GB2044982A

GB2044982A - Chemical decontamination of reactor parts

Info

Publication number: GB2044982A
Application number: GB7935287A
Authority: GB
Original assignee: Kraftwerk Union AG
Current assignee: Kraftwerk Union AG
Priority date: 1978-11-03
Filing date: 1979-10-11
Publication date: 1980-10-22
Also published as: JPS601600B2; JPS5566800A; GB2044982B; DE2847780A1; FR2440601A2; DE2847780C2; CH642475A5; FR2440601B2

Abstract

Structural or component parts, for example of a nuclear reactor, are decontaminated from radioactive contamination by a preliminary oxidising treatment with an alkaline permanganate solution for approximately 1 hour and are then exposed for 5-20 hours to an inhibited citrate-oxalate decontamination solution. Subsequently, an after- treatment is carried out with a citric acid-hydrogen peroxide solution which contains suspendable inert material such as cellulose fibres or sponge rubber balls.

Description

SPECIFICATION Method for the chemical decontamination of structural parts.

The present invention relates to a method for the chemical denontamination of structural parts and components, for example of water-cooled nuclear reactors.

After a short operating period there is produced in the primary circuit of a nuclear power station a cohesive, dense layer of oxide resulting from corrosion of the structural materials. During operation this initially inactive oxide layer becomes contaminated, thus becoming radioactive, even in components which do not lie in the direct radiation field of the core area. This contamination can be traced back to the incorporation af activated corrosion products into the oxide layer of the strucural materials. Since this process runs continuously, the result is a concentration, particularly of long-lived nuclides, in the oxide layer. It is thus imperative that possible ways be sought of eliminating this superficial contamination by appropriate decontamination means.This requirement becomes increasingly urgent as the operating period increases, as maintenance of the installations and above all repair work result in increasing radiation loading of the operation personel.

Previously, attempts were made to decontaminate contaminated surfaces with aqueous solutions of mineral and organic acids. However, the results thereby obtained were completely unsatisfactory, as, more particularly, damage to the structural material itself was at the same time notedm Only the two-stage APAC method (Alkaline- Permaganate - Ammonium - Citrate) had relatively good decontamination results, yet likewise resulted in selective corrosion phenomena and, according to the structural material, in unduly great attacks on the base metal. Moreover, the decontamination solution is inhibited in the case of the APAC method by substances containing sulphur. However, sulphur compounds are exceptionally undesirable in the primary systems of water-cooled nuclear reactors, since in subsequent operation with Ni-alloys sulphur can result in selective manifestations of corrosion.

British Patent Application No. 9093/77 Serial No. 1572867 has resulted from research in this field by the present applicants. In the meantime, testing and practical application of the method has been continued and according to the present invention there is provided a method of chemical decontamination comprising the steps of: subjecting the parts to be decontaminated to a preliminary oxidising treatment with an alkaline permanganate solution; corroding the parts with an inhibited citrate-oxalate decontamination solution; and treating the decontaminated parts with a citric acid-hydrogen peroxide solution which contains suspendable inert material.

For a clearer understanding of the entire method, it will now be described in detail.

The contaminated structural parts are first subjected to an oxidation pre-treatment in an alkaline permanganate solution containing 10-50 g sodium hydroxide solution and 5-30 g potassium permanganate per 1000 ml water. With this preoxidation it is important that the treatment period is not more than 2 hours, since otherwise there is a danger of precipitation of manganese dioxide (MnO2) which is difficult to dissolve once more.

The decontamination solution contains, in 1000 ml water, 25-50 g citric acid, 20-40 g oxalic acid, 2-4 g ethylenediamine-tetraacetic acid and 5 g Fe-Ill-formate. The first three constituents named represent a combination of complexing agents and organic acids; the decontamination factor is increased with thesem The oxalic acid content is above all important for the decontamination factorm The value of 40 g oxalic acid per 1000 ml water specified already represents the uppermost limit. With higher concentrations of oxalic acid, there is a danger of oxalate forming on the surfaces of the working parts. Moreover, by increasing this oxalic acid value, the decontamination factor can no longer be appreciably increased.

When this decontamination solution is formulated, it is also important that the citric acid must form a greater portion in the solution than the oxalic acid since this and the ethylene-diaminetretraacetic acid have the task of keeping oxalates, which are difficult to dissolve, away from the surfaces of the working parts. The citric acidloxalic acid/ethylenediaminetetraacetic acid ratio should preferably be 12.5 :10 1. Without the addition of the ethylenediaminetetraacetic acid the proportion of citric acid would have to be increased by the factor 2. In addition to the oxalic acid concentration, the pH value has a considerable influence on the decontamination factor. It is important for the decontamination treatment that the pH value is kept constant at 3.5 + 0.5.With a pH value above pH 4 the decontaminating effect would be greatly diminished, at a pH value under pH 3 on the other hand, the danger of selective damage to the base metal would be greatly increased. Ammonia is used in a manner known per se for adjusting the pH value. In order to inhibit the decontamination solution 2- and 3-valent metal salts of organic acids may be used. The given value of 5 g Fe-lll-formate represents a lower limit which should not be reduced any further. If smaller quantities of the inhibitor are added, this results in an attack on the base metal and in selective damage to the structural materials.

Maintaining the treatment temperatures specified is of importance for the decontamination result. Below 85"C, the Fe-, CR-, Ni-oxides (spinels) are hydrolyzed by the alkaline permanganate solution only in an incomplete and very slow manner. Similarly, below 85"C, the decontamination solution dissolves the Fe-, Crand Ni-oxides in a very slow and incomplete manner. 100"C is convenient because it is the boiling temprature of water. By increasing the pressure this temperature and conseqently also the decontamination factor can be increased. However, a temperature of 1 25"C ought not to be exceeded, as otherwise the organic constituents of the decontamination solution decompose.

The treatment period of a maximum of 20 hours should be observed, since with longer treatments with the decontamination solution the grain boundary zones of the structural materials can be attacked. The treatment period is dependent upon the respective structural materials in question and upon the type of contamination. In general, a 6-12 hour decontamination treatment suffices to remove present contaminations.

The after treatment may be carried out with a suspended solution which contains in each 1000 ml water not less than 1.0 g citric acid not less than 0.5 g hydrogen peroxide, 0.1-0.5 g perfluorocarboxylic acid and 0.1-5 g cellulose fibres. With this decontamination after-treatment stage, it is important that the fibrous suspended solution is agitated vigorously. This is necessary in order for the fibres to coat the surface of the working part. This vigorous agitation of the solution can be effected in a manner known per se, for example, by means of a pump or air injection. The fibrous substance, which is inert, has the task of removing any loosely adhering, residual oxide film still remaining on the surface after the preceding 2-stage treatment by gentie, mechanical centrifugal action.Organic and/or inorganic fibres and also fabric clippings of these fibres may be used as the inert agent. In the case of narrow pipe systems and heat exchangers, balls of rubber sponge may be used in the place of organic, fibrous substances. These soft balls should be 0.1 - 0.3 mm greater in diameter than the rated diameter of the pipes to be decontaminated. The concentration of 0.1 - 5.0 g fibrous material specified should be observed since, when the concentration is too low, the centrifugal action is too slight and, when the concentration is too high, the agitating and pumping capacity of the solution is no longer guaranteed.

Hydrogen peroxide is added to the suspended solution so that in this decontamination aftertreatment any Fe-ll-oxalates which may be formed in the preceding 2-stage treatment and which are difficult to dissolve may also be removed by converting them into easily soluble Fe-Ill-oxalates. This danger of Fe-ll-oxalates forming exists above all in the case of 13% - and 17% CR-steels. However, since the hydrogen peroxide simultaneously oxidizes the oxalate to CO2, an organic carboxylic, dicarboxylic-, oxycarboxylic- or hydroxycarboxylic acid is also added to the suspended solution in order to form a complex with the liberated ferrous ion. Without the addition of this acid, there would be fresh cementation of the iron. By adding a wetting agent, the surface tension of the suspended solution may be greatly reduced.The fibres can thus coat the surface more intensively. The starting concentration of the wetting agent should be determined in each case by the concentration specificed by the manufacturer. All those organic wetting agents which are free from compounds containing sulphur may be used. The suspended solution may also contain conventional, commercially-available organic products as an expanding agent.

It is important in all 3 stages in the method - oxidation pretreatment, decontamination treatment and decontamination aftertreatment-thatthese soutions are free from compounds containing sulphur. In the primary system of nuclear reactors products containing sulphur are highly undesirable, since with Ni-alloys nickel/sulphur compounds are formed at higher temperatures, which result in brittle phases in the structural material. Furthermore, as a result of different operating conditions, polythionic acids can form in the steam generators of the primary system, which acids induce intercrystalline corrosion in inconel 600 at ambient temperatures.

The decontamination method described has already been used in practice for large-scale decontamination in nuclear power stations with very good results. Simultaneous procedural tests during this decontamination showed in subsequent metallographic tests that as a result of this decontamination treatment according to the invention no selective damage whatever to the materials of the nuclear power station has occurredm In every case the material loss was less than 0.1 ltm. Examples from the results of large-scale decontamination carried out and also the materials tested are set forth in the following.

This compilation is to show that with the decontamination method described the entire range of highly allowed Cr: Ni-steels, Ni-alloys and highly alloyed Cr steels can be decontaminated with high decontamination factors without the base metal being damaged.

TABLE 1 Decontamination of the main coolant pumps in Biblis (KWB-A and KWB-B) Plant Length of Component Material Decontamination Decontamination Result Operation Treatment Dose Dose Decon.

Capacity Capacity Factor Before After KBW-A 1 Cycle Wheel YD10 1.4313 11 h Decon. 7000 75 93 Wheel YD30 1.4313 8.5 h Decon. 7000-10000 50-70 100-140 Tightening 1.4550 14 h Decon. 6000 60 100 Disc YD30 Tightening 1.4550 7 h Decon. 2000-3000 30-80 25-100 Disc YD20 Inlet YD10 1.4552 13 h Decon. 9000-7000 50-70 100 Inlet YD30 1.4552 15 h Decon. 6000 60-100 60-100 KWB-B Wheel YD10 1.4313 3 h Decon. 7000 25 28 Wheel YD20 1.4313 2 h Decon. 700 15-18 45 Wheel YD40 1.4313 2 h Decon. 700 25 28 Tightening 1.4550 3 h Decon. 400 2-4 100-200 Disc YD10 Tightening 1.4550 3 h Decon. 400 2-4 100-200 Disc YD30 TABLE 2 Decontamination of bundle of heating rods in pressure vessel in Biblis (KWB-A) and Borssele (KCB) Plant Length Component Material Decontamination Decontamination Result Operation Treatment Dose Dose Decon.

Capacity Capacity Factor Before After KCB 28 months bundle II 1.4435 10.5 h Decon. 2000-3000 80-300 7-38 bundle III 1.4435 20.5 h Decon. 2500-300 45-300 8-67 bundle IV 1.4435 10 h Decon 3500-6000 50-200 17-120 KWB-A 2 cycles bundle I 1.4435 11.5 h Decon. 3000-5000 5-7 430-1000 bundle III 1.4435 6 h Decon. 500-2000 15-20 10-130 TABLE 3 Decontamination of the axial pumps in Brunsbüttel (KKB) and also of the DE manhole cover in Gundremmingen (KRB-I) Plant Length of Component Material Decontamination Decontamination Result Operation Treatment Dose Dose Decon.

Capacity Capacity Factor Before After KKB 192 days Wheel P2 x6CrNiMo 16.6 9 h Dec. 6000 50 120 Wheel P3 " 10 h Dec. 1000 15 66 Wheel P5 " 5 h Dec. 5000-15000 150-1500 10-100 Wheel P6 " 3.5 h Dec. 20000-45000 200-3000 15-100 Wheel nut P2 1.4021 9 h Dec. 300 3 100 Bearing Cover P2 1.4550 9 h Dec. 700 3 230 Hydrost.

bearing P3 1.4122 6 h Dec. 1000 10 100 KRB-I 10 years DE-manhole cover 1.4301 18 h Dec. 850 30-150 6-28 In the case of the large-scale decontamination in nuclear power stations specified above, parts of the primary system had to be decontaminated in order to reduce the disturbing radiation loading during repair work. With this decontamination, entire components as well as partial areas of systems were decontaminated. Parts which were easily dismantled were treated in external tanks by immersion in baths. Areas of the primary system unable to be dismantled were localized by shut-off devices and attacked with solution with the aid of an external decontamination circuit.When the 3-stage decontamination process was carried out, it was revealed that the third treatment stage can increase the decontamination factor again by a factor of 5 to 10 according to the structural materials in question and the type of contamination in question.

Since the used decontamination solutions themselves have become radioactive, these must be given over to radioactive waste. It is important in this connection that a substantial reduction in volume is obtained. In the present case, the two solutions, namely the oxidising solution and the decontamination solution, are mixed together, and the oxalic acid is thereby oxidized to form CO2 and the KMnO4 is reduced to Mn. With a mixture ratio of 1:1 a solution thus pre-treated can be concentrated by vaporization by approximately 80% without this resulting in the precipitation of salts. For processing this concentrate until it is finally to be stored, further chemical and physical methods known per se can then be applied.

The method according to the invention thus allows not only thorough decontamination of radioactively contaminated structural parts of nuclear reactors, and more particularly virtually without any disadvantageous effect on the base metal, but also allows the used solutions to be concentrated in a relatively simple manner.

Claims

1. A method for chemical decontamination comprising the steps of: subjecting the parts to be decontaminated to a preliminary oxidising treatment with an alkaline permanganate solution; corroding the parts with an inhibited citrate-oxalate decontamination solution; and treating the decontaminated parts with a citric acid-hydrogen peroxide solution which contains suspendable inert material.

2. A method according to claim 1, wherein the parts are rinsed with a deionate between the oxidising treatment and the corrosion treatment.

3. A method according to claim 1 or 2, wherein the parts are rinsed with a deionate between the corrosion treatment and the after-treatment.

4. A method according to claim 1 or 2, wherein the parts are subjected to the oxidising treatment for substantially one hour.

5. A method according to any preceding claim, wherein the alkaline permanganate solution is at a temperature of from 85'Cto 1250C.

6. A method according to any preceding claim, wherein the permanganate solution is alkalised by means of an alkali metal hydroxide.

7. A method according to any preceding claim, wherein the alkaline permanganate solution contains the following substances added to each 1000 ml water: 10-50 g sodium hydroxide; and 5-30 g potassium permanganate.

8. A method according to any preceding claim, wherein the parts are subjected to the corrosion treatmentforfrom Sto 20 hours.

9. A method according to any preceding claim wherein the pH value of contamination solution is substantially 3.5.

10. A method according to claim 9, wherein the pH value of the decontamination solution is 3.5 + 0.5.

11. A method according to any preceding claim, wherein the temperature of the decontamination solution is from 85into 125"C.

12. A method according to any preceding claim, wherein the decontamination solution contains the following substances added to each 1000 ml water: 25-50 g citric acid; 20-40 g oxalic acid; 2-4 g ethylenediaminetetraacetic acid; not less than 5 g Fe-lll-formate; and ammonia to adjust the pH value.

13. A method according to claim 12, wherein the ratio of oxalic acid: citric acid: ethylenediaminetetraacetic acid is substantially 10:12.5:1.

14. A method according to any preceding claim, wherein the decontamination solution is inhibited by means of di- and tri- valent metal salts of organic metals.

15. A method according to any preceding claim, wherein the parts are subjected to the aftertreatment for from 2 to 8 hours.

16. A method according to any preceding claim wherein the temperature of the citric acid-hydrogen peroxide solution is from 20"C to 800C.

17. A method according to any preceding claim wherein the citric acid-hydrogen peroxide solution contains for each 1000 ml water:

1.0 g citric acid; 0.5 g hydrogen peroxide; and 0.1-0.5 g perfluorocarboxylic acid.

18. A method according to any preceding claim, wherein the solution contains 0.1 to 5 g cellulose fibres as the inert material.

19. A method according to any preceding claim, wherein the inert material is in the form of organic or inorganic fibres having a length of 0.5 to 15 mm, a diameter of 0.05 to 1 mm and a density of at least 1 g/cm3.

20. A method according to any preceding claim, wherein the inert material is in the form of fabric clippings having a size of from 0.2 to 4 cm2.

21. A method according to any one of claims 1 to 17, wherein the inert material is in the form of a foamed organic material.

22. A method according to claim 21, wherein the inert material is in the form of sponge rubber balls having a diameter of from 5 to 35 mm.

23. A method according to any preceding claim, wherein the citric acid-hydrogen peroxide solution contains one or more organic carboxylic-, dicarboxylic- or hydrocarboxylic acids.

24. A method according to any preceding claim, wherein the citric acid-hydrogen peroxide solution contains an expanding agent in the form of a conventional, commercially-obtainable organic product.

25. A method according to any preceding claim, wherein none of the solutions contains sulphur or sulphur-containing compounds.

26. A method for chemical decontamination according to claim 1 and substantially as hereinbefore described.