EP3244418B1 - Chemical decontamination of radioactive metallic surfaces - Google Patents

Description

The present invention relates to a method for decontaminating radioactive metal surfaces using a decontamination solution. Furthermore, the invention relates to the use according to the invention of a decontamination solution.

In the field of nuclear reactor technology, radioactive contamination of metal components occurs, among other things. Such contamination occurs regularly in the regular operation of reactors and affects in particular metal components that are in the primary circuit, for example a pressurized water reactor. Here, among other things, radioactive substances, mostly metal oxides, are deposited on the surface of the components, causing them to become radioactively contaminated.

In the case of an inspection of the nuclear power plant, it is regularly necessary to remove the radioactivity from the contaminated components, i.e. the deposits on the metal surface, in order to protect the inspection personnel from radiation. After this, the components in the nuclear power plant continue to be operated.

In principle, mechanical means can be used to remove such deposits, e.g. grinding down the contaminated areas. This is particularly disadvantageous in the case of components which, because of their dimensions or their positioning, are difficult to access for the grinding tool.

Furthermore, a decontamination of the components with oxalic acid is known, which, however, requires numerous rinsing steps are. Furthermore, this process is a "gentle decontamination process" that is designed to only dissolve the deposits and not to attack the component itself, which should continue to be operated after the decontamination and should perform its proper function again. One of the consequences of this is that although the radiation level of the component is generally lowered, this reduction would not be sufficient for the component to be able to be released. In other words, the previously known methods are always based on the core idea that the nuclear reactor should go back into operation after the overhaul, ie the components should not be damaged beyond a calculated material removal.

Furthermore, in all known methods it is always assumed that the essential systems of the nuclear reactor, such as pumps or ion exchangers, can be used. Because ion exchange resins are always used for cleaning in the state of the art, which leads to a considerable amount of radioactive waste that has to be disposed of with immense - also financial - effort. This problem is exacerbated if, in addition to the radioactively contaminated metal oxide layer, the base material of the component is also to be removed, some of which is also radioactively contaminated. This is the case because the dissolution of the metal base material results in a high ion load that also has to be disposed of via the ion exchange resins.

Of course, the problems described above do not only occur in nuclear power plants, but in principle in situations in which metal components come into contact with radioactivity.

Accordingly, there is a need for an improved chemical decontamination method of radioactively contaminated metal surfaces. In particular, there is a need for an improved decontamination process which does not rely on the use of additional system components--for example ion exchangers or pumps--and in which the amount of contaminated waste produced is reduced.

the U.S. 5,752,206 A describes a method for in situ decontamination and metal recovery from radioactively contaminated metal contained in process plants, including process plant ancillaries, comprises two basic steps. In the first step, an acidic decontamination solution is circulated through the facility and brought into contact with the radioactively contaminated metal to remove the radioactive contaminants and a first surface portion of the metal from the metalliferous facility. In the second step, an acidic digestion solution is circulated through the plant to remove at least a second portion of the metal, which is essentially free of radioactive contaminants.

the WO 00/51135 A relates to the treatment of radioactively contaminated metal objects with an acidic solution to cause the dissolution of a surface layer of the objects. The pH of the solution is then raised with calcium hydroxide and a compound containing magnesium to separate the dissolved metal from the solution in solid form.

According to the invention, this object is achieved by a method having the features specified in claim 1 . Furthermore, this object is achieved through the use of a decontamination solution having the features specified in claim 13 .

Advantageous configurations are specified in its dependent claims.

More specifically, the method according to the invention is a method for decontaminating a radioactive metal surface, comprising the step of bringing at least a section of the radioactive metal surface into contact with a decontamination solution containing at least one inorganic acid, with at least insoluble radioactive solids from the section of the metal surface being introduced into the decontamination solution be released.

A core idea of the present invention is the use of an inorganic acid in the decontamination solution, which leads to the release of insoluble radioactive solids from the section of the radioactively contaminated metal surface that has been brought into contact with the decontamination solution.

The release occurs here by "blasting off" the insoluble radioactive solids, ie areas of the contaminated deposits present on the metal surface, by the hydrogen gas formed during the reaction of the inorganic acid with the metal of the metal surface. In other words, the reaction of the inorganic acid contained in the decontamination solution with the metal of the radioactive metal surface to be decontaminated leads to the development of hydrogen gas bubbles below and/or within the radioactive layer of deposits formed on the metal surface. The hydrogen gas bubbles formed in this way then cause the detachment, ie the removal, of individual areas of the contaminated deposits from the metal surface.

The areas of deposit thus removed are typically insoluble solids, such as the metal oxides mentioned above, and, as will be described hereinafter, can be easily removed from the decontamination solution. Since the deposits on the metal surface usually have a significantly higher level of radioactivity than the underlying metal surface of the component, a large proportion of the activity of the component can be removed. Accordingly, in contrast to the use of oxalic acid known in the prior art, the aim is not to dissolve the deposits but to blast them off, whereby they are released as insoluble solids into the decontamination solution and can then be easily removed from it.

In contrast to the use of oxalic acid known in the prior art, which is a weak and organic acid, an inorganic acid is used in the process according to the invention, which is also preferably a strong acid. As a result, on the one hand, the generation of hydrogen described above is achieved when the acid reacts with the metal, and on the other hand, the classic inorganic acids have a significantly smaller molecular volume compared to the organic acids (such as oxalic acid). This advantageously means that the decontamination solution according to the invention can more easily penetrate through, for example, cracks in the deposit layer on the metal surface. Furthermore, the use of a weak organic acid usually also requires that the decontamination process be carried out at temperatures of at least 80°C, which entails considerable effort. Such high temperatures are advantageously not required with the method according to the invention.

Another general advantage of the method according to the invention is that, as described above, the inorganic acid, after penetrating the deposit layer on the metal surface, is reacted with the metal of the metal surface itself (the base material) and radioactive metal ions are removed from the metal surface in the these are also released into the decontamination solution. In other words, controlled removal of the radioactively contaminated metal surface itself is also possible using the method according to the invention. As will be described below, these metal ions/metal salts can then advantageously be removed from the decontamination solution and disposed of—and preferably together with the insoluble solids that have been blown off from the deposit layer. As a result, the amount of contaminated waste to be disposed of is significantly reduced compared to the known methods.

The concept of decontamination is known to those skilled in the art. This is to be understood in particular as meaning the reduction and/or removal of radioactivity on the metal surface. In particular, this should be understood to mean the removal of a radioactive layer of deposits on a component and/or the removal of radioactive isotopes from the base material of the radioactively contaminated metal surface. The decontamination method of the present invention can preferably also be referred to as chemical decontamination. More preferably, the decontamination process can be a decontamination process for a nuclear reactor that is to be dismantled.

The release of solid and liquid substances is regulated according to the Radiation Protection Ordinance (StrlSchV) and essentially divided into unrestricted release and the Approval for disposal on landfills. After decontamination of the metal surface, it is preferably a component that is released for disposal on landfills. Even more preferably, after the metal surface has been decontaminated, it is a component that is suitable for unrestricted release.

The term metal surface is to be understood below as meaning both the actual surface of the metal component brought into contact with the decontamination solution and a radioactive layer of deposits thereon, which forms, for example, during normal use of the component in a pressurized water reactor. Such a deposit layer preferably consists of sparingly soluble metal oxides. In other words, the radioactive metal surface to be decontaminated preferably consists of at least one surface made of metal base material and a layer of deposits arranged thereon.

The surface of the metal base material preferably has a layer thickness of >0 μm and ≦50 μm, more preferably >0 μm and ≦20 μm. The layer of deposits arranged on this surface of the metal base material is preferably diffusion-permeable and/or a non-continuous layer. More preferably, the layer of deposits has cracks and/or pores. The inorganic acid of the decontamination solution according to the invention penetrates through this layer to the surface of the metal base material.

The metal of the metal surface to be decontaminated can in principle be any suitable metal. The metal is preferably a base metal, ie in other words a metal with one of its redox pairs have a negative standard potential with respect to the standard hydrogen electrode. More preferably the metal is a transition metal, ie a metal with atomic numbers 21-30, 39-48, 57-80 and 89-112. Even more preferably, the transition metal is a first-row transition metal. Also more preferably, the transition metal is a divalent transition metal. The metal is very particularly preferably selected from the group consisting of nickel, iron, manganese, chromium, titanium, copper, cobalt and combinations of at least two of these metals. Even more preferably, the metal is selected from the group consisting of nickel, chromium, cobalt, iron, and combinations of at least two of these metals. Even more preferably, the metal is nickel or a nickel alloy .

According to the invention, at least one section of the metal surface is also brought into contact with the decontamination solution. Preferably multiple sections, and more preferably an entire metal surface, are contacted with the decontamination solution. In other words, at least one surface of the component to be decontaminated is preferably brought into contact with the decontamination solution. Even more preferably, one or all surfaces of the component to be decontaminated are brought into contact with the decontamination solution. For better understanding, reference is made below to the radioactive metal surface, although this always means a section of the same.

The radioactive metal surface can be brought into contact with the decontamination solution according to the invention in any suitable manner. The metal surface to be decontaminated is preferably wetted with the decontamination solution. More preferably, the metal surface to be decontaminated is immersed in the decontamination solution, more preferably fully immersed. Also preferably - as will be explained below - the metal surface to be decontaminated is the inner lateral surface of a metal and cylindrical component (such as a tube of a recuperator) and the decontamination solution is introduced into the cavity of the cylindrical component.

According to the invention, the decontamination solution comprises at least one inorganic acid. The decontamination solution preferably also includes at least water, with which the inorganic acid is present in an aqueous solution. The decontamination solution particularly preferably consists of water and an inorganic acid. In other words, the decontamination solution is then an aqueous acid.

According to the invention, the use of any suitable inorganic acid is possible. As described above, the vast majority of these inorganic acids are characterized by having a small molecular size, in other words, a small molecular volume. This advantageously allows - and in particular in contrast to the organic acids used in the prior art - penetration of the water-insoluble layer on the surface of the base material and thus the inventive formation of hydrogen gas bubbles within and/or below this water-insoluble layer.

As is generally known, the term inorganic acid should be understood to mean an acid which, with the exception of carbonic acid, has no carbon atoms. The term inorganic acid should preferably be understood to mean an acid which, with the exception of carbonic acid, does not include any carbon, sulfur or nitrogen atoms. This points has the further advantage that no problematic end products (e.g. nitrate or sulfate compounds) can arise.

More preferably, the inorganic acid is a strong acid. More preferably, the inorganic acid is a hydrogen halide. These acids are characterized in particular by a low pKa value and a small molecular size and are therefore particularly suitable for the process according to the invention. In addition, the halides are particularly suitable for salt formation, which—as will be explained below—is likewise particularly advantageous for the process according to the invention. The acid is very particularly preferably an acid selected from the group consisting of hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), hydrofluoric acid (HF), or mixtures thereof. More preferably, the acid is hydrochloric acid or hydrofluoric acid. Most preferably the acid is hydrochloric acid. This is particularly advantageous since particularly harmless end products are formed during the implementation and comparatively simple handling in terms of occupational safety is possible.

The insoluble radioactive solids released from the metal surface into the decontamination solution are essentially components of the blasted-off layer on the metal surface. In other words, these are smaller areas of this layer, ie fragments thereof. These insoluble solids are preferably water-insoluble and/or acid-insoluble solids. Furthermore, the released insoluble solids are metal oxides, more preferably spinels. Spinels are Hardly soluble minerals from the mineral class of oxides and hydroxides, usually present in crystal form, and preferably oxides with a molar ratio of metal: oxygen = 3:4.

The method according to the invention also has the further step of separating the insoluble solids from the decontamination solution. Separation processes suitable for this purpose are discussed further below.

In addition to the insoluble radioactive solids, the method according to the invention also releases water-soluble radioactive metal salts from the section of the metal surface into the decontamination solution. In other words, at least insoluble radioactive solids and water-soluble radioactive metal salts are released from the metal surface portion into the decontamination solution.

As previously described, this additional release of the radioactive metal salts is due to the reaction of the surface of the metal base material with the acid, leading to the formation of hydrogen. Advantageously, such a reaction also leads to the continuous dissolution of the surface of the metal base material, which is usually also radioactively contaminated. This removal of the surface of the metal base material also contributes to the decontamination of the surface or of the component whose surface is being treated. As will be explained further below, the extent of this removal can preferably be controlled via process parameters such as, for example, temperature or time of bringing into contact.

The metal salts are preferably in dissolved form. In particular, the metal salts are particularly preferably a metal salt selected from the group consisting of nickel chloride, chromium chloride, iron chloride and cobalt chloride.

The method according to the invention also has the further step of adding a base to the decontamination solution, with the metal salts contained in the decontamination solution being precipitated as metal hydroxides. The metal hydroxides thus precipitated can then--just like the insoluble solids--be separated from the decontamination solution. Before adding the base, the metal surface is also preferably brought out of contact with the decontamination solution. This can be achieved, for example, by removing the component from the decontamination solution. The metal surface which has been brought out of contact with the decontamination solution can more preferably be subjected to a rinsing step. This can be done, for example, by rinsing the component with water.

The base can be any suitable base that causes the decontamination solution to become alkaline and the previously dissolved metal salts to precipitate as metal hydroxides. The base used is particularly preferably sodium hydroxide (NaOH) or calcium hydroxide (Ca(OH) ₂ ). This has the advantage that the end products remaining in solution in the decontamination solution, in this case NaCl or CaCl ₂ , are essentially water-soluble and harmless, and the decontamination solution can be disposed of as waste water after the insoluble solids and precipitated metal hydroxides have been separated off. NaOH is the most preferred base of the process of this invention.

The separation according to the invention of the insoluble radioactive substances (solids or precipitated metal salts) from the decontamination solution can preferably take place after the addition of the base to the contamination solution, i.e. only after the precipitation of the metal salts. This has the advantage that the insoluble solids and the precipitated metal salts are removed simultaneously, i.e. by means of one separation step.

However, it is particularly preferred to separate the insoluble radioactive solids from the decontamination solution before adding the base, i.e. before precipitating the metal salts. This has the advantage that the radioactive solids, which carry the main part of the radioactivity, are separated separately and thus in a smaller volume, which has to be disposed of under special precautions and, for example, has to be transferred to a repository. More preferably, the precipitated metal salts are separated from the decontamination solution by means of a further separation step, which then takes place after the base has been added to the decontamination solution. Advantageously, in such a procedure, two fractions of separated radioactive materials are obtained, the first fraction comprising the intermediately active solids and the second fraction comprising the significantly less active precipitated metal salts. The latter fraction, which has a significantly greater proportion by weight than the first fraction, can then be disposed of separately from the first fraction with significantly less effort.

The separation of both the insoluble solids and the precipitated metal salts from the decontamination solution can be accomplished by any suitable method or expedient. For example and preferably, such a separation takes place by filtering, suction, centrifugation, sedimentation and/or mechanical collection. Separation via filtering or sedimentation is very particularly preferred. The former leads to a particularly thorough separation with subsequent easy disposal of the filter cake. The latter is an extremely cost-effective method that does not require any additional tools.

The insoluble solids separated from the decontamination solution are also preferably reduced in weight and/or volume in a further step before disposal. Such a reduction in weight and/or volume advantageously achieves easier and more cost-effective further processing and disposal/disposal. This is preferably done by drying, incinerating and/or incinerating the solids. This procedure is particularly advantageous in the case of separation via filtering and drying, incineration and/or incineration of the resulting filter cake. The solids separated from the decontamination solution are also preferably burned and/or incinerated in addition to or instead of drying.

The insoluble solids can preferably be stored in a repository in a further step after drying, incineration and/or incineration.

The weight and/or volume of the precipitated metal hydroxides separated from the decontamination solution is also preferably reduced in a further step before disposal. Again, these can be dried, burned and/or incinerated. Advantageously, such a reduction in weight and/or volume makes further processing and disposal/disposal easier and more cost-effective achieved. The precipitated metal hydroxides separated from the decontamination solution are also preferably burned and/or incinerated in addition to or instead of drying.

The precipitated metal hydroxides can preferably be stored in a repository in a further step after drying, incineration and/or incineration.

As already stated above, the insoluble radioactive solids and/or the precipitated metal hydroxides are preferably disposed of in a repository in a further step of the method according to the invention. The decontamination solution in turn can advantageously be disposed of as waste water in a further step after the insoluble solids and the precipitated metal hydroxides have been separated off. The decontaminated metal component, which after carrying out the decontamination according to the invention preferably has an activity that entitles it to be released without restrictions, can also preferably be disposed of as scrap metal and sold to a metal recycler, for example.

The metal of the metal surface to be decontaminated particularly preferably comprises at least nickel, more preferably an alloy. Particularly in the case of nickel or nickel alloys, a particularly high level of decontamination was found using the method according to the invention.

More particularly preferably, the radioactive metal surface to be decontaminated is the surface of a component of a nuclear installation, for example a nuclear power plant. More preferably around the surface of a component of a pressurized water reactor, even more preferably around the surface of a component of the primary circuit of a pressurized water reactor.

The component is also preferably a component of a nuclear reactor to be dismantled and/or a component which is to be replaced, i.e. is to be disposed of after decontamination.

The component is very particularly preferably a component of the steam generator, in particular of a shell-and-tube heat exchanger (recuperator), as can be found, for example, in the primary circuit of a pressurized water reactor. Even more preferably, the component is at least one tube of a tube bundle heat exchanger. Accordingly, the radioactive metal surface to be decontaminated is in particular the surface of a tube of a tube bundle heat exchanger, more preferably the inner surface, i.e. the inner lateral surface, of at least one tube of a tube bundle heat exchanger.

A shell-and-tube heat exchanger is generally made up of a hollow cylinder made of sheet steel with hundreds to thousands of tubes inside. The first medium flows through the metal cylinder and the second through the tubes. During this process, the hotter medium cools down while the colder medium is heated. During operation of the nuclear reactor, the inner surfaces of the tubes in particular are radioactively contaminated. However, since these have a very small nominal width of, for example, approximately 14 mm and a relatively large length, for example, approximately 16 m, decontamination using mechanical methods known in the prior art cannot be implemented, so that when they are replaced or dismantled, significant amounts of contaminated waste are produced attack.

Another fundamental advantage of the method according to the invention is that it controls diffusion is, ie that, for example, an intensive pumping of the decontamination solution is not required, as is the case, for example, in the prior art. Accordingly, the method according to the invention is preferably characterized in that it is diffusion-controlled or that pumping/circulating the decontamination solution is not required.

Nevertheless, according to the invention it is not ruled out and even preferred that the decontamination solution is circulated. This achieves an increase in the decontamination process. Furthermore, the decontamination solution is preferably filtered, for example in order to remove the insoluble solids and/or the precipitated metal hydroxides from the decontamination solution, as described above. Circulation and filtering of the decontamination solution are very particularly preferably combined, more preferably circulation and/or filtering of the decontamination solution take place continuously.

Any suitable method known to the person skilled in the art and any suitable device known to the person skilled in the art, for example pumps, can be used for circulating the decontamination solution. In this case, however, it is particularly preferred to use already existing apparatus of a nuclear reactor. In particular, the circulation is preferably achieved by at least one pump which is part of the primary circuit of a pressurized water reactor. Accordingly, in a particularly preferred embodiment of the method according to the invention, the decontamination solution can be introduced into the primary circuit of a nuclear reactor, preferably a pressurized water reactor.

Although the method according to the invention can be carried out at any suitable temperature, it turned out that a temperature range from ≧40° C. to below the evaporation temperature of the decontamination solution led to particularly good results. Accordingly, the method according to the invention is preferably carried out at a temperature in the range from ≥ 40 °C to below the evaporation temperature of the decontamination solution, more preferably in a range from ≥ 40 °C to ≤ 90 °C, more preferably ≥ 40 °C to ≤ 70 °C and most preferably ≥ 50°C to ≤ 60°C.

Furthermore, it turned out that--preferably in the aforementioned temperature ranges--bringing in contact for a period of ≥3 h achieved particularly good results. The contacting therefore preferably takes place for a period of ≧3 hours, more preferably for a period of ≧3 hours and ≦50 hours. Within this period of time, there was advantageously sufficient material removal and thus sufficient decontamination, so that the contaminated surface or the decontaminated component can be classified as harmless.

As already explained above, the decontamination method according to the invention can be terminated at any time and in particular when sufficient material has been removed without major problems. To this end, the method according to the invention comprises the further step of rinsing the metal surface to be decontaminated with a rinsing solution. More preferably, this rinsing solution comprises water and/or consists of water.

The decontamination solution very particularly preferably comprises 37% HCl in an aqueous dilution of ≧1:2 and ≦1:10. More preferably, the decontamination solution consists of 37% HCl in an aqueous dilution of ≧1:2 and ≦1:10.

The use of hydrochloric acid in this aforementioned concentration range led to particularly good results.

Furthermore, in the method according to the invention, ultrasonic treatment is preferably carried out at least during the bringing into contact. Here, the decontamination solution and/or the metal surface to be decontaminated can be exposed to ultrasound. Such ultrasonic sonication also leads to an improved efficiency of the decontamination.

The invention further relates to the use of a decontamination solution comprising at least one inorganic acid for the decontamination of a radioactive metal surface. The further explanations given with regard to the method according to the invention apply analogously.

The individual embodiments of the invention described above can be freely combined with one another as long as nothing to the contrary has been explicitly stated or an obvious exclusion is opposed.

Further advantages, details and features of the invention result from the exemplary embodiments below.

Example 1

A pressurized water reactor type nuclear reactor is being dismantled. Metal components that were radioactively contaminated during the operation of the nuclear reactor are chemically decontaminated in order to then be able to remove them from the safety area and also be able to dispose of/sell them as harmless scrap metal.

For reasons of improved readability, reference is only made to a few specific metal components, namely the tubes of the tube bundle heat exchanger (RWÜ) of the pressurized water reactor to be dismantled. Of course, however, the process according to the invention can be transferred to all other components of the reactor.

The RHE of the pressurized water reactor to be dismantled is located in its primary circuit and has around 16,000 pipes with an internal diameter of around 14 mm and a length of 17 m each. The tubes are still made of a nickel alloy, more precisely an Alloy 600.

Due to the regular operation of the pressurized water reactor, an insoluble radioactively contaminated layer, which mainly consists of metal oxides, has deposited on the inside of these tubes, i.e. on their inner shell surfaces. Furthermore, the metallic base material, i.e. an approximately 20 µm thick layer on the inside of these pipes, was also radioactively contaminated. The layer thickness depends on the operating time and driving style and will be significantly thicker during long periods of operation.

To decontaminate these pipes of the RWÜ, they were removed from the RWÜ in a first step and cut into sections of shorter length of about 1 m. Thereafter, these sections were transferred upright into a container in which the decontamination solution according to the invention was presented. The contaminated surfaces on the inside of the sections were completely brought into contact with the decontamination solution.

A 37% hydrochloric acid (HCl) in a 1:10 dilution in water (10% aqueous hydrochloric acid) was used as a decontamination solution. used. Furthermore, the decontamination solution had a temperature of 60 °C. This temperature was essentially maintained during the further course of the decontamination.

After the parts had been placed in the decontamination solution, the generation of hydrogen bubbles could be detected almost immediately. Accordingly, the acid in the decontamination solution was reacted with the metal contained in the pipes.

The areas of the insoluble layer of deposits on the pipes blown off by the hydrogen bubbles continued to be released into the decontamination solution and sank to the bottom of the container by gravity.

After a period of approximately 4-5 hours, the base material of the pipes had been sufficiently removed so that they could be removed from the container. Thereafter, the sections were subjected to a rinsing step with a rinsing solution consisting essentially of water to remove adhering activity. The sections/pipes only showed such a low level of contamination that they could be disposed of in a landfill.

The decontamination solution still in the container was transferred to a second container, with the sedimented insoluble solids of the spinel layer remaining in the first container and being able to be collected in a simple manner. In the approximately 16,000 pipes of the RWÜ, only about 10 kg of medium-active solids accumulated. After drying and incineration, these were packaged according to regulations.

In a further step, sodium hydroxide was then added as a base to the decontamination solution transferred to the second container in order to make the decontamination solution alkaline. The metal salts in the solution then precipitated out as insoluble metal hydroxides and sedimented again on the bottom of the second container due to gravity.

The remaining decontamination solution could now be pumped off after neutralization and, since it no longer showed any activity above the unrestricted release limit, disposed of with the waste water. The sedimented precipitated metal hydroxides, which had approximately 2000-fold lower activity than the insoluble solids of the spinel layer, were collected again. Around 3-4 t of (wet) metal hydroxide accumulated in the approximately 16,000 pipes of the RHE. After drying and incineration, these less active waste products were disposed of according to regulations.

Example 2

Exemplary embodiment 2 essentially corresponds to exemplary embodiment 1 and only the differences between the two exemplary embodiments will be discussed below.

In exemplary embodiment 2, the sections of the pipes to be decontaminated were placed horizontally in the first container, in which the decontamination solution was continuously circulated by a pump. In such a configuration, the division of the tubes into longer sections is also conceivable, or division can be dispensed with entirely.

The efficiency of the purely diffusion-controlled process described in exemplary embodiment 1 was increased by the continuous circulation of the decontamination solution, since the decontamination solution is constantly exchanged in the area of the surface to be decontaminated.

A further advantage of the circulation of the decontamination solution is particularly evident when, as in exemplary embodiment 2, an exchangeable particle filter is also introduced into the circulation circuit. This replaceable particle filter served to filter out the solids released into the decontamination solution prior to adding the base to the decontamination solution.

After completion of the first phase of the decontamination reaction (before adding the base), which could be reduced to about 3-4 hours in the case of the exemplary embodiment, the replaceable particle filter could then be replaced by a second particle filter and the first particle filter according to the weight and volume reduction are transferred to a repository.

Using the second particle filter, the precipitated metal hydroxides were then removed from the decontamination solution in the same way and then disposed of.

Example 3

In example 3, the decontamination solution (again 10% aqueous hydrochloric acid) was introduced directly into the primary circuit of the pressurized water reactor. In other words, there was no prior dismantling of the same into sections and the entire inner surface of the primary circuit was brought into contact with the decontamination solution according to the invention.

Furthermore, the decontamination solution was continuously circulated with the aid of a pump that was already part of the primary circuit of the nuclear reactor. A replaceable particle filter was again integrated into this circulating circuit of the decontamination solution. With which first the insoluble solids and - after replacement - the precipitated metal hydroxides were removed from the decontamination solution.

Exemplary embodiment 3 shows that decontamination is also advantageously possible without dismantling/dividing the components.

Example 4

Exemplary embodiment 4 essentially corresponds to exemplary embodiment 1 and only the differences between the two exemplary embodiments will be discussed below.

Various other decontamination solutions were tested. Specifically, a 30% aqueous hydrochloric acid at a temperature of 50°C was used. Furthermore, a 50% aqueous hydrochloric acid at a temperature of 40°C was used. Hydrofluoric acid was also tried, as well as combinations of HCl and HF.

Comparable results as in example 1 could be observed.

Claims (13)

1. Method for decontaminating a radioactive metal surface comprising the steps of:

   - bringing at least one portion of the metal surface into contact with a decontamination solution comprising at least one inorganic acid,

   wherein at least insoluble radioactive solids and water-soluble radioactive metal salts are released from the portion of the metal surface into the decontamination solution and wherein the released insoluble solids comprise metal oxides, in particular spinels;

   - adding a base to the decontamination solution, wherein the metal salts contained in the decontamination solution are precipitated as metal hydroxides; and

   - separating the insoluble solids from the decontamination solution and/or separating the insoluble solids and the precipitated metal hydroxides from the decontamination solution.
2. Method according to claim 1, wherein the weight and/or volume of the insoluble solids and/or of the precipitated metal hydroxides are reduced in a further step.
3. Method according to either of the preceding claims 1-2, wherein, in a further step, the separated insoluble solids and/or the separated precipitated metal hydroxides are disposed of in a repository and/or the decontamination solution is disposed of as wastewater.
4. Method according to at least one of the preceding claims, wherein the metal surface is at least a base metal, preferably at least nickel or a nickel alloy.
5. Method according to at least one of the preceding claims, wherein the surface is the surface of a component of the primary circuit of a nuclear reactor, in particular a tube of a tube bundle heat exchanger.
6. Method according to at least one of the preceding claims, wherein the decontamination solution is continuously circulated and/or filtered.
7. Method according to at least one of the preceding claims, wherein the decontamination solution is introduced into the primary circuit of a nuclear reactor.
8. Method according to at least one of the preceding claims, wherein the acid is selected from the group consisting of hydrochloric acid (HCl), hydrobromic acid (HBr), hydriodic acid (HI), hydrofluoric acid (HF), or mixtures thereof.
9. Method according to at least one of the preceding claims, wherein the method is carried out at a temperature in the range of from ≥ 40°C to below the evaporation temperature of the decontamination solution.
10. Method according to at least one of the preceding claims, wherein the process of bringing a portion of the metal surface into contact with the decontamination solution takes place for a duration of ≥ 3 hours, preferably for a duration in the range of ≥ 3 hours and ≤ 50 hours.
11. Method according to at least one of the preceding claims, wherein the decontamination solution comprises 37% HCI in an aqueous dilution of ≥ 1:2 and ≤ 1:10.
12. Method according to at least one of the preceding claims, wherein at least during the process of bringing a portion of the metal surface into contact with a decontamination solution, treatment with ultrasound is carried out.
13. Use of a decontamination solution comprising at least one inorganic acid for decontaminating a radioactive metal surface in a method according to any of claims 1 to 12.