Induced nuclear reactions: processes – systems – and elements – Fission reactor material treatment – Impurity removal
Patent
1989-07-07
1992-03-03
Walsh, Donald P.
Induced nuclear reactions: processes, systems, and elements
Fission reactor material treatment
Impurity removal
376306, 376309, 376305, 376313, G21C 1942
Patent
active
050930737
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the decontamination of surfaces, in particular on components of cooling circuits of nuclear reactors, by treatment of the radioactively contaminated surface layers with an aqueous, acid-containing decontamination solution.
In the cooling circuits of nuclear reactors, layers in which radioactive contaminants such as, for example, activated corrosion products, and also fission products, are incorporated are formed on the surfaces of the cooling circuit components. With increasing age of the nuclear power stations, this leads to an increase in the activity, the proportion of longer-lived nucleides rising in particular. With increasing age of the nuclear power stations, however, maintenance work and repairs must also be carried out more frequently and modifications must be made, so that the radiation exposure of the personnel increases. In order to facilitate work on radioactively contaminated plants or even to make it possible, decontaminations are necessary. The contaminated surface layers must then be removed as completely as possible, but the base materials of the cooling circuit components must be protected.
The composition of the surface layers does not have to be the same as that of the materials of the cooling circuit components. Physical conditions and water chemistry determine the corrosion of the materials and the transport and deposition of the resulting corrosion products and hence the composition and structure of the surface layers. For example under the conditions of a pressurized water reactor (PWR), oxide layers of high chromium content with spinel-type mixed oxides, which dissolve only extremely slowly in acids, form at a temperature of about 570 K in cooling water containing boric acid and lithium hydroxide.
All known processes for the decontamination of the surfaces of components of pressurized water reactors therefore comprise two or more treatment steps, the insoluble Cr(III) oxide being converted in a first step in an oxidizing phase into soluble 6-valent chromium, and the entire oxide layer being loosened at the same time. In a second treatment step, in most cases after intermediate rinsing, the loosened oxide layer is then dissolved in an acidic, reducing and complex-forming solution and removed.
For the first treatment step, that is to say the oxidative treatment step, a number of processes are usual, such as, for example, the so-called "AP" processes which consist of a treatment with alkaline permanganate solution, or the "NP" processes in which nitric acid solutions are used for the oxidation. Further known processes envisage the use of permanganic acid, hydrogen peroxide, cerium(IV) salts or other oxidizing agents. The current state of the art is extensively described, for example, in the following two publications: Maintenance, Modification or Plant Decommissioning", Technical Reports Series No. 249, International Atomic Energy Agency, Vienna 1985; "Dekontamination - Stand der Technik und aktuelle Entwicklungsziele [Decontamination State of the Art and Current Development--Targets]", VGB Kraftwerkstechnik 66 (1986) 579-588.
All the known processes have the common feature that they must be employed at relatively high temperatures, in most cases between 350 K. and 400 K. This involves various serious disadvantages, such as the necessity of relatively expensive and complicated auxiliary equipment, an increase in corrosivity, pressure build-up due to steam at treatment temperatures above 370 K., and others.
Attempts have therefore already been made on various occasions to develop oxidation treatments which work satisfactorily at lower temperatures, preferably at usual room temperature. As an example, a Swedish process may be mentioned here, in which the oxidation is carried out by means of ozone-containing nitric acid. This process has, however, the disadvantage that control of a process with a gas-containing liquid as the reagent is difficult and that ozone is not easy to handle and, in addition, is toxic and moreover can lead to explosions.
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Decontamination of Pressurized Water Reactors. PCT WO84/03170.
ABB Reaktor GmbH
Chelliah Meena
Greenberg Laurence A.
Lerner Herbert L.
Paul Scherrer Institut
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