Radioactive organic waste treatment method

Disclosed is a method for treating a radioactive organic waste, the radioactive organic waste including a cation exchange resin adsorbing radionuclide ions, the method including the step of bringing the radioactive organic waste into contact with an organic acid salt aqueous solution containing an organic acid salt and whereby desorbing the radionuclide ions from the cation exchange resin, in which the organic acid salt contained in the organic acid salt aqueous solution includes a cation that is more readily adsorbable by the cation exchange resin than hydrogen ion is. This enables reduction in concentration of a radioactive substance in the radioactive organic waste and reduction in amount of a high-dose radioactive waste.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent application serial Nos. 2013-130070 and 2013-179670, each filed on Jun. 21, 2013, and Aug. 30, 2013, the contents of which are hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to radioactive organic waste treatment methods and systems. Specifically, it relates to methods and systems suitable for treating a radioactive organic waste such as a spent ion exchange resin and filter sludge which contain radionuclides, the radioactive organic waste being generated in nuclear power plants.

2. Description of Related Art

Reactor water cleanup systems and fuel pool cooling cleanup systems of nuclear power plants generate a radioactive organic waste such as filter sludge including a cellulosic filter aid and anion exchange resin. Such radioactive organic waste is hereinafter also referred to as “radioactive spent resin” or simply referred to as “spent resin” or “organic waste”. The radioactive organic waste is stored in a storage tank over a long period of time. The radioactive organic waste is generated steadily with the operation of a nuclear power plant. And the radioactive organic waste is due to be subjected to treatments such as stabilization and volume reduction and to be ultimately disposed of by burial in the ground after the storage.

The ion exchange resins include styrene-divinylbenzene as a base material, are chemically stable, and can be stored safely over a long period of time. The ion exchange resins, however, are hardly decomposable due to their stability and generally require a thermal treatment at a high temperature in order to reduce their volume.

Exemplary methods for treating a radioactive spent ion exchange resin by a thermal decomposition (thermal treatment) can be found as a treatment method using plasma in Japanese Unexamined Patent Application Publication No. 2001-305287 (Patent Document 1); and as a treatment method using microwaves in Japanese Unexamined Patent Application Publication No. Sho 59-46899 (Patent Document 2). The treatment methods in Patent Document 1 and Patent Document 2 respectively promote volume reduction of the spent ion exchange resin.

To solve the problem, there proposed are treatment methods for the volume reduction of the radioactive spent ion exchange resin by another technique than thermal decomposition. Examples of them are as follows.

There are treatment methods of decomposing organic substances in the spent ion exchange resin with hydrogen peroxide. Typically, Japanese Unexamined Patent Application Publication No. Sho 61-270700 (Patent Document 3) describes a radioactive waste treatment method, in which the cellulosic filter sludge is hydrolyzed and liquefied with a cellulolytic enzyme to give a liquid, and the liquid is acted upon by hydrogen peroxide in the presence of an iron ion to oxidize and decompose the organic substances. Ferrous sulfate is used to give the iron ion in the working examples of this document. Japanese Unexamined Patent Application Publication No. Sho 58-161898 (Patent Document 4) discloses a method of bringing a radioactive spent ion exchange resin into contact with hydrogen peroxide in a ferric sulfate aqueous solution and whereby oxidizing and decomposing the ion exchange resin.

Japanese Unexamined Patent Application Publication No. Sho 63-40900 (Patent Document 5) describes a treatment method of the radioactive spent ion exchange resin. By the treatment method, radionuclides contained in a spent ion exchange resin are eluted with a sulfuric acid aqueous solution to remove most of the radioactive substances (radionuclides) from the spent ion exchange resin; the spent ion exchange resin is then converted into an inorganic substance and solidified by an incineration or a chemical decomposition; an eluate containing the radionuclides is incorporated with a divalent iron ion and a base to form ferrite particles; and the radionuclides are taken into the formed ferrite particles and thus separated from the eluate.

Japanese Unexamined Patent Application Publication No. Sho 63-188796 (Patent Document 6) describes a treatment method of a decontamination waste liquid. In the treatment method, a radioactive decontamination waste liquid is treated with a cation exchange resin, and whereby iron and radionuclides in the decontamination waste liquid are scavenged by the cation exchange resin and removed from the waste liquid. The decontamination waste liquid from which the radionuclides have been removed is solidified with cement in a metal drum. Independently, the iron and radionuclides scavenged by the cation exchange resin are eluted out with an organic acid (e.g., oxalic acid or formic acid) to give an eluate containing the eluted iron and radionuclides; and the eluate is given a liquid which converts the iron and radionuclides each into an oxide or hydroxide to be oxidized and decomposed. The oxide or hydroxide is separated from the eluate by a precipitation, and the separated oxide or hydroxide is stored for a radioactive decay. The eluate after the removal of iron and radionuclides becomes a clear water and reused in the nuclear power plant.

Japanese Unexamined Patent Application Publication No. Sho 57-9885 (Patent Document 7) discloses a composition for removing a metal oxide using oxalic acid and hydrazine. The technology is disclosed as not a volume reduction treatment technology, but a chemical cleaning technology relating to such volume reduction treatment.

Japanese Unexamined Patent Application Publication No. 2013-44588 (Patent Document 8) describes a treatment method for a spent resin in a nuclear power plant. The method is described as a treatment method for the volume reduction of filter sludge including a spent ion exchange resin and/or a filter aid. In the method, adsorbed radioactive metal ions are eluted out from the ion exchange resin by an action of oxalic acid (a kind of organic acids); and radionuclides included in crud including an iron oxide are dissolved and removed together with the crud, the crud being deposited on the resin surface. The organic acid (oxalic acid) for use in the treatment is decomposable typically by an oxidizing agent, and this enables the volume reduction of a waste liquid generated as a secondary waste.

SUMMARY OF THE INVENTION

The present invention provides a method for treating a radioactive organic waste, the radioactive organic waste including a cation exchange resin adsorbing radionuclide ions, the method including the step of bringing the radioactive organic waste into contact with an organic acid salt aqueous solution containing an organic acid salt and whereby desorbing the radionuclide ions from the cation exchange resin, in which the organic acid salt contained in the organic acid salt aqueous solution includes a cation that is more readily adsorbable by the cation exchange resin than hydrogen ion is.

This enables reduction in concentration of a radioactive substance in the radioactive organic waste and reduction in amount of a high-dose radioactive waste.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Disadvantages in known technologies to be improved are as follows.

The thermal decomposition treatment method in Patent Document 1 promotes the volume reduction of the spent resin. The method is, however, applied to such spent ion exchange resin containing radionuclides in a relatively high concentration. A high temperature treatment is required in order to decompose the spent resin which is chemically stable. For example, the high temperature treatment is a thermal treatment at 500° C. or high. This requires a remote control system typically for pressure reduction and atmosphere control, requires a sophisticated exhaust gas treatment system, and causes a treatment system for use in the method to have a complicated structure as a whole.

The decomposition treatment method in Patent Document 2 using hydrogen peroxide can employ a simple system, but gives a residual waste liquid containing a large amount of sulfate group as a result of the treatment. Hence, the method requires a neutralization treatment. Therefore, the volume reduction performance of the method is lower than the thermal treatment method using the plasma.

The volume reduction treatment by decomposition with hydrogen peroxide as in Patent Document 3 and Patent Document 4 gives a residual radioactive waste liquid containing a large amount of sulfate group derived from the exchange group of the ion exchange resin. Hence, the method requires the neutralization treatment. Therefore, the volume reduction performance of the radioactive waste by the method is lower than the thermal treatment method.

The spent ion exchange resin treatment method in Patent Document 5 employs an aqueous sulfuric acid solution as a desirable eluent for eluting radionuclides from the spent ion exchange resin. The method therefore disadvantageously suffers from the formation of a large amount of waste sulfuric acid. This requires a treatment such as collection and reuse of sulfuric acid typically by electrodialysis.

When iron and radionuclides adsorbed by a cation exchange resin are eluted out using an organic acid (e.g., oxalic acid or formic acid) as in the decontamination waste liquid treatment method in Patent Document 6, the radionuclides are insufficiently desorbed from the cation exchange resin and remain partially in the cation exchange resin. This has been experimentally verified by the present inventors.

The composition for the removal of the metal oxide in Patent Document 7 is adapted to be used not in the volume reduction treatment of the spent resin, but in the cleaning of a metal material.

The nuclear-power-plant spent resin treatment method using oxalic acid alone as described in Patent Document 8 requires a large amount of an oxalic acid solution because the method performs crud dissolution and elusion of adsorbed radioactive metal ions from the resin concurrently.

An object of the present invention is to reduce a concentration of a radioactive substance in a radioactive organic waste and to reduce an amount of a high-dose radioactive waste.

Embodiments of the present invention will be illustrated below.

First Embodiment

Initially, First Embodiment will be illustrated with reference toFIGS. 1 and 2.

FIG. 1illustrates the structure of a radioactive organic waste treatment system according to First Embodiment.

A radioactive organic waste treatment system1according to the present embodiment has a first cleaning tank3, a second cleaning tank4, an organic acid tank5, a transfer water tank6, an organic acid salt tank7, a transfer water tank8, and a cleaning waste liquid treatment tank9.

The first cleaning tank3includes an agitating equipment that includes agitator blades14and a motor15. The agitator blades14and the motor15are connected with a rotating shaft. An organic waste supply pipe12equipped with a transfer pump13is connected between a high-dose resin storage tank2and the first cleaning tank3. An organic acid supply pipe16is connected between a bottom of the organic acid tank5and a selector valve18; whereas a transfer water supply pipe17is connected between the bottom of the transfer water tank6and the selector valve18. The organic acid tank5is charged with an oxalic acid aqueous solution; whereas the transfer water tank6is filled with water acting as transfer water. A liquid supply pipe20is connected between the selector valve18and the first cleaning tank3and is equipped with a transfer pump19.

The second cleaning tank4includes agitating equipment that includes agitator blades23and a motor24. The agitator blades23and the motor24are connected with a rotating shaft. An organic waste transfer pipe22equipped with a transfer pump21is connected between the first cleaning tank3and the second cleaning tank4. An organic acid salt supply pipe25is connected between the bottom of the organic acid salt tank7and a selector valve27; whereas a transfer water supply pipe26is connected between the bottom of the transfer water tank8and the selector valve27. The organic acid tank5is filled with an ammonium formate aqueous solution; whereas the transfer water tank8is filled with water acting as transfer water.

A liquid supply pipe29is connected between the selector valve27and the second cleaning tank4and is equipped with a transfer pump28. An organic waste transfer pipe31is inserted into the second cleaning tank4and one end of the organic waste transfer pipe31extends to the vicinity of the bottom of the second cleaning tank4. The organic waste transfer pipe31is equipped with a transfer pump30.

An ozone injection pipe37having a multiplicity of nozzles is arranged at the bottom in the cleaning waste liquid treatment tank9. The ozone injection pipe37is connected via an ozone supply pipe38to an ozone supplier36. A waste liquid transfer pipe33is mounted into the first cleaning tank3and is connected to the cleaning waste liquid treatment tank9. The waste liquid transfer pipe33is equipped with a transfer pump32. A waste liquid transfer pipe35is mounted into the second cleaning tank4and is connected to the cleaning waste liquid treatment tank9. The waste liquid transfer pipe35is equipped with a transfer pump34. A gas exhaust pipe39is connected to the cleaning waste liquid treatment tank9. A waste liquid discharge pipe41is equipped with a transfer pump40and is mounted into the cleaning waste liquid treatment tank9.

A nuclear power plant generates a radioactive organic waste typically in a reactor water cleanup system and a fuel pool cooling cleanup system. The radioactive organic waste includes filter sludge including a cellulosic filter aid and an ion exchange resin. The radioactive organic waste is stored in the high-dose resin storage tank2over a long period of time. A transfer water tank10filled with water is connected via a transfer water supply pipe11to the high-dose resin storage tank2. The radioactive organic waste stored in the high-dose resin storage tank2includes crud removed from cooling water typically in the reactor water cleanup system and the fuel pool cooling cleanup system. The crud includes radionuclides such as cobalt-60. The ion exchange resin stored in the high-dose resin storage tank2includes adsorbed ions of radionuclides such as cobalt-60, cesium-137, carbon-14 and chlorine-36.

FIG. 2illustrates a procedure of a radioactive organic waste treatment method according to the present embodiment using the radioactive organic waste treatment system1inFIG. 1. In the following explanation, reference signs indicated by numbers alone correspond to the reference signs inFIG. 1.

Initially, a step of supplying the radioactive organic waste from the high-dose resin storage tank2to the first cleaning tank3will be illustrated. The step is performed upstream from a first cleaning step S51inFIG. 2.

A boiling water nuclear power plant generates filter sludge (radioactive organic waste) including a cellulosic filter aid and an ion exchange resin typically from the reactor water cleanup system and fuel pool cooling cleanup system. The filter sludge is stored in the high-dose resin storage tank2over a long period of time. To treat the radioactive organic waste stored in the high-dose resin storage tank2, the water in the transfer water tank10is supplied through the transfer water supply pipe11into the high-dose resin storage tank2to convert the radioactive organic waste in the high-dose resin storage tank2into slurry that is easily transferable.

The transfer pump13is driven to supply the slurry containing the radioactive organic waste from the high-dose resin storage tank2through the organic waste supply pipe12to the first cleaning tank3. The transfer pump13is stopped so as to stop the supply of slurry to the first cleaning tank3at the time when the level of the slurry containing the radioactive organic waste reaches a predetermined level in the first cleaning tank3. The transfer pump32is then driven to supply water contained in the slurry from the first cleaning tank3through the waste liquid transfer pipe33into the cleaning waste liquid treatment tank9. The water is handled as a waste liquid. The waste liquid brought into the cleaning waste liquid treatment tank9is treated in an after-mentioned cleaning waste liquid treatment step S52as with a cleaning waste liquid. The transfer pump40is driven to bring the waste liquid through the waste liquid discharge pipe41to a storage tank. The transfer pump32is stopped upon the completion of transfer of water contained in the slurry in the first cleaning tank3.

The first cleaning step S51(an organic acid treatment process) is performed thereafter. The first cleaning step S51mainly performs the dissolution of crud such as iron oxide by injecting an organic acid. The crud has been transferred together with the radioactive organic waste to the first cleaning tank3. The organic acid is used for reasons as follows.

Such organic acid includes carbon, hydrogen, oxygen and nitrogen as main constitutive elements and does not give a non-volatile residue in a waste liquid when an organic acid aqueous solution generated as a cleaning waste liquid in the first cleaning step S51is treated by oxidization with ozone (an organic acid oxidization treatment process). The organic acid for use herein is preferably at least one selected typically from formic acid, oxalic acid, carbonic acid, acetic acid, and citric acid.

The organic acid tank5is filled with an aqueous solution of oxalic acid as the organic acid. The oxalic acid aqueous solution may be a saturated aqueous solution and may have an oxalic acid concentration of about 0.8 mol/L. The first cleaning step S51performs operations as follows.

The selector valve18is operated to allow the organic acid supply pipe16to communicate with the liquid supply pipe20, and the transfer pump19is driven. The oxalic acid aqueous solution in the organic acid tank5is supplied through the organic acid supply pipe16and the liquid supply pipe20to the first cleaning tank3. In this process, the water in the transfer water tank6is not supplied to the first cleaning tank3because the transfer water supply pipe17does not communicate with the liquid supply pipe20. The transfer pump19is stopped so as to stop the supply of the oxalic acid aqueous solution to the first cleaning tank3at the time when the liquid level of the oxalic acid aqueous solution in the first cleaning tank3reaches a preset level. The oxalic acid aqueous solution may be supplied into the first cleaning tank3in an amount 10 times the amount of the radioactive organic waste in the first cleaning tank3.

A heater (not shown) is arranged on an outer surface of the first cleaning tank3and heats the oxalic acid aqueous solution in the first cleaning tank3to a temperature typically of 60° C. The temperature of the oxalic acid aqueous solution is held at 60° C. by controlling the thermal dose by the heater. While holding the temperature at 60° C., the motor15is driven to rotate the agitator blades14to thereby agitate the radioactive organic waste and the oxalic acid aqueous solution with each other in the first cleaning tank3. The radioactive organic waste is immersed in the oxalic acid aqueous solution for duration typically of 6 hours with agitation in the first cleaning tank3. Thus, the crud mixed with the radioactive organic waste is dissolved by the action of oxalic acid in the first cleaning tank3. The crud dissolution allows the radionuclides such as cobalt-60 contained in the crud to migrate into the oxalic acid solution. An iron component in the crud, when dissolved, forms iron (II) ion. The iron (II) ion may react with oxalic acid to form iron oxalate, and the iron oxalate might precipitate. To suppress the formation of iron oxalate, a small amount of an oxidizing agent (e.g., hydrogen peroxide) that converts the iron(II) ion to iron(III) ion may be fed to the first cleaning tank3according to necessity.

In the first cleaning step S51, the ion exchange resin forming part of the radioactive organic waste is immersed in oxalic acid as the organic acid. This allows part of the adsorbed radionuclides to be desorbed from the ion exchange resin. Specifically, oxalic acid dissociates into hydrogen ion and oxalic acid ion, and radionuclides adsorbed by a cation exchange resin and an anion exchange resin undergo ion exchange with the hydrogen ion and oxalic acid ion, respectively, and are desorbed from the ion exchange resins.

The first cleaning step S51is completed upon the lapse of 6 hours, i.e., the immersion time of the radioactive organic waste in the oxalic acid aqueous solution in the first cleaning tank3. The motor15and the heating of the first cleaning tank3by the heater are respectively stopped, and the transfer pump32is driven to supply, as a cleaning waste liquid, the oxalic acid aqueous solution containing the radionuclides from the first cleaning tank3through the waste liquid transfer pipe33into the cleaning waste liquid treatment tank9. The transfer pump32is stopped upon the completion of the transfer of the oxalic acid aqueous solution from the first cleaning tank3to the cleaning waste liquid treatment tank9.

A cleaning waste liquid treatment step S52is performed after the completion of the transfer of the oxalic acid aqueous solution to the cleaning waste liquid treatment tank9. In the cleaning waste liquid treatment step S52, ozone is supplied from the ozone supplier36through the ozone supply pipe38to the ozone injection pipe37for a predetermined time and is injected through the multiplicity of nozzles formed in the ozone injection pipe37into the oxalic acid aqueous solution in the cleaning waste liquid treatment tank9. Oxalic acid contained as an organic component in the oxalic acid aqueous solution is decomposed by the injected ozone. The oxalic acid reacts with ozone and is decomposed into carbon dioxide and water. The carbon dioxide and the remainder of ozone injected into the cleaning waste liquid treatment tank9are supplied through the gas exhaust pipe39to an off-gas treatment equipment (not shown), and a radioactive gas contained in the gas discharged to the gas exhaust pipe39is removed by the off-gas treatment equipment.

After the stop of ozone supply, the transfer pump40is driven to discharge the radionuclide-containing waste liquid in the cleaning waste liquid treatment tank9to the waste liquid discharge pipe41and is temporarily stored in a storage tank (not shown). A concentration-powdering step S54as follows is then performed. The waste liquid in the storage tank is powdered typically with a thin film dryer, housed in a metal drum, and solidified with cement. Such radioactive solidified article is handled as a high-dose waste and is stored in a predetermined storage area. The radioactive waste liquid discharged from the cleaning waste liquid treatment tank9may be concentrated by heating, thus reduced in volume, charged into a metal drum, and solidified with cement.

After the completion of the discharge of the oxalic acid aqueous solution from the first cleaning tank3to the cleaning waste liquid treatment tank9, the selector valve18is operated to allow the transfer water supply pipe17to communicate with the liquid supply pipe20; and the transfer pump19is driven to supply, as transfer water, water in the transfer water tank8through the transfer water supply pipe17and the liquid supply pipe20to the first cleaning tank3. In this process, the oxalic acid aqueous solution in the organic acid tank5is not supplied to the first cleaning tank3because the organic acid supply pipe16does not communicate with the liquid supply pipe20. The transfer pump19is stopped so as to stop the water supply to the first cleaning tank3at the time when a predetermined amount of water is supplied from the transfer water tank8to the first cleaning tank3, and the water level in the first cleaning tank3reaches a preset level.

The motor15is driven to rotate the agitator blades14to thereby agitate the radioactive organic waste and the water with each other in the first cleaning tank3. Thus, the radioactive organic waste is converted into slurry. The transfer pump21is driven to supply the slurry containing the radioactive organic waste from the first cleaning tank3through the organic waste transfer pipe22to the second cleaning tank4. When the slurry containing the radioactive organic waste is transferred from the first cleaning tank3, the water amount in the first cleaning tank3reduces, and this may impede the transfer of the radioactive organic waste from the first cleaning tank3. In this case, the transfer pump19may be driven according to necessity so as to supply water from the transfer water tank8into the first cleaning tank3. The transfer pump21is stopped and the transfer pump34is driven upon the completion of the transfer of the radioactive organic waste from the first cleaning tank3to the second cleaning tank4. The water in the second cleaning tank4is then discharged through the waste liquid transfer pipe35to the cleaning waste liquid treatment tank9. The water brought from the second cleaning tank4to the cleaning waste liquid treatment tank9is treated in the cleaning waste liquid treatment step S52as with the cleaning waste liquid. The transfer pump40is then driven to bring the treated water through the waste liquid discharge pipe41to a storage tank.

A second cleaning step S53(an organic acid salt treatment process) is performed when the transfer pump34is stopped so as to complete the water discharge from the second cleaning tank4to the cleaning waste liquid treatment tank9. The second cleaning step S53employs an organic acid salt to more efficiently desorb radionuclides adsorbed by the ion exchange resin (e.g., a cation exchange resin). The organic acid salt for use in the second cleaning step S53is desirably one capable of dissociating in an aqueous solution to form a cation that is more readily adsorbable by a cation exchange resin than the hydrogen ion is. Specifically, the organic acid salt is preferably such an organic acid salt that includes carbon, hydrogen, oxygen, and nitrogen as main constitutive elements and does not form a non-volatile residue in a waste liquid when the organic acid salt aqueous solution as a cleaning waste liquid after the completion of the second cleaning step S53is treated by oxidation typically with ozone (an organic acid salt oxidization treatment process). The organic acid salt is preferably a salt of an organic acid, where the salt is selected typically from ammonium salt, barium salt, and cesium salt; and the organic acid is selected typically from formic acid, oxalic acid, carbonic acid, acetic acid, and citric acid. The ammonium salt is decomposed into nitrogen gas and water by the oxidization treatment and can contribute to reduction in amount of radioactive waste more than barium salt and cesium salt do. The ammonium salt, barium salt, or cesium salt of formic acid, oxalic acid, carbonic acid, acetic acid, or citric acid dissociates in the aqueous solution into NH4+, Ba2+, or Cs+, respectively. The cations NH4+, Ba2+, and Cs+are more readily adsorbable by the cation exchange resin than hydrogen ion is.

The organic acid salt tank7is filled with an aqueous solution of ammonium formate as the organic acid salt. The ammonium formate aqueous solution may have an ammonium formate concentration of 1.2 mol/L. The second cleaning step S53performs operations as follows. The selector valve27is operated to allow the organic acid salt supply pipe25to communicate with the liquid supply pipe29; and the transfer pump28is driven. The ammonium formate aqueous solution is thus supplied from the organic acid salt tank7through the organic acid salt supply pipe25and the liquid supply pipe29to the second cleaning tank4. In this process, the water in the transfer water tank8is not supplied to the second cleaning tank4because the transfer water supply pipe26does not communicate with the liquid supply pipe29. The transfer pump28is stopped so as to stop the supply of the ammonium formate aqueous solution to the second cleaning tank4at the time when the liquid level of the ammonium formate aqueous solution in the second cleaning tank4reaches a preset level.

A heater (not shown) is arranged on an outer surface of the second cleaning tank4and heats the ammonium formate aqueous solution in the second cleaning tank4to a temperature typically of 60° C. The temperature of the ammonium formate aqueous solution is held at 60° C. by controlling the thermal dose applied by the heater. While holding the temperature at 60° C., the motor24is driven to rotate the agitator blades23to thereby agitate the radioactive organic waste and the ammonium formate aqueous solution with each other in the second cleaning tank4. While being agitated, the radioactive organic waste is immersed in the ammonium formate aqueous solution in the second cleaning tank4for duration typically of 2 hours. The radioactive organic waste includes a cation exchange resin adsorbing radionuclide ions. The adsorbed radionuclide ions are exchanged with ammonium ion and efficiently desorbed into the ammonium formate aqueous solution in the second cleaning tank4, where the ammonium ion is present in the ammonium formate aqueous solution and is more readily adsorbable by the cation exchange resin than hydrogen ion is. This remarkably reduces the amount of radionuclides adsorbed by the cation exchange resin.

The second cleaning step S53is completed upon the lapse of the immersion time, i.e., 2 hours, of the radioactive organic waste in the ammonium formate aqueous solution in the second cleaning tank4. The motor24and the heating of the second cleaning tank4by the heater are respectively stopped, the transfer pump34is driven to supply, as a cleaning waste liquid, the ammonium formate aqueous solution containing radionuclides from the second cleaning tank4through the waste liquid transfer pipe35into the cleaning waste liquid treatment tank9. The transfer pump34is stopped upon the completion of the transfer of the ammonium formate aqueous solution from the second cleaning tank4to the cleaning waste liquid treatment tank9.

The cleaning waste liquid treatment step S52is performed after the completion of the transfer of the ammonium formate aqueous solution to the cleaning waste liquid treatment tank9. In the cleaning waste liquid treatment step S52, ozone is supplied by the ozone supplier36to the ozone injection pipe37for a predetermined time and is injected into the ammonium formate aqueous solution in the cleaning waste liquid treatment tank9. Thus, ammonium formate contained as an organic component in the ammonium formate aqueous solution is decomposed by ozone. The ammonium formate reacts with ozone and is decomposed into carbon dioxide (gas), nitrogen gas, and water. Such gases are supplied through the gas exhaust pipe39to the off-gas treatment equipment (not shown).

After the stop of ozone supply, the transfer pump40is driven to discharge the waste liquid containing radionuclides from the cleaning waste liquid treatment tank9to the waste liquid discharge pipe41. The radionuclide-containing waste liquid is then temporarily stored in a storage tank (not shown). The concentration-powdering step S54is then performed, and the waste liquid in the storage tank is powdered typically with a thin film dryer, housed in a metal drum, and solidified with cement. The resulting radioactive solidified article is also handled as a high-dose waste and stored in a predetermined storage area. After ammonium formate is decomposed by ozone in the cleaning waste liquid treatment tank9, a radioactive waste liquid is discharged from the cleaning waste liquid treatment tank9. The radioactive waste liquid may be concentrated by heating and reduced in volume, and then charged into a metal drum and solidified with cement.

After the completion of the transfer of the ammonium formate aqueous solution to the cleaning waste liquid treatment tank9, the selector valve27is operated to allow the transfer water supply pipe26to communicate with the liquid supply pipe29; and the transfer pump28is driven to supply water from the transfer water tank8to the second cleaning tank4. The transfer pump28is stopped so as to stop the water supply from the transfer water tank8to the second cleaning tank4after a predetermined amount of water is supplied to the second cleaning tank4. The agitator blades23are rotated to agitate the radioactive organic waste and the water with each other in the second cleaning tank4to thereby form slurry containing the radioactive organic waste. The transfer pump30is driven to discharge the slurry containing the radioactive organic waste after cleaning from the second cleaning tank4to the organic waste transfer pipe31. The radioactive organic waste after cleaning and being discharged to the organic waste transfer pipe31includes substantially no crud, contains radionuclide ions adsorbed by the cation exchange resin in a still reduced amount, and thereby has a remarkably lower radiation dose rate.

The radioactive organic waste discharged to the organic waste transfer pipe31is temporarily stored in a storage tank (not shown). The radioactive organic waste taken out from the storage tank is incinerated typically in an incinerator. Ash formed by incineration is solidified with cement in a metal drum. The resulting solidified article is handled as a low-level radioactive waste.

In the present embodiment, the first cleaning step S51may employ one selected from formic acid, carbonic acid, acetic acid, and citric acid instead of oxalic acid; whereas the second cleaning step S53may employ an ammonium salt, barium salt, or cesium salt of one selected from oxalic acid, carbonic acid, acetic acid, and citric acid; or barium salt or cesium salt of formic acid, instead of ammonium formate.

The present embodiment enables reduction in amount of a high-dose radioactive waste and reduction in concentration of a radioactive substance contained in a radioactive organic waste. This is because the first cleaning step S51employs the oxalic acid aqueous solution and thereby enables the dissolution of an iron oxide component mixed with the radioactive organic waste; and the second cleaning step S53exchanges adsorbed radionuclide ions in the cation exchange range with ammonium ion contained in the ammonium formate aqueous solution, where the cation exchange resin is present as the radioactive organic waste. Even after the treatment with the oxalic acid aqueous solution, some radionuclide ions may be not desorbed from, but still adsorbed by the cation exchange resin. Particularly in this case, the present embodiment can efficiently desorb the residual adsorbed radionuclide ions from the cation exchange resin by bringing the ammonium formate aqueous solution into contact with the radioactive organic waste.

Specifically, the present embodiment utilizes the action of an organic acid salt aqueous solution such as the ammonium formate aqueous solution and can desorb a larger amount of adsorbed radionuclide ions from the cation exchange resin than that of the method in Patent Document 6 in which adsorbed radionuclide ions are desorbed from the cation exchange resin by the organic acid aqueous solution (e.g., the oxalic acid aqueous solution).

The present embodiment can still reduce the concentration of a radioactive substance contained in the radioactive organic waste such as the cation exchange resin and can reduce the amount of a high-dose radioactive waste (amount of the cation exchange resin adsorbing radionuclide ions). In addition, the present embodiment employs the oxidization treatment to decompose organic components in the cleaning waste liquid and performs concentration or dry powdering of the residual waste liquid. The organic components are oxalic acid contained in the oxalic acid aqueous solution; and ammonium formate contained in the ammonium formate aqueous solution. Thus, the embodiment can still further reduce the amount of the high-dose radioactive waste.

In an embodiment of the radioactive organic waste treatment system1, the liquid supply pipe29and the organic waste transfer pipe31may be connected to the first cleaning tank3without employing the second cleaning tank4, the transfer pumps21and34, and the organic waste transfer pipes22and35. When the radioactive organic waste treatment system1having the structure according to this embodiment is employed, the first cleaning step S51and the second cleaning step S53can be performed by supplying the radioactive organic waste from the high-dose resin storage tank2into the first cleaning tank3; and then supplying the oxalic acid aqueous solution and the ammonium formate aqueous solution sequentially to the first cleaning tank3. The radioactive organic waste treatment system can undergo size reduction because of not using the second cleaning tank4, the transfer pumps21and34, and the organic waste transfer pipes22and35. In addition, the system can perform the radioactive organic waste treatment in a shorter time because the system eliminates the need of transferring the radioactive organic waste from the first cleaning tank3to the second cleaning tank4.

Second Embodiment

A radioactive organic waste treatment method according to Second Embodiment will be illustrated below as another preferred embodiment of the present invention. The radioactive organic waste treatment method according to the present embodiment may be adapted to the treatment of a radioactive organic waste generated in a boiling water nuclear power plant.

FIG. 3illustrates a radioactive organic waste treatment system for use in the present embodiment.

The radioactive organic waste treatment system1A inFIG. 3corresponds to the radioactive organic waste treatment system1inFIG. 1, except for not using the second cleaning tank4, the transfer pumps21and34, and the organic waste transfer pipes22and35; arranging a cleaning tank3A instead of the first cleaning tank3inFIG. 1; and arranging an aqueous ammonia supply tank42instead of the organic acid salt tank7inFIG. 1. The aqueous ammonia supply tank42is filled with aqueous ammonia as a basic aqueous solution.

How the radioactive organic waste treatment system1A differs from the radioactive organic waste treatment system1inFIG. 1will be specifically described below.

An organic acid supply pipe16is connected to the bottom of the organic acid tank5. A transfer water supply pipe17is connected to the bottom of the transfer water tank6. An aqueous ammonia supply pipe45is connected to the bottom of aqueous ammonia supply tank42. The pipes16,17, and45are connected to a liquid supply pipe20that is in turn connected to the cleaning tank3A. The pipes16,17, and45are equipped with on-off valves43,44, and46, respectively. An organic waste transfer pipe31is connected to the cleaning tank3A. The other structure (configuration) of the radioactive organic waste treatment system1A is the same as with the radioactive organic waste treatment system1inFIG. 1.

The radioactive organic waste treatment method according to the present embodiment using the radioactive organic waste treatment system1A will be illustrated below.

According to the present embodiment, the first cleaning step S51and the second cleaning step S53are performed in the cleaning tank3A. A radioactive organic waste as slurry is supplied from the high-dose resin storage tank2through the organic waste supply pipe12to the cleaning tank3A. The transfer pump32is driven to discharge water in the cleaning tank3A though the waste liquid transfer pipe33to the cleaning waste liquid treatment tank9, as in First Embodiment. After the discharge of the water from the cleaning tank3A, the transfer pump32is stopped, the on-off valve43is opened, and the transfer pump19is driven to supply the oxalic acid aqueous solution from the organic acid tank5into the cleaning tank3A. After the supply of a predetermined amount of the oxalic acid aqueous solution to the cleaning tank3A, the on-off valve43is closed and the transfer pump19is stopped so as to stop the supply of the oxalic acid aqueous solution to the cleaning tank3A.

The agitator blades14are rotated to start agitation of the oxalic acid aqueous solution and the radioactive organic waste with each other in the cleaning tank3A; the oxalic acid aqueous solution is heated to 60° C.; and the first cleaning step S51is started. The radioactive organic waste is immersed in the oxalic acid aqueous solution for 6 hours in the cleaning tank3A, and thereby crud mixed with the radioactive organic waste is dissolved by the action of oxalic acid. In addition, some of adsorbed radionuclide ions are desorbed from the cation exchange resin.

After the lapse of 6 hours, the on-off valve46is opened, and the transfer pump19is driven. The aqueous ammonia is supplied from the aqueous ammonia supply tank42through the liquid supply pipe20into the cleaning tank3A. In the cleaning tank3A, the oxalic acid aqueous solution is neutralized with the aqueous ammonia and thereby forms ammonium oxalate as an organic acid salt. This results in immersion of the radioactive organic waste in an ammonium oxalate aqueous solution in the cleaning tank3A, and the second cleaning step S53is thus started. The transfer pump19is stopped and the on-off valve46is closed after the supply of a predetermined amount of the aqueous ammonia to the cleaning tank3A.

A part of the radioactive organic waste is a cation exchange resin adsorbing radionuclide ions. The adsorbed radionuclide ions are exchanged with ammonium ion in the ammonium oxalate aqueous solution and desorbed into the ammonium oxalate aqueous solution, as in First Embodiment. The step of immersing the radioactive organic waste in the ammonium oxalate aqueous solution may be performed for 2 hours. The desorption of the adsorbed radionuclide ions from the cation exchange resin is continuously performed during the step, and the amount of the radionuclide ions adsorbed by the cation exchange resin is significantly reduced.

The second cleaning step S53is completed upon the completion of the immersion of the radioactive organic waste in the ammonium oxalate aqueous solution for 2 hours. At this time, the rotation of the agitator blades14is stopped, and the transfer pump32is driven to transfer the ammonium oxalate aqueous solution from the cleaning tank3A to the cleaning waste liquid treatment tank9. Ozone is supplied to the ammonium oxalate aqueous solution in the cleaning waste liquid treatment tank9to decompose ammonium oxalate into nitrogen gas, carbon dioxide gas, and water.

After the completion of the cleaning waste liquid treatment step S52by the ozone supply into the cleaning waste liquid treatment tank9, a waste liquid is discharged from the cleaning waste liquid treatment tank9to the waste liquid discharge pipe41and temporarily stored in a storage tank (not shown). The waste liquid in the storage tank is powdered typically with a thin film dryer, housed in a metal drum, and solidified with cement.

The present embodiment offers advantageous effects as given by First Embodiment.

In addition, the radioactive organic waste treatment system1A for use in the present embodiment can have a size smaller than that of the radioactive organic waste treatment system1. This is because the system1A does not require the second cleaning tank4, the transfer pumps21and34, and the organic waste transfer pipes22and35. The present embodiment enables the treatment of the radioactive organic waste using the downsized radioactive organic waste treatment system1A. The present embodiment can perform the radioactive organic waste treatment in a shorter time. This is because the present embodiment can perform the first cleaning step S51and the second cleaning step S53both in the cleaning tank3A and, unlike First Embodiment, does not require the transfer of the radioactive organic waste from the first cleaning tank3to the second cleaning tank4.

In addition, the present embodiment can perform the radioactive organic waste treatment in a still shorter time. The reason is as follows. According to the present embodiment, aqueous ammonia is added to the oxalic acid aqueous solution in the cleaning tank3A after the completion of the first cleaning step S51. The oxalic acid aqueous solution is thereby neutralized and forms an ammonium oxalate aqueous solution as an organic acid salt aqueous solution. This eliminates the need of the transfer of the oxalic acid aqueous solution acting as the organic acid aqueous solution from the cleaning tank3A to the cleaning waste liquid treatment tank9. This also eliminates the need of the cleaning waste liquid treatment step S52for an oxalic acid aqueous solution in the cleaning waste liquid treatment tank9.

In another embodiment, a formic acid aqueous solution may be employed as the organic acid aqueous solution for use in the first cleaning step S51; and an ammonium formate aqueous solution may be employed as the organic acid salt aqueous solution for use in the second cleaning step S53. Even this embodiment can be performed as with the present embodiment. Specifically, after the completion of the first cleaning step S51, aqueous ammonia is added to the formic acid aqueous solution in the cleaning tank3A in which the radioactive organic waste is immersed; and an ammonium formate aqueous solution is formed as the organic acid salt aqueous solution in the cleaning tank3A as a result of formic acid neutralization. The second cleaning step S53for the radioactive organic waste is performed using the ammonium formate aqueous solution in the cleaning tank3A.

According to the present embodiment, the organic acid for use in the first cleaning step S51may correspond to (be identical to) the base component of the organic acid salt (formic acid ion moiety of formic acid, or oxalic acid ion moiety of oxalic acid) for use in the second cleaning step S53. In this case, the solid-liquid separation (separation of the radioactive organic waste from the organic acid aqueous solution) is not performed after the first cleaning step S51, but the organic acid in contact with the radioactive waste liquid is neutralized with a basic aqueous solution (e.g., aqueous ammonia) to form an organic acid salt aqueous solution, and the formed organic acid salt aqueous solution is used to clean the radioactive organic waste in the second cleaning step S53. As used herein the term “base component” refers to a Broensted base, namely, a component that receives hydrogen ion.

Third Embodiment

A radioactive organic waste treatment method according to Third Embodiment will be illustrated below as still another preferred embodiment of the present invention. The radioactive organic waste treatment method according to the present embodiment may be adapted to the treatment of a radioactive organic waste generated in a pressurized water nuclear power plant.

FIG. 4illustrates a radioactive organic waste treatment system for use in the present embodiment.

The radioactive organic waste generated in the pressurized water nuclear power plant does not include crud such as iron oxide, unlike the radioactive organic waste generated in the boiling water nuclear power plant. The treatment for the radioactive organic waste generated in the pressurized water nuclear power plant does not require the first cleaning step S51for dissolving crud using an organic acid aqueous solution.

The radioactive organic waste treatment system1B is used for the radioactive organic waste treatment according to the present embodiment so as to treat the radioactive organic waste generated in the pressurized water nuclear power plant. As illustrated inFIG. 4, the system1B corresponds to the radioactive organic waste treatment system1inFIG. 1, except for not employing the first cleaning tank3, the organic acid tank5, the transfer water tank6, the transfer pumps19,21, and32, the liquid supply pipe20, and the organic waste transfer pipes22and33; and except for connecting the second cleaning tank (hereinafter also simply referred to as “cleaning tank”)4to the organic waste supply pipe12. The other configurations of the radioactive organic waste treatment system1B are as with the radioactive organic waste treatment system1inFIG. 1.

The radioactive organic waste generated in the pressurized water nuclear power plant is stored in the high-dose resin storage tank2. The radioactive organic waste is supplied from the high-dose resin storage tank2through the organic waste supply pipe12to the cleaning tank4and undergoes the radioactive organic waste treatment method according to the present embodiment. The method according to the present embodiment subjects the radioactive organic waste not to the first cleaning step S51as in First Embodiment, but to the second cleaning step S53; and subjects a waste liquid generated in the second cleaning step S53to the cleaning waste liquid treatment step S52. Slurry containing the radioactive organic waste is supplied to the cleaning tank4, and water in the cleaning tank4is discharged to the cleaning waste liquid treatment tank9. An organic acid salt aqueous solution such as an ammonium formate aqueous solution is then supplied from the organic acid salt tank7into the cleaning tank4. The radioactive organic waste in the cleaning tank4is immersed in the ammonium formate aqueous solution for 2 hours. The radioactive organic waste includes a cation exchange resin adsorbing radionuclide ions. The adsorbed radionuclide ions are exchanged with ammonium ion in the ammonium formate aqueous solution and thereby desorbed from the cation exchange resin into the ammonium formate aqueous solution.

After the completion of the second decontamination step (second cleaning step) for 2 hours, the ammonium formate aqueous solution is discharged from the cleaning tank4to the cleaning waste liquid treatment tank9. The cleaning waste liquid treatment step S52is performed in the cleaning waste liquid treatment tank9by supplying ozone to the ammonium formate aqueous solution to decompose ammonium formate into nitrogen gas, carbon dioxide gas, and water. After the completion of the cleaning waste liquid treatment step S52, a radioactive waste liquid may be discharged from the cleaning waste liquid treatment tank9, powdered typically with a thin film dryer, housed in a metal drum, and solidified with cement. The radioactive waste liquid may also be concentrated by heating, housed in a metal drum, and solidified with cement.

The method according to the present embodiment treats the radioactive organic waste with an organic acid salt aqueous solution such as an ammonium formate aqueous solution. As in First Embodiment, the use of ammonium formate aqueous solution enables the desorption of adsorbed radionuclide ions from the cation exchange resin in a larger amount than that of the technique disclosed in Patent Document 6 where adsorbed radionuclide ions are desorbed from a cation exchange resin by the action of an organic acid aqueous solution (e.g., an oxalic acid aqueous solution). The method can still reduce the concentration of radionuclides in a radioactive organic waste typified by a cation exchange resin and can reduce the amount of a high-dose radioactive waste (amount of the cation exchange resin adsorbing radionuclide ions). Here, radionuclide ions adsorbed by an anion exchange resin can be removed by an oxalate ion contained in the oxalic acid aqueous solution. And the radionuclide ions can be removed by the formate ion contained in the ammonium formate aqueous solution. In addition, the method employs the oxidization treatment to decompose organic components in the cleaning waste liquid and employs the concentration or dry powdering of the residual waste liquid. The organic components are exemplified by oxalic acid contained in the oxalic acid aqueous solution; and ammonium formate contained in the ammonium formate aqueous solution. The method can thereby still reduce the amount of a high-dose radioactive waste.

The radioactive organic waste treatment system1B for use in the present embodiment can have a smaller size than that of the radioactive organic waste treatment system1. This is because the system1B does not require the facilities such as the first cleaning tank3and the organic acid tank5to be arranged in the radioactive organic waste treatment system1, as described above.

How to reduce the amount of a cleaning agent for use in chemical cleaning of an organic waste generated from nuclear facilities will be illustrated.

FIG. 5is a flow chart schematically illustrating a treatment method for an organic waste such as a spent ion exchange resin or filter sludge.

The organic waste treatment method illustrated inFIG. 5includes a first cleaning step S101, a second cleaning step S102, and a waste liquid decomposition step S103. The first cleaning step S101decomposes crud with an aqueous solution of a reducing organic acid, where the crud is deposited on the organic waste. The second cleaning step S102is performed after the step S101and elutes adsorbed radioactive metal ions from the organic waste using an organic acid salt aqueous solution. The waste liquid decomposition step S103decomposes organic substances by heat or an oxidizing agent such as hydrogen peroxide or ozone, where the organic substances are contained in a crud solution and a radionuclide eluate generated in the first cleaning step S101and the second cleaning step S102, respectively.

The first cleaning step S101is performed in order to dissolve and remove radionuclides such as Co-60 (cobalt-60) together with the crud by the action of the reducing organic acid aqueous solution, where the radionuclides are incorporated in the crud deposited on the organic waste. In addition, the step is expected to advantageously elute part of adsorbed radioactive metal ions from the ion exchange resin.

The second cleaning step S102is performed in order to efficiently elute adsorbed radioactive metal ions from the organic waste with a solution of an organic acid salt. The organic acid salt for use herein is desirably one that forms an ion having ion selectivity for the organic waste higher than those of hydrogen ion and the organic acid ion; or forms an ion capable of forming a stable complex with a radioactive metal ion adsorbed by the organic waste. In an embodiment, a non-volatile ion may be added in an amount approximately corresponding to the ion exchange capacity of the ion exchange resin. This enables still efficient elution of the radioactive metal ions. Here, the ion having the ion selectivity for the organic waste higher than those of the hydrogen ion is typically hydrazine ion. The organic acid ion is typically oxalate ion. Further, the ion having the ion selectivity for the organic waste higher than those of the oxalate ion is formate ion or carbonate ion, for example. Furthermore, the ion capable of forming the stable complex is typically oxalate ion or citrate ion.

The organic acid and organic acid salt for use in embodiments of the present invention preferably include at least one element selected typically from carbon, hydrogen, oxygen, nitrogen and do not give a non-volatile residue in a waste liquid after oxidization decomposition or thermal decomposition of the cleaning waste liquid. The organic acid is exemplified by oxalic acid and citric acid. The organic acid salt is exemplified by hydrazine salts of oxalic acid, citric acid, formic acid, carbonic acid, and acetic acid. The organic acid salt is preferably hydrazine oxalate or hydrazine citrate which includes an organic acid having reducibility.

The non-volatile ion may be added to the organic acid salt in an amount corresponding approximately to the ion exchange capacity of the ion exchange resin. The non-volatile ion is added in an amount of less than 1% of the resin organic waste amount and may probably not substantially affect the volume reduction of the resulting waste. The non-volatile ion is exemplified by potassium ion, zinc ion, calcium ion, and cobalt ion.

The second cleaning step S102elutes the adsorbed radioactive metal ions from the organic waste by the action of the organic acid salt and thereafter gives a waste. The waste is subjected to incineration or solidification (S104). The waste liquid decomposition step S103decomposes the organic substances in the crud solution and radionuclide eluate and thereafter gives a radionuclide solution. The radionuclide solution is subjected to volume reduction (S105), and the residue of which is charged into a container or solidified (S106). Here, the volume reduction (S105) is carried out by a concentration process or a dry powdering process.

The treatment method according to the present embodiment basically includes the steps as mentioned above, but may be modified as follows. Initially, the first cleaning step S101and the second cleaning step S102may be performed step by step in an identical cleaning tank (facilities in the same block).

The organic waste may be heated during the first cleaning step S101and the second cleaning step S102. The solutions of the organic acid and organic acid salt may be supplied continuously or intermittently in the two steps during the immersion treatment of the organic waste in the solutions of the organic acid and organic acid salt, respectively.

The first cleaning step S101can be omitted when the organic waste includes substantially no crud such as iron oxide. The first cleaning step S101can also be omitted when the second cleaning step S102employs an organic acid salt capable of dissolving the crud.

Independently, the second cleaning step S102can be omitted when the first cleaning step S101employs an organic acid capable of efficiently eluting adsorbed radioactive metal ions from the organic waste.

The first cleaning step S101and the second cleaning step S102generate a crud solution and a radionuclide eluate, respectively. The crud solution and the radionuclide eluate may be subjected to the waste liquid decomposition step S103in an identical tank (facilities of the same block) at different times or simultaneously.

Fourth Embodiment

FIG. 6illustrates an organic waste treatment system according to Fourth Embodiment.

The treatment system inFIG. 6includes a chemical cleaning unit101that treats an organic waste; and a waste liquid decomposing unit102that treats a cleaning waste liquid. A first cleaning step S101and a second cleaning step S102are performed in the chemical cleaning unit101(facilities of the same block). The first cleaning step S101dissolves crud; whereas the second cleaning step S102elutes radioactive metal ions from the organic waste.

The chemical cleaning unit101includes a first receiver tank202, a chemical reaction tank204, and a cleaning liquid supply tank206. The waste liquid decomposing unit102includes an ozone decomposition system209, a treated water collection tank210, a dry powdering system211, and a solidification system212.

A chemical cleaning organic waste is stored in an organic waste storage tank201. Slurry containing about 10 percent by weight of the organic waste is drawn from the organic waste storage tank201and transferred in a predetermined amount to the first receiver tank202in the chemical cleaning unit101. The organic waste is then transferred by a transfer pump221to the chemical reaction tank204. An oxalic acid aqueous solution is supplied in an amount of about 72 g/L from the cleaning liquid supply tank206to the transferred organic waste in the chemical reaction tank204by a transfer pump222. Thus, the dissolution treatment of crud deposited on the organic waste is performed in the chemical reaction tank204. Oxalic acid is used herein as an exemplary organic acid.

The oxalic acid solution to be supplied from the cleaning liquid supply tank206to the chemical reaction tank204may be a saturated solution and have a concentration of about 0.8 mol/L. An aqueous citric acid solution may be used instead of the oxalic acid aqueous solution. The organic acids have reducibility. Temperature control equipment205is arranged so as to heat the chemical reaction tank204. The heating may be performed to a temperature of lower than 100° C.

In an embodiment, oxalic acid alone may be collected by precipitating a crud contained in a crud solution generated in the treatment and thereafter separating its supernatant liquid etc., and the collected oxalic acid may be transferred to the cleaning liquid supply tank206by a transfer pump223and reused in the crud dissolution. The ultimately generated crud solution is handled as a cleaning waste liquid and transferred to the ozone decomposition system209in the waste liquid decomposing unit102.

A hydrazine formate aqueous solution is continuously supplied in an amount of about 40 to about 400 g/L from the cleaning liquid supply tank206to the residual organic waste after crud dissolution in the chemical reaction tank204. Thus, an elution treatment of adsorbed radioactive metal ions from the organic waste is performed. The hydrazine formate aqueous solution for use herein may be a neutral solution having a pH of about 7. Here, concentration of the hydrazine formate aqueous solution is a mass of its solute (the hydrazine formate) per 1 liter of the aqueous solution.

The treatment generates a radionuclide eluate. In an embodiment, the hydrazine formate aqueous solution alone may be collected from the radionuclide eluate, and the collected hydrazine formate aqueous solution may be transferred to the cleaning liquid supply tank206and reused in the elution of radioactive metal ions. A hydrazine salt of oxalic acid, acetic acid, or citric acid may be used herein instead of hydrazine formate. The ultimately generated radionuclide eluate is handled as a cleaning waste liquid and transferred to the ozone decomposition system209.

When performed with respect to Co-60 adsorbed by the organic waste, the decontamination process (cleaning process) offers a decontamination performance in terms of decontamination factor DF of about 4 when employing oxalic acid as the cleaning agent; and offers better decontamination performance in terms of a DF of 20 or more when employing the hydrazine formate as the cleaning agent. It is necessary to add the oxalic acid many times in order to obtain the DF of 20 or more when employing only the oxalic acid as the cleaning agent. On the other hand, it is not necessary to add the hydrazine formate many times when employing the hydrazine formate as the cleaning agent. Thus, it is possible to decrease the amount used of the cleaning agent. As used herein the term “decontamination factor DF” refers to a numerical value as determined by dividing the counting rate before decontamination by the counting rate after decontamination. In addition, the decontamination process (an ion elution) employing the hydrazine formate is carried out after the decontamination process (a crud dissolution) employing oxalic acid. Thus, the ion elution is not carried out when employing oxalic acid as the cleaning agent. Therefore, the term “decontamination factor DF” refers to a numerical value as determined by dividing the counting rate before the decontamination by the counting rate after decontamination of only the crud dissolution. On the other hand, when the ion solute is carried out, the term “decontamination factor DF” refers to a numerical value as determined by dividing the counting rate before the decontamination by the counting rate after decontamination of the crud dissolution and the ion solute.

The organic waste after the cleaning is drawn as slurry by weight from the chemical reaction tank204and transferred to a second receiver tank207, where the slurry has an organic waste concentration of about 10 percent. The organic waste is transferred in a certain amount to an incineration system or cement solidification system208and is incinerated or solidified with cement.

Oxalic acid and hydrazine formate contained in the cleaning waste liquid transferred to the ozone decomposition system209are decomposed typically into carbon dioxide, nitrogen, and water by ozone decomposition. This converts organic substances in the cleaning waste liquid into inorganic substances and allows solids components in the waste liquid to be crud, eluted radioactive metal ions, and other salts.

A radionuclide solution formed by ozone decomposition is collected into the treated water collection tank210, transferred in a certain amount to the condensation system or dry powdering system211by a pump224, and is subjected to a concentration or dry powdering treatment.

The resulting residue is transferred to the container filling system or solidification system212and stored as filled in the container. The residue may be solidified with cement or another solidification agent.

Fifth Embodiment

FIG. 7illustrates an organic waste treatment system according to Fifth Embodiment.

The treatment system inFIG. 7includes a chemical cleaning unit103that supplies a cleaning liquid containing a non-volatile ion to an organic waste; and a waste liquid decomposing unit102that treats a cleaning waste liquid. Using the treatment system, the first cleaning step S101and the second cleaning step S102are performed in the same block as in Fourth Embodiment.

The organic waste is drawn as slurry from the organic waste storage tank201, transferred to the first receiver tank202, and transferred to the chemical reaction tank204by a pump221. An oxalic acid solution is fed to the chemical reaction tank204, followed by crud dissolution. The concentration and amount of the oxalic acid solution, and the temperature in the process are as in Fourth Embodiment.

After the crud dissolution, cobalt (as ion) is fed from non-volatile ion supply tank213(non-volatile ion storage tank) and added to hydrazine formate for use in the elution of radioactive metal ions. The cobalt (ion) is added in an amount corresponding to about 3 meq/L of the ion exchange capacity of the organic waste to be treated. The resulting mixture is supplied as an eluent to the chemical reaction tank204, followed by elution of radioactive metal ions. The eluent for use herein may be a neutral liquid having a pH of 7 and may be supplied in an amount as in Fourth Embodiment. The treatment method according to the present embodiment offers decontamination performance with respect to Co-60 in terms of a DF of 1000 or more, indicating significantly better decontamination performance than that in Fourth Embodiment. Equivalent or better decontamination performance can be obtained by using an aqueous solution containing potassium ion, zinc ion or calcium ion instead of cobalt ion (an aqueous solution of cobalt sulfate, cobalt nitrate or cobalt chloride) to be added to hydrazine formate.

A cleaning waste liquid generated in the chemical cleaning unit103is transferred to the ozone decomposition system209and is treated as in Fourth Embodiment.