EP3607562B1 - Dosing of zinc for decontamination of light water reactors - Google Patents

Description

The present invention relates to a method for the decontamination of radioactive metal surfaces by means of the decontamination solution and the use of the decontamination solution for this purpose.

In the field of nuclear reactor technology, radioactive contamination of metal components occurs. Such contamination occurs regularly in the regular operation of reactors and particularly affects metal components that are in the primary circuit, for example a pressurized water reactor. Radioactive substances are deposited in the oxide layers formed on the surface of the components, causing them to become radioactively contaminated.

In the event of an overhaul of the nuclear power plant, it is regularly necessary to remove the contaminated components from radioactivity, i.e. from the deposits on the metal surface, in order to protect the inspection staff from radiation. After this, the components can continue to be operated in the nuclear power plant. The same applies if the nuclear power plant is to be dismantled.

In principle, mechanical means can be used to remove such deposits, in which case, for example, the oxide layers and thus the contaminated areas are abraded. This is particularly disadvantageous in the case of components which, due to their dimensions or their positioning, are difficult to access for the grinding tool.

Furthermore, a decontamination of the components with a decontamination solution having a complexing agent, including various carboxylic acids, such as oxalic acid, is known. In an upstream step, the sparingly soluble components of the oxide layers are first oxidized or reduced, with Cr-III being oxidized to Cr-VI using permanganates (potassium permanganate, permanganic acid), for example. The oxide layer, consisting mainly of iron and nickel ions, is then dissolved with the aid of the complexing agent and the cations released, including ⁶⁰ Co ²⁺ or ⁵⁸ Co ²⁺ , are removed from the decontamination solution by ion exchange. This decontamination process is usually carried out in several rounds, with the oxide layer being broken down in layers.

In addition to these radioactive isotopes, inactive ions are always released into the decontamination solution, which are also removed from the decontamination solution via the ion exchange resins. Furthermore, the radioactive ions in the decontamination solution cause recontamination of the components during the decontamination process. This reduces the efficiency of the decontamination process, which on the one hand leads to the need for a larger number of decontamination cycles, which are time-consuming and costly, and on the other hand results in an increased amount of contaminated ion exchange resins that have to be disposed of with immense effort .

Of course, the problems outlined above do not only occur in nuclear power plants, but principally in situations in which metal components come into contact with radioactivity and decontamination is required.

Accordingly, there is a need for an improved method of decontaminating radioactively contaminated metal surfaces. In particular, there is a need for a decontamination process with increased efficiency, in which decontamination can be carried out with a reduced number of decontamination cycles and / or a reduced amount of contaminated ion exchange resins.

From the US 2015/114845 A1 a method for decontaminating radioactively contaminated metal surfaces is known, using zinc and a complexing agent such as oxalic acid. The use of elemental metal is expressly required.

the WO 00/78403 A1 describes a process for the decontamination of radioactively contaminated metal surfaces in which zinc and a complexing agent are used. Diphosphonic acids, in particular etidronic acid, are used as complexing agents.

the US 2003/070731 A1 teaches the production of aqueous solutions from complexing agents and transition metals such as zinc.

According to the invention, this object is achieved by a method having the features specified in claim 1. Furthermore, this object is achieved by the use according to claim 11. Advantageous refinements are given in the subclaims.

More precisely, the method according to the invention is a method for decontaminating a radioactively contaminated metal surface, comprising the step of bringing at least a section of the radioactively contaminated metal surface into contact with a decontamination solution, comprising a complexing agent and a transition metal. As has surprisingly been shown, when a transition metal is added to the decontamination solution, the recontamination of the metal surface that occurs during the decontamination process is effectively reduced.

Without being restricted to this, it is assumed that the transition metal added to the decontamination solution competes with the released radioactive isotopes for (renewed) storage in the metal surface (or the oxide layer on it). As a result, a larger amount of radioactive isotopes can advantageously be removed from the decontamination solution via the ion exchange process, which in turn leads to a reduction in the number of rounds of decontamination steps required and / or a reduction in the amount of ion exchange resins to be disposed of.

The decontamination solution is preferably an aqueous solution. It contains an ion of a transition metal, more preferably a cation of the transition metal, even more preferably a divalent or trivalent cation of the transition metal. Most preferably the transition metal is a divalent cation of the transition metal.

More preferably, the transition metal is a depleted transition metal, ie a transition metal with a reduced proportion of isotopes that can be easily activated by neutrons compared to the natural occurrence. The use of a depleted transition metal is particularly advantageous if the metal to be decontaminated, for example the component of a reactor, is not to be disposed of after decontamination, but is to be reused and exposed to neutron flux.

The transition metal is also preferably selected from the group consisting of zinc, nickel, cobalt or mixtures thereof. More preferably, the transition metal is selected from the group consisting of zinc and nickel. Most preferably the transition metal is zinc. The use of zinc in the decontamination solution surprisingly showed the greatest effect in decontaminating the metal surface.

The transition metal in the decontamination solution is preferably in a concentration in the range from 0.5 mg / kg and 15 mg / kg, more preferably 0.5 mg / kg and 10 mg / kg, more preferably 1.5 mg / kg and ≤ 5 mg / kg or ≥ 2 mg / kg and ≤ 5 mg / kg and most preferably about ≥ 3 mg / kg and ≤ 4 mg / kg. Instead of the mg / kg, the mmol / L can also be given, whereby the given mg / kg value has to be divided by the atomic mass of the respective transition metal. The transition metal in the decontamination solution is preferably in a concentration in the range of ≥ 7 µmol / L and ≤ 230 pmol / L, more preferably ≥ 7 µmol / L and ≤ 155 pmol / L, more preferably ≥ 23 µmol / L and ≤ 70 µmol / L or ≥ 30 µmol / L and ≤ 80 µmol / L and most preferably approximately ≥ 46 µmol / L and ≤ 62 µmol / L.

The specified concentration ranges for the concentration of the transition metals at the time when the metal surface is brought into contact with the decontamination solution preferably apply. The stated concentrations are likewise preferably the mean concentrations.

In the following, instead of "transition metals", reference is only made to the element zinc as an example. As far as applicable, the statements made also apply analogously to transition metals in general and preferably also to nickel and / or cobalt.

The term “zinc” should preferably be understood to mean the ^{zinc ions present in the decontamination solution, more preferably Zn 2+.} This can, even more preferably, be depleted zinc, in particular zinc depleted ^{in 64 Zn.}

More preferably, the zinc is introduced into the decontamination solution by means of a soluble zinc compound. Preferred soluble zinc compounds are selected from the groups of acids used and / or the complexing agents used with zinc, including zinc methanesulfonate (Zn (CH ₃ SO ₃ ) ₂ ), zinc nitrate (Zn (NO ₃ ) ₂ ), zinc permanganate (Zn (MnO ₄ ) ₂ ), zinc sulfate (ZnSO ₄ ) and / or a soluble zinc complex. The zinc complex is more preferably a complex of zinc and the complexing agent used.

The term decontamination is known to the person skilled in the art. This is to be understood in particular as the reduction and / or removal of radioactivity on the metal surface. In particular, this is to be understood as the removal of a deposited layer of metal oxides on a metal component, the deposited layer having radioactive isotopes, preferably cobalt. In other words, radioactive isotopes are removed from the metal surface to be decontaminated by means of the method according to the invention. These radioactive isotopes are preferably selected from the group consisting of ⁵⁵ Fe ions, ⁶³ Ni ions, ⁵⁴ Mn ions, ⁶⁵ Zn ions, ¹²⁵ Sb ions, ¹³⁷ Cs ions, ⁵⁸ Co ions and ⁶⁰ Co ions. The radioactive isotopes are more preferably selected from the group consisting of ⁵⁴ Mn ions, ¹²⁵ Sb ions, ¹³⁷ Cs ions, ⁵⁸ Co ions and ⁶⁰ Co ions. Most preferably, these radioactive isotopes are ⁵⁸ Co ions and / or ⁶⁰ Co ions, even more preferably ⁶⁰ Co ions. 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 to be dismantled or for a nuclear reactor to be continued to operate.

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

In the following, the term radioactively contaminated metal surface should preferably be understood to mean the surface of a metal component including the radioactively contaminated deposit layer located on it, which is formed, 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 comprises at least one radioactively contaminated layer of sparingly soluble metal oxides arranged on the surface of metal base material. The deposit layer is even more preferably spinels, preferably Cr-Ni spinels and / or Cr-Fe spinels. Spinels are, usually in crystal form, sparingly soluble minerals from the mineral class of oxides and hydroxides and preferably oxides with the molar ratio
Metal: oxygen = 3: 4.

The metal of the metal surface to be decontaminated can in principle be any suitable metal. Preferred the metal is a metal selected from the group consisting of iron, nickel, chromium, manganese, titanium, niobium, copper, cobalt and combinations of at least two of these metals. Even more preferably, the metal is selected from the group consisting of iron, chromium, nickel, cobalt, and combinations of at least two of these metals.

According to the invention, at least a section of the metal surface is also brought into contact with the decontamination solution. Preferably, several sections, and even more preferably the entire metal surface, are brought into contact with the decontamination solution. For better understanding, reference is made in the following to the radioactively contaminated metal surface, although this also always refers to a section of the same.

The radioactively contaminated metal surface can be brought into contact with the decontamination solution in any suitable manner. The metal surface to be decontaminated is preferably wetted with the decontamination solution. The decontamination solution is more preferably introduced into the primary circuit of a reactor.

The decontamination solution can more preferably be circulated. This advantageously avoids concentration gradients in the area of the metal surface and increases the efficiency of the decontamination process. The circulation is more preferably carried out continuously and, likewise preferably, using pumps.

The metal surface to be decontaminated is also preferably the inner jacket surface of a metal and cylindrical component (such as a pipe of a recuperator) and the decontamination solution is introduced into the cavity of the cylindrical component.

The method according to the invention preferably has an additional method step for oxidation or reduction of the radioactively contaminated metal surface before the method step of bringing the at least one section of the metal surface into contact with the decontamination solution, i.e. as a first method step. In the case of oxidation, this process step can also be referred to as pre-oxidation of the radioactively contaminated metal surface. It is further preferred that Cr-III is oxidized to Cr-VI in the pre-oxidation. The pre-oxidation is preferably carried out by bringing the radioactively contaminated metal surface into contact with nitric acid and potassium permanganate, with sodium hydroxide and potassium permanganate, a vanadium compound (preferably vanadium formate) or with permanganic acid, with permanganic acid treatment being the most preferred. In the case of an upstream process step for reduction, the oxidation layer is preferably reduced with the aid of a vanadium compound. In the subsequent process step, the dissolved products are preferably complexed with picolinic acid.

More preferably, after the pre-oxidation step and before the at least one section of the metal surface is brought into contact with the decontamination solution, an additional process step for reducing the excess oxidizing agent, for example the permanganates (potassium permanganate, permanganic acid), can be carried out.

Likewise preferably, after the at least one section of the metal surface has been brought into contact with the decontamination solution, the method according to the invention has the further method step of at least partial removal of the radioactive isotopes or their ions contained in the decontamination solution. These radioactive isotopes are preferred selected from the group consisting of ⁵⁵ Fe, ⁶³ Ni, ⁵⁴ Mn, ⁶⁵ Zn, ¹²⁵ Sb, ¹³⁷ Cs, ⁵⁸ Co and ⁶⁰ Co. More preferably, the radioactive isotopes are selected from the group consisting of ⁵⁴ Mn, ¹²⁵ Sb, ¹³⁷ Cs , ⁵⁸ Co and ⁶⁰ Co. Most preferably these radioactive isotopes are ⁵⁸ Co and / or ⁶⁰ Co, more preferably ⁶⁰ Co.

The radioactive isotopes are preferably removed by binding to an ion exchange resin, more preferably a cation exchange resin and / or a synthetic resin ion exchanger. Most preferably, the ion exchanger is a strongly acidic cation exchanger in which protons are exchanged for the bound cations. Such ion exchange resins are sufficiently known to the person skilled in the art.

More preferably about 50%, even more preferably about 70%, 80%, 90% or 99% of the radioactive isotopes in the decontamination solution are removed. Most preferably, approximately ≥ 99% and <100% of the isotopes in the decontamination solution are removed.

The method according to the invention is more preferably carried out cyclically. In other words, at least the method steps of bringing the metal surface into contact with the decontamination solution and the subsequent at least partial removal of the radioactive isotopes in the decontamination solution are repeated at least once. Of course, individual or all of the other method steps listed above can also be repeated here. The method according to the invention is preferably repeated until a decontamination factor has been reached which corresponds to a reduction in the activity of the radioactively contaminated metal surface by 1 to 3 order (s), more preferably approximately 2 orders of magnitude. the The decontamination factor is preferably determined by measuring the activity of the ion exchange resin used to remove the radioactive isotopes in the decontamination solution, or comparing the activity of the ion exchange resin before and after performing the method according to the invention.

The method according to the invention is likewise preferably repeated cyclically approximately 1 to 30 times, more preferably 10 to 25 times, even more preferably 13 to 20 times. A range from 13 to 17 cycles showed particularly good results when using oxalic acid.

According to the invention, the decontamination solution comprises at least one complexing agent in addition to the transition metal. The complexing agent can also be referred to as a chelating agent. Complexing agents form chelate complexes with metal ions. Exemplary complexing agents include nitriloacetic acid, ethylenediaminetetraacetic acid, hydrofluoric acid, oxalic acid, tartaric acid, citric acid and their salts.

The decontamination solution further comprises water, so that the water-soluble constituents of the decontamination solution can be present in their dissolved form. In other words, the decontamination solution is an aqueous solution.

The acid is selected from the group consisting of carboxylic acid, methanesulfonic acid, oxalic acid, picolinic acid, and citric acid. The acid is preferably a mixture of methanesulfonic acid and oxalic acid. Most preferably the acid is oxalic acid. More preferably, the decontamination solution also comprises an oxidizing agent, more preferably permanganic acid, or a reducing agent. In further preferred embodiments, the decontamination solution comprises zinc methanesulfonate, Zinc nitrate, zinc permanganate, zinc sulfate and / or a zinc complex of the complexing agent used. The complex of the transition metal and the complexing agent used is particularly preferred.

The use of the decontamination solution for carrying out the method according to the invention is also part of this invention.

Examples

• Figur 1 die Korrelation von Zn-Konzentration der Dekontaminationslösung und ⁶⁰Co Dekontamination.
• Figur 2 die Korrelation von Zn-Konzentration der Dekontaminationslösung und ⁶⁰Co Dekontamination.
• Figur 3 die Korrelation von Fe-Konzentration der Dekontaminationslösung und ⁶⁰Co Dekontamination.

The figures show in detail:

• Figure 1 the correlation of the Zn concentration of the decontamination solution and ⁶⁰ Co decontamination.
• Figure 2 the correlation of the Zn concentration of the decontamination solution and ⁶⁰ Co decontamination.
• Figure 3 the correlation of the Fe concentration of the decontamination solution and ⁶⁰ Co decontamination.

Example 1: Correlation of Zn concentration and ⁶⁰ Co decontamination

Primary circuit decontamination of a light water reactor was carried out, with the mean Zn and Fe concentration in the decontamination medium and the ⁶⁰ Co removed from the decontamination solution via the ion exchange resin (strongly acidic cation exchanger) being determined. The primary circuit decontamination was carried out over 15 cycles.

How with the Figures 1 and 2 (Determination of the ⁶⁰ Co decontamination depending on the Zn concentration) can be seen, there is a very good correlation between this transition metal and the amount of ⁶⁰ Co.

In comparison, such a very good correlation between the Fe concentration and ⁶⁰ Co could not be demonstrated (see Figure 3 ).

Example 2: Correlation of Ni concentration or Cr concentration and ⁶⁰ Co decontamination

Example 1 was repeated, the Ni concentration or the Cr concentration being considered instead of the Zn concentration. In each case, a correlation was also found between the concentration of the transition metal and the activity carried out ^{over 60 Co.} The correlation determined tended to decrease from Ni to Cr compared to Zn.

Claims (11)

1. Method for the decontamination of a radioactively contaminated metal surface, comprising the step of:

   - bringing at least a portion of the metal surface into contact with a decontamination solution comprising a complexing agent selected from hydrofluoric acid, methanesulfonic acid and carboxylic acids such as nitriloacetic acid, ethylenediaminetetraacetic acid, oxalic acid, tartaric acid, citric acid and picolinic acid and the salts thereof, and an ion of a transition metal.
2. Method according to claim 1, wherein the ion of the transition metal is selected from the group consisting of zinc, nickel, cobalt, or mixtures thereof.
3. Method according to at least one of the preceding claims, wherein the concentration of the transition metal is in a range of ≥ 0.5 and ≤ 15 mg/kg.
4. Method according to at least one of the preceding claims, wherein the ion of the transition metal is zinc and is present in a concentration in a range of ≥ 2 and ≤ 5 mg/kg.
5. Method according to at least one of the preceding claims, wherein ⁵⁸Co ions and/or ⁶⁰Co ions are removed from the metal surface.
6. Method according to at least one of the preceding claims, wherein the decontamination solution is introduced into the primary circuit of a nuclear reactor.
7. Method according to at least one of the preceding claims, wherein the decontamination solution is circulated.
8. Method according to at least one of the preceding claims, wherein the method comprises, as a first method step, a pre-oxidation step or a reduction step for respectively oxidizing or reducing the radioactively contaminated metal surface.
9. Method according to at least one of the preceding claims, wherein the method further comprises the step of:

   - at least partially removing the radioactive isotopes present in the decontamination solution.
10. Method according to claim 9, wherein all of the method steps are repeated at least once.
11. Use of an aqueous solution comprising a complexing agent selected from hydrofluoric acid, methanesulfonic acid and carboxylic acids such as nitriloacetic acid, ethylenediaminetetraacetic acid, oxalic acid, tartaric acid, citric acid and picolinic acid and the salts thereof, and an ion of a transition metal, in a concentration in the range of ≥ 0,5 mg/kg and ≤ 15 mg/kg for decontaminating radioactively contaminated metal surfaces.

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