GB1572867A

GB1572867A - Method for chemical decontamination of structural parts

Info

Publication number: GB1572867A
Application number: GB909377A
Authority: GB
Original assignee: Kraftwerk Union AG
Current assignee: Kraftwerk Union AG
Priority date: 1976-03-29
Filing date: 1977-03-03
Publication date: 1980-08-06
Also published as: FR2346819A1; JPS52118200A; CH626741A5; FR2346819B1; DE2613351A1; DE2613351C3; JPS608479B2; DE2613351B2

Abstract

A process for the chemical decontamination of nuclear reactor components in two steps comprises an approximately one hour-long oxidative pretreatment with an alkaline permanganate solution and, after intermediate rinsing with deionised water, pickling with an inhibited citrate-oxalate decontamination solution which has a pH set to approximately 3.5 and is applied for from 5 to 20 hours. The temperatures in the process, as well as during the final rinsing with deionized water, are at 95-100 DEG C.

Description

(54) A METHOD FOR CHEMICAL DECONTAMINATION OF STRUCTURAL PARTS (71) We, KRAFTWERK UNION AKTIENGESELLSCHAFT, a German company of 433 Miilheim (Ruhr), Wiesenstrasse 35, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a method for chemical decontamination of structural parts in two stages.

In water-cooled nuclear reactor for example, after a short running period, a cohesive dense oxide layer forms in the primary circulation of a nuclear power plant as a result of corrosion of the structural materials. This initially inactive oxide layer is contaminated during operation even in components which do not lie in the direct radiation field of the nuclear zone, thus becoming radioactive. This contamination can be attributed to the incorporation of activated corrosion products into the oxide layer of the structural materials. Since this process runs continuously, the result is a concentration, particularly of long-lived nuclides, in the oxide layer. It is therefore imperative that possibilities of eliminating this surface contamination by suitable decontaminating agents be sought.This demand becomes more and more pressing as the running period increases, as maintenance of the installations and above all repair work result in ever increasing radiation loading of the operating personnel.

Attempts have been made hitherto to decontaminate contaminated surfaces with aqueous solutions of mineral and organic acids. The- results thereby obtained, however, were totally inadequate since damage to the structural material itself, particularly, was caused at the same time.

Only the two stage APAC method (Alkaline - Permanganate - Ammonium Citrate) provided a good degree of decontamination, but it similarly resulted in selective corrosion phenomena and, depending on the structural material, to unduly high base metal attacks.

According to the present invention there is provided a method for the chemical decontamination of structural parts, wherein the structural parts are subjected to a preliminary oxidising treatment with an alkaline permanganate solution and are then corroded with an inhibited citrateoxalate decontamination solution having a pH value of 3.5+0.5.

Preferably, the structural parts are rinsed with a deionate between the oxidising treatment and the corrosion treatment.

The structural parts may be subjected to the oxidising treatment for substantially one hour.

The alkaline permanganate solution is preferably at a temperature of from 95 to 100"C.

Preferably, the structural parts are corroded for from 5 to 20 hours.

Preferably, the pH of the decontamination solution is substantially 3.5.

The decontamination solution may contain the following substances added to each 1000 ml water: 50 g di-ammonium hydrogen citrate; 40 g oxalic acid; 4 g ethylenediamine tetraacetic acid; 5 g Fe-III-Formate; together with sufficient ammonia to bring the pH of the solution to substantially 3.5.

The first three constituents represent a combination of complexing agents and organic acids; with these the decontamination factor is increased.

The decontamination solution is preferably at a temperature of from 95 to 100"C.

Preferably, the structural parts are rinsed with deionate after the corrosion treatment.

The permanganate solution may contain the following substances added to each 1000 ml water: 10 g sodium hydroxide solution; and 5 g potassium permanganate.

Preferably, the decontamination solution is inhibited by means of di- and tri- valent metal salts of organic acids. The inhibitor may be, for example, a temperature resistant inhibitor in the form of Fe-II and Fe-III salts of organic acids and is added to prevent attack on the base material thus preventing selective damage to the structural materials.

With the preliminary oxidising treatment and subsequent corrosion by the decontamination solution, radioactive materials may be brought to the following dose rate values within 1" -20 hours: Austenite 1.4550 from 1000 mR/h to 0.1 lmR/h Inconel 600 from 150 mR/h to 0.5-1 mR/h Inconel X 750 from 300 mR/h to 0.5-1 mR/h (Inconel is a Registered Trade Mark) Subsequent metallographic tests on these substances show that the decontamination treatment according to the invention does not cause selective damage of any kind to occur on these substances. The loss of material was less than 0.1y in all cases.

Similar results were obtained in the decontamination of other substances such as, for instance, Incoloy 800, Incoloy 901, chromium steel 1,4523 and deposit weldings (Incoloy is a Registered Trade Mark).

As already mentioned in the introduction, decontamination of this type is often necessary on parts of the primary circulation system, such as pipelines, steam generators etc. so that repairs can be carried out without an unduly high radiation load.

Obviously it is not necessary to treat the entire primary circulation system in this way, on the contrary it is sufficient to decontaminate the areas in the vicinity of the repair sites. All that is needed for this are blocking devices insertable from the exterior and which rest against the inner pipe walls, forming a seal, with the aid, for instance, of compressed air. Obviously the seal could also be made with the aid of purely mechanical means via elastic and acid resistant plastic seals.

Since the decontamination solutions used have themselves become radioactive, these must be discarded as radioactive waste. It is important here that a considerable reduction in volume of the used solutions may be obtained. In the present case, the two solutions, i.e. the oxidising solution and the decontamination solution are mixed together, by which the oxalic acid is oxidised to CO2 and KMnO4 reduced to Mn. With a mixture ratio of 1:1, a solution pretreated in this way may be concentrated by vaporisation by about 80% without it resulting in the precipitation of salts. For the further processing of this concentrate up to final storage, further chemical and physical methods, known per se, may be used.

The method according to the invention thus makes not only thorough decontamination of radioactively contaminated nuclear reactor structural parts possible, and virtually without any adverse effect on the base material, but also relatively simple concentration of the used solutions.

WHAT WE CLAIM IS: 1. A method for the chemical decontamination of structural parts, wherein the structural parts are subjected to a preliminary oxidising treatment with an alkaline permanganate solution and are then corroded with an inhibited citrateoxalate decontamination solution having a pH value of 3.5+0.5.

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

4. A method according to claim 1, 2 or 3, wherein the alkaline permanganate solution is at a temperature of from 95 to 1000C.

5. A method according to any preceding claim, wherein the structural parts are corroded for from 5 to 20 hours.

6. A method according to any preceding claim, wherein the pH of the decontamination solution is substantially 3.5.

7. A method according to claim 6, wherein the decontamination solution contains the following substances added to each 1000 ml water: 50 g di-ammonium hydrogen citrate; 40 g oxalic acid; 4 g ethylenediamine tetraacetic acid; 5 g Fe-III-formate; together with sufficient ammonia to bring the pH of the solution to subtantially 3.5.

8. A method according to any preceding claim, wherein the decontamination solution is at a temperature of from 95 to 100"C.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

The permanganate solution may contain the following substances added to each 1000 ml water:

10 g sodium hydroxide solution; and

5 g potassium permanganate.

Preferably, the decontamination solution is inhibited by means of di- and tri- valent metal salts of organic acids. The inhibitor may be, for example, a temperature resistant inhibitor in the form of Fe-II and Fe-III salts of organic acids and is added to prevent attack on the base material thus preventing selective damage to the structural materials.

With the preliminary oxidising treatment and subsequent corrosion by the decontamination solution, radioactive materials may be brought to the following dose rate values within 1" -20 hours: Austenite 1.4550 from 1000 mR/h to 0.1 lmR/h Inconel 600 from 150 mR/h to 0.5-1 mR/h Inconel X 750 from 300 mR/h to 0.5-1 mR/h (Inconel is a Registered Trade Mark) Subsequent metallographic tests on these substances show that the decontamination treatment according to the invention does not cause selective damage of any kind to occur on these substances. The loss of material was less than 0.1y in all cases.

Similar results were obtained in the decontamination of other substances such as, for instance, Incoloy 800, Incoloy 901, chromium steel 1,4523 and deposit weldings (Incoloy is a Registered Trade Mark).

As already mentioned in the introduction, decontamination of this type is often necessary on parts of the primary circulation system, such as pipelines, steam generators etc. so that repairs can be carried out without an unduly high radiation load.

Obviously it is not necessary to treat the entire primary circulation system in this way, on the contrary it is sufficient to decontaminate the areas in the vicinity of the repair sites. All that is needed for this are blocking devices insertable from the exterior and which rest against the inner pipe walls, forming a seal, with the aid, for instance, of compressed air. Obviously the seal could also be made with the aid of purely mechanical means via elastic and acid resistant plastic seals.

Since the decontamination solutions used have themselves become radioactive, these must be discarded as radioactive waste. It is important here that a considerable reduction in volume of the used solutions may be obtained. In the present case, the two solutions, i.e. the oxidising solution and the decontamination solution are mixed together, by which the oxalic acid is oxidised to CO2 and KMnO4 reduced to Mn. With a mixture ratio of 1:1, a solution pretreated in this way may be concentrated by vaporisation by about 80% without it resulting in the precipitation of salts. For the further processing of this concentrate up to final storage, further chemical and physical methods, known per se, may be used.

The method according to the invention thus makes not only thorough decontamination of radioactively contaminated nuclear reactor structural parts possible, and virtually without any adverse effect on the base material, but also relatively simple concentration of the used solutions.

WHAT WE CLAIM IS: 1. A method for the chemical decontamination of structural parts, wherein the structural parts are subjected to a preliminary oxidising treatment with an alkaline permanganate solution and are then corroded with an inhibited citrateoxalate decontamination solution having a pH value of 3.5+0.5.
2. A method according to claim 1, wherein the structural 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 structural parts are subjected to the oxidising treatment for substantially one hour.
4. A method according to claim 1, 2 or 3, wherein the alkaline permanganate solution is at a temperature of from 95 to 1000C.
5. A method according to any preceding claim, wherein the structural parts are corroded for from 5 to 20 hours.
6. A method according to any preceding claim, wherein the pH of the decontamination solution is substantially 3.5.
7. A method according to claim 6, wherein the decontamination solution contains the following substances added to each 1000 ml water:

50 g di-ammonium hydrogen citrate;

40 g oxalic acid;

4 g ethylenediamine tetraacetic acid;

5 g Fe-III-formate; together with sufficient ammonia to bring the pH of the solution to subtantially 3.5.
8. A method according to any preceding claim, wherein the decontamination solution is at a temperature of from 95 to 100"C.
9. A method according to any preceding

claim, wherein the structural parts are rinsed with deionate after the corrosion treatment.

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

5 g potassium permangante.
11. A method according to any preceding claim, wherein the decontamination solution is inhibited by means of di- and trivalent metal salts of organic acids.
12. A method for the chemical decontamination of structural parts according to claim 1 and substantially as hereinbefore described.