EP2758966B1 - Method for decomposing an oxide layer - Google Patents

Description

The invention relates to a process for the decomposition of a chromium, iron, nickel, if necessary, zinc oxide and radionuclide-containing oxide layer by means of a permanganic acid and a mineral acid-containing aqueous oxidative decontamination solution, which flows in a circuit (K1), wherein the oxidative decontamination solution to a pH Value ≤ 2.5, in particular for the decomposition of oxide layers deposited on inner surfaces of areas or components of a nuclear power plant.

In particular, the invention relates to a method for the extensive degradation of radionuclides in the primary system and the auxiliary systems in a nuclear power plant using the existing operating medium and the power plant operating systems.

During the power operation of a PWR (pressurized-water reactor) nuclear power plant, oxidic protective layers (Fe0.5Ni1.0Cr1.5O4, NiFe2O4) are formed at operating temperatures of> 180 ° C and reducing conditions on the wetted inner surfaces of the systems and components. Radionuclides are incorporated into the oxide matrix. The aim of chemical decontamination methods is to dissolve this oxide layer in order to be able to remove the incorporated radionuclides. This is intended to ensure that in the case of a revision, the radiation exposure of the inspection personnel kept as low as possible or in the case of dismantling of the nuclear reactor, the components can be easily fed into a recycling cycle.

Due to their composition and structure, the oxide protective layers are considered to be chemically insoluble. By a preliminary oxidative chemical treatment of the oxide structure, this can be broken and the sparingly soluble oxide matrix in easily soluble metal oxides are transferred. This breakdown of the oxide matrix occurs by oxidation of the trivalent chromium into the hexavalent chromium:

Fe _0.5 Ni _1.0 Cr _1.5 O ₄ / NiFe ₂ O ₄ / Fe ₃ O ₄ → oxidation → CrO ₄ ^2- , FeO, NiO, Fe ₂ O ₃ equation (1)

• "NP" Oxidation = Salpetersäure + Kaliumpermanganat (nitric acid, permanganate) (s. z. B. EP-B-0 675 973 )
• "AP" Oxidation = Natriumhydroxid + Kaliumpermanganat (alkaline, permanganate) "HP" Oxidation = Permangansäure (s. z. B. WO-A-2007/062743 ).

        Mn-VII + Cr-III → Mn-IV + Cr-VI     Gleichung (2)

As an oxidation treatment, the so-called "permanganate pre-oxidation" according to equation (2) has become established worldwide, the following three oxidation treatments being available:

• "NP" oxidation = nitric acid + potassium permanganate ( n itric acid, permanganate) (see eg. EP-B-0 675 973 )
• "AP" oxidation = sodium hydroxide + potassium permanganate ( a lkaline, p ermanganate) "HP" oxidation = permanganic acid (see eg. WO-A-2007/062743 ).

Mn-VII + Cr-III → Mn-IV + Cr-VI Equation (2)

The manganese ion is present in the permanganate in the oxidation state 7 and is reduced according to equation (2) in the oxidation state 4, at the same time present in the trivalent oxidation state chromium is oxidized to the oxidation state 6. For the oxidation of 1 MOL Cr ₂ O ₃ , according to equation (2), 2 MOL MnO ^{4- are} needed.

A chemical decontamination of an entire primary system, including all activity-carrying auxiliary systems, has so far only been carried out in a few nuclear power plants. Worldwide, over 50 different decontamination processes have been developed in recent years. Of all these processes, only the technologies have been established that build on a preliminary pre-oxidation with permanganates (MnO ⁴ ) (eg. EP 0 071 336 . EP 0 160 831 B1 . EP 242 449 B1 . EP 0 355 628 B1 . EP 0753 196 B1 . EP 1082 728 B1 ).

Schritt I:

Voroxidations-Schritt

Schritt II.

Reduktions-Schritt

Schritt III.

Dekontaminations-Schritt

Schritt IV:

Zersetzungs-Schritt

Schritt V:

Endreinigungs-Schritt.

Currently available chemical decontamination processes are currently carried out with the following process sequence:

Step I:

Preoxidation step

Step II.

Reduction step

Step III.

Decontamination step

Step IV:

Decomposition step

Step V:

Final Cleaning step.

All processes use permanganate (potassium permanganate, permanganic acid) for preoxidation (I.) and oxalic acid for reduction (II.). The methods only show differences in the decontamination step (III.). Here different chemicals and chemical mixtures are used.

The previous decontamination methods build on the previously discussed concept. The sparingly soluble oxide protective layers are converted into more soluble oxide compounds during a preoxidation step and remain on the surface of the system. During the pre-oxidation therefore no activity discharge from the systems to be decontaminated. Degradation of the dose rate does not take place in this period of decontamination.

Only after the second process step (II.) The reduction of the permanganate and the formed manganese dioxide by means of oxalic acid and in the decontamination step (III.) The oxides are dissolved and the dissolved cations / radionuclides discharged and bound to ion exchange resins.

During pre-oxidation (I.), manganese oxihydrate [MnO (OH) ₂ ] or manganese dioxide (MnO ₂ ) is formed in all decontamination technologies used so far, as equations (3) and (4) illustrate.

2 MnO ₄ ^- + Cr ₂ O ₃ + H ₂ O → 2MnO (OH) ₂ + 2 CrO ₄ ^2- + H ₂ O Equation (3)

2MnO (OH) ₂ → 2MnO ₂ + 2H ₂ O

(AP / HP-oxidation)

4 KMnO ₄ + 4 HNO ₃ + 2 Cr ₂ O ₃ + 4H ₂ O → 4 MnO (OH) ₂ + 4 KNO ₃ + 2 H ₂ Cr ₂ O ₇ Equation (4)

4MnO (OH) ₂ → 4 MnO ₂ + 4H ₂ O

(NP-oxidation)

The manganese dioxide is insoluble and deposits on the inner surface of the components / systems. With increasing manganese oxyhydrate / manganese dioxide deposition, the desired oxidation of the oxide protective layer is hindered. In addition, the converted iron and nickel oxides remain undissolved on the surface, so that the barrier layer on the surface further amplified.

At the end of the pre-oxidation step, the following new chemical compounds introduced or produced in process step (I) are present in the system to be decontaminated: on the system interface: MnO ₂ , NiO, FeO, Fe ₂ O ₃ , Fe ₃ O ₄ in the pre-oxidation solution: KMnO ₄ , NaOH or HNO ₃ , MnO (OH) ₂ , CrO ₄ ^2- or Cr ₂ O ₄ ^2- . colloidal

Accordingly, at the end of the pre-oxidation step, all the metal oxides, including the radionuclides, are still present in the system to be decontaminated. The manganese oxyhydrate [MnO (OH) ₂ ] formed was partially introduced into non-perfused system areas and can no longer be discharged / removed in the subsequent process steps.

According to the state of the art, in the course of the oxidation of the oxide layer no degradation of radioactivity occurs, ie no decontamination, since virtually no cations are dissolved out of the oxide layer, which could be removed with the aid of a cation exchanger. Rather, the oxide layer is dissolved in a second process step with the aid of oxalic acid, which is preceded by a reduction step for reducing excess permanganic acid and manganese oxide hydrate. Only after these process steps are cations removed by ion exchange from the cleaning solution (decontamination solution).

Object of the present invention is to avoid the disadvantages of the prior art, in particular to allow a simplification of the process flow, the formation of manganese dioxide and oxalate to be avoided. The formation of CO ₂ should at least be reduced. Also, the release of oxide particles should be largely avoided.

Of the US-A-3,873,362 is a method for removing an oxide layer in nuclear facilities by pretreatment with aqueous alkali permanganate solution.

The EP-A-1 054 413 describes in the cited prior art the use of permanganic acid without sulfuric acid to decontaminate radioactive materials.

Object of the present invention is to avoid the disadvantages of the prior art, in particular to allow a simplification of the procedure, the formation of manganese dioxide and oxalate to be avoided. The formation of CO ₂ should at least be reduced. Also, the release of oxide particles should be largely avoided.

According to the invention, the object is essentially achieved in that the oxidation of the oxide layer and its dissolution takes place in a single treatment step with the aid of the aqueous decontamination solution, that sulfuric acid is used as the mineral acid, by means of which the pH is adjusted, and that after degradation of the Permanganic acid the solution flows through a bypass line of the circuit while maintaining the circulation operation of a cation exchanger in which existing in the solution 2-valent cations and 2-valent radionuclides are fixed with simultaneous release of sulfate and dichromate anions.

It is inventively provided that at the beginning of the procedure, the pH is set by metered addition of sulfuric acid. During the oxidative degradation of the layer and the process steps carried out in this context, there is no need for further addition of sulfuric acid.

In particular, it is provided that after enrichment of the solution with dichromic acid of a predetermined concentration, in particular 300 ppm or less, preferably 100 ppm or less, the solution flows through a bypass line an anion exchanger in which the dichromate is fixed with simultaneous release of SO ₄ - ions.

In this case, the amount of anion exchange resin used is designed to amount of dichromate ions to be fixed.

According to the invention, it is provided that the permanganate concentration in the oxidative decontamination solution is adjusted in such a way that, when the predetermined dichromate concentration is reached, the permanganate ions are consumed by chemical oxidation reactions, the relationship being particularly true:

Total demand for HMnO ₄ [kg] = Cr-III inventory [kg] x U

with 1.35 ≦ U ≦ 1.40, in particular U = 1.38.

According to the invention, a method is provided for reducing the activity inventory in components and systems, wherein the oxide layers of the wetted inner surfaces are removed with an oxidative decontamination solution. In this case, the oxidative decontamination can be carried out with power plants own systems without the help of external decontamination auxiliary systems, the activity reduction without manganese formation and other cation precipitations and without CO ₂ -Anfall and without release of oxidic particles take place and the metal oxides are simultaneously dissolved chemically and as Cations / anions are fixed together with the manganese and the nuclides (Co-60, Co-58, Mn-54, etc.) on ion exchange resins.

In contrast to the previously discussed decontamination concepts, according to the invention, the chemical conversion of the sparingly soluble oxides into readily soluble oxides, the dissolution of the oxides / radionuclides and the discharge and fixing of the dissolved cations to cation exchangers take place in a single process step, which acts as an oxidative decontamination step referred to as.

Furthermore, and in contrast to the prior art, according to the invention, the permanganic acid used is completely converted to the Mn ²⁺ cation in the course of the preoxidation step. Manganese oxyhydrate precipitation does not occur.

Through the reaction of Mn-VII to Mn-II, 5 equivalents (electrons) are available for the oxidation of Cr ₂ O ₃ . This means that in comparison with the previous decontamination according to the teaching of the invention almost twice the amount of Cr ₂ O ₃ can be oxidized to chromate / dichromate.

• 43 g Cr-III zu Cr-VI aufoxidiert
• 72,5 g MnO(OH)₂ fallen aus.

In the past permanganate-based decontamination concepts, per 100 g of permanganate ions used:

• 43 g Cr-III oxidized to Cr-VI
• 72.5 g of MnO (OH) ₂ precipitate.

• 73 g Cr-III zu Cr-VI aufoxidiert
• es treten keine MnO(OH)₂ / MnO₂ - Ausfällungen auf.

In the decontamination concept according to the present invention, per 100 g of permanganate ions used

• 73 g Cr-III oxidized to Cr-VI
• no MnO (OH) ₂ / MnO ₂ precipitations occur.

• auf der Systemoberfläche: Fe₂O₃
• in der Voroxidationslösung: H₂Cr₂O₇ und H₂SO₄.

In contrast to the previous decontamination processes, only the following chemical compounds resulting from the "oxidative decontamination step" remain in the system:

• on the system surface: Fe ₂ O ₃
• in the pre-oxidation solution: H ₂ Cr ₂ O ₇ and H ₂ SO ₄ .

The whereabouts of H ₂ Cr ₂ O ₇ and H ₂ SO _{4 are} advantageous since both compounds are needed in the ongoing process and are thus desired in terms of process engineering. The dichromate protects the basic material of the system or the component from chemical attacks and the sulfuric acid ensures the required low pH value throughout the process, as is also the case with the Fig. 1 is clarified.

The remaining hematite (Fe ₂ O ₃ ) in the system can not be dissolved by mineral acids that have oxidizing properties (such as nitric acid). The Fe ₂ O ₃ is therefore dissolved in a subsequent step, a so-called hematite step, and then the dissolved Fe ion is bound to cation exchanger.

• kein Braunstein entstehen kann
• die durch den Zerfall der schwerlöslichen Spinell- / Magnetit-Oxide entstehenden Einzeloxide (FeO, NiO) zeitgleich chemisch gelöst werden
• die sich bildende Eisen- und Nickel-Salze eine hohe Löslichkeit aufweisen
• die gelösten Kationen (Fe³⁺, Ni²⁺ und Mn²⁺) auf Ionenaustauscher fixiert werden.

According to the teaching of the invention, both the pH and the permanganic acid and the proton supplier (sulfuric acid) are coordinated according to a fixed logistic scheme so that in the course of carrying out the "oxidative decontamination step":

• no brownstone can arise
• the individual oxides (FeO, NiO) formed by the decomposition of the sparingly soluble spinel / magnetite oxides are simultaneously dissolved chemically
• the forming iron and nickel salts have a high solubility
• the dissolved cations (Fe ³⁺ , Ni ²⁺ and Mn ²⁺ ) are fixed on ion exchangers.

1. a)
   
           6HMnO₄ + 5Cr₂O₃ + 2H⁺ 6Mn²⁺ + 5Cr₂O₇ ^2- + 4H₂O     Gleichung (5)
   
   
2. b)
   
           Mn²⁺ + H₂KIT [Mn²⁺-KIT] + 2 H⁺
   
   

The above-described formation of manganese dioxide after NP, AP or HP oxidation is avoided according to the invention by using permanganic acid in the acidic range (pH ≦ 2.5, preferably pH ≦ 2.2, in particular pH ≦ 2). That is in the Acid medium-forming Mn ²⁺ is removed according to the invention by means of cation exchanger already during the "oxidative decontamination step" from the solution according to equation (5):

1. a)
   
   6HMnO ₄ + 5Cr ₂ O ₃ + 2H ⁺ 6Mn ²⁺ + 5Cr ₂ O ₇ ^2- + 4H ₂ O Equation (5)
   
   
2. b)
   
   Mn ²⁺ + H ₂ KIT [Mn ²⁺ -KIT] + 2H ⁺
   
   

According to the invention, the chemical reaction according to equation (5) reliably leads to the formation of Mn ²⁺ . The reaction is controlled by protons (H ⁺ ions).

If insufficient protons (H ⁺ ) are available during the oxidative decontamination step, the chemical reaction proceeds in accordance with equations (3) and (4). It forms as the end product manganese oxyhydrate / manganese dioxide.

The Fig. 1 shows the interaction between the pH (= acid concentration) and the permanganate content. If the pH is exceeded in the curve shown, brownstone is formed in the oxidation reaction [equations (3) and (4)]. If the curve is undercut, the reaction proceeds to the Mn ²⁺ cation [Equation (5)].

• Schwefelsäure ist gegenüber Permanganat beständig,
  sie wird weder oxidativ zerstört noch chemisch verändert
• Permangansäure wird durch Schwefelsäure nicht reduziert,
  eine Braunsteinbildung (MnO₂) findet nicht statt
• Metalloxide werden aufgelöst und bilden leichtlösliche Sulfate
• die gelösten Kationen werden auf Kationenaustauscherharze gebunden,
  die Schwefelsäure steht dem Prozess wieder zur Verfügung
• ein Grundmaterialangriff findet nicht statt.

According to the present invention, the required pH of <2.5, preferably ≤ 2.2, preferably pH ≤ 2.0, is adjusted by adding sulfuric acid. Of all the mineral acids available, sulfuric acid satisfies the conditions required for the decontamination process according to the invention, such as

• Sulfuric acid is resistant to permanganate,
  it is neither oxidatively destroyed nor chemically altered
• Permanganic acid is not reduced by sulfuric acid,
  A brown formation (MnO ₂ ) does not take place
• Metal oxides are dissolved and form easily soluble sulfates
• the dissolved cations are bound to cation exchange resins,
  Sulfuric acid is available to the process again
• a base material attack does not take place.

Due to the properties listed above, sulfuric acid is still available at the end of the "oxidative decontamination step" for the subsequent steps.

In the course of the "oxidative decontamination step" resulting oxides (NiO, Ni ₂ O _3, FeO,) are already dissolved by the sulfuric acid during the oxidation step.

According to the present invention, sulfuric acid is used for the pH adjustment. The amount of sulfuric acid required to prevent MnO (OH) ₂ formation is based on the permanganate concentration. With increasing permanganate concentration, the pH must be lowered, that is, a higher acid concentration must be set (s. Fig. 1 ).

• bei 0,1 MOL Permangansäure pro Liter ein pH-Wert von ca. 1,
• bei 0,01 MOL Permanganat pro Liter ein pH-Wert von ca. 2.

The following pH values apply as a guideline:

• at 0.1 MOL permanganic acid per liter, a pH of about 1,
• at 0.01 MOL permanganate per liter, a pH of about 2.

• Die Berechnung des H₂SO₄ - Bedarfs ohne Einbezug der gelösten Kationen erfolgt nach den Gleichungen (6 und 7):
  
          pH = X - [ (mg/kg eingesetzte HMnO₄) x 9E-05 ]     Gleichung (6)
  
  mit 2,0 ≤ X ≤ 2,2, insbesondere X = 2,114
  
          mg /kg H"SO₄ = Y x pH -z     Gleichung (7)
  
  mit 16 ≤ Y ≤ 18, insbesondere Y = 16,836
  und 4,5 ≤ Z ≤ 6,5, insbesondere Z = 5,296.

According to the present invention, depending on the permanganate content of the solution, the sulfuric acid requirement is calculated via the pH as follows:

• The calculation of the H ₂ SO ₄ requirement without inclusion of the dissolved cations takes place according to the equations (6 and 7):
  
  pH = X - [(mg / kg HMnO _{4 used} ) x 9E-05] Equation (6)
  
  with 2.0 ≦ X ≦ 2.2, in particular X = 2.144
  
  mg / kg H "SO ₄ = Y x pH-z equation (7)
  
  with 16 ≤ Y ≤ 18, in particular Y = 16.836
  and 4.5 ≦ Z ≦ 6.5, especially Z = 5.296.

During the "oxidative decontamination step", the concentration of free protons (H ⁺ ) is reduced by the formation of metal sulfates. The amount of dissolved Fe, Ni, Zn, Mn cations are therefore included in the determination of the additional mineral acid requirement according to the following formulas:

mg SO ₄ ^-2 / liter = [mg cation / liter] * [cation specific factor].

The calculation of the H ₂ SO ₄ demand, taking into account the dissolved cations, takes place according to the equation (7 ').

mg / kg H ₂ SO ₄ = [yx pH ^-z ] + [(K ₁ * F ₁ ) + (K ₂ F ₂ ) + ...... (K _n * F _n )] Equation (7 ')

wherein K ₁ , K ₂ ... K _n each mg cation / liter and F ₁ , F ₂ ... F _n specific factor of the respective cation.

• F1 (Fe-II) zwischen 1,70 und 1,74, insbesondere 1,72
• F2 (Fe - III) zwischen 2,55 und 2,61, insbesondere 2,58
• F3 (Ni-II) zwischen 1,62 und 1,66, insbesondere 1,64
• F4 (Zn-II) zwischen 1,45 und 1,50, insbesondere 1,47
• F5 (Mn-II) zwischen 1,70 und 1,80, insbesondere 1,75 .

The following applies to the following cations:

• F1 (Fe-II) between 1.70 and 1.74, especially 1.72
• F2 (Fe - III) between 2.55 and 2.61, especially 2.58
• F3 (Ni-II) between 1.62 and 1.66, especially 1.64
• F4 (Zn-II) between 1.45 and 1.50, in particular 1.47
• F5 (Mn-II) between 1.70 and 1.80, especially 1.75.

Noteworthy Zn components are present in the protective layer when the so-called Zn mode of operation was performed in the power operation of the nuclear power plant.

Depending on the Fe / Cr / Ni / Zn composition of the protective layer, it is possible, according to the present invention, to calculate exactly the amount of the individual cations depending on the amount of HMnO ₄ used. which are released in the "oxidative decontamination step". This is possible because the amount of HMnO ₄ used converts to 100% in Mn ²⁺ and stoichiometrically produces the amount of dichromate produced. The amount of oxidized Cr-III is 06. November 2014/54066 amended description pages Again, the amount of converted Fe / Cr / Ni / Zn - oxides and thus the resulting in the oxidative decontamination step Fe / Ni / Zn / Mn - ions before.

Fig. 2 shows by way of example the temporal degradation of permanganic acid and the concomitant simultaneous construction of the cations (Fe-II, Ni-II, Mn-II) and the anion Cr ₂ O ₇ ^2- in the "oxidative decontamination solution" in a system with high chromium contents.

During the oxidative oxide conversion and the simultaneously occurring dissolution of the new oxide structures, the system to be decontaminated is operated in the circulation without ion exchange integration. This should in principle be based on the Fig. 6 be clarified. The oxidative decontamination step, which is carried out in the circulation until the conversion of the HMnO ₄ amount to 100% in Mn ²⁺ (cycle operation K1), without the solution passing through a cation exchanger (KIT).

• Im "oxidativen Dekontaminations-Schritt" pro HMnO₄-Dosierung gelöste Kationen:
  
          [g] Fe-II = [g eingesetzte HMnO₄-Menge] x 0,72 x [Gew.%Fe] / [Gew.%Cr] x 0,33
  
          [g] Ni-II = [g eingesetzte HMnO₄-Menge] x 0,72 x [Gew.%Ni] / [Gew.%Cr]
  
          [g] Zn-II = [g eingesetzte HMnO₄-Menge] x 0,72 x [Gew.%Zn] / [Gew.%Cr]
  
          [g] Mn-II = [g eingesetzte HMnO₄-Menge] x 0,46.
  
  

The calculation of the amount of dissolved cation and of the remaining Fe ₂ O ₃ is carried out, depending on the amount of permanganic acid used and the composition of the oxide matrix, according to the following formulas:

• Cations dissolved in the "oxidative decontamination step" per HMnO ₄ dosage:
  
  [g] Fe-II = [g HMnO ₄ amount used] x 0.72 x [wt.% Fe] / [wt.% Cr] x 0.33
  
  [g] Ni-II = [g HMnO ₄ amount used] × 0.72 × [% by weight Ni] / [% by weight Cr]
  
  [g] Zn-II = [g HMnO ₄ amount used] × 0.72 × [% by weight Zn] / [% by weight Cr]
  
  [g] Mn-II = [g HMnO ₄ amount used] x 0.46.
  
  

In the "oxidative decontamination step" per HMnO ₄ dosage to Fe ₂ O ₃ converted iron oxide is dissolved in the "hematite step":

[g] Fe ₂ O ₃ = [amount of HMnO _{4 used} ] x 0.72 x [wt.% Fe] / wt.% Cr *] x 0.67 x 1.43

According to the invention, the amount of HMnO _{4 used is} the amount of the oxide layer which can be dissolved from the oxide matrix of the Fe / Cr / Ni protective layer. Fig. 7 shows this relationship with an example of a performed system decontamination. The averaged oxide protective layer thickness was about 5.5 μm. In total, the "oxidative decontamination step" including the "hematite step" was performed 11 times. This in Fig. 7 shown graph shows that the average oxide layer degradation per HMnO ₄ dosage was reproducibly on the order of about 0.5 microns.

According to the present invention, a maximum permanganic acid concentration of 150 ppm has to be used per oxidative decontamination step, which is repeated as a function of the previously determined or estimated chromium concentration, as explained above.

• Oxidieren und Lösen des in der Schutzschicht (Fe_0.5Ni_1.0Cr_1.5O₄) eingebundenen Cr₂O₃:
  
          6HMnO₄ + 5Cr₂O₃ + 6H₂SO₄ → 6MnSO₄ + 5H₂Cr₂O₇ + 4H₂O     Gleichung (8)
  
  

In order to minimize the required use of sulfuric acid, hereinafter also referred to as mineral acid, the "oxidative decontamination step" is preferably carried out with an HMnO ₄ concentration of ≦ 50 ppm HMnO ₄ . During the "oxidative decontamination step", the following partial chemical reactions take place:

• Oxidizing and dissolving the Cr ₂ O ₃ incorporated in the protective layer (Fe _0.5 Ni _1.0 Cr _1.5 O ₄ ):
  
  6HMnO ₄ + 5Cr ₂ O ₃ + 6H ₂ SO ₄ → 6MnSO ₄ + 5H ₂ Cr ₂ O ₇ + 4H ₂ O Equation (8)
  
  

The oxidation of Cr-III-oxide to water-soluble dichromate Ni is released from the protective coating and is then as Ni-II oxide (NiO) or Ni-III oxide (Ni ₂ O ₃ ) before. The Ni oxides are then dissolved in an intermediate by HMnO ₄ and formation of Ni (MnO ₄ ) ₂ according to equation (9):

NiO + 2HMnO ₄ + 5H ₂ O → [Ni (MnO ₄ ) ₂ × 6 H ₂ O] (Equation (9)

With increasing consumption of HMnO ₄ , the rearrangement of Ni-II from Ni-permanganate to Ni-dichromate takes place (equation 10) or Ni-sulfate (equation 11).

[3Ni (MnO _{4) 2} 6H ₂ O] + 5Cr ₂ O ₃ + 6H ₂ SO ₄ + 2NiO equations (10)

6MnSO ₄ + 5NiCr ₂ O ₇ + 12H ₂ O

NiCr ₂ O ₇ ⁺ H ₂ SO ₄ → NiSO ₄ ⁺ H ₂ Cr ₂ O ₇ Equation (11)

By the oxidation of the Cr-III-oxide to form water-soluble dichromate additionally Fe is released from the oxide matrix and is then as Fe-II oxide (FeO) or Fe-III oxide (Fe ₂ O ₃ ) before. FeO is easily dissolved by sulfuric acid (Equation 12). On the other hand, Fe ₂ O ₃ is not sufficiently dissolved by sulfuric acid and therefore remains in the system and is dissolved in the subsequent "hematite step" process step (see below) and fixed on cation exchange resins.

FeO + H ₂ SO ₄ → FeSO ₄ ⁺ H ₂ O Equation (12)

To accelerate the "oxidative decontamination step", a process temperature of preferably 60 ° C to 120 ° C is set.

According to the present invention, the oxidative decontamination is preferably carried out in a temperature range of 95 ° C to 105 ° C.

After the conversion of the permanganate according to equations (8) to (12), the integration of the particular power plant-owned cation ion exchanger (KIT) takes place.

This should also be based on the Fig. 6 be clarified. During the conversion of the permanganate to Mn ²⁺ , the solution is circulated in the system (K1) to be decontaminated. After conversion of the permanganate, the solution is passed through the cation exchanger KIT in the bypass via a purification circuit K2.

The prerequisite for the incorporation of a cation exchanger is that the permanganate has completely or substantially completely converted to Mn ²⁺ and the solution is free of MnO ₄ ^- ions (standard value <2 ppm MnO ₄ ).

During operation of the cation exchanger KIT, the divalent cations (Mn-II, Fe-II, Zn-II and Ni-II) as well as the divalent radionuclides (Co-58, Co-60, Mn-54) are removed from the solution. At the same time, the corresponding anions (sulfate and dichromate) are released and are available to the process again. See equations (13) and (14).

Release of the sulfate to form sulfuric acid:

MnSO ₄ + H ₂ KIT → H ₂ SO ₄ + [Mn ²⁺ -KIT] Equation (13)

NiSO ₄ + H ₂ KIT → H ₂ SO ₄ + [Ni ^{2 + -} KIT]

FeSO ₄ + H ₂ KIT → H ₂ SO ₄ + [Fe ²⁺ -KIT]

Fe (SO ₄ ) ₃ + 3H ₂ KIT → 3H ₂ SO ₄ + [Fe ³⁺ -KIT]

Release of the dichromate to form dichromic acid:

NiCr ₂ O ₇ + H ₂ KIT → H ₂ Cr ₂ O ₇ + [Ni ²⁺ -KIT] Equation (14)

The operation of the cation exchanger KIT takes place at a process temperature of ≤ 100 ° C.

The operation of the cation exchanger KIT takes place until all dissolved cations are fixed on the cation exchange resin.

According to the present invention, after cation cleaning, permanganic acid is again added and the above-described process steps are repeated until the dichromic acid concentration has reached a predetermined value such as 300 ppm or less.

• D1 = Aufbrechen und Auflösen der Oxidmatrix
• D2 = Fixieren der gelösten Kationen an Kationenaustauscher KIT und
• D3 = Fixieren des Dichromats an Anionenaustauscher AIT .

Fig. 3 shows purely in principle the individual phases of the "oxidative decontamination step", wherein the individual phases D1 to D3 are defined as follows:

• D1 = break up and dissolution of the oxide matrix
• D2 = fixing of the dissolved cations to cation exchangers KIT and
• D3 = fix the dichromate to anion exchanger AIT.

In Fig. 5 the courses of the cation concentration of an "oxidative decontamination step" are shown by way of example with the aid of a three-time HMnO ₄ dosing.

This sequence ( Fig. 3 and Fig. 5 , Phase D1 and D2) can be repeated until the dichromic acid concentration has reached a value of about 300 ppm.

Preferably, the maximum dichromic acid concentration is limited to 100 ppm.

Once the specified dichromic acid concentration has been reached, the dichromate is removed from the solution by means of the anion exchanger AIT (see p. Fig. 6 - Cleaning circuit K3).

The prerequisite for the integration of the anion exchanger is that all permanganate ions are consumed by the chemical oxidation reactions and the solution is free of permanganate ions (s. Fig. 6 - Cleaning circuit K3).

The amount of anion exchanger used is based on the dichromate inventory of the solution to be purified. Only the amount of anion exchanger is provided whose capacity is sufficient for a recording of the dichromate. This ensures that the sulfuric acid concentration does not change in the solution.

In the first phase of the anion exchanger purification, both the sulfate ions of the sulfuric acid and the dichromate ions of the dichromic acid are bound to the anion exchange resins. If the anion exchanger is charged to 100% with dichromate and sulfate, the sulphate ions already fixed are displaced by the dichromate ions during further loading of the anion exchanger with sulfate ions and dichromate ions. This process continues until the anion exchanger is 100% loaded with dichromate ions and all sulfate ions are again available for oxidative decontamination.

If the dichromate ions are removed from the solution, permanganic acid is added again and the process starts again as described above ( Fig. 3 , Phases D1, D2 and D3).

The repetition of the step sequences is carried out until cation discharge no longer takes place. If all cations and anions are fixed on ion exchangers after carrying out previous steps, only sulfuric acid is present in the solution.

According to the prior art, it is customary, after the expiration of the pre-oxidation, the to reduce excess permanganate with oxalic acid (step II.) and then to initiate the decontamination step (step III.) by adding further decontamination chemicals.

At the time of the reduction (step II), all the ingredients of the pre-oxidation step (residual permanganate, colloidal MnO (OH) ₂ , chromate and nickel permanganate) and all converted metal oxides are present in the system in the solution. or component surface.

Since the metal ions partially in dissolved form (MnO ₄ -, CrO ₄ ^2-) as well as slightly soluble metal oxides (NiO, FeO, Mn ₂ / MnO (OH ₂₎ are present, already of the reduction occur in the course of the second process step (step II.) High cation contents in the solution.

At the same time, the reduction of permanganate, chromate and manganese dioxide with oxalic acid produces large amounts of CO ₂ (see Equations (15)). This CO ₂ formation taking place on the surface leads to a mobilization of oxide particles, which then settle in low-flow zones of the system and lead there to an increase in the dose rate.

2 HMnO ₄ + 7 H ₂ C ₂ O ₄ → 2 MnC ₂ O ₄ + 10CO ₂ + 8 H ₂ O Equations (15)

MnO ₂ + 2H ₂ C ₂ O ₄ → MnC ₂ O ₄ + 2 CO ₂ + 2H ₂ O

Cr ₂ O ₇ ^2- + 3 H ₂ C ₂ O ₄ + 8 (H ₃ O) ⁺ → Cr ³⁺ + 6 CO ₂ + 15 H ₂ O

The above-described release of oxide particles does not occur in the invention. At the time of oxalic acid metering, both the solution and the system surface are free of permanganate, manganese dioxide and chromate / dichromate. The unwanted evolution of CO ₂ and the release of oxide particles does not occur.

The oxalate compounds formed from divalent cations and the reduction chemical "oxalic acid" have limited solubility in water. Depending on the process temperature, the solubility of the divalent cations is: 50 ° C 80 ° C unit NiC ₂ O ₄ about 3 about 6 mg Ni-II / liter FeC ₂ O ₄ can.15 about 45 mg Fe-II / liter MnC ₂ O ₄ about 120 about 170 mg Mn-II / liter

Computationally, large quantities of cations are released during a primary system decontamination when using the previous decontamination methods per decontamination cycle. This leads already in the reduction step to oxalate precipitations on the inner surfaces of the systems.

The oxide protective layers of a primary system of a pressurized-water nuclear power plant usually result in a total total NOx inventory of 1,900 kg to 2,400 kg [Fe, Cr, Ni oxide].

• Chrom → 70 bis 80 kg Cr
• Nickel → .100 bis 120 kg Ni
• Eisen → 190 bis 210 kg Fe

When decontaminating a primary system of a pressurized water reactor, the following maximum cation release must therefore be expected:

• Chromium → 70 to 80 kg Cr
• Nickel → .100 to 120 kg Ni
• Iron → 190 to 210 kg Fe

• Nickel → 67 ppm Ni
• Eisen → 117 ppm Fe

In primary system decontamination, usually 3 decontamination cycles are performed. With a total volume of about 600 m ³ and a uniform distribution of the cations over 3 cycles, the following concentrations of divalent cations are to be expected per cycle:

• Nickel → 67 ppm Ni
• Iron → 117 ppm Fe

This rough calculation shows that in all previous decontamination processes, which used oxalic acid for reduction and / or decontamination, Fe ²⁺ and Ni ^{2 +} oxalate formation can not be avoided.

If, as described above, oxalate residues remain in the system after completion of a decontamination cycle, more permanganate must be used in the subsequent cycle, as shown by Equations (16):

3NiC ₂ O ₄ + 2HMnO ₄ + H ₂ O + 2MnO 3 NiO (OH) ₂ + 6CO ₂ equations (16)

3FeC ₂ O ₄ + 2HMnO ₄ + H ₂ O 3 FeO + 2MnO (OH) ₂ + 6CO ₂

This leads, without improving the decontamination result, to a higher permanganate requirement and, as a result, to an increased MnO (OH) ₂ deposition on the surfaces and ultimately to a higher accumulation of radioactive waste. In addition, the cation addition increases in the subsequent cycle, the risk of re-oxalate formation increases and the accumulation of ion exchange resins is further increased.

The already dissolved radionuclides (Co-58, Co-60, Mn-54) are incorporated into the oxalate layer. This leads to a recontamination in the systems.

As already described above, according to the present invention, the liberated divalent cations (Ni, Mn, Fe, Zn) and the dichromate are dissolved in the "oxidative decontamination step", and the fixation of the cations and anions takes place promptly on ion exchange resins. The oxalate deposits conventionally used in conducting chemical decontamination do not take place.

At the end of the oxidative decontamination step sequences, the "hematite step" is performed. In this process step the hematite (Fe ₂ O ₃ ) is dissolved according to equation (17):

Fe ₂ O ₃ + 6 OH ₂ C ₂ O ₄ → 2 [Fe (C ₂ O ₄ ) ₃ ] ^3- + 3 H ₂ O Equation (17)

Due to the sulfuric acid underlayment, the solubility of Me-II oxalates in the "hematite step" is significantly higher than that of other decontamination technologies.

Under the chemical conditions of the present invention, the solubility limit for Ni-II oxalate and Fe-II oxalate is as follows:

Ni ²⁺ - oxalate → approx. 80 mg Ni-II / liter

Fe ²⁺ - oxalate → about 150 mg Fe-II / liter.

The formation of oxalates and their deposition on the system inner surfaces does not take place due to the low Me-II cation concentrations and the much higher solubility of the Me-oxalates.

The oxalic acid concentration should be 50 to 1000 ppm H ₂ C ₂ O ₄ at the hematite step.

It is preferable to adjust an oxalic acid concentration of ≦ 100 ppm.

During the "hematite step", the dissolved cations are bound to cation exchangers, the dissolution of the hematite and the fixing of the dissolved Fe ions being carried out simultaneously (see Fig. 4 - Phases of the "hematite step").

The "hematite step" is carried out until no iron is removed from the system.

Upon completion of the "hematite step", the oxalic acid remaining in the solution is decomposed by permanganic acid to form carbon dioxide (equation (18)).

5 H ₂ C ₂ O ₄ + 2 HMnO ₄ + 2 H ₂ SO ₄ → 2 MnSO ₄ + 10 CO ₂ + 8 H ₂ O Equation (18)

After performing the "hematite step", the entire "oxidative decontamination" sequence can be repeated. This repetition is based on the remaining Cr ₂ O _{3 to} be broken down in the system. After carrying out the second "oxidative decontamination step", a "hematite step" is subsequently carried out again.

Each nuclear power plant has its own specific oxide structure, oxide composition, oxide dissolving behavior, and oxide / activity inventory. For the preliminary planning of a decontamination only assumptions can be made. Only in the course of carrying out the decontamination then shows whether the preliminary assumptions were correct.

A decontamination concept must therefore be able to adapt to the respective changes during execution.

With the present invention, it is possible to respond specifically to all conceivable new requirements. The detailed steps outlined above can be repeated as desired, depending on the type and amount of the oxide / activity inventory present in the system.

Decontamination, in accordance with the present invention, requires a very low level of chemicals compared to previous processing techniques.

The required quantities of chemicals can therefore be dosed with existing in nuclear power plants (NPP) own dosing and the resulting cations are removed by means of NPP own cleaning systems (ion exchanger). Large external decontamination facilities do not need to be installed.

By controlling the overall process from the nuclear power plant's power plant, the process parameters can be quickly adapted to the respective new requirements (chemical dosing, chemical concentrations, process temperature, time of KIT and AIT exchanger integration, step sequences, etc.).

If necessary, the process variations can be carried out until the desired activity output or the desired dose rate reduction has been achieved.

The sulfuric acid present in the solution remains in the solution during the execution of all process steps. The concentration is not changed. Only at the end of the total decontamination procedure are the sulfate ions bound to the anion exchanger AIT during the final purification (see Fig. 4 , AIT cleaning step D6).

Further details, advantages and features of the invention will become apparent not only from the claims - alone and / or in combination - but also from the figures already described above and also explained below, which are self-explanatory.

Fig. 1

der pH-Arbeitsbereich nach der Erfindung im Vergleich zum Stand der Technik,

Fig. 2

Änderung der Permangatsäure-Konzentration und der Kationen- und Dichromsäure-Konzentration in Abhängigkeit von der Prozessdauer,

Fig. 3

die Prozessabfolge im oxidativen Dekontaminationsschritt,

Fig. 4

die Prozessfolge im Hämatit-Schritt einschließlich des Endreinigungs-Schrittes,

Fig. 5

aufeinanderfolgende oxidative Dekontaminationsschritte und Zunahme der Dichromatsäure in Abhängigkeit von der Anzahl der aufeinanderfolgenden oxidativen Dekontaminationsschritte bei in der Lösung verbleibender Dichromatsäure,

Fig. 6

Prinzipdarstellung des Dekontaminationskreislaufes sowie der Ionenaustauscher- Reinigungsheisläufe und

Fig. 7

Abtrag einer Oxidschicht in Abhängigkeit von der Anzahl der durchgeführten oxidativen Dekontaminationsschritte.

The figures show:

Fig. 1

the pH working range according to the invention in comparison to the prior art,

Fig. 2

Change in the permanganic acid concentration and the cation and dichromic acid concentration as a function of the duration of the process,

Fig. 3

the process sequence in the oxidative decontamination step,

Fig. 4

the process sequence in the hematite step including the final cleaning step,

Fig. 5

successive oxidative decontamination steps and increase of the dichromic acid as a function of the number of successive oxidative decontamination steps in the presence of dichromate remaining in the solution,

Fig. 6

Schematic representation of the decontamination cycle as well as the ion exchange cleaning heaves and

Fig. 7

Removal of an oxide layer as a function of the number of oxidative decontamination steps carried out.

Based on Fig. 1 is clarified that, when the pH depending on the permanganate concentration below the in Fig. 1 drawn oblique straight line, it is ensured that brownstone can not form. The prior art operates at a pH and permanganate acid concentration above the line. As a result of this, brownstone forms. The straight line is determined according to equations (6) and (7) or (7`).

The Fig. 2 illustrates that, depending on the process time and the conversion of the permanganate to Mn ^2+, the concentration of cations and dichromic acid increases.

The inventive oxidative decontamination is purely in principle of Fig. 3 refer to. In method step D1, permanganate acid is metered into the solution as a function of the pH adjusted by means of sulfuric acid according to equations (6, 7, 7 ') in order to dissolve the metal oxides and form readily soluble sulfates. The Cr-III oxide is oxidized to Cr-VI and is present in the solution as dichromic acid. After the permanganate is complete or essentially has been completely converted to Mn ²⁺ and the solution is substantially free of MnO ⁴ -Inonen, the solution flows through a bypass, the cation exchanger KIT, in which the cations are fixed. In the solution remains sulfuric acid and dichromic acid.

Then again the solution, which no longer flows through the cation exchanger, according to the to be oxidized Cr ^-3 oxide again added permanganic acid. An addition of sulfuric acid is not required as long as the amount per kg of solution has been calculated according to Equation (7) or (7 '). After the dichromic acid concentration has reached a predetermined value, the solution flows through a bypass through the anion exchanger AIT, in which dichromate ions are fixed in the manner described above. The solution then leaves sulfuric acid and hematite.

The hematite is used according to the Fig. 4 separated from the solution. For this purpose, oxalic acid is first added (process step D4). The solution flows through a cation exchanger KIT, the dissolution of the hematite and the fixing of the Fe ions being carried out simultaneously. This process step D4 is carried out until no more iron is discharged. Subsequently, in the process step D5 permanganic acid is added to decompose the oxalic acid to form carbon dioxide and the forming manganese sulfate removed by means of cation exchanger. In the solution then remains only sulfuric acid.

Of the Fig. 7 In principle, it can be seen that the oxide layer can be reproducibly removed in layers, depending on the number of oxidative decontamination steps carried out, that is to say the metered addition of HMnO ₄ . It can be seen that oxide layer thicknesses in the order of 0.3 .mu.m to 0.6 .mu.m are removed per oxidative decontamination step.

In method step D1, the poorly soluble Fe, Cr, Ni structure is converted chemically into slightly soluble oxide forms by means of permanganic acid. Dissolution of the converted oxide forms is carried out with sulfuric acid. In terms of process technology, this is done in a cycle operation K1 ( Fig. 6 ) in a sulfuric acid / permanganic acid solution. The circuit operation K1 remains maintained until the permanganic consumed completely and has been converted to Mn ^2+. Typically, if at the beginning of the process the permanganic acid concentration is set to less than 50 ppm, more preferably between 30 and 50 ppm, the conversion of permanganic acid to Mn ²⁺ takes 2 to 4 hours. The conversion of the oxide structure and the dissolution of the converted oxides takes place at the same time. The final products of the dissolution process are sulfate salts. Upon completion of phase D1, phase D2 begins. Here are present as sulfate salts present metal cations on the cation exchanger KIT and fixed there. In this exchange process, the sulfate is released again and the decontamination solution is available again.

During the phase D2 - as during the phase D1 - the circulation mode K1 is maintained unchanged and the connection of the cation exchanger takes place in the bypass mode. The purification rate (flow rate) through the cation exchanger (m ³ / h) to the total volume of the system to be decontaminated [m ³ ] is specified by the respective system design of the nuclear power plant. The bypass operation K2 in the case of continuing circulation operation K1 of the cation exchanger is carried out until all the cations have been bound to the cation exchanger KIT. The total time required for this is given by the available cleaning rate.

After completion of the phases D1 and D2 a procedural breakpoint is specified. The further process steps are based on the total oxide content of the system to be decontaminated. If there are large amounts of chromium in the oxide matrix, it is advisable to repeat phases D1 and D2. This repetition procedure D1 + D2 can be carried out until the Dichromate concentration in the decontamination solution has a value of z. B. has reached 100 ppm dichromate. Then the process step D3 takes place. At the time of phase D3, the decontamination solution contains sulfuric acid and dichromic acid. The dichromic acid is removed from the solution by bypassing an anion exchanger. During phase D3, the cycle operation K1 of the system to be contaminated continues to operate unchanged. The connection of the anion exchange circuit K3 takes place in bypass mode. The bypass operation of the cation exchange circuit K2 can continue to operate. The bypass operation K3 of the anion exchanger is carried out until the dichromate ions are bound to the anion exchanger AIT. The time required for this is given by the available cleaning rate. The degradation of the dichromate concentration is advantageously carried out to a final concentration of less than 10 ppm. The presence of small amounts of dichromate in the solution preserves the basic protective property of the dichromate.

After completion of phase D3 a procedural 2nd breakpoint is specified. In the course of breakpoint 2, the further procedure is determined, taking into account the consideration described below. The further process steps are based on the total oxide inventory of the system to be decontaminated. If there is a large oxide inventory, the process sequence D1 to D3 must be repeated several times before the beginning of the hematite step, wherein the number of sequences D1 to D3 is preferably limited to a maximum of 4 passes. The hematite step, to be referred to as phase D4, causes the hematite Fe ₂ O _{3 formed} in the oxidative decontamination step to be dissolved in a sulfuric acid-oxalic acid solution. At the same time, the dissolved iron is fixed on the cation exchanger KIT. Sulfuric acid and oxalic acid are released from the beginning by removal of cations and are constantly available for the hematite dissolution process. During the entire phase D4, both the circulation operation K1 of the system to be decontaminated and the cation exchange circuit K2 are operated. The integration of the cation exchange circuit K2, in which the iron is fixed, takes place in bypass mode. The hematite initiation phase, ie phase D4, is carried out until no appreciable iron discharge occurs.

In the subsequent process step D5, in which sulfuric acid and oxalic acid is present, the oxalic acid is oxidatively degraded to CO ₂ . The oxidative degradation takes place by means of HMnO ₄ . In this case, only the circuit K1 is operated without passing through the cation exchanger K2 or the anion exchanger K3. After the degradation of the oxalic acid, sulfuric acid and Mn sulfate are present in the solution. Only after the degradation has taken place is the Mn ²⁺ bound to the cation exchanger by switching on the circulation K 2.

After completion of the hematite step, a procedural breakpoint 3 is specified. In the course of the stop 3 the further procedure is determined. The further process steps are based on the total oxide inventory of the system to be decontaminated. If there is a large amount of oxide inventory, process steps D1 to D5 must be repeated until the desired decontamination result (dose rate reduction) has been achieved. If so, the final cleaning step is performed. Chemically, this means that sulfuric acid is removed from the system. This is done by means of anion exchange resins D6. During process step D6, both the large cycle operation K1 of the system to be decontaminated and the anion exchange cycle K3 are operated. The bypass operation K3 of the anion exchanger is carried out until the sulfate ions are bound to the anion exchanger AIT. The total time required for this is given by the available cleaning rate.

The repetition of the individual phases D1 to D6 alone does not take place. Rather, the method steps D1 + D2 or D1 + D2 + D3 or D1 + D2 + D3 + D4 or D1 + D2 + D3 + D4 + D5 are repeated several times.

Claims (18)

1. Method for decomposing an oxide layer containing chromium, iron, nickel, and optionally zinc and radionuclides using an aqueous oxidative decontamination solution containing permanganic acid and a mineral acid, flowing in a circuit (K1), wherein the oxidative decontamination solution is adjusted to a pH ≤ 2.5, especially for decomposing oxide layers deposited on the interior surfaces of areas or components of a nuclear power plant,
   wherein
   the oxidation of the oxide layer and its dissolution takes place in a single treatment step with the aid of the aqueous decontamination solution, that sulfuric acid is used as the mineral acid for adjusting the pH, and that after degradation of the permanganic acid, while maintaining the flow in the circuit,
   the solution flows over a bypass line of the circuit to a cation exchanger in which divalent cations and divalent radionuclides present in the solution are fixed with simultaneous liberation of sulfate and dichromate anions.
2. Method according to claim 1,
   wherein
   after enriching the solution with dichromic acid at a predetermined concentration, especially 300 ppm or less, preferably 100 ppm or less, the solution flows over a bypass line to an anion exchanger in which the dichromate is fixed with simultaneous liberation of the SO₄ ^- ions.
3. Method according to claim 1 or 2,
   wherein
   the quantity of anion exchange resin used is selected based on the quantity of dichromate ions to be fixed.
4. Method according to at least one of the preceding claims,
   wherein
   the permanganate concentration in the oxidative decontamination solution is set such that when the prespecified dichromate ion is reached, the permanganate ions are consumed by chemical oxidation reactions, wherein in particular the following relation applies:
   
           Total demand of HMnO₄ [kg] = Cr-III load [kg] x U
   
   with 1.35 ≤ U ≤ 1.40, especially U = 1.38.
5. Method according to at least one of the preceding claims,
   wherein
   the quantity of sulfuric acid to be used can be calculated according to the pH in the oxidative decontamination solution depending on the permanganic acid to be used and the permanganic acid quantity requirement can be calculated based pm the expected amount of chromium to be oxidized according to the equations
   
           pH = X - [ (mg/kg HMnO₄ used) x 9E-05 ]     Equation (6)
   
   with 2.0 ≤ X ≤ 2.2, especially X = 2.114
   and
   
           mg/kg H2SO₄ = Y x pH^-z     Equation (7)
   
   with 16 ≤ Y ≤ 18, especially Y = 16.836
   and 4.5 ≤ Z ≤ 6.5, especially Z = 5.296,
   if the dissolved cations in the oxidative decontamination solution are not taken into consideration, or
   
           mg /kg H₂SO₄ = [YxpH^.2]+[(K₁*F₁) + (K₂F₂)+......(K_n*F_n)]     Equation (7')
   
   if the dissolved cations in the oxidative decontamination solution are taken into consideration,
   wherein 16 ≤ Y ≤ 18, especially Y = 16.836 and 4.5 ≤ Z ≤ 6.5, especially Z = 5.296, and F₁, F₂ ... F_n is the specific factor of the respective cations.
6. Method according to claim 5,
   wherein
   the specific factor (F) for the cations below is determined as follows: following

   - F1 (Fe-II) between 1.70 and 1.74, especially 1.72

   - F2 (Fe-III) between 2.55 and 2.61, especially 2.58

   - F3 (Ni-II) between 1.62 and 1.66, especially 1.64

   - F4 (Zn-II) between 1.45 and 1.50, especially 1.47

   - F5 (Mn-II) between 1.70 and 1.80, especially 1.75.
7. Method according to at least one of the preceding claims,
   wherein
   in the oxidative decontamination solution, the permanganic acid is set at a maximum concentration of 150 ppm, preferably 50 ppm.
8. Method according to at least one of the preceding claims,
   wherein
   after the hematite present in the oxidative decontamination solution after fixation of the dichromate in the anion exchanger is dissolved by addition of a carboxylic or dicarboxylic acid, especially oxalic acid, and the dissolved Fe cations are bound in a cation exchanger.
9. Method according to at least one of the preceding claims,
   wherein
   the oxalic acid concentration should be set at a value of between 50 ppm and 1000 ppm, especially a maximum of 100 ppm.
10. Method according to at least one of the preceding claims,
    wherein
    oxalic acid remaining in the oxidative decontamination solution after complete removal of the Fe ions is decomposed with permanganic acid, forming CO₂ and Mn²⁺, and the Mn²⁺ ions are fixed on the cation exchanger.
11. Method according to at least one of the preceding claims,
    wherein
    the permanganic acid concentration is set such that an oxide layer with a thickness of between 0.3 µm and 0.6 µm is removed until the permanganic acid is completely consumed wherein preferably the thickness of the oxide layer to be removed is governed by the quantity of permanganic acid used.
12. Method according to at least one of the preceding claims,
    wherein
    the oxidative decontamination step (D1, D2, D3) is performed at a temperature between 60°C and 120°C, particularly preferably between 95°C and 105°C.
13. Method according to at least one of the preceding claims,
    wherein
    the removal of the hematite is performed at a temperature between 60°C and 120°C, particularly preferably between 95°C and 105°C.
14. Method according to at least one of the preceding claims,
    wherein
    the sulfuric acid in the oxidative decontamination step is regenerated by withdrawing the Mn-II- /Fe-II- /Fe-III- /Ni-II ions using a cation exchanger.
15. Method according to at least one of the preceding claims,
    wherein
    the dichromic acid forming during the degradation of the oxide layer is actively involved in the decontamination process.
16. Method according to at least claim 1,
    wherein
    the oxide layer formed on the inner surfaces of a coolant circuit of a nuclear power plant or the components thereof is oxidized in layers and dissolved by circulating permanganic acid and sulfuric acid (D1), that after complete consumption of the permanganic acid, with the circulation continuing, the oxidative decontamination is carried over a bypass through a cation exchanger for binding the divalent Fe, Ni, Zn and Mn cations present in the solution (D2), that then permanganic acid is again added to the oxidative decontamination solution, especially to re-establish the initial concentration, that previous process steps (D 1, D2) are repeated to an extent until a preset dichromic acid value is present in the oxidative decontamination solution, that then, while continuing the circulation, the solution is sent over a bypass line to an anion exchanger for binding the dichromate (D3), that previous process steps (D1, D2, D3) are repeated to an extent until a preset thickness of the oxide layer has been removed, that then in a subsequent process step (D4), by adding a carboxylic or dicarboxylic acid such as oxalic acid, the circulating sulfuric acid solution containing carboxylic or dicarboxylic acid such as oxalic acid is conveyed over a bypass through a cation exchanger in which iron ions are bound with simultaneous liberation of carbonate or dicarbonate or oxalate and sulfate ions.
17. Method according to claim 16,
    wherein
    oxalic acid is used as the dicarboxylic acid and after complete removal of the iron ions, the oxalic acid is oxidized to carbon dioxide with permanganate, and the Mn^2+- ions formed are fixed on cation exchangers.
18. Method according to at least claim 1 or claim 16,
    wherein
    at the beginning of degradation of the oxide layer, the pH is adjusted with sulfuric acid and no more sulfuric acid is added during the degradation of the oxide layer and performance of the further process steps wherein preferably the pH is adjusted with sulfuric acid to a value < 2.2, especially ≤ 2.0.

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