Nuclear Surface Decontamination via pH Cycling
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Solution Overview
Problem
The existing surface decontamination methods for nuclear power plant components, such as those in a pressurized water reactor, are inefficient in reducing radioactivity due to the accumulation of activated nuclides in oxide layers, which requires multiple treatment cycles and limited solubility of chromium III oxides under acidic conditions.
Innovation Solution
A method involving alternating pH changes between acidic and alkaline solutions during decontamination cycles, using specific oxidizing agents like ozone, permanganic acid, and potassium permanganate, significantly enhances the decontamination factor by oxidizing and reducing chromium III to VI and back, facilitating the removal of metal ions and radioactive isotopes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple treatment cycles are used to achieve sufficient decontamination, then the decontamination factor increases, but the time and resources required increase significantly
Solution Approach 1:
The patent applies parameter changes by systematically varying pH values (from acidic to alkaline and vice versa) and oxidation potentials across treatment cycles. This transforms the chemical environment to alternately dissolve different oxide components, achieving higher decontamination factors more efficiently than conventional single-condition treatments.
Solution Approach 2:
The patent implements periodic action through alternating oxidation and reduction steps in sequential treatment cycles. Each cycle reverses the chemical conditions (oxidizing then reducing), creating periodic chemical environments that progressively remove different oxide layers, thereby increasing the decontamination factor while managing treatment time.
2Reliability
If chromium III oxides are treated under acidic conditions, then the decontamination process can proceed, but the solubility of chromium III oxides is limited reducing effectiveness
Solution Approach 1:
The patent overcomes the solubility limitation of chromium III oxides by changing the pH parameter from acidic to alkaline conditions. Under alkaline conditions, chromium III oxides exhibit significantly enhanced solubility, allowing effective removal of chromium-containing oxide layers that cannot be adequately dissolved under acidic conditions alone.
Solution Approach 2:
The patent applies inversion by reversing the conventional approach: instead of attempting to dissolve all oxide components under acidic conditions, it alternates between acidic and alkaline environments. This inversion of chemical conditions allows each pH regime to target specific oxide components optimally, including chromium III oxides under alkaline conditions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in a substantial increase in the decontamination factor, achieving higher radioactivity reduction with fewer cycles by exploiting pH changes, particularly at extreme pH values, effectively managing oxidizing agents and their residues.
Implementation Method 1
the oxide layer is first treated by oxidation (oxidation step)... The oxidizing agent used in the previous oxidation step is therefore removed or neutralized
Implementation Method 2
it is also possible to reduce or neutralize an oxidizing agent such as HMnO 4 with the aid of a reducing agent added to the decontamination acid, for example ascorbic acid, citric acid or hydrogen peroxide. As a result, the Cr-VI formed in the oxidation step is also reduced back to Cr-III.
Implementation Method 3
The decontamination step that follows the oxidation is used to dissolve the previously oxidatively treated oxide layer with the aid of one or mixtures of complex-forming organic acids.
Implementation Method 4
These metal ions can be removed from the cleaning solution with an ion exchanger.
Data Source
Figure 1
AI summary
The invention relates to a method for chemical decontamination of an oxide-coated surface of a metal structural part or of a system in a nuclear power plant with several cleaning cycles, each involving an oxidation step, in which the oxide layer is treated with an aqueous solution containing an oxidation agent, and a subsequent decontamination step, in which the oxide layer is treated with an aqueous solution of an acid. At least one oxidation step is carried out in an acid solution and at least one oxidation step in an alkaline solution.