Dissolution and decontamination process

Details Dissolution and decontamination process Dissolution and decontamination process

2002-01-22

2004-04-06

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Platinum group metal

C376S308000, C976SDIG003, C976SDIG003, C976SDIG003

Reexamination Certificate

active

06716402

ABSTRACT:

TECHNICAL FIELD
The present invention concerns a process for dissolving ruthenium deposits that are present on a surface and a process for decontaminating the internal circuits of nuclear fuel reprocessing plants using the said dissolution process.
The present invention therefore concerns, in particular, the decontamination of internal circuits in irradiated nuclear fuel reprocessing plants. The process according to the present invention makes it possible to remove deposits composed, in part, of ruthenium from the pipes and other equipment in such plants.
Radioactive ruthenium, in other words the isotopes
106
Ru and
103
Ru, or stable ruthenium, make up part of the fission products in nuclear fuels after irradiation in a reactor. In a general manner, in fuel irradiated at 34,000 megawatts/day, the level of ruthenium is around 0.2% of the initial weight of uranium.
During the various reprocessing stages for irradiated fuels, most of this ruthenium is found in the circuits and equipment of the reprocessing plants in various chemical forms. Some species are soluble in the medium generally used to attack the fuel, in other words nitric acid. These species are, for example, complexes of nitrated nitro nitrosyl ruthenium RuNO (NO
3
)
3
(H
2
O)
2
.
Other species are particularly insoluble in this medium, such as the oxides of ruthenium RuO
2
and RuO
2
. x (H
2
O)
2<x<3
or even metallic ruthenium. The fraction of insoluble ruthenium generally represents between 25 and 90%, depending on the type of nuclear fuel.
Depending on their origin and chemical nature, these insoluble species are found in various places in the reprocessing plant such as in the dissolution circuits at the head of the process, at the point of the first extraction cycle or the concentration and storage circuits for the fission products.
In terms of cleaning up reprocessing plants, the treatment of ruthenium is important for two main reasons:
due to its specific activity, deposited active ruthenium accounts for a significant part of the residual activity and removing this ruthenium constitutes a favoured method of reducing the radioactive dose rate of installations before they are dismantled.
when the ruthenium precipitates, it entraps radioactive elements such as U or Pu, for example. These irradiating deposits cover the metal walls and, as a result, limit the efficiency of the sequences used for cleaning the metal walls, which represent the ultimate phases of clean up.
The elimination of these various types of ruthenium deposit is therefore a vital step in the clean up of irradiated fuel reprocessing plants.
STATE OF THE PRIOR ART
Patent application EP-A-O 247 933 describes a decontamination process for solid materials that have been contaminated with radioactive elements such as ruthenium. In this process, the part that needs decontaminated is firstly brought into contact with a decontaminating solution of sodium hydroxide at a concentration of 8 mol.l
−1
, and K
2
S
2
O
8
at a concentration of 25 g.l
−1
, then rinsed with a sulphuric acid solution, at a concentration of 0.5 mol.l
−1
.
This method is efficient but has two major disadvantages:
it uses a very strong sodium hydroxide solution, containing around 184 g.l
−1
of sodium, which leads to a high number of glass vessels being required to treat the effluent that is generated, and
it uses sulphates, which is a disadvantage in operational terms and can cause problems of corrosion in acid media in certain grades of steel such as austenitic steel.
Other oxidisers in alkaline media have been considered, such as Mn
VII
in the form of KMnO
4
, but the precipitation phenomena of reduced Mn
II
leads to part of the Ru being entrapped in the radioactive sludge, which is difficult to treat.
Oxidation processes in acidic media have the disadvantage of leading to high volatilisation of the ruthenium in the form of ruthenium tetra-oxide RuO
4
, which leads to aerial contamination of installations by reductive polymerisation in the form of (RuO
2
)x. (RuO
4
)y. zH
2
O. This phenomenon is also encountered in the treatment of effluents containing ruthenium by evaporation in nitric media or the dissolution of used fuels. Moreover, oxidising processes in acidic media have the disadvantage of being more sensitive in terms of the corrosion resistance of installations made out of austenitic steel with a quite narrow passivation range.
Document JP-A-63 243 232 describes a process for decontaminating parts that have been contaminated with Ru by bringing them into contact with a flow of ozone gas, forming volatile RuO
4
, trapping the RuO
4
on a specific absorbent, and reducing the ruthenium into the form of Ru metal. The use of ozone as the oxidising agent allows the ruthenium to be maintained in a volatile form, in other words, there are no polymerisation phenomena on the aerial metal parts up to the recovery reactor.
This process cannot be used for cleaning up reprocessing plants for several reasons:
the decontamination is carried out at high temperature. However, it is not possible to heat, without high cost, all of the circuits that need to be decontaminated.
this process means that a trapping reactor has to be created, which constitutes an awkward modification to the installation.
this process means that a Ru recovery unit has to be created for every part involved in the clean up operation.
The oxidation of ruthenium by ozone in liquid phase is, moreover, used for the purification and the recovery of ruthenium from catalysts and electrodes.
Document U.S. Pat. No. 5,248,496 describes a method for volatilising ruthenium in the form of RuO
4
through the successive formation of ruthenate, perruthenate then tetra-oxide at a pH close to 8 and a temperature between 80 and 100° C.
Document JP-A-10 273 327 describes a method for synthesising RuO
4
by the reaction of rutheniated species, Ru metal, tri-halide, sulphate, oxide or in the form of soluble complexes, with ozone in water with a pH less than or equal to 7 and at a temperature of, ideally, between 5 and 50° C.
In both processes, going through a gaseous form allows the ruthenium to be purified so that it can be re-used. Maximum volatilisation conditions are therefore an objective.
This is not an objective in the case of cleaning up reprocessing plants. In fact, in these plants, it is necessary to solubilise the ruthenium in a decontaminating solution before sending the said solution to the outlets for vitrification.
The precise objective of the present invention is to provide a process that meets this objective and which does not have the disadvantages mentioned above.
DESCRIPTION OF THE INVENTION
In order to attain this objective, the present invention provides a process for dissolving ruthenium deposits that are present on a surface, comprising bringing the said surface into contact with an aqueous solution of perruthenate, with the said aqueous solution having a pH equal to or greater than 12.
The ruthenium deposits are, for example, of ruthenium metal or oxides of ruthenium such as the oxides of ruthenium RuO
2
and RuO
2
. x (H
2
O)
2<x<3.
The reaction involved can be expressed schematically as follows:
Ru
solid
+RuO
4
−
aqueous
→2 RuO
4
2−
aqueous
with Ru
solid
being the ruthenium making up the deposit that has to be dissolved, RuO
4
−
aqueous
the perruthenate of the basic aqueous solution used to dissolve the deposits of ruthenium according to the present invention, and RuO
4
2−
aqueous
the ruthenium that has been dissolved in the aqueous solution in its ruthenate form.
The perruthenate is the oxidant of the perruthenate/ruthenate couple:
RuO
4
2−
/RuO
4
2−
E
0
=0.593V/ENH at pH=14
In the process according to the present invention, this redox couple presents the following advantages:
it has quite a high oxidation potential for oxidising the ruthenium deposits targeted in the process according to the present invention.
it is soluble in its oxidised form and in its reduced form, and this avoids precipitation phenomena

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