Induced nuclear reactions: processes – systems – and elements – Fission reactor material treatment – Impurity removal
Patent
1990-09-13
1992-03-03
Walsh, Donald P.
Induced nuclear reactions: processes, systems, and elements
Fission reactor material treatment
Impurity removal
376305, 376309, G21C 1942
Patent
active
050930729
DESCRIPTION:
BRIEF SUMMARY
The present invention relates in general terms to the field of in particular pressurized, water-cooled nuclear reactors.
During the prolonged operation of such reactors, the primary circuit carrying the hot water leaving the core and which is usually largely made from stainless steels (such as e.g. steel 304 L, steel 316 L and/or Inconel 600) becomes internally covered with a layer of metal oxides containing radioactive elements which must be removed as a result of the radioactivity contained therein and which can be prejudicial to personnel during maintenance operations carried out with the reactor shutdown and also during the dismantling thereof.
The elimination or reduction of such a metal oxide layer containing such radioactive elements constitutes the operation called the decontamination of the surface and the essential object of the present invention is to provide a more efficient decontamination process than those of the prior art.
A very small amount of the metallic elements constituting the alloys of the primary circuit of a pressurized water reactor is in solution or suspension in the water of said circuit in the form of oxides (corrosion products). The repeated passage of these elements in the neutron flux of the reactor core activates them.
As a result of the solubilization-precipitation process due to the temperature gradient existing in the primary circuit between its hottest point at approximately 350.degree. C. and its coldest point on reactor shutdown and which can drop to 40.degree. or 50.degree. C., a deposit of corrosion products is formed on the walls of the pipes.
These deposits contain activated elements such as manganese 54, cobalt 58 and cobalt 60 (.gamma. radiation emitters), which can be largely responsible for the irradiation of personnel during reactor maintenance work and particularly work carried out on its primary circuit. It is therefore important to be able to eliminate or reduce this background activity by eliminating the oxide layer formed.
In the prior art the internal surface of a primary circuit of a water reactor is decontaminated by a chemical process known as the Citrox process, which involves an oxidizing pretreatment using potassium permanganate and soda, followed by a reducing phase with oxalic and citric acid at pH 3 to 4 and as is described more particularly in the British document "Review of dilute chemical decontamination processes for water cooled in nuclear reactors" NP 1033, Research Project 828-1, Mar. 1979.
However, although this known process is effective from the decontamination standpoint, it can lead to intergranular corrosion in certain sensitive areas of the treated surface which can lead to local, but relatively serious corrosion particularly in the case of structures which have to be exposed to high temperatures and pressures (e.g. in REP: T 350.degree. C., P 160 bars).
The present invention specifically relates to a process for the radioactive decontamination of metal surfaces, which is as simple and effective as the aforementioned prior art process, but which has the additional advantage of having no corrosive action on the surfaces to be decontaminated.
This decontamination process is mainly characterized in that it consists of subjecting said surfaces to the successive stages of an oxidizing pretreatment with the aid of a solution of potassium permanganate KMnO.sub.4 and nitric acid, rinsing with demineralized water and a reducing treatment in a basic medium with the aid of a solution of alkali metal gluconate of formula HOH.sub.2 C--CHOH--CHOH--CHOH--CHOH--COOM, in which M is an alkali metal chosen from among Na and K, as well as soda NaOH.
The interest of this process is essentially based on the reducing treatment phase in the basic medium with the aid of an alkali metal gluconate solution. Thus, whereas the prior art reducing treatment was carried out in a highly acid medium (pH 3 to 4), through working in accordance with the Inventive process in a basic medium with a gluconate acid salt, it is possible to utilize to the maximum the chela
REFERENCES:
patent: 4476047 (1984-10-01), Bonnici et al.
patent: 4587043 (1986-05-01), Murray et al.
patent: 4686067 (1987-08-01), Veysset et al.
Chabrand Christine
Gauchon Jean P.
Guy Brunel
Chelliah Meena
Commissariat a l''Energie Atomique
Walsh Donald P.
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