Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR COMPLEX DECONTAMINATION OF METAL SURFACES
WITH DEEP AND SURFACE CONTAMINATION AND OF LIQUID
RADIOACTIVE WASTES
The invention refers to electrochemical decontamination of contaminated metal
surfaces,
for example, components of nuclear power plants, and is based on contacting
the surface being
decontaminated with a special electrolytic solution. Decontamination is
carried out with the help
of a system of electrodes, which is followed by treatment of the used
solution.
In the process of operation of nuclear power plants, as well as plants for
mining and
processing of rare earth elements, deep and surface contamination of
equipment, contacting
radioactive substances, takes place. In the case of complicated deep
contamination chemical
methods for decontamination by means of aggressive media such as those
disclosed in Russian
Patent No. 1,830,149, c1.G21 F9/00, 1993 or Russian Patent No. 2,017,244, cl.
G21 F9/36, 1994
are very expensive and ecologically hazardous for personnel. Electropolishing
method such as
those disclosed in Russian Patent No. 2,009,557, c1.G21 F9/34, 1994 or German
Patent No.
3,343,396, c1.G21 F9/30, 1985 used for such purposes, consist in removal of a
rather thick layer
from the metal surface, thickness of which depends on the depth of
radionuclides diffusion layer.
This results in considerable consumption of power and change of dimensions of
objects being
decontaminated.
The objective ofthe invention is to provide a method and compound for
decontamination
of radioactive contaminated metal objects by means of electrochemical
destruction of oxide films
impeding diffusion of radionuclides to metal surface. After the destruction of
oxide films the
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positive potential, applied to the object being decontaminated, promotes
movement of
radionuclides to the surface. Deep contamination presents in fact a more
complex case of surface
decontamination. In fact, destruction and removal of oxide films transforms
deep contamination
into surface contamination. (see. A. D. Zimon, U.K. Pikalov, Decontamination,
Izdat (Publishing
House), Moscow, 1994 p. 40,96.)
Humus substances have been found to be effective means for decontamination of
metals
with contaminated surfaces and they are added to the electrolytic compound for
intensification
of the decontamination process.
Anions of humus substances, added to electrolyte composition, interact with
canons of
radionuclides situated on the surface being decontamination. Electrochemical
processes lead al so
to changes in chemical structure of humus substances and result in formation
of fractions with
still higher decontaminating effect.
In the method of the present invention, a solution of liquid radioactive
wastes is obtained
electrolytically in decontamination of metal objects. A resulting insoluble
residue consists of
humic acids and positive radionuclides (for example, such as Cs, Sr, Co, Mn,
Fe, Ni, Ce). Thus,
in this case, humic acid plays the role of a flocculent. For intensification
of decomposition and
reduction of content of radionuclides down to a required level, the
introduction of a humus
substances suspension may be repeated with the use of special additives. For
example, for
removal of radioactive cesium it is necessary to add to the solution a
microquantity of potassium
ferrocyanide which, being adsorbed on colloidal particles of humus substances,
additionally
bonds cesium ions by transforming them into an insoluble state. In case the
proposed method,
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it is not necessary to prepare adsorbents in advance, as their formation and
the absorption of
radionuclides in the solution proceed simultaneously. Iron salts may be used
as flocculants for
treatment of liquid wastes for final removal of multivalent radionuclides from
liquid wastes. In
case of decontamination of iron containing objects (steel, pig iron) by the
proposed method the
solution is saturated by iron ions in the process of decontamination at the
expense of dissolution
of oxide films and partially surface itself, which considerably decreases the
amount of reagent
used.
The residue of organic substances containing radionuclides formed according to
the
present invention is preferably ashed with the aim of reducing the amount of
solid radioactive
wastes isolated from the solution. Decontaminated aqueous solution may be used
repeatedly.
The invention is explained by the following examples:
Example 1
Part of deeply contaminated 10 X 10 cm steel plate, which had been in a
durable contact
with uranium concentrate and was unamenable to decontamination by submersion
into regular
decontaminating solutions based on surfactant and organic acids, was subject
to decontamination
by the proposed method. The object being tested served as anode while a
titanium plate was used
as cathode, and alkaline suspension of humus substances with sodium sulfate
additives was used
as an electrolyte. After two hours off decontamination ~3-activity reduced 10
times, and y-
activity reduced from 1000 ,uR/h to the background after three hours.
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Example 2
Samples of steam generator pipe and of pipeline of autonomous pump circuit
made of
1 X 18H9T and 08X 18H 1 OT (Russian standard) steel, correspondingly, were
decontamination by
the proposed method. During 20 years of operation they were in contact with a
heat carrier of
the primary circuit which had the following parameters: temperature
:260°C, specific activity:
106 Bq/1 and pH approximately 6.5, which promoted the formation of a solid
oxide film with
radionuclides sorbed in it.
Decontamination results are given in Table 1.
'table 1
Results of gamma-spectrometric measurement
Name of Sample Initial Specific Specific ActivityDecontamination
Gravity after
activity for Co-60,decontamination, Factor
Bq/cmz
Bq/cmz
Steam Generator 8.1 E + S 1. I E + 2 733
Pipe
Pipe of autonomous4.4E + 5 1.6E + 2 272
circuit
Ferrous metal pipes with the level of contamination about 400 ~cR/h were
decontaminated
by the method described in Example 1. y-activity reduced to background after
10 minutes of
decontamination.
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Part of a lead screen with the level of contamination about 150 ,uR/h, which
had been
used for biological protection at nuclear power plant, was decontaminated by
the above-described
method. Radioactivity was reduced down to the background after 5 minutes.
A working piece of the primary circuit of nuclear reactor (a return valve),
which had (3-
radiation activity of 3000 particles/cmz min, was subject to decontamination
by the above-
mentioned method. After two and a half hours of decontamination, the
decontamination factor
made up approximately 400.
Decontamination of liquid radioactive wastes, obtained by the above-mentioned
method,
was carried out in the following way. Solution containing up to 0.1 g of
potassium ferrocyanide
was added to 1.0 litre of the solution being decontaminated, then the mixture
was stirred and, by
means of introduction of a mineral acid, solution pH was reduced down to
coagulation of humus
acids. The formed complex residue, mainly containing radioactive cesium and
other
radionuclides, was separated and iron sulphate solution was added to acid
filtrate. Then, by
means of adding alkali, pH was increased up to formation of flocculent, thus
multivalent
radionuclides were finally removed from the solution.
Liquid radioactive wastes, not containing humus substances, e.g., trap water
or liquid
radioactive wastes from special storage, may be also utilized by the proposed
method by means
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of their alkalization (if necessary) and introduction of humus substances with
subsequent
utilization according to the scheme given in Example 6. In this case, liquid
radioactive wastes
from a special storage were decontaminated by the given method.
Results obtained in Examples 6 and 7 are given in Table 2.
Table 2
Results of decontamination of Liquid Radioactive Wastes
SAMPLE Contents
of Radionuclides,
Bq/1
A total Cs-137 Co-60 Sr-90
Initial liquid radioactive 1.3E+4 9.3E+3 2.OE+3 6.OE+2
wastes (Example 6) 4.8E+2 7.4 3.3E+2 9.3
After decontamination
Initial liquid radioactive 9.3E+4 4.8E+4 l .0E+3 2.2E+4
wastes (Example 7) 5.2E+2 1.3E+2 56 9.3
After decontamination
Permissible concentration 5.6E+2 2.OE+3 14.8
of radionuclides
in potable water
Note: contamination levels for Mn-54 and Ru-106 are not given as in initial
liquid radioactive
wastes they were found to be within the permissible concentration of
radionuclides in potable
water.
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