IL27663A - Method and composition for decontamination of stainless steel surfaces of the cooling systems of nuclear reactors - Google Patents

Description

METHOD AND CCMPOSITIOH FOR DECONTAMINATION OF STAINLESS STEEL SURFACES OF THE COOLING SYSTEMS OF HUGLEAR REACTORS This invention was made in the course of or under a contract with the United States Atomic Energy Commission.

The invention relates to a method of and a composition for the removal of radioactive contamination from ir^tal surfaces* It is effective for removing the extremely tenacious radioactive filbn 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 remote and dissolve radioactive films formed during extended periods of continuous operation (up to at least two or three years). Prom 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. 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, aw this is particularly true if the components are. in a critical location, e.g., a drain1 valve plug, seat or stem.

T e 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.

Inct¾ procedure E 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 We 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: Oxalic acid 20 - 30 g/ Dibasic ammonium citrate kO - 60 g/l Ferric sulfate or nitrate 1.7 2Λ g/l Diethyl thiourea 0.8 ■» 1.2 g/l The ratio of oxalic acid to dibasic ammonium citrate should not be greater than l/2, otherwise secondary film formation may occur.

The preferred composition at present is as follows: Oxalic acid 25 g/l Dibasic ammonium citrate 50 g/l Ferric sulfate 2 g l the reactor decontamination subsequently described. If significant surfaces of carbon steel or of stainless steel of the kOO series are present, it is preferable to use 1.2 g/l of diethyl thiourea in the RDW-3 solution.

The following examples are based on decontamination of the primary cooling system of the Plutonium Recycle Test Reactor located near Richland, Washington. It is a heavy water moderated and cooled reactor of the pressure tube, pressurized water type. A complete description of the FRTR is given in U. S. Atomic Energy Commission Report HW-61236, "Plutonium Recycle Test Reactor - Final Safeguards Analysis." Except for the zircaloy-2 pressure tubes, in which the fuel elements are mounted, the primary cooling system is composed principally of type 3 h stainless steel.

Example I Experimental Tests Samples were cut from various "Jumpers" in the primary cooling system of the FRTR. 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 100°— 105°C with 3$ KMn04 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 HDW-3 SOLUTION Jumper Number 1U55 1546 181 7-2 1847-1 I356 12 9 1253 2049-T Months of Filming 6 6 12 16 20 22 22 27 3$ ΚηΟ^-Ι^ NaOH IAa 5 4 31 12 24 28 25 55 FA* 1 1 8 2 5 7 4 6 DFC 5 4 4 6 .8 4 6.3 9.2 3$ i KMnO -10?. NaOH IA 5 4 26 10 38 35 27 1* FA 1 1 2 7 8 3 4 DF 5 4 6.5 5 5.5 4.5 9 11.2 3 ΚΜηΟ½-ΐ8# NaOH IA 5 4 29 13 2 27 24 50 FA 1 1 1 2 3 4 4 5 DF 5 4 29 6.5 8 6.75 6 10 a β IA-Initial activity mrad/hr "b = FA-Final activity mrad/hr c = DF-Decontamination Factor = IA/FA No secondary films or precipitates were formed. It will be noted that the treatment was particularly effective 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 was 0.22 mils 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 2¾~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 !AO-A and 1&0-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 A 12 carbon steel. Pitting did not occur on any of the carbon steel alloys, but slight pitting up to 6 mils diameter occurred on the kOO series stainless steels.

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

Dull grey passive oxides formed on the carbon steel and ¾00 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-2* No corrosion was apparent on the samples of this alloy.

TABLE II RESULTS OF RDW-3 CORROSION EVALUATION Alloy UIXJLQ .(1) 8 hours l6 hours 2k hours 8 hours 1 kkO-C SS 0.09 0.13 O.I98 A few scattered D 2-3 mil. p diameter pits.

Stellited A212 CSsA(3) O.Qk6 0.10 0.208 Dull grey D A212 CS Welded to 30k SS 0.02k 0.029 0.0¾·5 Dull grey D A2k$ CS 0.0k6 0.05k 0.092 Dull grey D A106 CS o.okk Ο.Ο78 0.103 Dull grey D (1) Total mils penetration in time period given (2) SS - stainless steel (3) CS - carbon steel Example II Reactor Decontamination The primary cooling system of the Plutonium Recycle Test Reactor was drained. It was then flushed by pumping water through it and draining. This was repeated several times. Next, an aqueous solution of 3$ KMhO¼ and 1Q# HAOH was introduced into the system and circulated for six hours. It was maintained at 100° - 105°C by a heat exchanger for two of those hours. The heating and cooling each took two hours. It was then repeatedly flushed with water until the latter had attained a pH between 10 and 11 The RDW-3 solution was then circulated through the system for three hours. It was maintained at 80°G 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 III DECONTAMINATION FACTORS OBTAINED ON PRTR PRIMARY SYSTEM System Location Activity Levels (mR/hr) Decontamine Before After Factor Inlet header, injection 60 5 12 pumps Outlet header, injection 100 5 20 pumps Outlet line, DT-2 Tank 90 5 18 Inlet line, DT-2 Tank 100 5 20 Pressurizer, outlet piping 190 5 38 Inlet, primary pump - 2 220 5 i* Inlet, primary pump - 1 210 5 k2 Inlet, lower ring header 300 10 30 Bypass, HX-1 & pressurizer 100 5 20 Piping, HX-1 to pressurizer 110 5 22 Volute, primary pump - 2 150 5 30 Volute, primary pump - 3 120 5 2k Inlet to Degasser 100 5 20 Bottom of HX-5 header 190 5 38 HX-1 general background on 25 6 k enclosure grating HX-1 Secondary side: 6" inside manhole 1200 5 2kO 1* from the tubes 1800 5 360 Maximum reading on tubes k50 25 180 In the above table the abreviations used have the following meanings: DT-2 storage tank The zircaloy-2 pressure tubes were coated with an iron film containing contaminants. This film was completely removed, leaving a bright surface. To the best of oJ¾r knowledge this represents the most successful nuclear reactor decontamination ever recorded.

W ileic have described specific embodiments of ettifi. process, it will be understood that various changes are possible. *}Έ, therefore, wis otKfl. invention to be limited only by the scope of the appended claims. 1

Claims (1)

1. Earing now particularly described and ascertained nature of our said invention and in what manner the same is to be we declare that what we claim A composition for removing radioisotopes deposited on metal surfaces of the cooling systems of nuclear reactors consisting of an aqueous solution having the following Oxalic acid 20 30 Dibasic amonium citrate AO 60 Ferric sulfate or nitrate Methyl thiourea g l the ratio of oxalic acid to dibasic ammonium oltrate being not greater than A composition as in Claim wherein the solutes have substantially the following Oxalic acid 25 Dibasic ammonium citrate 50 Ferric sulfate 2 Diethyl thiourea 1 A method of decontaminating a nuclear reactor cooling system composed primarily of stainless which system has to extended circulation of hot water during reactor which method circulating through said system a hot potassium permanganate solution and thereafter circulating through said system a hot aqueous solution of the following Oxalic acid 20 Dibasic ammonium citrate 40 60 Ferric sulfate or nitrate Diethyl thiourea the ratio of oxalic acid to dibasic ammonium citrate being not greater than A method as defined in Claim wherein the solution A method as defined in Claim wherein the solution employed in Step has the following Oxalic acid 25 Dibasic citrate Ferric sulfate 2 Diethyl thiourea 1 FOR OB OF insufficientOCRQuality