EP2923360B1 - Process for decontamination of surfaces of parts of the cooling circuit of a nuclear reactor - Google Patents

Process for decontamination of surfaces of parts of the cooling circuit of a nuclear reactor Download PDF

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Publication number
EP2923360B1
EP2923360B1 EP13815419.0A EP13815419A EP2923360B1 EP 2923360 B1 EP2923360 B1 EP 2923360B1 EP 13815419 A EP13815419 A EP 13815419A EP 2923360 B1 EP2923360 B1 EP 2923360B1
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decontamination
acid
aqueous solution
metal ions
oxide layer
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German (de)
French (fr)
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EP2923360A1 (en
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Luis Sempere Belda
Jose Pedro MOREIRA DO AMARAL
Christian Topf
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Areva GmbH
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Areva GmbH
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    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
    • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
    • G21F9/001—Decontamination of contaminated objects, apparatus, clothes, food; Preventing contamination thereof
    • G21F9/002—Decontamination of the surface of objects with chemical or electrochemical processes
    • G21F9/004—Decontamination of the surface of objects with chemical or electrochemical processes of metallic surfaces
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
    • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
    • G21F9/28—Treating solids
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
    • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
    • G21F9/28—Treating solids
    • G21F9/30—Processing

Definitions

  • the invention relates to a method for surface decontamination of components of the coolant circuit of a nuclear reactor, that is, a pressurized water or boiling water reactor.
  • the core of the coolant circuit is a reactor pressure vessel in which nuclear fuel-containing fuel elements are arranged.
  • At the reactor pressure vessel several cooling loops are usually connected, each with a coolant pump.
  • a pressurized water reactor with temperatures in the range of 300 ° C show even stainless austenitic FeCrNi steels, which, for example, the tube system of the cooling loops, Ni alloys, of which, for example, the exchanger tubes of steam generators and others about used for coolant pumps, eg cobalt-containing components, a certain solubility in water.
  • Metal ions liberated from the abovementioned alloys pass with the coolant flow to the reactor pressure vessel, where they are partially converted into radioactive nuclides by the neutron radiation prevailing there.
  • the nuclides are dispersed by the coolant flow throughout the coolant system and are stored in oxide layers that form on the surfaces of coolant system components during operation.
  • the oxide layers contain iron oxide with di- and tri-valent iron and oxides of other metals, especially chromium and nickel, which are present as alloying constituents in the steels mentioned above, depending on the type of alloy used for a component.
  • Nickel is always present in divalent form (Ni 2+ ), chromium in trivalent (Cr 3+ ) form.
  • the oxide layer is first treated oxidatively in the case of components containing chromium (oxidation step) and then the oxide layer is dissolved under acidic conditions in a so-called decontamination step with the aid of an acid, which is referred to below as decontamination or deconic acid.
  • decontamination or deconic acid The metal ions passing into the solution in the course of treatment with a deconic acid are removed from the solution by passing them through an ion exchanger.
  • An excess of oxidizing agent optionally present after the oxidation step is neutralized or reduced in a reduction step by addition of a reducing agent.
  • the dissolution of the oxide layer or the dissolution of metal ions in the decontamination step thus takes place in the absence of an oxidizing agent.
  • the reduction of the excess oxidant may be an independent treatment step, wherein the cleaning solution is metered into a reduction agent serving for the purpose of reduction, for example ascorbic acid, citric acid or oxalic acid for the reduction of permanganate ions and manganese dioxide.
  • a reduction agent serving for the purpose of reduction
  • the reduction of excess oxidizing agent can also take place in the context of the decontamination step, wherein an amount of organic decontamination acid is added which is sufficient to neutralize or reduce excess oxidant on the one hand and to cause oxide dissolution on the other hand.
  • a treatment or decontamination cycle comprising the treatment sequence "Oxidation Step Reduction Step Decontamination Step” or “Oxidation Step Decontamination Step with Simultaneous Reduction” is performed several times to achieve sufficient decontamination or reduction of the radioactivity of the component surfaces.
  • Decontamination methods of the type described above are known, for example, under the name CORD (chemical oxidation, reduction and decontamination).
  • the oxidative treatment of the oxide layer is necessary because chromium-III oxides and trivalent chromium-containing mixed oxides, especially of the spinel type, are difficult to dissolve in the decontaminates in question for decontamination.
  • an oxidizing agent such as Ce 4+ , HMnO 4 , H 2 S 2 O 8 , KMnO 4 , KMnO 4 with acid or alkali or ozone.
  • the result of this treatment is that Cr-III is oxidized to Cr-VI, which goes into solution as CrO 4 2- .
  • the cleaning solution contains essentially Cr-III, Fe-II, Fe-III, Ni-II and, in addition, radioactive isotopes, e.g. Co-60th These metal ions can be removed from the cleaning solution with an ion exchanger.
  • Deconic acid which is commonly used in the decontamination step, is oxalic acid because it effectively dissolves the oxide layers to be removed from component surfaces.
  • deconic acids especially oxalic acid
  • oxalate precipitates with bivalent metal ions such as Ni 2+ , Fe 2+ , and Co 2+ sparingly soluble precipitates, in the case of oxalic acid, to which reference is made below.
  • the said precipitates can be distributed throughout the coolant system, depositing on the inner surfaces of pipelines and components, such as steam generators.
  • the rainfall complicates the entire process implementation.
  • a further disadvantage is that in the course of the formation of, in particular, oxalate precipitates for coprecipitation of radionuclides contained in the aqueous solution and thus to a Re-contamination of the component surfaces comes.
  • the risk of recontamination is particularly high for components with a large surface to volume ratio. This is especially the case with steam generators which have a very large number of small diameter exchanger tubes.
  • recontamination preferably occurs in zones with low flow.
  • a further disadvantage of the formation of oxalate and other precipitates is that they can clog filter devices, such as the filter upstream of an ion exchanger and sieve trays or the protection filters of circulating pumps.
  • a further disadvantage arises when a treatment cycle comprising an oxidation step and a decontamination step described above is repeated, that is to say when a renewed oxidation step follows a deconstruction step. If precipitation has occurred in the previous decontamination step, the corresponding metal ions, such as Ni in the case of a nickel oxalate precipitate, can not be removed from the cleaning solution by means of ion exchangers.
  • the oxalate residue of the precipitates is oxidized to carbon dioxide and water, thereby consuming oxidizing agent uselessly.
  • the oxalate in solution, that is not bound in the form of a precipitate, the oxalate in a simple manner, such as before the cleaning solution is passed into an ion exchanger, destroyed in a simple and cost-effective manner, for example by means of UV light, ie converted to carbon dioxide and water.
  • This object is achieved in a decontamination process of the type mentioned above by metal ions that have passed into the aqueous solution during the oxidation step, be removed before performing the decontamination step, ie before the addition of an organic deconic acid, using a cation exchanger from the solution.
  • the procedure is advantageously such that the aqueous solution is passed through a cation exchanger.
  • the removal of nickel is particularly advantageous since it forms salts or precipitates which are particularly sparingly soluble with organic acids.
  • the oxide layer is treated with a deconic acid and solid metal ions from the oxide layer can be solved, the resulting metal ion concentrations are lower than in conventional decontamination, since at least a portion of the gone in the oxidation step in the metal ions have been removed before, so is no longer in the solution.
  • the risk of the solubility product of a metal salt of a deconic acid (the product of the activities of the metal cation and the acid anion) being exceeded and a sparingly soluble precipitate forming is thus reduced.
  • the formation of sparingly soluble nickel oxalate precipitates is critical because nickel oxalate has a relatively low solubility product.
  • ion exchangers are generally organic in nature, they are sensitive to oxidizing agents, in particular to the preferred used in a process according to the invention permanganic acid or their alkali metal salts, which are very strong oxidizing agent. Therefore, in the case of organic ion exchangers in particular, it is expedient to neutralize an oxidant still present in the aqueous solution with the aid of a reducing agent before the solution is passed over the cation exchanger to remove metal ions.
  • the reducing agent used is the deconic acid used in the subsequent decontamination step. It is advantageous that this acid is already on site, so that an additional effort for example for procurement and warehousing and for an additional authorization, which would be when using a different of the deconic acid reducing agent, such as glyoxylic required.
  • An inventive method can be used, for example, for decontamination of all or part of the coolant system of a nuclear reactor, such as a boiling water reactor.
  • Fig. 1 schematically the coolant system and the primary circuit of a pressurized water reactor is shown. It comprises, in addition to the pressure vessel 1, in which at least in operation a plurality of fuel elements 2 are present, a conduit system 3 which is connected to the pressure vessel 1, and various installations such as a steam generator 4 and a coolant pump 5.
  • the aim of the cleaning in question or the decontamination is to dissolve an existing on the inner surfaces 7 of the components of the primary circuit oxide layer and to remove their gone into solution components from the aqueous solution.
  • the entire coolant system is filled with an aqueous solution containing, for example, a complex-forming organic acid such as oxalic acid, to which reference will be made hereinafter by way of example.
  • a filling this is also to be understood as a procedure in which the coolant present after switching off the power operation, ie after the shutdown of the system in the coolant system forms the aqueous solution in question, this being used to carry out the oxidation step an oxidizing agent, preferably permanganic acid or potassium permanganate, is added.
  • an oxidizing agent preferably permanganic acid or potassium permanganate
  • the oxidation was carried out in acidic solution with permanganic acid as the oxidizing agent with a concentration of about 200 ppm at a temperature of about 90 ° C.
  • permanganic acid as the oxidizing agent with a concentration of about 200 ppm at a temperature of about 90 ° C.
  • the concentration or amount of nickel ions increased to about 6,000 g in about 10 hours and then remained substantially the same.
  • oxalic acid as reducing agent was metered into the aqueous solution in a slightly more than stoichiometric amount to neutralize unconsumed permanganic acid.
  • nickel is retained by the cation exchanger, so that its present in the total system amount or its concentration decreases accordingly.
  • the decontamination step (III) was initiated by the addition of oxalic acid.
  • the metered addition was carried out in such a way that an oxalic acid concentration of 2000 ppm was not exceeded in the solution. It can be seen in the diagram that the amount of nickel first increased greatly due to the dissolution of the oxide layer, but then decreased due to the switched cation exchanger 8. If the amount of nickel accumulated in Phase I had not been removed in accordance with the present invention, Phase III, rather than a 7,000 gram nickel, would have resulted in a substantially greater total nickel in the solution of approximately 13,000 grams, resulting in solubility problems and the risk of precipitation ,

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • General Engineering & Computer Science (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Electrochemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Chemical & Material Sciences (AREA)
  • General Chemical & Material Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Food Science & Technology (AREA)
  • Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)
  • Cleaning By Liquid Or Steam (AREA)
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Description

Die Erfindung betrifft ein Verfahren zur Oberflächen-Dekontamination von Bauteilen des Kühlmittelkreislaufs eines Kernreaktors, also eines Druckwasser- oder Siedewasserreaktors. Kernstück des Kühlmittelkreislaufs ist ein Reaktordruckbehälter, in dem Kernbrennstoff enthaltende Brennelemente angeordnet sind. Am Reaktordruckbehälter sind meist mehrere Kühlschleifen mit jeweils einer Kühlmittelpumpe angeschlossen.The invention relates to a method for surface decontamination of components of the coolant circuit of a nuclear reactor, that is, a pressurized water or boiling water reactor. The core of the coolant circuit is a reactor pressure vessel in which nuclear fuel-containing fuel elements are arranged. At the reactor pressure vessel several cooling loops are usually connected, each with a coolant pump.

Unter den Bedingungen des Leistungsbetriebes beispielsweise eines Druckwasserreaktors mit Temperaturen im Bereich von 300°C zeigen selbst rostfreie austenitische FeCrNi-Stähle, aus denen beispielsweise das Röhrensystem der Kühlschleifen besteht, Ni-Legierungen, aus denen beispielsweise die Austauscher-Rohre von Dampferzeugern bestehen und sonstige etwa für Kühlmittelpumpen verwendete, z.B. Cobalt enthaltende Bauteile, eine gewisse Löslichkeit in Wasser. Aus den genannten Legierungen herausgelöste Metallionen gelangen mit dem Kühlmittelstrom zum Reaktordruckbehälter, wo sie durch die dort herrschende Neutronenstrahlung teilweise in radioaktive Nuklide umgewandelt werden. Die Nuklide werden wiederum vom Kühlmittelstrom im gesamten Kühlmittelsystem verteilt und werden in Oxidschichten, die sich während des Betriebs auf den Oberflächen von Bauteilen des Kühlmittelsystems bilden, eingelagert. Mit zunehmender Betriebsdauer reichern sich die aktivierten Nuklide in und/oder auf der Oxidschicht an, so dass die Radioaktivität bzw. die Dosisleistung an den Bauteilen des Kühlmittelsystems zunimmt. Die Oxidschichten enthalten je nach Art der für ein Bauteil verwendeten Legierung als Hauptbestandteil Eisenoxid mit zwei- und dreiwertigem Eisen und Oxide anderer Metalle, vor allem Chrom und Nickel, die als Legierungsbestandteile in den oben erwähnten Stählen vorhanden sind. Dabei liegt Nickel stets in zweiwertiger Form (Ni2+), Chrom in dreiwertiger (Cr3+) Form vor.Under the conditions of power operation, for example, a pressurized water reactor with temperatures in the range of 300 ° C show even stainless austenitic FeCrNi steels, which, for example, the tube system of the cooling loops, Ni alloys, of which, for example, the exchanger tubes of steam generators and others about used for coolant pumps, eg cobalt-containing components, a certain solubility in water. Metal ions liberated from the abovementioned alloys pass with the coolant flow to the reactor pressure vessel, where they are partially converted into radioactive nuclides by the neutron radiation prevailing there. The nuclides, in turn, are dispersed by the coolant flow throughout the coolant system and are stored in oxide layers that form on the surfaces of coolant system components during operation. With increasing operating time, the activated nuclides accumulate in and / or on the oxide layer, so that the radioactivity or the dose rate increases on the components of the coolant system. The oxide layers contain iron oxide with di- and tri-valent iron and oxides of other metals, especially chromium and nickel, which are present as alloying constituents in the steels mentioned above, depending on the type of alloy used for a component. Nickel is always present in divalent form (Ni 2+ ), chromium in trivalent (Cr 3+ ) form.

Bevor Kontroll-, Wartungs-, Reparatur- und Rückbaumaßnahmen am Kühlmittelsystem vorgenommen werden können ist eine Reduzierung der radioaktiven Strahlung der jeweiligen Bauteile bzw. Komponenten erforderlich, um die Strahlungsbelastung des Personals zu verringern. Dies geschieht dadurch, dass die auf den Oberflächen der Bauteile vorhandene Oxidschicht mittels eines Dekontaminationsverfahrens möglichst vollständig entfernt wird. Bei einer derartigen Dekontamination wird entweder das gesamte Kühlmittelsystem bzw. ein davon etwa durch Ventile abgetrennter Teil mit einer wässrigen Reinigungslösung befüllt oder es werden einzelne Bauteile des Systems in einem separaten, die Reinigungslösung enthaltenden Behälter behandelt.Before control, maintenance, repair and demolition measures can be carried out on the coolant system, it is necessary to reduce the radioactive radiation of the respective components in order to reduce the radiation exposure of the personnel. This happens because the oxide layer present on the surfaces of the components is removed as completely as possible by means of a decontamination process. In such a decontamination, either the entire coolant system or a part separated therefrom by valves is filled with an aqueous cleaning solution or individual components of the system are treated in a separate container containing the cleaning solution.

Die Oxidschicht wird bei Chrom enthaltenden Bauteilen zunächst oxidativ behandelt (Oxidationsschritt) und anschließend die Oxidschicht unter sauren Bedingungen in einem sog. Dekontaminationsschritt mit Hilfe einer Säure, die im Folgenden mit Dekontaminations- oder Dekontsäure bezeichnet wird, aufgelöst. Die im Zuge der Behandlung mit einer Dekontsäure in die Lösung übertretenden Metallionen werden aus der Lösung entfernt, indem diese über einen Ionentauscher geleitet wird. Ein gegebenenfalls nach dem Oxidationsschritt vorhandener Überschuss an Oxidationsmittel wird in einem Reduktionsschritt durch Zugabe eines Reduktionsmittels neutralisiert bzw. reduziert. Die Auflösung der Oxidschicht bzw. das Herauslösen von Metallionen im Dekontaminationsschritt erfolgt somit bei Abwesenheit eines Oxidationsmittels. Die Reduktion des überschüssigen Oxidationsmittels kann ein eigenständiger Behandlungsschritt sein, wobei der Reinigungslösung ein nur dem Zwecke der Reduktion dienendes Reduktionsmittel, beispielsweise Ascorbinsäure, Zitronensäure oder Oxalsäure zur Reduktion von Permanganat-Ionen und Braunstein zudosiert wird. Die Reduktion überschüssigen Oxidationsmittel kann aber auch im Rahmen des Dekontaminationsschritts erfolgen, wobei eine Menge an organischer Dekontaminationssäure zugesetzt wird, die ausreicht um einerseits überschüssiges Oxidationsmittel zu neutralisieren bzw. zu reduzieren und andererseits eine Oxidauflösung zu bewirken. In der Regel wird ein die Behandlungsfolge "Oxidationsschritt-Reduktionsschritt-Dekontaminationsschritt" oder "Oxidationsschritt-Dekontaminationsschritt mit gleichzeitiger Reduktion" umfassender Behandlungs- oder Dekontaminationszyklus mehrmals durchgeführt, um eine ausreichende Dekontamination bzw. Verringerung der Radioaktivität der Bauteiloberflächen zu erzielen. Dekontaminationsverfahren der oben beschriebenen Art sind z.B. unter der Bezeichnung CORD (chemische Oxidation, Reduktion und Dekontamination) bekannt.The oxide layer is first treated oxidatively in the case of components containing chromium (oxidation step) and then the oxide layer is dissolved under acidic conditions in a so-called decontamination step with the aid of an acid, which is referred to below as decontamination or deconic acid. The metal ions passing into the solution in the course of treatment with a deconic acid are removed from the solution by passing them through an ion exchanger. An excess of oxidizing agent optionally present after the oxidation step is neutralized or reduced in a reduction step by addition of a reducing agent. The dissolution of the oxide layer or the dissolution of metal ions in the decontamination step thus takes place in the absence of an oxidizing agent. The reduction of the excess oxidant may be an independent treatment step, wherein the cleaning solution is metered into a reduction agent serving for the purpose of reduction, for example ascorbic acid, citric acid or oxalic acid for the reduction of permanganate ions and manganese dioxide. However, the reduction of excess oxidizing agent can also take place in the context of the decontamination step, wherein an amount of organic decontamination acid is added which is sufficient to neutralize or reduce excess oxidant on the one hand and to cause oxide dissolution on the other hand. Typically, a treatment or decontamination cycle comprising the treatment sequence "Oxidation Step Reduction Step Decontamination Step" or "Oxidation Step Decontamination Step with Simultaneous Reduction" is performed several times to achieve sufficient decontamination or reduction of the radioactivity of the component surfaces. Decontamination methods of the type described above are known, for example, under the name CORD (chemical oxidation, reduction and decontamination).

Die oxidative Behandlung der Oxidschicht ist erforderlich, weil sich Chrom-III-Oxide und dreiwertiges Chrom enthaltende Mischoxide vor allem des Spinelltyps in den für eine Dekontamination in Frage kommenden Dekontsäuren nur schwer lösen. Um die Löslichkeit zu erhöhen, wird daher zunächst die Oxidschicht mit einer wässerigen Lösung eines Oxidationsmittels wie Ce4+, HMnO4, H2S2O8, KMnO4, KMnO4 mit Säure oder Lauge oder Ozon behandelt. Ergebnis dieser Behandlung ist, dass Cr-III zu Cr-VI oxidiert wird, welches als CrO4 2- in Lösung geht.The oxidative treatment of the oxide layer is necessary because chromium-III oxides and trivalent chromium-containing mixed oxides, especially of the spinel type, are difficult to dissolve in the decontaminates in question for decontamination. In order to increase the solubility, therefore, first the oxide layer treated with an aqueous solution of an oxidizing agent such as Ce 4+ , HMnO 4 , H 2 S 2 O 8 , KMnO 4 , KMnO 4 with acid or alkali or ozone. The result of this treatment is that Cr-III is oxidized to Cr-VI, which goes into solution as CrO 4 2- .

Aufgrund der Anwesenheit eines Reduktionsmittels im Dekontschritt, was stets der Fall ist, wenn eine organische Dekontaminationssäure verwendet wird, wird das im Oxidationsschritt entstandene Cr-VI, das als Chromat in der wässrigen Lösung vorliegt, wieder zu Cr-III reduziert. Am Ende eines Dekontschrittes befinden sich in der Reinigungslösung im Wesentlichen Cr-III, Fe-II, Fe-III, Ni-II und daneben radioaktive Isotope wie z.B. Co-60. Diese Metallionen können aus der Reinigungslösung mit einem Ionentauscher entfernt werden. Eine im Dekontschritt häufig verwendete Dekontsäure ist Oxalsäure, weil sich mit ihr die von Bauteiloberflächen zu entfernenden Oxidschichten effektiv auflösen lassen.Due to the presence of a reducing agent in the decontamination step, which is always the case when an organic decontamination acid is used, the resulting in the oxidation step Cr-VI, which is present as chromate in the aqueous solution is reduced again to Cr-III. At the end of a decontamination step, the cleaning solution contains essentially Cr-III, Fe-II, Fe-III, Ni-II and, in addition, radioactive isotopes, e.g. Co-60th These metal ions can be removed from the cleaning solution with an ion exchanger. Deconic acid, which is commonly used in the decontamination step, is oxalic acid because it effectively dissolves the oxide layers to be removed from component surfaces.

Nachteilig ist jedoch, dass manche Dekontsäuren, insbesondere auch Oxalsäure, mit zweiwertigen Metallionen wie Ni2+, Fe2+, und Co2+ schwerlösliche Niederschläge, im Falle von Oxalsäure, auf die im Folgenden exemplarisch Bezug genommen wird, Oxalat-Niederschläge bildet. Die genannten Niederschläge können im gesamten Kühlmittelsystem verteilt werden, wobei sie sich auf den Innenflächen von Rohrleitungen und von Komponenten, beispielsweise von Dampferzeugern, ablagern. Hinzu kommt, dass die Niederschläge die gesamte Verfahrensdurchführung erschweren.However, it is disadvantageous that some deconic acids, especially oxalic acid, form oxalate precipitates with bivalent metal ions such as Ni 2+ , Fe 2+ , and Co 2+ sparingly soluble precipitates, in the case of oxalic acid, to which reference is made below. The said precipitates can be distributed throughout the coolant system, depositing on the inner surfaces of pipelines and components, such as steam generators. In addition, the rainfall complicates the entire process implementation.

Nachteilig ist weiterhin, dass es im Zuge der Bildung insbesondere von Oxalatniederschlägen zur Mitfällung von in der wässrigen Lösung enthaltenen Radionukliden und somit zu einer Rekontamination der Bauteiloberflächen kommt. Die Gefahr einer Rekontamination ist besonders groß bei Komponenten mit einem großen Verhältnis von Oberfläche zu Volumen. Dies ist vor allem bei Dampferzeugern der Fall, die eine sehr große Anzahl von Austauscherrohren mit geringem Durchmesser aufweisen. Weiterhin treten Rekontaminationen bevorzugt in Zonen mit geringem Durchfluss auf.A further disadvantage is that in the course of the formation of, in particular, oxalate precipitates for coprecipitation of radionuclides contained in the aqueous solution and thus to a Re-contamination of the component surfaces comes. The risk of recontamination is particularly high for components with a large surface to volume ratio. This is especially the case with steam generators which have a very large number of small diameter exchanger tubes. Furthermore, recontamination preferably occurs in zones with low flow.

Ein weiterer Nachteil der Bildung von Oxalat- und sonstigen Niederschlägen besteht darin, dass sie Filtereinrichtungen, etwa die einem Ionentauscher vorgeschalteten Filter und Siebböden oder die Schutzfilter von Umwälzpumpen verstopfen können. Ein weiterer Nachteil ergibt sich schließlich, wenn ein oben beschriebener, einen Oxidationsschritt und einen Dekontschritt umfassender Behandlungszyklus wiederholt wird, wenn sich also an einen Dekontschritt ein erneuter Oxidationsschritt anschließt. Wenn in dem vorausgegangenen Dekontschritt Niederschläge entstanden sind, so können die entsprechenden Metallionen, etwa Ni im Falle eines Nickeloxalat-Niederschlags, nicht mit Hilfe von Ionentauschern aus der Reinigungslösung entfernt werden. Die Folge ist, dass im anschließenden Oxidationsschritt der Oxalatrest der Niederschläge zu Kohlendioxid und Wasser oxidiert wird und dadurch Oxidationsmittel nutzlos verbraucht wird. Wenn dagegen das Oxalat sich in Lösung befindet, also nicht in Form eines Niederschlags gebunden ist, kann das Oxalat auf einfache Weise, etwa bevor die Reinigungslösung in einen Ionentauscher geleitet wird, auf einfache und kostengünstige Weise beispielsweise mit Hilfe von UV-Licht zerstört, d.h. zu Kohlendioxid und Wasser umgesetzt werden.A further disadvantage of the formation of oxalate and other precipitates is that they can clog filter devices, such as the filter upstream of an ion exchanger and sieve trays or the protection filters of circulating pumps. Finally, a further disadvantage arises when a treatment cycle comprising an oxidation step and a decontamination step described above is repeated, that is to say when a renewed oxidation step follows a deconstruction step. If precipitation has occurred in the previous decontamination step, the corresponding metal ions, such as Ni in the case of a nickel oxalate precipitate, can not be removed from the cleaning solution by means of ion exchangers. The consequence is that in the subsequent oxidation step, the oxalate residue of the precipitates is oxidized to carbon dioxide and water, thereby consuming oxidizing agent uselessly. In contrast, if the oxalate is in solution, that is not bound in the form of a precipitate, the oxalate in a simple manner, such as before the cleaning solution is passed into an ion exchanger, destroyed in a simple and cost-effective manner, for example by means of UV light, ie converted to carbon dioxide and water.

Wenn also Niederschläge der oben beschriebenen Art während eines Dekontaminationsverfahrens aufgetreten sind, ist ein großer Zeit- und Kostenaufwand erforderlich, um diese zumindest teilweise aus einer wässrigen Lösung bzw. einem zu dekontaminierenden Kühlmittelsystem wieder zu entfernen und das Dekontaminationsverfahren fortsetzen zu können. Bisher wurde dazu versucht, die Geschwindigkeit der Entfernung von Nickel aus der wässrigen Lösung während des Dekontschritts durch eine entsprechend große Austauscherkapazität zu erhöhen. Bei der Reinigung bzw. Dekontamination größerer Systeme, etwa des kompletten Kühlmittelkreislaufs steht diese Möglichkeit aus technischen Gründen nur eingeschränkt zur Verfügung.Thus, when precipitates of the type described above have occurred during a decontamination process, a great deal of time and expense is required to at least partially remove them from an aqueous solution or system to be decontaminated and continue the decontamination process. So far, attempts have been made to increase the rate of removal of nickel from the aqueous solution during the deconcentration by a correspondingly large exchange capacity. When cleaning or decontaminating larger systems, such as the complete coolant circuit, this option is limited for technical reasons.

Davon ausgehend ist es die Aufgabe der Erfindung, ein Dekontaminationsverfahren vorzuschlagen, das hinsichtlich der geschilderten Nachteile verbessert ist.On this basis, it is the object of the invention to propose a decontamination method, which is improved in terms of the described disadvantages.

Diese Aufgabe wird bei einem Dekontaminationsverfahren der eingangs genannten Art gelöst, indem Metallionen, die während des Oxidationsschrittes in die wässrige Lösung übergetreten sind, vor der Durchführung des Dekontaminationsschritts, also vor der Zugabe einer organischen Dekontsäure, mit Hilfe eines Kationentauschers aus der Lösung entfernt werden. Dazu wird in verfahrenstechnisch vorteilhafter Weise so vorgegangen, dass die wässrige Lösung über einen Kationentauscher geführt wird. Besonders vorteilhaft ist dabei die Entfernung von Nickel, da dieses mit organischen Säuren besonders schwerlösliche Salze bzw. Niederschläge bildet.This object is achieved in a decontamination process of the type mentioned above by metal ions that have passed into the aqueous solution during the oxidation step, be removed before performing the decontamination step, ie before the addition of an organic deconic acid, using a cation exchanger from the solution. For this purpose, the procedure is advantageously such that the aqueous solution is passed through a cation exchanger. The removal of nickel is particularly advantageous since it forms salts or precipitates which are particularly sparingly soluble with organic acids.

Wenn dann in einem nachfolgenden Dekontschritt, wie oben bereits ausgeführt, die Oxidschicht mit einer Dekontsäure behandelt wird und dabei massiv Metallionen aus der Oxidschicht heraus gelöst werden, sind die sich einstellenden Metallionenkonzentrationen geringer als bei herkömmlichen Dekontaminationsverfahren, da zumindest ein Teil der beim Oxidationsschritt in Lösung gegangenen Metallionen schon vorher entfernt wurden, sich also nicht mehr in der Lösung befindet. Die Gefahr, dass das Löslichkeitsprodukt eines Metallsalzes einer Dekontsäure (das Produkt aus den Aktivitäten des Metallkations und des Säureanions) überschritten wird und sich ein schwerlöslicher Niederschlag bildet, ist somit verringert. Besonders im Falle von Nickel und Oxalsäure ist die Bildung von schwerlöslichen Nickeloxalat-Niederschlägen kritisch, da Nickeloxalat ein relativ geringes Löslichkeitsprodukt aufweist.If, in a subsequent decontamination step, as already explained above, the oxide layer is treated with a deconic acid and solid metal ions from the oxide layer can be solved, the resulting metal ion concentrations are lower than in conventional decontamination, since at least a portion of the gone in the oxidation step in the metal ions have been removed before, so is no longer in the solution. The risk of the solubility product of a metal salt of a deconic acid (the product of the activities of the metal cation and the acid anion) being exceeded and a sparingly soluble precipitate forming is thus reduced. Especially in the case of nickel and oxalic acid, the formation of sparingly soluble nickel oxalate precipitates is critical because nickel oxalate has a relatively low solubility product.

Da Ionentauscher in aller Regel organischer Natur sind, sind sie empfindlich gegenüber Oxidationsmitteln, insbesondere gegenüber der bei einem erfindungsgemäßen Verfahren bevorzugt eingesetzten Permangansäure oder deren Alkali-Salze, die sehr starke Oxidationsmittel sind. Daher ist es insbesondere im Falle organischer Ionenaustauscher zweckmäßig, ein in der wässrigen Lösung noch vorhandenes Oxidationsmittel mit Hilfe eines Reduktionsmittels zu neutralisieren, bevor die Lösung zur Entfernung von Metallionen über den Kationentauscher geführt wird.Since ion exchangers are generally organic in nature, they are sensitive to oxidizing agents, in particular to the preferred used in a process according to the invention permanganic acid or their alkali metal salts, which are very strong oxidizing agent. Therefore, in the case of organic ion exchangers in particular, it is expedient to neutralize an oxidant still present in the aqueous solution with the aid of a reducing agent before the solution is passed over the cation exchanger to remove metal ions.

Vorzugsweise wird als Reduktionsmittel die im nachfolgenden Dekontschritt eingesetzte Dekontsäure verwendet. Dabei ist vorteilhaft, dass diese Säure ohnehin vor Ort ist, so dass ein zusätzlicher Aufwand etwa für Beschaffung und Lagerhaltung und für eine zusätzliche Zulassung, welche bei Einsatz eines von der Dekontsäure unterschiedlichen Reduktionsmittel, etwa Glyoxylsäure, erforderlich wäre, entfällt.Preferably, the reducing agent used is the deconic acid used in the subsequent decontamination step. It is advantageous that this acid is already on site, so that an additional effort for example for procurement and warehousing and for an additional authorization, which would be when using a different of the deconic acid reducing agent, such as glyoxylic required.

Ein erfindungsgemäßes Verfahren kann beispielsweise zur Dekontamination des gesamten oder eines Teils des Kühlmittelsystems eines Kernreaktors, etwa eines Siedewasserreaktors benutzt werden.An inventive method can be used, for example, for decontamination of all or part of the coolant system of a nuclear reactor, such as a boiling water reactor.

In der beigefügten Abbildung Fig. 1 ist schematisch das Kühlmittelsystem bzw. den Primärkreislauf eines Druckwasserreaktors dargestellt. Es umfasst neben dem Druckbehälter 1, in dem zumindest im Betrieb eine Vielzahl von Brennelementen 2 vorhanden sind, ein Leitungssystem 3, das an den Druckbehälter 1 angeschlossen ist, sowie diverse Einbauten wie z.B. einen Dampferzeuger 4 und eine Kühlmittelpumpe 5. Der Sekundärkreislauf 11, der unter anderem einen Generator 12 antreibende Dampfturbinen 13 umfasst, ist in Fig. 1 ebenfalls dargestellt. Ziel der in Rede stehenden Reinigung bzw. der Dekontamination ist es, eine auf den inneren Oberflächen 7 der Bauteile des Primärkreislaufs vorhandene Oxidschicht aufzulösen und deren in Lösung gegangene Bestandteile aus der wässrigen Lösung zu entfernen. Das gesamte Kühlmittelsystem wird mit einer wässrigen Lösung gefüllt, die beispielsweise eine komplexbildende organische Säure wie Oxalsäure enthält, auf die im Folgenden exemplarisch Bezug genommen wird. Wenn hier von einer Befüllung gesprochen wird, so ist darunter auch ein Vorgehen zu verstehen, bei dem das nach dem Abschalten es Leistungsbetriebs, also nach dem Herunterfahren der Anlage im Kühlmittelsystem vorhandende Kühlmittel die in Rede stehende wässrige Lösung bildet, wobei dieser zur Durchführung des Oxidationsschritts ein Oxidationsmittel, vorzugsweise Permangansäure oder Kaliumpermanganat, zugesetzt wird. Im Falle einer Komplettdekontamination wird das gesamte Kühlsystem gefüllt, ansonsten können auch nur Teile, beispielsweise nur ein Abschnitt des Leitungssystems, behandelt werden.In the attached picture Fig. 1 schematically the coolant system and the primary circuit of a pressurized water reactor is shown. It comprises, in addition to the pressure vessel 1, in which at least in operation a plurality of fuel elements 2 are present, a conduit system 3 which is connected to the pressure vessel 1, and various installations such as a steam generator 4 and a coolant pump 5. The secondary circuit 11, the including a generator 12 driving steam turbines 13 includes, is in Fig. 1 also shown. The aim of the cleaning in question or the decontamination is to dissolve an existing on the inner surfaces 7 of the components of the primary circuit oxide layer and to remove their gone into solution components from the aqueous solution. The entire coolant system is filled with an aqueous solution containing, for example, a complex-forming organic acid such as oxalic acid, to which reference will be made hereinafter by way of example. If this is referred to as a filling, this is also to be understood as a procedure in which the coolant present after switching off the power operation, ie after the shutdown of the system in the coolant system forms the aqueous solution in question, this being used to carry out the oxidation step an oxidizing agent, preferably permanganic acid or potassium permanganate, is added. In the case of complete decontamination, the entire cooling system is used Otherwise, only parts, for example only a portion of the piping system, can be treated.

Im Folgenden wird nun die Anwendung des erfindungsgemäßen Verfahrens bei der Dekontamination des kompletten Kühlmittelsystems eines Druckwasserreaktors beschrieben, wobei nur der erste Reinigungszyklus betrachtet wird.In the following, the application of the method according to the invention in the decontamination of the complete coolant system of a pressurized water reactor will now be described, wherein only the first cleaning cycle is considered.

Die Oxidation erfolgte in saurer Lösung mit Permangansäure als Oxidationsmittel mit einer Konzentration von etwa 200 ppm bei einer Temperatur von etwa 90 °C. Wie dem beigefügten Diagramm zu entnehmen ist, stieg während des Oxidationsschritts (I) die Konzentration bzw. die Menge von Nickelionen innerhalb von etwa 10 Stunden auf Wert im Bereich von 6000 g an und blieb dann im wesentlichen gleich. Nach etwa 17 Stunden seit Beginn des Oxidationsschritts wurde zur Neutralisation nicht verbrauchter Permangansäure Oxalsäure als Reduktionsmittel in die wässrige Lösung in einer leicht überstöchiometrischen Menge zudosiert. Nach einer Einwirkdauer von etwa 3 Stunden wurde zum Zeitpunkt 20h mit der Entfernung der Nickelionen (II) und natürlich auch sonstiger Metallionen durch Zuschalten des Kationentauschers 8 begonnen, d.h. es wurde das Ventil 10 des Bypasses 9 geöffnet, so dass eine Teilstrom der im Kühlmittelsystem zirkulierenden wässrigen Lösung über den Kationentauscher 8 geführt wurde, was in technisch stark schematisierter und technisch vereinfacht dargestellter in Fig. 1 angedeutet ist.The oxidation was carried out in acidic solution with permanganic acid as the oxidizing agent with a concentration of about 200 ppm at a temperature of about 90 ° C. As shown in the attached diagram, during the oxidation step (I), the concentration or amount of nickel ions increased to about 6,000 g in about 10 hours and then remained substantially the same. After about 17 hours from the beginning of the oxidation step, oxalic acid as reducing agent was metered into the aqueous solution in a slightly more than stoichiometric amount to neutralize unconsumed permanganic acid. After an exposure time of about 3 hours, the removal of the nickel ions (II) and of course other metal ions by switching the cation exchanger 8 was started at the time 20h, ie it was the valve 10 of the bypass 9 is opened, so that a partial flow of circulating in the coolant system aqueous solution was passed through the cation exchanger 8, which in technically highly schematized and technically simplified in Fig. 1 is indicated.

Wie in dem Diagramm erkennbar ist, wird Nickel vom Kationentauscher zurückgehalten, so dass dessen im Gesamtsystem vorhanden Menge bzw. dessen Konzentration entsprechend sinkt. Im vorliegenden Beispiel näherte sich die in der wässrigen Lösung gelöste Nickelmenge während der Nickelentfernung (II) asymptotisch einem unteren Wert von etwa 500 g.As can be seen in the diagram, nickel is retained by the cation exchanger, so that its present in the total system amount or its concentration decreases accordingly. In the present example, that approached in the aqueous Solution dissolved amount of nickel during nickel removal (II) asymptotically a lower value of about 500 g.

Etwa ab diesem Zeitpunkt, d.h. nach etwa 35 Stunden seit dem Beginn des Reinigungszyklus wurde der Dekontschritt (III) durch Zudosierung von Oxalsäure eingeleitet. Die Zudosierung erfolgte so, dass in der Lösung eine Oxalsäurekonzentration von 2000 ppm nicht überschritten wurde. In dem Diagramm ist erkennbar, dass die Nickelmenge durch die Auflösung der Oxidschicht zunächst stark anstieg, sich dann aber aufgrund des zugeschalteten Kationentauschers 8 verringert. Wäre die in Phase I angefallene Nickelmenge nicht auf erfindungsgemäße Weise entfernt worden, hätte sich in Phase III statt einer Nickelmange von etwa 7000g eine wesentlich höhere Gesamt-Nickelmenge in der Lösung von etwa 13000 g ergeben, was zu Löslichkeitsproblemen und der Gefahr von Niederschlägen geführt hätte.From about this time, i. After about 35 hours from the beginning of the cleaning cycle, the decontamination step (III) was initiated by the addition of oxalic acid. The metered addition was carried out in such a way that an oxalic acid concentration of 2000 ppm was not exceeded in the solution. It can be seen in the diagram that the amount of nickel first increased greatly due to the dissolution of the oxide layer, but then decreased due to the switched cation exchanger 8. If the amount of nickel accumulated in Phase I had not been removed in accordance with the present invention, Phase III, rather than a 7,000 gram nickel, would have resulted in a substantially greater total nickel in the solution of approximately 13,000 grams, resulting in solubility problems and the risk of precipitation ,

Claims (6)

  1. Method for chemical decontamination of a surface, having an oxide layer, of a metallic component of the coolant system of a nuclear power plant comprising at least one oxidation step during which the oxide layer is treated with an aqueous solution containing an oxidising agent, and a subsequent decontamination step during which the oxide layer is treated with an aqueous solution of a decontamination acid which has the property to form a poorly soluble precipitation with metal ions, in particular with nickel ions,
    characterised in that
    before the implementation of the decontamination step, metal ions which entered into the solution during the oxidation step are removed from the aqueous solution with the aid of a cation exchanger.
  2. Method according to claim 1, that a reduction step is implemented before the removal of the metal ions in which an oxidising agent present in the aqueous solution is neutralised with the aid of a reducing agent.
  3. Method according to claim 2, characterised in that the decontamination acid used in the subsequent decontamination step is used as a reducing agent.
  4. Method according to one of the preceding claims, characterised in that at least one part of the aqueous solution is guided via a cation ion exchanger and metal ions contained in the aqueous solution are thereby removed.
  5. Method according to one of the preceding claims, characterised in that permanganic acid or a salt of permanganic acid is used in the oxidation step.
  6. Method according to one of the preceding claims, characterised by the use of oxalic acid as a decontamination acid.
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