Electrolytic decontamination process and process for reproducing decontaminating electrolyte by electrodeposition and apparatuses therefore

This disclosure relates to electrolytic decontamination of radioactively contaminated objects such as equipment or parts. The objects to be decontaminated are divided into two types: First, wastes resulting from dismantlement of radioactively contaminated equipment and parts, and second, equipment, vessels, pipes and tools that are to be reused. The electrolyte used for decontamination of the first type may be an inorganic acid aqueous solution of relatively low concentration that is inexpensive and rapid in polishing. A suitable inorganic acid is sulfuric acid that does not generate harmful gases in the process of electrolysis. The concentration of the sulfuric acid should be high to achieve polishing efficiency. About 5 Vol. % is the most suitable for uniform polishing and disposal of waste electrolyte. An electrolyte of this concentration is effective in macroscopic polishing but not in microscopic polishing (mirror finish), however. Therefore, an electrolyte for decontamination of the second type that requires microscopic polishing must be a high concentration acid solution, preferably 70% or higher phosphoric acid content. The electrolyte is reproduced by an electrodeposition process in diaphragm electrolysis.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for radioactive decontamination 
of metal by electrolytic polishing of the metal surface of radioactively 
contaminated equipment or parts used, for example, in nuclear plants or 
other facilities handling radioactive substances. It also relates to a 
process for recovering, by electrodeposition capture, radioactive metal 
ions in the form of solid metal, which ions dissolve in an electrolyte 
during the process of the electrolytic decontamination, and reproducing 
decontaminating electrolyte having the initial concentration. 
Equipment, parts and piping used in nuclear plants are frequently 
contaminated by diffusion and deposition of radioactive conjugated oxides 
(which may be called "CRUD") and other radioactive substances as the 
plants are operated. 
Radioactively contaminated equipment can be decontaminated by blasting the 
equipment with ice or dry ice, a high pressure jet of water, ultrasonic 
cleaning, chemical polishing or electrolytic polishing. The electrolytic 
polishing is the most advantageous method in respect to decontamination 
and prevention of recontamination, but it presents some problems in the 
disposal of waste electrolyte. 
The major portion of the radioactive substances that contaminate metals is 
contained in the CRUD. The CRUD is composed of radioactive conjugated 
oxides which are hard to dissolve in an electrolyte. In an electrolytic 
polishing process, it is possible to separate the CRUD by allowing a DC 
current to flow between one or more cathodes and an anode formed by the 
contaminated metal part, the anode and the cathode being dipped in an 
electrolyte, so that a very thin surface layer of the metal part under the 
CRUD dissolves in the electrolyte. In the course of the electrolysis, 
metallic ions are eluted from the surface of the metal part, oxygen 
bubbles are generated and the electrolyte permiates into the CRUD, so that 
the CRUD is loosened from the metal part and dispersed in the electrolyte 
as suspended substances. Various radioactive substances dissolve and 
accumulate in the electrolyte during such electrolytic decontamination. 
Among them, metal oxides separated from the contaminated part and 
suspended in the electrolyte can be relatively easily taken out of the 
electrolyte in a recycling system by employing a solid-liquid separation 
method such as filtration of the electrolyte or separation by 
sedimentation. 
On the other hand, radioactive substance eluted from the contaminated part 
and existing in the form of metallic ions in the electrolyte cannot be 
removed by the solid-liquid separation methods mentioned above and 
therefore they gradually accumulate in the electrolyte, thus increasing 
the radiation level of the electrolyte. If electrolytic decontamination is 
continued using an electrolyte in this state, workers are possibly exposed 
to the radiation and the service life of the electrolyte comes to an end 
because the electrolytic polishing efficiency is reduced as the 
concentration of metallic ions dissolved in the electrolyte is increased. 
A dilute aqueous solution of a strong acid such as diluted surfuric acid 
may be used as an electrolyte for electrolytic polishing decontamination 
as described. This solution effects rapid polishing and it is easily 
disposed of after use. The surface polished using this solution is, 
however, rough and consequently easily contaminated again. Therefore, use 
of this solution is limited only to contaminated parts that are to be 
disposed of rather than reused. Of the high concentration acid solutions 
generally used for electrolytic polishing high concentration sulfuric 
acids yield a reduced glossy polished surface, but high concentration 
phosphoric acids and high concentration phosphoric acids-sulfuric acids 
yield a more glossy surface. Therefore, they are quite effective in 
preventing recontamination of equipment desired to be reused, though there 
has been a problem in disposal of the waste electrolyte. Specifically, 
various methods have been presented for isolating metallic ions 
accumulated in high concentration in the electrolyte during 
decontamination, although it has been considered difficult to isolate 
concentrated metallic ions dissolved in the electrolyte when a high 
concentration acid solution is used as the electrolyte. 
One of the known isolation methods is to separate metallic ions by allowing 
them to deposit on a cathode capture electrode in an electrolytic cell 
provided with a partitioning diaphragm such as an unglazed plate or an ion 
exchange membrane. In this case, the reaction that generates hydrogen gas 
at the capture cathode electrodes takes place prior to the reaction for 
metal deposition on the capture electrode in the cathode chamber 
partitioned by the diaphragm, and therefore it is necessary to lower the 
hydrogen ion concentration to such an extent as to permit metal 
deposition. In an electrolyte of high concentration acid solution, 
however, acid is diffused into the cathode chamber due to large gradient 
of concentration between the anode and the cathode chambers partitioned by 
the diaphragm. As a result, it is not possible to lower the hydrogen ion 
concentration to such an extent as to permit metal deposition, and the 
diaphragm cannot have an expected effect. 
Because of the above reasons, a high concentration acid solution used as a 
decontaminating electrolyte is conventionally solidified in plastic or 
cement for disposal, when the concentration of metallic ions dissolved in 
the electrolyte or the radiation level of the electrolyte increases to a 
certain value. Such disposal of the waste electrolyte presents another 
problem from the increasing quantity of waste which causes secondary 
contamination. 
It is a primary object of this invention to present various methods of 
isolating metallic ions dissolved in an electrolyte during the process of 
electrolytic decontamination, in the state of the highest possible 
concentration, and of reproducing the electrolyte so as to minimize the 
volume of the secondary waste. 
BRIEF SUMMARY OF THE INVENTION 
In electrolytic decontamination of radioactively contaminated equipment or 
parts, various methods well known for general electrolytic polishing can 
be employed. In electrolytic decontamination, the volume of the secondary 
waste can be reduced by selecting a suitable electrolyte in accordance 
with the objects to be decontaminated. 
The objects to be decontaminated are divided into two types: First, wastes 
resulting from dismantlement of radioactively contaminated equipment and 
parts, and second, equipment, vessels, pipes and tools that are to be 
reused. 
The electrolyte used for decontamination of the first type may be an 
inorganic acid aqueous solution of relatively low concentration that is 
inexpensive and rapid in polishing. A suitable inorganic acid is sulfuric 
acid that does not generate harmful gases in the process of electrolysis. 
The concentration of the sulfuric acid should be high to achieve polishing 
efficiency. About 5 Vol. % is the most suitable for uniform polishing and 
disposal of waste electrolyte. An electrolyte of this concentration is 
effective in macroscopic polishing but not in microscopic polishing 
(mirror finish), however. Therefore, an electrolyte for decontamination of 
the second type that requires microscopic polishing must be a high 
concentration acid solution, preferably 70% or higher phosphoric acid 
content. According to this invention, the electrolyte is reproduced by an 
electrodeposition process in diaphragm electrolysis.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 shows an example of a system in accordance with this invention for 
reproducing an electrolyte from an electrolytic decontamination using 
inorganic acid aqueous solution of a relatively low concentration. Unlike 
a system for the electrolytic decontamination using a high concentration 
phosphoric acid-sulfuric acid electrolyte that is generally used for 
electrolytic polishing, in the electrolytic decontamination using an 
inorganic acid aqueous solution of relatively low concentration, metallic 
ions dissolved in the electrolyte are easily deposited on a capture 
electrode in the form of solid metal, and therefore are isolated in the 
state of the highest possible concentration. This is advantageous in that 
waste electrolyte, the secondary waste to be disposed of after isolating 
metallic ions, is in a small amount. 
In this example, however, the hydrogen ion concentration is still high 
during the process of electrolytic decontamination. The reaction which 
generates hydrogen gas takes place prior to the reaction for metallic 
deposition on the capture electrode, but when an electrolyte is diluted to 
the hydrogen ion concentration to permit metallic deposition, polishing 
efficiency substantially reduces, making electrolytic decontamination 
impossible. Accordingly, when metallic ions in the electrolyte have 
increased to a certain level, it is necessary to transfer the electrolyte 
to another cell where metallic ions are isolated by deposition and as the 
pH is adjusted by injecting an alkali. Another method . available for 
isolating metallic ions by deposition is to install an unglazed plate or a 
similar porous electrolytic diaphragm between the anode and cathode in the 
electrolytic cell so as to effect a diaphragm electrolysis. 
In the deposition and isolation of metallic ions by diaphragm electrolysis, 
the hydrogen ion concentration in the electrolyte drops due to the 
generation of hydrogen gas in the cathode chamber, and metallic ions in 
the electrolyte deposit on the capture electrode. Therefore, the 
electrolYtic cell is partitioned by an electrolytic diaphragm, and the 
capture electrode is provided in the cathode chamber. Otherwise, a capture 
electrode surrounded by an electrolytic diaphragm formed by a unglazed 
cylinder is installed in the electrolytic cell. When DC current is passed 
between the contaminated object, which is the anode, and the capture 
cathode electrode, metallic ions dissolved in the electrolyte can be 
isolated by deposition simultaneously with electrolytic decontamination of 
the cohtaminated object. This helps to prevent the concentration of the 
metallic ions dissolved in the electrolyte from increasing, thus extending 
the service life of the electrolyte. 
In such a case, however, the reaction for the elusion of metallic ions 
takes place prior to the reaction for generating oxygen gas on the surface 
of the object to be decontaminated in the anode chamber. As a result, the 
number of hydrogen ions as cations decreases so that acid ions are kept 
electrically neutral with respect to metallic ions. Therefore, the acid 
concentration drops, deteriorating the electrolyte. Meanwhile, hydrogen 
gas continues to be generated in the cathode chamber. When the hydrogen 
ion concentration reduces to the extent that metal hydroxide begins to be 
produced, the metallic ions cannot be isolated by deposition. This means 
that the useful or service life of the electrolyte is at an end. 
In the system shown in FIG. 1, in addition to the decontaminating 
electrolytic cell, a metallic ion isolating cell, that is divided into an 
anode chamber and a cathode chamber, is installed for circulation of the 
electrolyte in the anode chamber during the process of decontamination. In 
the metallic ion isolating cell, a DC current is allowed to flow between 
the insoluble electrode in the anode chamber and the capture electrode in 
the cathode chamber as the pH of the electrolyte in the cathode chamber is 
controlled by pouring the electrolyte from the anode chamber into the 
cathode chamber. Thus, the electrolyte is reproduced at the same time with 
isolation by deposition of metallic ions dissolved in the electrolyte and 
returned to the decontaminating electrolytic cell, so that electrolytic 
decontamination of contaminated objects and isolation by deposition of 
dissolved metallic ions can be continued semi-permanently. According to 
this method, metallic ions dissolved in the electrolyte are isolated by 
deposition while the pH of the electrolyte is controlled by injecting 
electrolyte having a high hydrogen ion concentration from the anode 
chamber into the cathode chamber. Therefore, deterioration of the 
deposition process due to an excessive rise in the pH value does not 
occur. Acid ions set free because of the deposition of metallic ions move 
through the electrolytic diaphragm into the anode chamber where they bond 
with hydrogen ions generated as oxygen gas is produced, on the insoluble 
electrode so that they are reproduced as acid. Thus, deterioration of the 
electrolyte is prevented in this example of the invention. 
Referring now to FIG. 1, a contaminated metal part or object 103 is 
connected to a positive DC potential and acts as an anode. The part 103 is 
submersed or dipped in an electrolyte 102 of a decontamination 
electrolytic cell 101, and a plurality of cathodes 104 are installed 
around the anode. A negative DC potential is connected to the cathodes 104 
and a DC current is passed between the electrodes in order to perform an 
electrolytic decontamination of the surface of the contaminated object 103 
as previously described. 
The electrolyte 102 is fed by a pump 105 from the electrolytic cell 101 to 
a filter 106 where suspended substances are removed, and returned through 
a pipe 107 into the cell 101, thus also agitating the electrolyte in the 
cell. Part of the circulating electrolyte is sent through a branch pipe 
108 into the lower level of a metallic ion isolating cell 109 which is 
divided into an anode chamber 111 and a cathode chamber 112 by an 
electrolytic diaphragm 110. The circulating electrolyte from the branch 
pipe 108 enters the anode chamber 111 and flows back into the 
decontamination electrolytic cell 101 through an overflow pipe 113 
connected to an upper level of the cell 109. 
An insoluble electrode 114 of, for example, platinum-plated titanium, and a 
capture electrode 115 of, for example, steel sheet are installed in the 
anode chamber 111 and cathode chamber 112, respectively. Positive and 
negative DC potentials are connected to the electrodes 114 and 115, and DC 
current is passed through the electrolyte of the cell and the electrolytic 
diaphragm 110 between the electrodes. The cathode chamber 112 is filled 
with the electrolyte 102 so that metallic ions dissolved in the 
electrolyte are deposited on the capture electrode 115. This causes a 
build-up 115a on the electrode 115. In the initial phase of current flow 
at the beginning of operation, the hydrogen ion concentration of the 
electrolyte in the cathode chamber 112 is high and a large volume of 
hydrogen gas is generated at the capture electrode 115. Therefore, the 
metallic ions do not deposit on the electrode 115. The amount of hydrogen 
gas being generated is decreased with an increase in pH value of the 
electrolyte, and then the metallic ions start to deposit on the electrode. 
The solution in the chamber 112 is agitated by an upward flow of the 
hydrogen gas generated on the capture electrode 115, and the solution is 
maintained at substantially pH 2. 
When the DC current is allowed to flow for a long time, however, the pH 
value of the electrolyte further increases. When it exceeds approximately 
pH 2, metal hydroxide begins to be generated in the cathode chamber 112, 
thereby slowing the deposition of metal. To avoid this, a pH meter 116 is 
installed in the cathode chamber 112. When pH 2 level is exceeded, more 
electrolyte having a lower pH value from the anode chamber 111 is fed by a 
pump 117 into the cathode chamber 112 to prevent an excessive rise in pH 
value. There is a return flow from the cathode chamber 112 to the anode 
chamber 111 through an overflow notch 110a formed in the upper edge of the 
diaphragm 110. A similar return flow arrangement may be provided in the 
other systems disclosed herein when necessary. A control 116a is connected 
to the pump 117 and to the pH meter 116 and it responds to the meter 
output such as to turn on the pump 117 when the pH level becomes 
excessive. 
As a specific example of the above apparatus, the cells were filled with a 
5% sulfuric acid aqueous solution as the electrolyte 102. 10A/dm.sup.2 DC 
current was allowed to flow for 15 minutes and stopped for 45 minutes to 
perform continuous electrolytic polishing of an SUS 304 plate as a 
contaminated object 103 in the decontamination electrolytic cell 101. In 
the metallic ion isolating cell 9, 5A/dm.sup.2 DC current was allowed to 
flow continuously, while automatically injecting electrolyte, using the 
meter 116 and the control 116a, from the anode chamber 111 into the 
cathode chamber 112, so that hydrogen ion concentration of the electrolyte 
in the cathode chamber did not exceed pH 2. Thus, the apparatus was 
operated continuously for two weeks to decontaminate the SUS 304 plate by 
electrolytic polishing and to perform isolation by deposition of metallic 
ions separated from the SUS 304 plate and dissolved into the electrolyte. 
The result was that the metallic ions deposited on the capture electrode 
115 in a stable manner and therefore iron ion concentration in the 
electrolyte never exceeded 25.2 g/l in the electrolytic cell 101. Sulfuric 
acid ions set free due to deposition of metallic ions in the cathode 
chamber 112 moved through the electrolytic diaphragm 110 into the anode 
chamber 111 and bonded with hydrogen ions generated on the insoluble 
electrode 114, thus being reproduced as metal-free sulfuric acid. 
Therefore, electric conductivity of the electrolyte did not decrease. 
On the other hand, when the apparatus was operated without injecting 
electrolyte from the anode chamber 111 into the cathode chamber 112, the 
hydrogen ion concentration of the electrolyte in the cathode chamber 112 
increased to pH 4 in about 18 hours and a large volume of metal hydroxide 
was generated. As a result, electrical conductivity of the electrolyte 
substantially reduced, thereby slowing the decontamination operation. 
In electrolytic decontamination with a dilute inorganic acid aqueous 
solution, polishing efficiency is not so much decreased as is the case 
with a high concentration phosphoric acid electrolyte, and disposal of 
spent electrolyte is relatively easy. The dilute inorganic acid aqueous 
solution electrolyte yields a smooth finished surface but not a mirror 
finished glossy surface which would be obtained with a high concentration 
phosphoric acid electrolyte. For example, if 10A/dm.sup.2 DC current is 
allowed to flow for 30 minutes to produce electrolytic polishing of the 
surface of a SUS 304 plate in phosphoric acid electrolyte containing 50% 
phosphoric acid and 25 % sulfuric acid, a surface with 0.45 .mu.m surface 
roughness and 418 gloss is obtained. On the other hand, if same 
experimentation is performed in a dilute aqueous solution electrolyte 
containing 5% sulfuric acid, a surface with 0.27 .mu.m surface roughness 
and 65 gloss is obtained. In short, high concentration phosphoric acid 
electrolyte can yield a glossy surface but it is difficult to dispose of, 
whereas a dilute aqueous solution electrolyte is relatively easy to 
dispose of but does not yield as glossy a surface. 
One of the convenient methods making use of the advantages of these two 
types of electrolyte is first to perform electrolytic decontamination in a 
dilute electrolyte in the first stage, and then to perform electrolytic 
polishing in a high concentration phosphoric acid solution electrolyte in 
the second stage so as to obtain a glossy surface. The contamination of 
high concentration electrolyte can be minimized by this method. 
FIG. 2 shows an example of reproducing or regenerating process of a high 
concentration acid decontamination electrolyte, by electrodeposition. 
According to this example, an electrolytic cell is divided by a diaphragm 
into an anode chamber and a cathode chamber. The cathode chamber is 
provided with a capture electrode and filled with electrolyte whose 
service life is spent. 
The anode chamber is provided with an anode formed by an insoluble 
electrode and filled with aqueous solution whose pH is adjusted to about 2 
by adding acid of the same components as the electrolyte. In this 
apparatus, DC current is allowed to flow through the diaphragm between the 
anode (the insoluble electrode) and the cathode (the capture electrode) so 
as to isolate metallic ions dissolved in the spent electrolyte by 
deposition on the cathode and at the same time to recover the electrolyte 
as strong acid solution of the initial concentration. 
In this method, in order to separate metallic ions dissolved in the spent 
electrolyte in the cathode chamber, it is required to remove hydrogen ions 
in the form of hydrogen gas from free acid that does not bond with 
metallic ions so as to lower the hydrogen ion concentration. Meanwhile, in 
order to reproduce acid solution electrolyte of the same volume and the 
same concentration with that decomposed in the cathode chamber, it is 
necessary to transfer anions separated in the cathode chamber into the 
anode chamber through the diaphragm so that they bond with hydrogen ions 
generated on the insoluble electrode or anode. In principle, the anode 
chamber should be filled with electrolyte of the same volume as that in 
the cathode chamber and should not contain acid. Such neutral electrolyte 
without acid content would, however, provide poor electric conductivity 
and make diaphragm electrolysis difficult. It is necessary, therefore, to 
employ a solution having such an acid content as to assure electric 
conductivity but not to affect the acid concentration of the reproduced 
electrolyte. In this sense, it s most desirable to use a solution of about 
pH 2 and with the same acid component as the electrolyte in the first 
batch so as to assure good electric conductivity, and to utilize the 
solution processed in the cathode chamber as anolyte for the subsequent 
batch. 
According to this batch system method, during the earlier phase of current 
flow, hydrogen ions disperse in the form of hydrogen gas from the acid 
solution in the cathode chamber so that anions are separated, while 
hydrogen ions are produced as oxygen gas is generated on the insoluble 
electrode in the anode chamber. The anions separated in the cathode 
chamber move into the anode chamber where they bond with hydrogen ions so 
that the acid is reproduced. In the course of this reaction cycle, the 
hydrogen ion concentration of the electrolyte in the cathode chamber drops 
to pH 2 when the dissolved metallic ions begin to deposit on the capture 
electrode. While metallic ions are depositing on the capture electrode, 
separated anions continue to move into the anode chamber so as to be 
reproduced as acid. Ultimately, therefore, a high concentration acid 
solution with dissolved oxygen ion removed is reproduced in the same 
amount as the initial electrolyte in the anode chamber. This reproduced 
acid solution is reusable as an electrolyte for electrolytic 
decontamination. Meanwhile, a solution of about pH 2 containing 
substantially no dissolved metallic ion is left in the cathode chamber, 
allowing metallic ions to deposit on the capture electrode. The solution 
in the cathode chamber may be moved into the anode chamber as pH adjusted 
anolyte for the subsequent batch. Through the repetition of this batch 
operation, dissolved metallic ions are separated in the form of solid 
metal and the electrolyte is reproduced without producing waste liquid. 
This method will be further described in detail, with reference to FIG. 2. 
An electrodeposition reproducing cell 201 is divided by a diaphragm 202 
into a cathode chamber 203 and an anode chamber 206. The cathode chamber 
203 contains a capture electrode 205 made of steel sheet and is filled 
with spent electrolyte 204, i.e. radioactive metallic ion-containing a 
high concentration acid electrolyte whose service life is over. The anode 
chamber 206 having substantially the same capacity as the cathode chamber 
contains an insoluble electrode 208 made, for example, of platinum-plated 
titanium net and is filled with an anolyte 207 having a hydrogen ion 
concentration adjusted to about pH 2 by an acid solution of the same 
components as the electrolyte so as to have good electric conductivity. 
Then, DC current is passed between the insoluble electrode 208 (the anode) 
and the capture electrode 205 (the cathode) so that dissolved metallic 
ions are deposited on the capture electrode 205, and so that electrolyte 
is reproduced or regenerated in the anode chamber 206. In the initial 
phase of the current flow, the hydrogen ion concentration of the 
electrolyte in the cathode chamber 203 is so high that a large volume of 
hydrogen gas is generated on the capture electrode 205, and therefore, 
metallic ions are not deposited on the capture electrode. When the 
hydrogen ion concentration of the electrolyte 204 decreases to about pH 2, 
however, the hydrogen gas is generated in a decreased amount and metallic 
ions begin to deposit on the capture electrode. Anions produced in the 
cathod.degree. chamber 203 then move through the diaphragm 202 into the 
anode chamber 206 where they bond with hydrogen ions produced by oxygen 
generation on the insoluble electrode 208 in the anode chamber 206 so as 
to be reproduced as an electrolyte. The current supply is continued until 
the desired result is obtained. 
In case the pH of the spent electrolyte in the cathode chamber 203 rises 
excessively in the course of the current flow, a pH meter 209 may be 
installed in the cathode chamber 203 and connected to a control 209a so 
that when an excessive rise in the pH is detected by the pH meter 209, a 
pump 210, actuated by the control 209a, is actuated to feed electrolyte 
reproduced in the anode chamber 206 into the cathode chamber 203, thereby 
controlling the hydrogen ion concentration of the spent electrolyte to 
about pH 2 so as to assure efficient electrodeposition. 
The following are specific examples of the foregoing system: 
EXAMPLE I 
The system was operated with spent electrolyte resulting from electrolytic 
decontamination of SUS 304 stainless steel in an electrolyte containing 75 
wt% phosphoric acid. 62.5 g/l of iron ions, 9.75 g/l of chromium ions, 
7.75 g/l of nickel ions and 0.21 g/l of cobalt ions were dissolved in the 
spent electrolyte. 
The cathode chamber 203 in FIG. 2 was filled with the spent electrolyte of 
the above composition and the anode chamber 206 was filled with an anolyte 
whose hydrogen ion concentration was adjusted to pH 2. Diaphragm 
electrolysis was conducted by supplying 10A/dm.sup.2 DC current until the 
total current supply reached 3,500 AH/l. As a result, 0.045 g/l of iron 
ions, 0.052 g/l of chromium ions, 0.067 g/l of nickel ions and 0.002 g/l 
of cobalt ions were left in the spent electrolyte in the cathode chamber 
203. In the anode chamber 206, the electrolyte was recovered as high 
concentration phosphoric acid solution with 75 wt% phosphoric acid content 
and containing 1250 g/l of phosphoric acid ions. Namely, electrolyte of 
substantially the same composition with the original electrolyte was 
reproduced except that about 20% metallic ions leaked through the 
diaphragm 202 into the reproduced electrolyte due to the diffusion of 
concentration. 
EXAMPLE II 
Similar to the above experiment, diaphragm electrolysis was conducted 
according to this example in a high concentration acid solution 
electrolyte composed of 70 Vol. % of 85% phosphoric acid and 30 Vol. % of 
98% sulfuric acid. The spent electrolyte resulted from electrolytic 
decontamination in the above electrolyte containing irons ions, chromium 
ions, nickel ions and cobalt ions in the same amount as for Example I. The 
result obtained was the same as that in Example I. The electrolyte 
reproduced in the anode chamber 206 contained phosphoric acid and sulfuric 
acid in the same mixing ratio as for the original electrolyte. 
FIG. 3 shows another system in accordance with the present invention in 
which electrolytic decontamination is performed in a high concentration 
acid solution electrolyte. According to this example, decontaminating 
electrolyte is successively reproduced by electrodeposition. 
In this example, an electrodeposition reproducing cell is divided by a 
diaphragm into an anode chamber and a cathode chamber. Electrolyte from an 
electrolytic decontamination cell is injected into the cathode chamber of 
the electrodeposition reproducing cell by using a pH controller so that 
hydrogen ion concentration in the cathode chamber is maintained at pH 2 at 
all times. To assure continuous injection, the electrodeposition 
reproducing cell and the electrolytic decontaminating cell are connected 
with each other. When DC current is allowed to flow through the diaphragm 
between the capture electrode in the cathode chamber and the insoluble 
electrode in the anode chamber, the pH value of the electrolyte in the 
cathode chamber increases as hydrogen ions are discharged as hydrogen gas 
from the electrolyte. According to this example, injection of the 
electrolyte from the electrolytic decontaminating cell into the cathode 
chamber starts when the pH value exceeds approximately 2, and stops when 
the pH value drops to approximately 2 or below. At the same time, to 
maintain a balance, a high concentration acid solution reproduced in the 
anode chamber of the electrodeposition reproduction cell is fed into the 
electrolytic decontamination cell in the same amount with the above 
injection for pH adjustment. This operation is automatically repeated 
under the control of a pH controller. 
In the electrodeposition reproducing cell of this example, radioactive 
metallic ions are separated without delay from the electrolyte with the pH 
value maintained at 2 and deposited on the capture electrode in the 
cathode chamber and hydrogen gas is generated. Anions separated by 
generation of hydrogen gas move through the diaphragm into the anode 
chamber where they bond with hydrogen ions generated on the insoluble 
electrode or anode so as to be reproduced as a high concentration acid 
solution. To initiate the electrodeposition reproducing process, 
therefore, it is only necessary to fill the cathode chamber with ordinary 
water and the anode chamber with the high concentration acid solution of 
the same concentration with the electrolyte used for the electrolytic 
decontamination process, only once at the beginning of operation. 
Electrodeposition reproduction is automatically continued because of a 
circulation of the solution. Moreover, the cathode and the anode chambers 
are automatically charged with water by means of level gauges each 
provided in the chambers so as to always maintain the levels constant. The 
makeup water is required only in the amount sufficient to compensate for 
the loss due to generation of hydrogen and oxygen as well as evaporation 
of water during operation. 
As mentioned above, this example has made possible a continuous system 
integrating electrolytic decontamination process and electrodeposition 
reproducing process. Specifi. cally, equipment or a part that is 
radioactive on its surface is decontaminated using a high concentration 
acid solution as electrolyte while the electrolyte is continuously fed to 
the electrodeposition reproduction process under a certain condition. In 
the electrodeposition reproducing cell, radioactive metallic ions 
separated from the electrolyte are allowed to deposit on the capture 
electrode in the form of radioactive metal which is easily disposed of. At 
the same time, the reproduced high concentration acid solution is fed back 
into the electrolytic decontamination cell in the same amount as the 
electrolyte fed to the electrodeposition reproducing cell for 
reproduction. According to this integrated continuous system, the 
radiation dose of the electrolyte is always maintained at a low level and 
metallic ion leakage to the reproduced electrolyte is minimized in the 
electrodeposition reproducing process. Besides, the radiation dose as well 
as the metallic ion content in the electrolyte are also maintained at low 
levels due to the renewal of the electrolyte in the electrolytic 
decontamination process. Moreover, compared with apparatus of a batch type 
of system, the apparatus according to this example requires a smaller 
number of devices and therefore is simpler in operation. 
One of the remarkable features of this example is that the hydrogen ion 
concentration of the electrolyte in the electrodeposition reproducing cell 
is maintained at pH 2 from the beginning of the operation. In the 
conventional electrodeposition reproducing process of batch system, it is 
necessary to operate the apparatus for a time to allow the hydrogen 
generating reaction to occur, before the pH value in the cathode chamber 
reaches 2 at which time the metallic ions begin to deposit on the capture 
electrode, whereas in this example, the apparatus is operated with the 
cathode chamber being filled with a solution of pH 2 and the anode chamber 
filled with high concentration acid solution from the beginning. 
The foregoing example is more specifically described with reference to FIG. 
3. The electrodeposition reproducing cell 301 used for the 
electrodeposition reproducing process according to this example is divided 
by a diaphragm 302 into a cathode chamber 303 and an anode chamber 304. 
The cathode chamber 303 contains a capture electrode 305 made, for 
example, of an iron sheet and is filled with ordinary water 307 at the 
beginning. The anode chamber 304 contains an insoluble electrode 306 made, 
for example, of platinum-plated titanium net and is filled with a high 
concentration acid solution 308 having the same components and 
concentration as the electrolyte used for electrolytic decontamination 
process at the beginning. DC current is passed between the electrodes 305 
and 306. 
In the electrolytic decontamination cell 309 used for the electrolytic 
decontamination process, equipment or a part bearing radioactivity on the 
surface is provided as an anode 310 and an insoluble electrode of the same 
type as the electrode 306 is provided as a cathode 311. The cell is filled 
with a high concentration acid solution used as electrolyte 312. DC 
current is passed between the electrodes 310 and 311 to perform 
electrolytic polishing so that at least part of the radioactive substance 
is removed from the anode surface and suspended in the electrolyte and the 
other part thereof is dissolved as radioactive metallic ions in the 
electrolyte, thus completing the decontamination process. 
According to this example, these two processes are operated in cooperation 
with each other. Namely, a pH detecting auxiliary bath 313 is installed on 
the upper part of the cathode chamber 303 of the electrodeposition 
reproducing cell 301. The auxiliary bath 313 is provided with a pH meter 
sensor or electrode 315 that is connected to a pH controller 314 and set 
at pH 2. Water 307 in the cathode chamber 303 is circulated by means of a 
circulation pump 316 through the auxiliary bath 313 for detection of pH 
value and then back to the chamber 303. 
An injection pipe 318 equipped with an injection pump 317 leads from the 
cell 309 to the auxiliary bath 313 (or the cathode chamber 303), the 
injection pump 317 being connected to be operated by the pH controller 314 
so as to continuously inject electrolyte 312 from the electrolytic 
decontaminating cell 309 into the cathode chamber 303. Further, a suction 
pipe 320 equipped with a suction pump 319 leads from the anode chamber to 
the electrolytic decontaminating cell 309, the suction pump 309 being 
connected to be operated by the pH controller 314. Unlike a batch type 
system, the capacity of the anode chamber 304 with the present circulating 
system may be moderately small. 
As mentioned earlier, the cathode chamber 303 and the anode chamber 304 are 
filled with ordinary water and a high concentration acid solution, 
respectively, at the beginning of operation. The circulation pump 316 is 
then operated to circulate water 307 in the cathode chamber 303 and the 
bath 313. Since the pH value of the water 307 in the cathode chamber is 
higher than 2 at this stage, the pH controller 314 actuates the 
interlocking injection pump 317, resulting in the electrolyte 312 in the 
electrolytic decontaminating process being injected into the cathode 
chamber 303 (or the auxiliary bath 313). Simultaneously, the suction pump 
319 is also actuated by the pH controller 314 whereby the solution 308 in 
the anode chamber is suctioned off and fed back into the electrolytic 
decontaminating cell 309 in the same amount as the electrolyte being 
injected from the electrolytic decontaminating cell 309 into the cathode 
chamber 303. 
This initial operation is continued for a certain period before DC current 
is allowed to flow between the electrodes 306 and 305. When the pH value 
in the cathode chamber 303 drops to 2, the injection pump 317 and the 
suction pump 319 are stopped by the functioning of the pH controller 314. 
When electrolysis is continued by supplying DC current to the electrodes 
305 and 306 in the electrodeposition reproducing cell 301 while the pH 
value in the cathode chamber is maintained at 2, radioactive metallic ions 
dissolved in the solution 307 in the cathode chamber 303 deposit and 
accumulate in the form of metal deposits on the capture electrode 305. 
Then, anions move through the diaphragm 302 into the anode chamber 304 
where the high concentration acid solution is reproduced. If, in the 
course of the electrolysis, the pH value in the cathode chamber 303 
exceeds 2, the injection pump 317 and the suction pump 319 are actuated 
again so as to adjust pH value in the cathode chamber 303 at 2. The above 
operation is automatically repeated so that electrolyte is reproduced by 
electrodeposition substantially continuously and constantly. 
In the operation of the above process, water evaporates due to exothermic 
reaction in the cathode chamber 303 and reduces due to decomposition of 
water in the anode chamber 304 during the electrolytic operation in the 
electrodeposition reproducing cell 301. It is necessary to compensate for 
such water loss so as to ensure continuous and steady operation. To this 
purpose, solenoidoperated valves 323 and 324 are connected to the 
respective chambers and are controlled by level gauges 321 and 322 
provided in the chambers 303 and 304. The valves 323 and 324 are connected 
in waterlines 325 and water is automatically supplied to maintain a 
constant level in the chambers. 
Specific examples illustrating the operation of the process shown in FIG. 3 
are given below: 
EXAMPLE III 
(A) Process of Electrolytic Decontamination 
Radioactively contaminated SUS 304 stainless steel was used as an anode 310 
to be decontaminated, and 75 wt % of phosphoric acid solution was used as 
an electrolyte to perform the electrolytic decontamination. 38 g/l of iron 
ions, 8.8 g/l of chromium ions, 6.8 g/l of nickel ions and 0.092 g/l of 
cobalt ions were dissolved in the spent electrolyte. 
(B) Process of Electrodeposition Reproduction 
The cathode chamber 303 of the electrodeposition reproducing cell 301 was 
initially filled with ordinary water and the anode chamber 304 was filled 
with 75 wt % phosphoric acid solution devoid of metallic ions at the 
beginning. After injecting said spent electrolyte into the cathode chamber 
303 by means of the injection pump 317, 8A/dm.sup.2 current was supplied 
to start the electrolytic operation of the cell 301 so as to check the 
change with time in the concentration of residual metallic ions in the 
solution 307 in the cathode chamber and the solution 308 in the anode 
chamber. The result was that the solution 307 in the cathode chamber 
contained 0.005 to 0.060 g/l of iron ions, 0.003 to 0.01 g/l of chromium 
ions, 0.001 to 0.005 g/l of nickel ions and 0.0001 to 0.0003 g/l of cobalt 
ions and that the solution 308 in the anode chamber contained 0.01 to 0.02 
g/l of iron ions, 0.006 to 0.007 g/l of chromium ions, 0.004 to 0.005 g/l 
of nickel ions and 0.0001 to 0.0002 g/l of cobalt ions due to leakage. 
As is obvious from these figures, the level of metallic ions was maintained 
very low both in the cathode chamber 303 and in the anode chamber 304. 
Moreover, current efficiency was stable around 10% during the above 
operation. 
EXAMPLE IV 
During operation as in Example III above, an electrolyte of a different 
composition from that given in Example III was temporarily injected into 
the cathode chamber 303 under the same condition as above. Namely, 4.84 
g/l of iron ions, 1.47 g/l of chromium ions, 0.34 g/l of nickel ions and 
0.0126 g/l of cobalt ions were contained in the electrolyte. These values 
are all smaller than those for the electrolyte above. As a result of 
examination of the change with time in the metallic ion concentration in 
the solution 307 in the cathode chamber as well as in the current 
availability, it was revealed that both the cathode and anode chambers 
contained metallic ions in smaller amounts than in Example I, as shown by 
the specific figures below. Current efficiency was not changed and was 
about 10%. 
Residual metallic ions in the solution 307 in the cathode chamber: iron ion 
0.0032 g/l, chromium ion 0.00096 g/l, nickel ion 0.0014 g/l, cobalt ion 
0.0003 g/l. Residual metallic ions in the solution 308 in the anode 
chamber: iron ion 0.016 g/l, chromium ion 0.0025 g/l, nickel ion 0.004 
g/l, cobalt ion 0.002 g/l. 
As shown by the above figures, metallic ions were contained in larger 
amount in the solution 308 in the anode chamber than in the solution 307 
in the cathode chamber, presumably because the electrolyte used in Example 
III remained in the anode chamber. 
Utilizing the foregoing process, it is possible to realize a novel and 
useful embodiment of the invention. Conventionally, a radioactively 
contaminated electrolyte remains on the surface of the object after it has 
been electrolytically decontaminated, and therefore it is necessary to 
rinse the object in water to remove electrolyte. Radioactive metallic ions 
then enter the rinsing water, causing secondary contamination and a 
troublesome secondary treatment is therefore required. 
According to a novel embodiment of this invention, however, the object is 
rinsed by spraying it with the solution from the electrodeposition 
reproducing cell or, preferably, the solution 307 in the cathode chamber 
that contains metallic ions having a lower level of radioactivity, or it 
is dipped in the solution 307 in the cathode chamber for a preliminary 
rinsing and then it is washed in water. The solution used for 
spray-rinsing is returned to the cathode chamber 303. The radioactively 
contaminated electrolyte thus entering the solution 307 in the cathode 
chamber presents no problem because it is part of the electrolyte to be 
injected for electrodeposition reproduction. In addition, according to 
this method, the level of radioactivity of the metallic ions dissolved in 
the scondary rinsing water is much lower than that encountered by 
conventional methods and is within the safety limit. 
The systems shown in FIGS. 4 to 7 are modifications of the systems shown in 
FIGS. 2 and 3 dealing with the process of electrodeposition reproduction 
of high concentration phosphoric acid decontaminating electrolyte, that is 
effective in preventing recontamination. The function of these systems is 
to improve processing capacity or reduce the time required for 
electrodeposition reproduction. In these embodiments, phosphoric acid is 
extracted, by a solvent, from the high concentration phosphoric acid 
decontaminating electrolyte used for an electrolytic decontamination 
process prior to feeding the electrolyte into the electrodeposition 
reproducing cell. The resultant solution after extraction (or electrolyte 
whose phosphoric acid content is decreased) is fed into the cathode 
chamber of the electrodeposition reproducing cell. Further, phosphoric 
acid is inversely extracted, by water, from the above solvent after 
extraction, and the resultant inverse extractive solution (or phosphoric 
acid aqueous solution containing substantially no metallic ion) is fed 
into the anode chamber of the electrodeposition reproducing cell. Thus, 
metallic ions in the solution is captured by electrodeposition in the 
cathode chamber, and the phosphoric acid concentration in the solution is 
increased to that of the initial electrolyte in the anode chamber so it 
may be reused as a decontaminating electrolyte. 
The solvent for liquid-liquid extraction of phosphoric acid from phosphoric 
acid aqueous solution may be taken from the group comprising isopropyl 
ether, ethylene menomethyl ether, normal butyl alcohol, isoamyl alcohol, 
methyl isobutyl ketone or butyl acetate. Since these organic solvents 
evaporate due to the heat of the decontaminating electrolyte and are 
inflammable, they are not suitable as a phosphorus extracting agent to be 
used in the process of electrodeposition reproduction. Among various 
phosphorus extracting agents studied, water-insoluble and noncombustible 
tributyl phosphate (TBP) is found to be most effective as a phosphorus 
extracting agent to be used in the process of electrodeposition 
reproduction of decontaminating electrolyte. 
Tributyl phosphate, known as a metal extracting agent, is usually used for 
extracting uranium from nitric acid. It is effective in extracting 
phosphoric acid but hardly extracts iron, nickel, chromium, cobalt or 
their metallic ions in a decontaminating electrolyte. By inverse 
extraction in water, phosphoric acid and metallic ions are almost 
completely extracted from this metal extracting agent. Besides, it can be 
used repeatedly without recharging because of its high boiling point and 
small evaporation loss. 
More specific details of the process for electrodeposition reproduction of 
decontaminating electrolyte based on extraction and inverse extraction of 
phosphoric acid by use of a solvent will be described in connection with 
FIGS. 4 to 7. 
Referring specifically to FIG. 4 which shows an example of 
electrodeposition reproduction process, a high concentration acid of 
phosphoric acid series is used as an electrolyte in the process of 
electrolytic decontamination. An object 402 which is radioactively 
contaminated on its surface is set in the electrolyte in an electrolytic 
decontamination cell 401 and connected to the anode of a DC source such as 
an AC-DC rectifier (not shown). DC current is passed between the object 
and cathodes 403 in the electrolyte so as to decontaminate the surface of 
the contaminated object 402. The decontaminating electrolyte containing 
radioactive metallic ions separated from the surface of the object 402 is 
suctioned off by a pump 404 into the extractive separating bath 412 where 
the electrolyte is separated into two solutions: one solution is an 
electrolyte with less phosphoric acid content obtained by extraction of 
phosphoric acid by a solvent and the other is a phosphoric acid aqueous 
solution substantially free from metallic ions obtained by inverse 
extraction of phosphoric acid by water. The above electrolyte and 
phosphoric acid aqueous solution thus obtained are fed into the cathode 
chamber 406 and the anode chamber 407, respectively, of the 
electrodeposition reproducing cell 405. Then, DC current is passed through 
a diaphragm 408 between the capture electrode 409 in the cathode chamber 
406 and an insoluble electrode 410 in the anode chamber 407 so as to 
effect electrodeposition reproduction of the decontaminating electrolyte. 
The solution in the anode chamber 407 whose phosphoric acid content is 
increased relative to that of the initial electrolyte is then fed back 
into the electrolytic decontaminating cell 401 by means of a pump 411. 
The extractive separating bath 412 where extraction and inverse extraction 
of phosphoric acid are performed is filled with a solvent (S) to about 
half level or volume. Operation of the extraction and the inverse 
extraction is described in the following, with reference to FIGS. 5a to 
5d. 
A decontaminating electrolyte 413 (FIG. 5a) of the same volume as the 
capacity of the anode chamber 407 is fed by the pump 404 from the 
electrolytic decontaminating cell 401 into the extractive separating bath 
412 where the electrolyte 413 is stirred for a time by a motor-driven 
agitator 414 (FIG. 4) so that phosphoric acid is extracted by the solvent 
(S). After the agitator 414 has been stopped, the resultant solution after 
the extraction 415, separated and settled in the lower layer in the bath 
412, is discharged through a discharge valve 416 into the cathode chamber 
406. At this time, care must be taken so as not to discharge the solvent 
(S) in the upper layer into the cathode chamber 406 and this may be done 
by observing an electric conductivity meter 417 mounted at the lower 
outlet of the bath 412. Since the amount of the resultant solution after 
the extraction 415 is discharged is smaller than the capacity of the 
cathode chamber 406, the feed water valve 418 must be opened to add water 
to the upper limit level of the cathode chamber 406. A liquid level 
control 418a may be provided to automatically control the valve 418 and 
the liquid level. 
After the feed water valve 420 at the outlet of a reservoir 419 is opened 
to supply the extractive separating bath 412 with an inverse extractive 
water 421, the agitator 414 is again actuated to mix the water with the 
solvent (S) in the bath 412, thereby inversely extracting phosphoric acid 
from the solvent (S). The agitator 414 is then stopped, and the resultant 
inverse extractive solution 422, separated and settled in the lower layer, 
is discharged through the discharge valve 423 into the anode chamber 407 
to its upper limit level. Since the volume of the resultant inverse 
extractive solution 422 is larger than that of the decontaminating 
electrolyte 413, a major portion of the solution is left undischarged in 
the extractive separating bath 412. 
When DC current is passed between the capture electrode 409 and the 
insoluble electrode 410 at this stage, hydrogen ions or cations in the 
solution are released in the form of a large amount of hydrogen gas on the 
capture electrode 409 in the cathode chamber 406 so that the pH value of 
the solution drops. Accordingly, phosphoric acid ions or anions move 
through the diaphragm 408 into the anode chamber 407 where they bond with 
hydrogen ions increased due to the generation of oxygen gas on the 
insoluble electrode 410, thus gradually raising the concentration of 
phosphoric acid in the solution. As the current flow is continued, the 
hydrogen ion concentration in the cathode chamber 406 is reduced to about 
pH 2. Then, the amount of hydrogen gas generated on the capture electrode 
409 decreases and metallic ions begin to deposit on the capture electrode 
409. 
During this operation, the liquid level drops both in the cathode chamber 
406 and anode chamber 407 because of the evaporation and decomposition of 
the water. Makeup water is automatically fed through the feed water valve 
418 into the cathode chamber 406 and the resultant inverse extractive 
solution 422 left in the extractive separating bath 412 is automatically 
fed through the discharge valve 423 into the anode chamber 407 so as to 
compensate for the liquid loss. A liquid level control 423a may be mounted 
in the chamber 407 and connected to automatically control the valve 423. 
The conductivity meter 417 may also be connected to an automatic 
controller 417a that is connected to control the operation of the valve 
423. The controller 417a would turn off the valve 423 when all of the 
solution 422 is drained from the bath 412. 
When the electric conductivity meter 417 detects that the resultant inverse 
extractive solution 422 has been substantially all discharged from the 
extractive separating bath 412, the level control function is released and 
the discharge valve 423 is closed. 
The current supply is continued, however, until substantially all of the 
metallic ions and the phosphoric acid ions are removed from the solution 
in the cathode chamber 406. The electrolyte 413 fed from the electrolytic 
decontaminating cell 401 is thus reproduced as a high concentration 
phosphoric acid solution of substantially the same volume. Water is 
supplied through a feed water valve 424 to the solution in the chamber 
407, as required, to adjust the phosphoric acid concentration to that of 
the initial electrolyte before feeding the whole volume of the solution in 
the anode chamber 407 by the pump 411 back into the electrolytic 
decontaminating cell 401, thus completing the electrodeposition 
reproducing process. 
Water almost as clear as fresh water is produced in the cathode chamber 406 
during the electrodeposition reproducing process. After completing 
electrodeposition reproducing process, this clear water is fed by the pump 
426 into the reservoir 419 so as to serve as part of the inverse 
extractive water to be used for the subsequent operation. Water is also 
supplied through the feed water valve 425 into the reservoir 419. 
According to this method, the resultant solution after extraction 415 in 
the bath 412 contains a small percentage of phosphoric acid when it is 
discharged into the cathode chamber 406. In addition, the solution is 
further diluted by mekeup water supplied through the feed water valve 418. 
Therefore, the current flow is started with the cathode chamber 406 filled 
with a solution having a decreased hydrogen ion concentration. As a 
result, compared with the preceding example in which electrodeposition 
reproduction is performed with an electrolyte having a high hydrogen ion 
concentration contained in the cathode chamber 406, the length of time 
required before electrodeposition starts is remarkably reduced in this 
example. 
In the system shown in FIG. 6, extraction and inverse extraction of 
phosphoric acid as well as electrodeposition reproduction are performed 
continuously. Parts common to the system shown in FIG. 4 are referred to 
by the same numerals, and a discussion of the common subject matter is 
omitted here. 
For the process of extraction and inverse extraction of phosphoric acid, an 
extracting bath 430, an extractive solution separating bath 431, an 
inverse extracting bath 432 and an inverse extractive solution separating 
bath 433 are separately installed in such a manner that the solution 
overflows from one bath to the next bath. The solvent (S) is continuously 
circulated by a pump 434 between the inverse extractive solution 
separating bath 433 and the extracting bath 430 and the solutions in the 
extracting bath 430 and the inverse extracting bath 432 are stirred at all 
times by the agitators 435, 436. 
Electrolyte is continuously fed by the pump 404 from the electrolytic 
decontamination cell 401 into the extracting bath 430 where phosphoric 
acid is continuously extracted by the solvent (S). The resultant solution 
after extraction 415, separated and settled in the lower layer of the 
extractive solution separating bath 431, is sent through a supply valve 
437 into the cathode chamber 406 so that the hydrogen ion concentration of 
the solution in the cathode chamber 406 is always maintained at pH 2. The 
solvent (S) flows from the extractive solution separating bath 431 to the 
inverse extracting bath 432 where phosphoric acid is back-extracted by 
water supplied through a feed water valve 420. The resultant inverse 
extractive solution 422, separated and settled in the lower layer of the 
inverse extractive solution separating bath 433, is sent through the 
supply valve 438 into the anode chamber 407, thereby controlling the 
liquid level in the anode chamber. The feed water valve 420 is operated in 
accordance with the level control in the inverse extractive solution 
separating bath 433 so that inverse extractive water is automatically 
supplied from the reservoir 419 into the inverse extracting bath 432 by 
the amount equivalent to the discharge of the resultant solution after the 
extraction 415 and the resultant inverse extractive solution 422. 
Since the solution in the cathode chamber 406 is continuously circulated 
for agitation by the pump 439 while the resultant solution after 
extraction 415 is automatically supplied into the cathode chamber 406 so 
as to maintain the hydrogen ion concentration at about pH 2 at all time, 
metallic ions deposit for capture on the capture electrode 409 without 
delay. To compensate for the water loss due to decomposition and 
evaporation, makeup water is supplied through the feed water valve 440 so 
as to control the liquid level in the cathode chamber 406. If the 
resultant solution after extraction 415 is injected in a larger amount 
than the water loss, the liquid level in the cathode chamber 406 gradually 
rises. In such a case, pH control is released so as to stop the injection 
of the resultant solution after extraction 415, when the liquid level 
reaches the uppermost limit. A liquid level control 437a and a pH control 
437b are connected to the chamber 406 and to a valve 437 in order to 
control the liquid level. While keeping current flowing, a valve 441 in 
the circulation line is opened to feed part of the solution from the 
cathode chamber 406 to the reservoir 419 so as to lower the liquid level 
when substantially no metallic ions and phosphoric acid ions are left in 
the liquid in the cathode chamber 406. 
The resultant inverse extractive solution 422 is automatically supplied to 
compensate for the water loss in the anode chamber 407 while the solution 
in the anode chamber 407 is fed back into the electrolytic decontaminating 
cell 401 as reproduced electrolyte, through a pump 411 with the feed rate 
being controlled by means of the electric conductivity meter 417. 
Meanwhile, electrolyte is fed from the electrolytic decontaminating cell 
401 into the extracting bath 430 in the same amount with the reproduced 
electrolyte fed back from the anode chamber 407 to the electrolytic 
decontaminating cell 401, thus continuously performing electrodeposition 
reproduction of electrolyte. 
Similar to the system shown in FIG. 4, the resultant solution after 
extraction 415, whose phosphoric acid content is decreased, is injected 
into the cathode chamber 406 in this system. Therefore, compared with the 
method as described earlier in which the electrolyte is directly injected 
from the electrolytic decontaminating cell 401 into the cathode chamber 
406, the electrodeposition reproducing capacity is improved in this 
example. 
Moreover, according to the systems as shown in FIGS. 4 and 6, phosphoric 
acid in the decontaminating electrolyte fed from the electrolytic 
decontaminating cell 401 flows into the anode chamber 407 through 
extraction and inverse extraction processes, thus saving electric energy 
required for moving the phosphoric acid from the cathode chamber 406 
through the diaphragm 408 to the anode chamber 407 during the process of 
electrodeposition reproduction. To raise the effect of extraction and 
inverse extraction of the phosphoric acid, a large volume of inverse 
extractive water may be used, which in turn leads the increase in the 
volume of the resultant inverse extractive solution 422 to be processed in 
the anode chamber 407, however. As a result, the speed of the 
electrodeposition reproduction is limited by the rate of water loss due to 
decomposition and evaporation in the anode chamber 407. This problem can 
be solved by incorporating evaporation into the process of concentrating 
phosphoric acid performed in the anode chamber 407. Heating the solution 
in the anode chamber 407, however, causes the functioning of the diaphragm 
408 to be lowered, resulting in the leakage of hydrogen ions from the 
anode chamber 407 into the cathode chamber 406. If incorporating 
evaporation into the process of concentrating phosphoric acid, therefore, 
it is necessary to concentrate the resultant inverse extractive solution 
422 prior to injecting it into the anode chamber 407 or to send the 
solution from the anode chamber 407 into an evaporator for concentrating 
the solution and to cool it before injection into the anode chamber 407. 
FIG. 7 shows a system in which evaporation is incorporated into the process 
of concentrating phosphoric acid in the resultant inverse extractive 
solution 422. In this system, the resultant inverse extractive solution 
422 discharged from the extractive separating bath (412 or 433) into the 
receiver 450, is fed to a vapor compression concentrating unit 451 so as 
to concentrate the phosphoric acid. The concentrator 452 is a vessel or a 
pipe that is glass lined on its inner wall. A compressor 453 suctions 
vapor from the concentrator 452 for depressurization as well as 
compressing suctioned vapor and transfers compression heat to the solution 
in the concentrator 452 through the jacket 454 or a similar heat 
transferring tube provided with a glass liner on its outer wall so that 
the resultant inverse extractive solution 422 is concentrated by 
evaporation at a low temperature. Thus, concentration by evaporation is 
achieved with little energy consumption and without the need for a heat 
source or cooling water. The concentrated solution in the concentrator is 
suctioned off by a pump 455 into the anode chamber 407. The condensate is 
sent to the reservoir 419 to be used as inverse extractive water. 
As described above, if evaporation is incorporated into the process of 
concentrating phosphoric acid in the resultant inverse extractive solution 
422, power consumption required for electric decomposition of water can be 
saved and ample inverse extractive water can be used to ensure 
satisfactory extraction and inverse extraction. As a result, speed or 
efficiency of electrodeposition reproduction is further increased. 
To verify the effect of the extraction and inverse extraction of phosphoric 
acid in this system, the following experiment was conducted with spent 
electrolyte obtained after the electrolytic decontamination of SUS 304 
plate in 75% phosphoric acid electrolyte and with tributyl phosphate (TBP) 
as solvent. 
40cc of TBP was poured into 10cc of the above-mentioned electrolyte so as 
to extract phosphoric acid. 50cc of water was poured into the resultant 
TBP obtained after extraction so as to inversely extract phosphoric acid. 
Then, 10cc of the above-mentioned electrolyte was poured into TBP 
recovered by the above inverse extraction. Thus, the process of extraction 
and inverse extraction was repeated. 10cc of electrolyte decreased to 7cc 
after extraction of phosphoric acid by 40cc of TBP which increased to 
43cc. The 43cc of TBP then decreased to about 40cc, the initial volume, 
when recovered by adding 50cc of water for inverse extraction of 
phosphoric acid so that about 53cc of resultant inverse extractive 
solution was obtained. 
Components as shown in the table below were contained in 10cc of the 
electrolyte (A) and 7cc of the resultant solution after extraction (B), 
53cc of the resultant inverse extractive solution (C) and 40cc of the TBP 
recovered (D) by the 2nd and the 5th extraction and inverse extraction. 
______________________________________ 
Extrac- 
Com- 
tion # ponent (A) mg (B) mg (C) mg (D) mg 
______________________________________ 
2 Fe 243.5 230 15.2 0.096 
Cr 80.5 77.8 5.6 &lt;0.2 
Ni 14.6 14.4 0.6 &lt;0.02 
Co 0.46 0.44 &lt;0.02 &lt;0.004 
Cu 0.6 0.5 0.04 &lt;0.02 
H.sub.3 PO.sub.4 
11700 5900 6830 -- 
5 Fe 243.5 192 17.4 0.168 
Cr 80.5 67.1 6.6 &lt;0.2 
Ni 14.6 12.7 0.65 &lt;0.02 
Co 0.46 0.37 &lt;0.026 &lt;0.004 
Cu 0.6 0.48 0.04 &lt;0.02 
H.sub.3 PO.sub.4 
11700 5030 7100 -- 
______________________________________ 
As is obvious from the table, if phosphoric acid is extracted from the 
decontaminating electrolyte with 75% phosphoric acid content by recovered 
TBP of the amount four times the volume of the electrolyte and if 
phosphoric acid extracted and dissolved in TBP is inversely extracted by 
water of the same amount with TBP, more than one-half of the phosphoric 
acid in the electrolyte is transferred to the resultant inverse extractive 
solution so that the TBP is almost completely recovered. Similar effects 
of extraction and inverse extraction were also obtained by experiments 
conducted in the same manner for low concentration sulfuric acid 
electrolyte and for high concentration phosphoric acidsulfuric acid 
electrolyte. 
If the resultant solution after extraction and the resultant inverse 
extractive solution thus obtained are fed into the cathode chamber and the 
anode chamber, respectively, of the electrodeposition reproducing cell, 
electrodeposition reproduction performance is more than two times that by 
the method in the example as mentioned earlier. 
The systems described herein are preferably operated with the temperature 
range of from approximately normal room temperature up to approximately 
50.degree. C. The diaphragms may be types that are well known to those 
skilled in the electrolysis art. In the system shown in FIG. 1, an 
unglazed ceramic plate may be used, and in the systems of FIGS. 2-4, 6 and 
7, an ion exchange membrane may be used. 
As described above, according to this example, phosphoric acid in the 
electrolyte is directly transferred into the anode chamber of the 
electrodeposition reproducing cell through the process of extraction and 
inverse extraction, thus reducing the time required for electrodeposition 
reproduction or increasing the electrodeposition reproduction capacity. In 
addition, reproduced electrolyte and solvent can be repeatedly used. This 
helps solve the problem of waste liquid disposal confronted by the 
electrolytic decontamination process involving high concentration acid 
electrolyte of phosphoric acid series which is effective in preventing 
contamination. Namely, the amount of radioactive secondary waste is 
significantly reduced. As described above, the methods of reproducing 
electrolyte in the electrolytic decontamination waste are described in 
various embodiments, but these methods are also available for the 
reproducing electrolyte in the chemical decontamination waste including 
radioactive metallic ions.