Patent Number: 054769892
Section: summary

FIELD OF THE INVENTION This invention relates to an adsorbent useful for the adsorption of radioactive nuclides which generate, for instance, in the course of reprocessing steps for the separation and recovery of valuable substances such as uranium, plutonium and the like from nuclear fuel used in a nuclear reactor, and to a process for the volume-reduction treatment of radioactive waste that contains radioactive nuclides. BACKGROUND OF THE INVENTION Various types of liquid waste accumulated at reprocessing facilities after treatment of spent nuclear fuel discharged from nuclear power stations contain many radioactive nuclides including long-lived .beta. and .gamma. nuclides of cesium and the like and transuranium elements such as uranium, plutonium and the like. For the treatment of radioactive liquid waste, it is necessary to reduce the amount of radiation by separating and removing radioactive nuclides from the liquid waste in order to reduce radiation exposure. In general, radioactive liquid waste is treated by means of evaporation concentration, coagulating sedimentation, ion exchanging and the like. In the evaporation concentration process, liquid waste to be treated is put in an evaporator and heated under atmospheric or reduced pressure to allow only moisture to evaporate, thereby concentrating the radioactive liquid waste to a reduced volume. The evaporated moisture is recovered using a condenser. On the other hand, the thus concentrated liquid waste is subjected to further treatment such as bituminization or the like depending on the radioactive nuclides present in the waste. The evaporation concentration process, however, has disadvantages in that because corrosion-resistant materials are required, the decontamination factor (DF) decreases due to evaporation of radioactive nuclides which also occurs and the volume-reducing effect is not sufficient. In the coagulating sedimentation process, radioactive nuclides in the liquid waste are removed after their coagulation and precipitation. Radioactive nuclide-including sludge in which the radioactive nuclides are incorporated is subjected to dehydration treatment, and the resulting residue is treated as solid waste, and the supernatant fluid is treated as low-level liquid waste. In the coagulating sedimentation process, however, the sludge formed has a high moisture content which causes a difficulty in carrying out the dehydration treatment, thus entailing a disadvantage in that the volume-reducing effect is not sufficient. In the ion exchanging process, ions of interest are removed from the liquid waste by conducting ion exchange using an ion exchange resin. The spent resin containing the ions of interest is treated as solid waste, and the liquid portion after the treatment is treated as low-level liquid waste. A chelate resin may be used instead of an ion exchange resin in a process similar to the ion exchange process. However, when an ion exchange or chelate resin which is commonly used for the removal of metals from general liquid waste is applied to the treatment of radioactive liquid waste, it is difficult to use such a resin because of the tendency toward deterioration of such an organic polymer resin due to the action of radiation. Even where such an application could be effected, a problem of selectivity occurs. For example, virtually nothing is known about an adsorbent useful for the selective separation and removal of transuranium elements such as plutonium and the like which are present in a small amount in liquid waste of high uranium content. Inorganic adsorbents may have radioactive resistance, but nothing is known on an inorganic adsorbent which has excellent adsorptivity. On the other hand, an adsorbent in which a ferrocyanate as an inorganic functional group is supported on an acrylic fiber as an organic support is disclosed in JP-B-63-24415 (the term "P-B" as used herein means an "examined Japanese patent publication"). This adsorbent, however, has disadvantages in that the functional group is not selective for the adsorption of transuranium elements and the support, being organic, has poor durability. Also, JP-B-60-51491 discloses a phenol-based chelate resin which has an aminomethylphosphonic acid-type functional group, and which is described as having excellent uranium-adsorbing ability. This resin, however, is not capable of selectively adsorbing plutonium present in radioactive liquid waste. In addition to the above described disadvantages, the ion exchanging and chelate resin processes have other problems in that each of these processes generates a large quantity of secondary wastes such as incombustible or flame retardant spent resin, liquid waste after resin washing and the like, and insufficient volume-reducing effect arises. Although the volume of spent organic ion exchange resin may be reduced by incineration, generation of harmful gas, formation of smoke dust and scattering of radioactive nuclides all occur. In addition to such problems, the resin cannot be incinerated completely, leaving a soft charcoal residue which causes another problem by scattering atomized dust containing radioactive nuclides when the residue is treated. As a result, incineration of this type of resin is practically impossible. Thus, as has been described above, the prior art adsorbents have common problems in that they have poor durability against radiation and transuranium elements in radioactive liquid waste are not adsorbed selectively. In addition, the prior art volume-reduction treatment methods have problems in that secondary wastes are generated in a large quantity, insufficient volume-reducing effect arises and the facility cost becomes high because of the necessity to use corrosion-resistant materials. SUMMARY OF THE INVENTION In view of the above, therefore, an object of the present invention is to provide an adsorbent for radioactive nuclides, which is durable against radiation and is capable of adsorbing transuranium elements selectively. Another object of the present invention is to provide a process for the volume-reduction treatment of radioactive waste, which enables significant volume reduction of radioactive nuclides-adsorbed waste and scattering of radioactive nuclides at the time of incineration does not occur. To achieve these objects, the inventors of the present invention have conducted intensive studies and they have found that an adsorbent of a fibrous active carbon system with an inorganic framework, especially with a specified equilibrium moisture regain, adsorbs radioactive nuclides excellently, that plutonium present in plutonium-bearing liquid waste can be separated and removed selectively and securely by using such an adsorbent and that, by incinerating the fibrous active carbon with excellent adsorptivity at a temperature higher than its ignition point, the fibrous active carbon alone can be gasified and scattered substantially completely while preventing adsorbed radioactive nuclides from being scattered. The present invention has been accomplished on the basis of these findings. Particularly, the present invention provides an adsorbent useful for the adsorption of radioactive nuclides which comprises fibrous active carbon having a specific surface area of 1,000 m.sup.2 /g or more and an equilibrium moisture regain of 10% or more at a relative humidity of 45%. The present invention further provides a process for the volume-reduction treatment of radioactive liquid waste which comprises subjecting radioactive liquid waste containing radioactive nuclides to an adsorption treatment using an adsorbent that comprises fibrous active carbon having a specific surface area of 1,000 m.sup.2 /g or more, and subsequently incinerating the adsorbent at a temperature higher than the ignition point of the fibrous active carbon. In this instance, the specific surface area is calculated by the so-called BET method based on nitrogen gas adsorption isotherm at liquid nitrogen temperature. These and other objects and advantages of the present invention will become apparent as the description progresses. DETAILED DESCRIPTION OF THE INVENTION Though not particularly limited, the material of fibrous active carbon which constitutes the adsorbent of the present invention may be selected, for example, from coal pitch, petroleum pitch, rayon, phenol fiber, acrylic fiber and the like. The fibrous active carbon may be produced by utilizing the conventional process as disclosed, for example, in U.S. Pat. No. 4,808,202, herein incorporated by reference. The specific surface area of the fibrous active carbon is not particularly limited, provided that the fibrous material has enough pores to adsorb the radioactive nuclides. However, since the adsorbed quantity of radioactive nuclides increases as the specific surface area increases, the fibrous active carbon has a specific surface area of preferably 1,000 m.sup.2 /g or more, more preferably from 1,500 to 2,500 m.sup.2 /g. The specific surface area may be controlled by changing the temperature or time with regard to the activating treatment. That is, the activating treatment at a higher temperature or for a longer period causes increase of the specific surface area. Also, the fibrous active carbon forming the adsorbent has an equilibrium moisture regain of preferably 10% or more, more preferably 15% or more, at a relative humidity of 45%. The equilibrium moisture regain at a relative humidity of 45% is measured in accordance with Kagakubinran, Kisohen, II, p. 143, edited by The Chemical Society of Japan, published by Maruzen Co. That is, a saturated KNO.sub.2 solution is placed in a sealed vessel and left at 20.degree. C. until the space of the vessel has a constant humidity (relative humidity: 45%). Then, a sample of fibrous active carbon is placed in the space and left until the amount of water adsorption is saturated. The moisture regain is measured, and the value thus measured is taken as the equilibrium moisture regain at a relative humidity of 45%. Since active carbon is mainly made of carbon, active carbon is generally non-polar and, therefore, its surface is hydrophobic. Accordingly, active carbon hardly adsorbs moisture at a low relative humidity of about 45%, thus showing a low equilibrium moisture regain. If active carbon is subjected to a carbonization treatment at a high temperature of from 500.degree.to 1,000.degree. C. during production, polar groups such as carboxyl, carbonyl, hydroxyl and the like groups are formed on the surface of the active carbon. This means that the active carbon becomes hydrophilic although its degree varies depending on the production process. Also, active carbon having high hydrophilic property can be produced by applying an addition treatment to commonly used active carbon as described later. The inventors of the present invention have conducted adsorption tests of radioactive nuclides using various fibrous active carbon preparations with different hydrophilic properties obtained in this method. As a result, they have found that the adsorptivity of fibrous active carbon becomes high as the hydrophilic degree of its surface increases. In other words, it is desirable for the fibrous active carbon to have an equilibrium moisture regain of 10% or more at a relative humidity of 45%. Although the reason for this is not clear while not desiring to be found, it appears that the capacity of fibrous active carbon to adsorb radioactive nuclides is improved when its surface is hydrophilic, because radioactive nuclides form complex compounds in radioactive liquid waste. A fibrous active carbon preparation having an equilibrium moisture regain of 10% or more at a relative humidity of 45% may be obtained by subjecting conventional fibrous active carbon to air oxidation, ozone oxidation or liquid phase oxidation, or by adding hydrophilic functional groups to the fibrous active carbon. The air oxidation treatment may be effected in the atmosphere at a temperature of from 300.degree.to 700.degree. C., preferably from 350.degree.to 600.degree. C. If the temperature is lower than 300.degree. C., a prolonged period of time necessary for the oxidation reaction is required, and if the temperature is higher than 700.degree. C., excess burning occurs reducing the volume of the fibrous active carbon. The oxidation time, although it varies depending on the heating temperature, is generally in the range of from 10 minutes to 5 hours. Alternatively, the air oxidation treatment may be carried out in the stream of heated air. The ozone oxidation may be effected by oxidizing fibrous active carbon in an ozone-containing atmosphere. The ozone concentration can be in the range of preferably from 100 to 500 ppm, more preferably from 250 to 450 ppm. If the ozone concentration is lower than 100 ppm, a prolonged period of time is necessary for the oxidation reaction, and if the concentration is higher than 500 ppm, the volume of the fibrous active carbon is reduced. The liquid phase oxidation may be effected by soaking fibrous active carbon in an oxidant solution for a period of from several hours to several days, followed by filtration, washing and drying in that order. The oxidizing agent may be selected from permanganates, chromates, hypochlorites, persulfates, bromic acid ion, chlorine, dilute nitric acid, concentrated nitric acid, hydrogen peroxide and the like. The functional groups to be added to fibrous active carbon are not particularly limited provided that they are hydrophilic in nature. Examples of suitable functional groups include hydroxyl, carboxyl, carbonyl, primary amino, secondary amino, tertiary amino, quaternary ammonium, sulfonic, phosphonic, ester, amide, nitroso, nitro, thiol, silanol, selenol and the like. Among these groups, hydroxyl, carboxyl, primary amino, secondary amino, tertiary amino, sulfonic, and phosphonic are preferable. These functional groups may be present alone or as a mixture of two or more. These functional groups can be added to fibrous active carbon for example: by adding a functional group-containing low or high molecular weight compound; by first adding a functional group-containing low molecular weight compound and then converting the thus obtained compound into a high molecular weight compound; by first adding a compound without any functional groups and then adding functional groups to the compound obtained; or by adding functional groups directly to the fibrous active carbon. When functional groups are added to the fibrous active carbon, the fibrous active carbon may have any optional shape and may therefore be molded into the form of web, sheet, cartridge or the like. Although the specific surface area of the fibrous active carbon decreases due to by the addition of functional groups, it is desirable to maintain the specific surface area at 1,000 m.sup.2 /g or more even after the addition reaction. According to the present invention, the fibrous active carbon which is used for the volume-reduction by incineration after adsorption of radioactive nuclides may have a composition in which the total amount of carbon, oxygen and hydrogen is 60% or more, preferably 80% or more. When a total of these components is less than 60%, flameless burning is not achieved appropriately at the time of incineration, and thus a harmful gas is sometimes generated. According to the present invention, liquid waste which contains radioactive nuclides may be subjected to an adsorption treatment using the adsorbent comprising fibrous active carbon with no pretreatment of the liquid waste, or after improving adsorptivity of the radioactive nuclides to the adsorbent by adding a complexing agent to the radioactive nuclides-bearing liquid waste to form complex compounds with the radioactive nuclides in the liquid waste. Examples of complexing agents which can be used include ethylenediaminetetraacetic acid (EDTA), tributyl phosphate, bis-(2-ethylhexyl) phosphate, mono-2-ethylhexyl 2-ethylhexylphosphonate, triethylamine, trioctylamine, phthalocyanine and the like. Among these agents, tributyl phosphate and bis-(2-ethylhexyl) phosphate are preferable. Some radioactive nuclides may change their ionic states and dispersion conditions depending on the acid concentration, thus changing their adsorptivity to the fibrous active carbon. For the purpose of improving the adsorptivity, the adsorption treatment may be carried out after adjusting the acid concentration to an appropriate level with the addition of an alkali or acid such as NaOH, HCl, HNO.sub.3 or the like, preferably with a 0.01-3N nitric acid solution. According to the present invention, practical treatment of radioactive liquid waste may be effected by employing any of the prior art means such as a batch method in an adsorption tank, a column process through an adsorption column and a combined means thereof. Alternatively, the fibrous active carbon may be molded into a sheet, a cartridge or the like, and a column process may be carried out using the molded product as an adsorbent. When the shape of adsorbent cannot be produced easily from the fibrous active carbon alone, an inorganic or organic binder may be blended with the fibrous active carbon. Also, the adsorbent may be formed by blending the fibrous active carbon with heat adhesive fibers and thermally fusing them at the time of molding. Examples of such fibers include copolymerized polyester fibers, polyolefin fibers made of polyethylene, polypropylene and the like, low melting point nylon and conjugated fibers made of polyester as the core and polyolefin as the sheath. These binders and fibers may be blended in an amount of 40% by weight or less, because amounts exceeding this range sometimes causes poor incineration. In the column process, radioactive liquid waste is passed through an adsorption column which has been packed with the adsorbent. The packed adsorbent layer may have a thickness of 200 mm or more, preferably in the range of from 500 to 2,000 mm. If the thickness of the packed layer is less than 200 mm, leakage occurs. Although the liquid passing rate varies depending on the properties of liquid waste to be treated, it is generally 0.5 hr.sup.-1 or more, preferably in the range of from 1 to 10 hr.sup.-1, as a space velocity (SV). In the cartridge process, liquid waste is passed through a cartridge of the fibrous active carbon which has been molded into a cylindrical or columnar form and set in a housing. The use of such a cartridge is quite effective especially when radioactive materials are handled, because it can be installed and detached easily and disposal of the spent cartridge is easy. Such a cartridge may be obtained, for example, by wet-molding the fibrous active carbon, or by firstly molding the fibrous active carbon into a sheet by a paper making process or a dry process and then tightly rolling the molded sheet. When molded into a cartridge, the fibrous active carbon may be blended with a small amount of inorganic and organic binders. According to the present invention, radioactive liquid waste, which is difficult to treat using prior art processes, can be processed easily, because plutonium can be adsorbed and removed selectively from plutonium-bearing liquid waste, by the use of an adsorbent which comprises fibrous active carbon having excellent selective adsorptivity and durability, especially having a specific surface area of 1,000 m.sup.2 /g or more and an equilibrium moisture regain of 10% or more at a relative humidity of 45%. According to the present invention, incineration of the radioactive nuclides-bearing adsorbent after the adsorption treatment can be carried out at a temperature which is equal to or higher than the ignition point of the adsorbent-constituting fibrous active carbon. The term "ignition point" as used herein means the temperature at which the temperature starts to increase sharply when a sample is heated in accordance with the ignition point measurement procedure of JIS-K-1474 (granulated active carbon test methods). When fibrous active carbon is heated at a temperature equal to or higher than its ignition point, it becomes red, starts flameless burning and its volume is reduced. Such a volume reduction seems to occur due to scattering of carbon converted into the form of carbon dioxide, because most portion of the fibrous active carbon is comprises carbon. As described above, incineration of the adsorbent is carried out at a temperature which is equal to or higher than the ignition point of the adsorbent-constituting fibrous active carbon, but preferably with an upper limit temperature of "ignition point +600.degree. C.". When fibrous active carbon is placed in a temperature atmosphere which is above a temperature of "ignition point +600.degree. C.", carbon bonds are severed by thermal decomposition simultaneously with the burning, thus burning with flames occurs. When fibrous active carbon burns with the emission of flames, some of radioactive nuclides scatter together with the flames, thus an additional gas-adsorbing filter is required. The flameless burning of fibrous activated carbon seems to occur due to its small apparent density (0.05 to 0.3 g/cm.sup.3) and high air content necessary for burning, because each fiber has a considerably small diameter (10 to 30 .mu.m) and innumerable pores. According to the present invention, the radioactive nuclides-bearing adsorbent removed after the adsorption treatment may be subjected to the incineration step as it is, but preferably after dehydration and drying treatments. As has been described above, according to the present invention, fibrous active carbon is used as an adsorbent and the radioactive nuclides-bearing waste removed after the adsorption treatment is subjected to an incineration treatment, thus making possible a marked reduction in the waste and prevention of the scattering of radioactive nuclides at the time of incineration, without bringing up a problem of the need for extra storage space due to increased amounts of waste. In addition, since only a burned residue, mainly containing non-volatile radioactive nuclides and co-present metal components, remains as a secondary waste in a very small amount, and the burned residue can be applied to any prior art processing, the process of the present invention does not require any additional special facility and therefore is quite effective from the economic point of view.