactive corrosion film because if it is only removed from the piping surfaces and not dissolved, it will settle out in the low velocity areas and traps.

A successful decontamination reagent must remove and dissolve radioactive films formed during extended periods of continuous operation (up to at least two or three years). From the limited data available in the AEC literature, it is probable that cleaning will not be required oftener unless under unusual circumstances or demands. It is desirous, in power reactor operation, to keep the down time at a minimum and possibly reach five years of continuous operation before cleaning. Therefore, the removal of these long term films is an essential requirement.

A successful decontamination reagent must not be excessively corrosive to the materials of construction in the primary system. The reactor system will be constructed of primarily one or two materials, but there will be small quantities of many other types of materials in the system. These lesser materials will be found in such places as valves and sampling devices. All of these materials must be compatible with the reagent if they are in either direct or indirect contact with the solution, this is particularly true if the components are in a critical location, e.g., a drain valve plug, seat or stem.

The film which forms on stainless steel surfaces in the cooling systems in water cooled nuclear reactors appears to be primarily magnetite mixed with oxides of nickel and chromium. Radioactive ions present in the water are included in or adsorbed on this film so that they cannot be removed without removing the film itself. It is known in the art to decontaminate stainless steel nuclear reactor cooling systems by treatment with alkaline potassium permanganate followed by oxalic acid. However, previously known oxalic acid containing solutions have tended to deposit a secondary film on the surfaces. Sometimes the secondary film takes the form of a precipitate settling out in the low velocity regions and dead legs of the cooling system. This secondary film contains a considerable amount of radioactivity and its formation partially nullifies the decontamination.

General Description of Invention In my procedure I first treat the system with the conventional alkaline potassium permanganate. This serves primarily for the removal of chromium and conditions the film so that it is dissolved in the acid medium. In the subsequent treatment I utilize a novel oxalic acid-containing aqueous solution which prevents the formation of the secondary film referred to above. This solution has the following composition:

G./l. Oxalic acid 20-30 Dibasic ammonium citrate 40-60 Ferric sulfate or nitrate 1.7-2.4 Diethyl thiourea 0.81.2

The ratio of oxalic acid to dibasic ammonium citrate should not be greater than /2, otherwise secondary film formation may occur.

The preferred composition at present is as follows:

G./l. Oxalic acid 25 Dibasic ammonium citrate 50 Ferric sulfate 2 Diethyl thiourea 1 This solution, which is designated RDW-3, was used in the experimental tests and the reactor decontamination subsequently described. If significant surfaces of carbon steel or of stainless steel of the 400 series are present, it is preferable to use 1.2 g./l. of diethyl thiourea in the RDW-3 solution.

Detailed Description The following examples are based on decontamination of the primary cooling system of the Plutonium Recycle Test Reactor located near Richland, Wash. It is a heavy water moderated and cooled reactor of the pressure tube, pressurized water type. A complete description of the PRTR is given in US. Atomic Energy Commission Report HW-61236, Plutonium Recycle Test Reactor Final Safeguards Analysis.

Except for the zircaloy-Z pressure tubes, in which the fuel elements are mounted, the primary cooling system is composed principally of type 304 stainless steel.

3 EXAMPLE I Experimental tests Samples were cut from various jumpers in the primary cooling system of the PRTR. These jumpers had been subjected to filming by the circulating coolant for various periods ranging from 6 to 32 months.

The samples were treated at l105 C. with 3% KMnO solution containing varying concentrations of NaOH and then at 80 C. with the RDW-3 solution. Results are shown in Table I.

TABLE I.DECONTAMINATION RESULTS FOR THE RDW-3 SOLUTION Jumper Number Months of Filming 6 6 12 16 20 22 22 27 27 29 32 32 a IA=Initial activity mrad/hr. FA=Final activity mrad/hr.

No secondary films or precipitates were formed. It will be noted that the treatment was particularly elfective on those samples exhibiting high initial activities.

Corrosion tests were then conducted on samples of various metals. One corrosion test was conducted with the RDW-3 solution at 80 C. for 120 hours; the total corrosion incurred by the carbon steel samples exposed 3 was 0.22 mil and by the stainless steel samples was 0.01 mil. A pale yellow film was deposited on the carbon steel samples when the test was terminated; the solution was a clear yellow color with no precipitates present.

Another corrosion test was conducted for a 24-hour period at 82 C.; numerous other types of materials were charged during this test. The corrosion data obtained are presented in Table II. The corrosion attack on the carbon steels and the 440-A and 440-C stainless steels increased with increased exposure while the attack on the remaining alloys did not increase after the initial attack. The attack to all alloys was very low; the most attack (0.2 mil) was on Stellited A212 carbon steel. Pitting did not occur on any of the carbon steel alloys, but slight pitting up to 6 mils diameter occurred on the 400 series stainless steels.

There was no deposition with the RDW-3 solution in contrast with other oxalic acid containing solutions. The solution was a clear yellow, with no precipitates.

Dull grey passive oxides formed on the carbon steel and 400 series stainless steels; these oxides proved to be resistant to subsequent rusting when the samples were rinsed in warm tap water.

Tests were also conducted on zircaloy-Z. No corrosion was apparent on the samples of this alloy.

s DF=Decontamination Factor=IA/FA.

flushed with water until the latter had attained a pH between 10 and 11.

The RDW-B solution was then circulated through the system for three hours. It was maintained at C. for one hour. The system was then flushed with deionized water until the latter attained a resistivity of 10,000 ohm centimeters. The results are given in Table III.

TABLE IIL-DECONTAMINATION FACTORS OBTAINED ON PRTR. PRIMARY SYSTEM Activity Levels (mR/hr.) Decontamination System Location Before After Factor Inlet header, injection pumps- 60 5 12 Outlet header, injection pumps 100 5 20 Outlet line, DT-2 Tank.- 5 l8 Inlet line, DT-Z Tank 5 20 Pressurizer, outlet piping- 190 5 38 Inlet, primary pump, 2 220 5 44 Inlet, primary pump, 1 210 5 42 Inlet, lower ring header- 300 10 30 Bypass, HX-l and pressurizer 100 5 20 Piping, HX-l to pressurizer. 5 22 Volute, primary pump, 2.- 5 30 Volute, primary pump, 3 120 5 24 Inlet to Degasser 100 5 20 Bottom of HX-5 header 190 5 38 HX-l general background on enclosure grating 25 6 4 HX-l Secondary side:

6 inside manhole 1, 200 5 240 1 from the tubes 1, 800 5 360 Maximum reading on tubes 4, 500 25 In the above table the abbreviations used have the following meanings: DT-2=storage tank; HX-1=primary heat exchanger; HX-5= auxiliagiy3 heat exchanger. There are three primary pumps, numbered 1, 2 an TABLE II.RESULTS OF RDW-3 CORROSION EVALUATION Penetration, mils 1 Sample Appearance Alloy 8 hours 16 hours 24 hours 16 hours 24 hours Haynes 25 0.015 0. 016 0.017 Shiny Shiny. 304 SS, stressed 0. 0061 0. 006 0. 0067 -do D0. 304 SS, sensitized 0.0051 0. 0065 0. 0069 do D0. 316 SS 0. 0060 0. 0042 0. 0058 -(10 D0. 17-4 PH SS 0.024 0.015 0.021 .d0 Do. 416 SS 0. 074 0. 086 0. 085 Dull grey Dlll grey-5 pits to 6 mils 18.111 91'. 440-A SS 0.063 0. 069 0. 11 Dull grey, slight attack at Dull grey, slight attack at Dull grey, surface roughencrevice area. crevice area. ing. 440-0 SS 0. 094 0. l3 0. 198 A few scattered 2-3 mil Dull grey, no pits Surface very rough, a few 3 diameter pits. mil diameter pits. Stellited A212 CS 3 0. 046 0.10 Dull grey Dull grey Dull grey. A212 CS Welded to 304 SS- 0. 024 0. 029 45 d do D0. CS 0. 046 0. 054 do. Do. 0. 044 0. 078 do Do.

3 GS carbon steel.

The zirca1oy-2 pressure tubes were coated with an iron film containing contaminants. This film was completely removed, leaving a bright surface. To the best of my knowledge this represents the most successful nuclear reactor decontamination ever recorded.

While I have described specific embodiments of my process, it will be understood that various changes are possible. I, therefore, wish my invention to be limited only by the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of removing a tenacious magnetite film containing included radioactive contaminants from a nuclear reactor cooling system composed primarily of stainless steel, which system has been subject to extended circulation of hot water during recator operation, which method comprises:

(a) circulating through said system of hot alkaline,

aqueous, potassium permanganate solution and thereafter (b) circulating through said system a hot aqueous solution of the following composition:

Oxalic acid 20-30 Dibasic ammonium citrate 40-60 Feric sulfate or nitrate 1.7-2.4

Diethyl thiourea 0.8-1.2

6 the ratio of oxalic acid to dibasic ammonium citrate being not greater than /2.

2. A method as defined in claim 1 wherein the solution employed in Step a. contains 3% potassium permanganate and 10% sodium hydroxide.

3. A method as defined in claim 2 wherein the solution employed in Step b. has the following composition:

LEON D. ROSDOL, Primary Examiner W. SCHULZ, Assistant Examiner US. Cl. X.R.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3664870 *Oct 29, 1969May 23, 1972Nalco Chemical CoRemoval and separation of metallic oxide scale
US3873362 *May 29, 1973Mar 25, 1975Halliburton CoProcess for cleaning radioactively contaminated metal surfaces
US4063962 *Apr 7, 1976Dec 20, 1977General Atomic CompanyMethod and apparatus for cleaning nuclear fuel elements
US4217192 *Jun 11, 1979Aug 12, 1980The United States Of America As Represented By The United States Department Of EnergyDecontamination of metals using chemical etching
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Classifications
U.S. Classification376/309, 134/38, 134/29, 510/110, 976/DIG.376, 134/22.19, 134/3, 134/27, 134/22.17, 134/2, 134/28, 376/310, 134/41
International ClassificationG21F9/00, C23G1/02, C23G1/00
Cooperative ClassificationG21F9/004, C23G1/00, C23G1/02
European ClassificationC23G1/02, C23G1/00, G21F9/00B2B