EP0313843B2 - Process for decontaminating surfaces - Google Patents

Description

The invention relates to a method for decontamination of surfaces according to the preamble of claim 1.

Such a method is known from WO 84/03170 known. This document is related in its oxidizing phase on the addition of Ozone and thus apparently reaches a low level Treatment temperature. Has such a procedure however, the disadvantage of controlling a process with a gaseous liquid as a reagent is difficult and the ozone is not too easy handle and is also toxic and also to Can cause explosions.

EP 0071336 A1 also describes a method for the chemical removal of deposits known in which using permanganate salt Decontamination is achieved in three steps becomes. Avoids the path taken there apparently the use of chromic acid.

Also according to EP 0160831 A3 for the first step used only permanganic acid. So that apparently a treatment temperature of 100 ° C is required. The solvent added directly to the primary coolant.

(1) "Decontamination of Nuclear Facilities to Permit Operation, Inspection, Maintenance. Modification or Plant Decommissioning", Technical Reports Series No. 249, International Atomic Energy Agency, Vienna 1985;

(2) Morell W., Bertold H.O., Opershall H., Fröhlich K.: "Dekontamination - Stand der Technik und aktuelle Entwicklungsziele, VGB Kraftwerkstechnik 66 (1986) 579-588.

Other known methods are listed in the following two publications.

(1) "Decontamination of Nuclear Facilities to Permit Operation, Inspection, Maintenance. Modification or Plant Decommissioning", Technical Reports Series No. 249, International Atomic Energy Agency, Vienna 1985;

(2) Morell W., Bertold HO, Opershall H., Fröhlich K .: "Decontamination - state of the art and current development goals, VGB Kraftwerkstechnik 66 (1986) 579-588.

This includes all known methods for decontamination the surfaces of components of pressurized water reactors two or more treatment steps, the insoluble in a first step Cr-III oxide in an oxidizing phase soluble hexavalent chromium and converted the whole oxide layer is loosened. In a second treatment step is then, usually after an intermediate rinse that loosened Oxide layer in an acid, refractory and complex-forming Solution resolved and removed.

For the first, i.e. the oxidative treatment step there are a number of methods in use so z. B. the so-called "AP" process, that in treatment with alkaline permanganate solution exist, or the "NP" procedures, in which nitric acid solutions for oxidation be used. Other known methods see the use of permanganic acid, Hydrogen peroxide, cerium IV salts or others Oxidizing agents before.

Most of the known methods have in common that at relatively high temperatures, mostly used between 350 and 400 K. Need to become. This is different serious disadvantages, such as the Need of relatively expensive and cumbersome auxiliary facilities, increase the corrosiveness and pressure build-up due to water vapor at treatment temperatures above 370 K.

Another serious disadvantage of all mentioned Process is the use of chemicals which contain elements that are neither in the materials of the components to be decontaminated still in the Coolant. Because complicated components or entire cooling circuits of nuclear reactors only very difficult and complete with considerable effort rinsed and thus after decontamination of all residues of the chemicals introduced can be cleaned, it is in practice unavoidable that residues of such chemicals remain in the circuits and possibly the continued operation of the nuclear reactors disturb permanently, be it through deposits, local corrosion or by activation.

It is therefore the object of the present invention a the known disadvantages known Process avoiding decontamination process to accomplish. at lower temperatures, even at normal room temperature, is effective and manages with relatively harmless chemicals, whose elements are not "foreign to the reactor". but also in the coolant and in the materials the cooling circuit components usually contain are.

This task is according to the procedure Claim 1 solved.

Contains according to the inventive method the one used in the first treatment step Chromic acid (chromium VI oxide) decontamination solution and permanganic acid. Both chrome and Manganese are also common in all reactor construction steels used as accompanying or alloying elements available. These chemicals are not only inexpensive, but in the used Concentrations are also relatively non-toxic and easy to handle. To the volumes of the as liquid radioactive waste to be considered Decontamination agents as low as possible to keep and continue the use of dangerous substances to minimize that in the first treatment step used decontamination solution afterwards other substances added to the solution for use in the second treatment step make suitable. As such, other substances come reducing agents, such as oxalic acid, Ascorbic acid, formic acid, etc. into consideration. The reducing agents cause the chromic acid, and the permanganic acid and its decomposition products, thus also the brown stone, in soluble Chromium III or manganese II salts converted will.

The permanganic acid can preferably be prepared by adding an aqueous solution of a Alkali or alkaline earth permanganate via a cation exchanger passed and so the free acid is formed after the addition of chromic acid is used as a decontamination agent. There are also solutions of chromic acid and salts of Permanganic acid suitable as a decontamination agent; however, the additional imported cation with the radioactive waste slightly higher salt loads arise. Characterizing for the effectiveness of the decontamination agent are the pH and the redox potential of the Solution. Therefore, this can be easily grasped The first treatment step is monitored and be controlled.

By reacting the permanganic acid with components the contaminated oxide layers and by spontaneous decomposition of the permanganic acid insoluble forms even at normal room temperatures Manganese dioxide ("manganese dioxide") that is on precipitates the surfaces. The discoloration shows the effectiveness of the decontamination solution on. Because of the presence of chromic acid in the decontamination solution there are no firmly adhering layers that form difficult to remove afterwards. Through the oxidative first treatment step the surfaces of the cooling circuit components not yet completely radioactive Free substances, which is why a second treatment step is also required to remove by oxidative Treatment of modified surface layers is necessary. The success of the second treatment step can also be checked visually since the brownish-red-violet colored surface layers from the decontaminated surfaces disappear.

The effect of the first treatment step used decontamination solution by pumping, stirring or by application of ultrasound significantly increase. Through the Chemical measures can also be the same Removal of the modified surface layers be accelerated in the second treatment step.

It has been shown that in the first treatment step modified surface layers for example carbon steels, rustproof Chrome steels, nickel alloys and others materials used in reactor construction alone by mechanical and or hydraulic action, for example in the middle of a high pressure water jet, can be removed to make a flawless To achieve decontamination.

According to a preferred training is on the second treatment step then the Solution in the circuit via an ion exchanger passed and rinsed the treated surface.

This measure eliminates an additional one Flushing medium.

a) Platten aus ferritischem Chromstahl (Werkstoff Nr. 1.4001 nach DIN) aus der Dichtung des Mannlochdeckels von Dampferzeugern;

b) Platten und Rohre aus austenitischen rostfreien Stählen;

c) Dampferzeugerrohre aus Eisen-Nickel-Chrom-Legierungen der Handelsbezeichnung INCOLOY 800 und aus Nickei-Chrom-Eisen-Legierungen der Handelsbezeichnung INCONEL 600. (INCOLOY und INCONEL sind eingetragene Warenzeichen der Firma International Nickel Company.)

The effectiveness of the described method according to the invention was tested on extensive sample material from the primary part of Swiss and foreign pressurized water reactors. There were mainly radioactive contaminated samples made of the following materials:

a) plates made of ferritic chrome steel (material no. 1.4001 according to DIN) from the seal of the manhole cover of steam generators;

b) plates and tubes made of austenitic stainless steels;

c) Steam generator tubes made of iron-nickel-chromium alloys of the trade name INCOLOY 800 and of nickel-chromium-iron alloys of the trade name INCONEL 600. (INCOLOY and INCONEL are registered trademarks of the company International Nickel Company.)

These samples a), b) and c) were major contaminated by the cobalt isotope Co-60.

Example 1:

Samples a) were made of ferritic chrome steel at room temperature (290 to 295 K) during 16 hours with a solution of 0.05 mol each Chromic and permanganic acid treated. After a Intermediate rinsing became a decontamination factor (Ratio of measured activity before and after treatment) determined from 2. Another Treatment at room temperature in an aqueous 0.1 mol solution of oxalic acid under action of ultrasound delivered after 15 minutes a decontamination factor of about 20 and after 6 hours to a decontamination factor of over 100. After treatment, the decontaminated surfaces of the samples metallic bare and neither macroscopic nor microscopic visibly attacked.

Example 2:

Samples c) made of nickel-chromium-iron alloys the trade name INCONEL 600 at room temperature for 16 hours a solution of 0.1 mol of chromic acid and 0.004 mol of potassium permanganate treated. After a Intermediate rinsing became a decontamination factor of only 1.2 found. After another Treatment at room temperature with a aqueous solution of 0.1 mol of oxalic acid during 6 hours of exposure to ultrasound became a decontamination factor determined from 12.

Example 3:

Samples a) made of ferritic chrome steel, samples b) austenitic stainless steels and samples c) from INCOLOY 800 and from INCONEL 600 were each at room temperature for 16 hours in aqueous solutions with 0.01 to 0.1 mol Chromic acid and 0.001 to 0.05 mol permanganic acid treated, the ratio of chromic acid to permanganic acid was between 1:10 and 25: 1. The samples were then each during 6 Hours at room temperature in an aqueous Solution with 0.1 mol oxalic acid under the influence of ultrasound treated further. Finally were on all samples, depending on the oxidative treatment and from the sample material, decontamination factors measured between 10 and 1000.

Example 4:

Samples a) made of ferritic chromium steel and samples c) made from INCONEL 600 were each treated for 16 hours at room temperature in a solution with 0.1 mol of chromic acid and 0.05 mol of permanganic acid. After a subsequent treatment with a water jet of 2.4 kbar (240 Pa) pressure at a treatment speed of 3.6 m ² / hour, decontamination factors of about 30 were obtained on samples a) made of ferritic chromium steel and 600 decontamination factors of on samples c) from INCONEL measured over 100. Extensive follow-up examinations showed that these treatments did not attack the surfaces of the base materials.

Example 5:

Samples c) from INCONEL 600 were during 16 hours at room temperature with a solution of 0.05 mol of chromic acid and 0.002 mol of permanganic acid sprayed. After a further one Treatment with a water jet, as in the Example 4, decontamination factors were between 20 and 80 determined.

Example 6:

From an aqueous solution of 0.4 mol Chromic acid and 0.1 mol permanganoic acid by adding a thickener, which under the trade name AEROSIL (registered Trademark of Degussa) on the market is available, a paste is made. The contaminated Surfaces of samples a) made of ferritic Chrome steel was coated with this paste. After an exposure time of 16 hours the samples with a water jet, as in the example 4, treated. Decontamination factors resulted between 5 and 15.

The experiments described for example and further extensive research showed that in reactor construction usually for the cooling circuits materials used through the treatments according to the inventive method not be harmed, regardless of whether they are so decontaminated components aged, heat treated, are welded or deformed.

Claims (5)

1. Method of decontaminating surfaces, in particular on components of coolant circuits in nuclear reactors, an oxidising agent being applied in a first treatment step to loosen the surface layers which are radioactively contaminated, and the loosened surface layers being removed in a second treatment step with the aid of a reducing agent added to the oxidising agent, characterised in that a decontamination solution containing chromic acid and permanganic acid or salts thereof is used as the oxidising agent, the decontamination solution not containing any ozone.
2. Method according to Claim 1, characterised in that the first and/or second treatment step is carried out under the effects of ultrasound.
3. Method according to Claim 1, characterised in that the surface layers are removed mechanically or hydraulically in the second treatment step.
4. Method according to Claim 1, characterised in that organic acids and/or complexing agents are added to the decontamination solution to which the reducing agents have been added.
5. Method according to one of Claims 1 to 4, characterised in that, after the second treatment step, the decontamination solution is circulated over an ion exchanger and, at the same time, the treating [sic] surfaces are rinsed.