RADIOACTIVE WASTE PELLETS IN SOLIDIFIED FORM AND A PROCESS FOR FORMING THE SAME

Abstract: The invention is concerned with radioactive waste pellets in solidified form in which radioactive waste pellets (4) including light waste pellets having specific gravities (ρ p ) smaller than that (ρ k ) of a filler (1) are solidified with the filler (1) in a container (3). A cover (5) disposed in an opening portion of the container (3) is provided with filler injection ports (8) which do not permit the passage of the light waste pellets. The invention is further related to a process for forming radioactive waste pellets in solidified form by injecting the filler (1) through the filler injection ports (8) formed in the cover (5), so that the radioactive waste pellets (4) and the filler (1) are uniformly solidified.

Description: BACKGROUND OF THE INVENTION The present invention relates to radioactive waste pellets in solidified form which are formed when the radioactive waste pellets include light waste pellets having specific gravities smaller than the specific gravity of a filler, and also relates to a process for forming the same. According to a known process, a concentrated waste liquid (consisting essentially of sodium sulfate Na₂SO₄) obtained by concentrating a regenerated waste liquid of used ion-exchange resin and a slurry of powdery ion-exchange resin, that are major radioactive wastes generated from boiling-water nuclear power plants, are dried, pulverized and pelletized, and the radioactive waste pellets thereof are charged into a container and are solidified with a filler. For example, Japanese Patent Laid-Open No. 197500/1982 (=US-A-4.505.851) discloses a process according to which radioactive waste pellets are charged into a drum, and a solution of a sodium silicate composition that serves as a filler is poured into the drum, in order to seal the drum (page 5, right upper column, line 3 to left lower column, line 5 of the published specification). The radioactive waste pellets may often include light waste pellets such as resin pellets having specific gravities smaller than the specific gravity of a filler, or may consist of light waste pellets only. According to the above-mentioned process, when the solution of sodium silicate composition which serves as a filler is poured into a drum filled with the radioactive waste pellets, resin pellets having small specific gravities float and concentrate in the upper portion of the drum. In this case, a layer consisting of the filler only is formed in the lower portion of the drum, and the filler is not sufficiently applied to the resin pellets that are radioactive waste pellets concentrated in the upper portion of the drum. In the radioactive waste pellets prepared in a solidified form, therefore, the filler is not uniformly applied to the radioactive waste pellets but is applied in a separated manner. Since the filler is not uniformly applied to the radioactive waste pellets as mentioned above, the radioactive waste pellets are solidified very weakly. Further, since the radioactive waste pellets are not sufficiently charged to the lower portion of the drum, the volume of the solidified radioactive waste pellets cannot be effectively reduced. Japanese Patent Laid-Open No. 73097/1975 discloses a container equipped with a cover which will be used for preparing radioactive waste pellets in solidified form (refer to the drawings of the published specification). That is, it has been known to construct a container by providing a concrete cover for a concrete container which is impregnated with a polymeric monomer or a resin solution. The above patent application, however, is concerned with the container only, but does not describe the radioactive waste pellets or the filler to be contained in the container. The above patent application does not teach to solidify radioactive wastes including light waste pellets having specific gravities smaller than that of the filler. SUMMARY OF THE INVENTION A first object of the present invention is to provide radioactive waste pellets in highly solidified form consisting of radioactive waste pellets and a filler that are uniformly charged into a container without being separated, the radioactive waste pellets including light waste pellets having specific gravities smaller than that of the filler, or the radioactive waste pellets being composed of light waste pellets only. A second object of the present invention is to provide a process for forming radioactive waste pellets in solidified form, said process being capable of uniformly charging the radioactive waste pellets and a filler into a container, and said radioactive waste pellets including at least light waste pellets having specific gravities smaller than that of the filler. These objects are accomplished with waste pellets as claimed in claim 1 and a method of forming the same as claimed in claim 6. According to the present invention, a filler and radioactive waste pellets including at least light waste pellets having specific gravities smaller than that of the filler are charged into a container that is provided with a cover which has a weight greater than the buoyancy which the light waste pellets receive in the filler, the cover further having filler injection ports that do not permit the passage of light waste pellets. Therefore, there are obtained radioactive waste pellets which are highly solidified since the gaps among the radioactive waste pellets are uniformly filled with the filler. Further, the filler is injected into the container through the filler injection ports of the cover that does not permit the passage of light waste pellets, the container containing radioactive waste pellets which include at least light waste pellets having specific gravities smaller than that of the filler. Therefore, the radioactive waste pellets and the filler are not separated from each other, and it is allowed to form the radioactive waste pellets in solidified form with the filler being uniformly injected into gaps among the radioactive waste pellets. Moreover, the radioactive waste pellets do not overflow when the filler is being injected, and contamination by radioactivity can be prevented. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram for illustrating the concept of the present invention; Fig. 2 is a diagram showing a relation between the specific gravity and the thickness of a cover; Fig. 3 is a diagram showing flow values of an alkali silicate composition and the lapse of time; Fig. 4 is a diagram showing a relation between the size of holes formed in the cover and the time required for injecting the filler; Figs. 5 and 6 are schematic diagrams showing a method of producing the cover according to an embodiment of the present invention; Fig. 7 is a schematic diagram illustrating a first example; Fig. 8 is a schematic diagram illustrating a third example; Fig. 9 is a schematic diagram illustrating a fourth example; Fig. 10 is a schematic diagram illustrating a fifth example; Fig. 11 is a diagram showing a charging system employed in the fifth example; and Fig. 12 is a schematic diagram illustrating a sixth example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the invention will be described below. In Fig. 1, a container 3 is disposed for containing radioactive wastes under a filler tank 2 which contains a filler 1. The container 3 is filled with radioactive waste pellets 4 including at least light waste pellets that have specific gravities smaller than that of the filler 1. A cover 5 is provided in an opening at an upper portion of the container 3 to cover the radioactive waste pellets 4. The cover 5 has a weight which is greater than the buoyancy which the light waste pellets receive in the filler 1. The cover 5 has small holes 8 which permit the passage of the filler 1 but which do not permit light waste pellets included in the radioactive waste pellets 4 to flow out. The container 3 and the cover 5 constitute a container in which the waste materials are to be solidified and are to be disposed of. Below is described how to solidify the radioactive waste pellets. First, the container 3 is densely filled with the radioactive waste pellets 4 which include at least light waste pellets up to the upper opening portion thereof. Next, the cover 5 is placed on the radioactive waste pellets 4 near the upper opening portion of the container 3. The filler 1 is poured onto the cover 5 from the filler tank 2. The filler 1 pass through the small holes 8 formed in the cover 5 and enter into the container 3 in sufficient amounts without permitting the radioactive waste pellets 4 to overflow. The filler 1 is poured in sufficient amounts into the container 3 up to the upper portion of the cover 5. From the requirement that the weight of the cover must be greater than the buoyancy which the light waste pellets receive in the filler, a specific gravity of the cover is given by the following relation, where ρ f denotes a specific gravity of the cover, ℓ denotes a height of the container, x denotes a thickness of the cover, Pr denotes the rate of volume occupied by the radioactive waste pellets relative to the total volume of the container, ρ K denotes a specific gravity of the filler, and ρ p denotes a specific gravity of the light waste pellets. Specifications of the cover used in the present invention will be discussed below. The following materials were used for the tests. The radioactive waste pellets included light waste pellets consisting essentially of a mixture of sodium sulfate that is a concentrated waste liquid and used ion-exchange resin. The radioactive waste pellets had been formed in almond shapes by a granulating machine. The filler was a solution containing an alkali silicate composition, and the container was a drum having a capacity of 200 liters. The radioactive waste pellets are prepared by mixing the sodium sulfate and the ion-exchange resin at a predetermined ratio. The majority portion of pellets consists of light waste pellets having specific gravities smaller than that of the solution of alkali silicate composition which serves as a filler. However, since sodium sulfate is partly contained at a large ratio, there are often contained pellets having specific gravities larger than that of the solution of alkali silicate composition that works as a filler. Fig. 2 shows a relation between the thickness and the specific gravity of the cover. A reduction ratio of charging amount of the radioactive waste pellets shown in Fig. 2 is given by the following relation. To restrain the reduction ratio of charging amount within 0.06% by weight, the thickness of the cover must be smaller than 50mm, and the specific gravity of the cover must be greater than 3.0. From the standpoint of crushing strength of the radioactive waste pellets, on the other hand, the radioactive waste pellets will not be crushed if the weight of the cover is smaller than about 3 tons. To inject the filler after the cover has been fitted, the small holes in the cover must permit the passage of the filler. In this case, the filler should not be hardened while it is being injected. Further, the light waste pellets should not overflow. Whether the filler can pass through the small holes in the cover is affected by its viscosity which also changes depending upon the temperature, time for solidification and lapse of time. Fig. 3 shows a relation between flow values (length (cm) which the filler (solution of alkali silicate composition) travels in one minute when it is poured on a glass plate tilted by 45°) and the lapse of time. After about 40 minutes from the injection of the filler, the paste-like filler starts to harden. Namely, the flow value decreases remarkably, and the filler cannot be injected into the gaps of the radioactive waste pellets any more. The flow value should desirably be greater than about 23 cm/min. Fig. 4 shows a relation between the size of small holes formed in the cover and the injection time (time until the injection of the filler (solution of alkali silicate composition) into the drum of radioactive waste pellets is completed). In this case, the small holes possessed the shape of a true circle or close to a true circle, a square shape or close to a square shape in cross section, the distance being equal or nearly equal from the periphery of the hole to the center thereof. When the size of the holes is too small, extended periods of time are required for injecting the filler. Namely, when the holes have a small size as indicated by A in Fig. 4, extended periods of time are required for the filler to fall into the container; i.e., the filler is cured while it is falling and can no longer be injected. Therefore, a minimum size of the holes of the cover is about 10 mm² as indicated by A. The larger the size of the holes, the greater the effect for injecting the filler. The holes, however, should have a size that does not permit the radioactive waste pellets to flow out even at the greatest. That is, the holes should have a size smaller than a minimum diameter (about 10 mm) of the radioactive waste pellets), i.e., should be smaller than about 80 mm² as indicated by a point B in Fig. 4. From the above consideration and experiments, the sectional area of each hole (having an equal or nearly equal distance from the periphery of the hole to the center thereof) in the cover should lie from about 10 mm² to about 80 mm². An optimum sectional area of the hole refers to a maximum sectional area that lies within the above-mentioned range and that is effective for injecting the filler or, in other words, that is effective for completing the injection before the curing proceeds. When the container is made of a concrete or of a composite material consisting of a concrete and other material, the cover having small holes should also be made of the same material as the container or should be made of a mixture containing the same material, so that the container and the cover are adhered together with an increased strength and that the container is obtained in a unitary structure. Working examples of the present invention will be described below concretely. Example 1: First, a gauze 6 consisting of wires, each wire being 5 mm in diameter, is prepared as shown in Fig. 5, and a concrete is blown onto the wire gauze 6 to produce a cover 5 having many small holes 8, each being about 10 mm in diameter (having a sectional area of 78.5 mm²), that are uniformly distributed. The radioactive waste pellets and the filler are charged and solidified as described below. First, about 160 kg of the radioactive waste pellets including light waste pellets which essentially consist of sodium sulfate and used ion-exchange resin, are densely charged into the 200-liter container 3 made of a concrete, as shown in Fig. 7. The cover 5 obtained as described above is placed thereon. The filler 1 consisting of an alkali silicate composition is allowed to flow in an amount of 158 kg onto the cover 5. The filler 1 flows through the small holes 8 of the cover 5, and is uniformly injected in sufficient amounts into gaps among the radioactive waste pellets 4 from the lower portion to the upper portion of the container 3. The product solidified according to this example was cut to observe the interior thereof. It was confirmed that the radioactive waste pellets 4 and the filler 1 had been solidified unitarily maintaining a sufficiently large strength. Further, the radioactive waste pellets in solidified form exhibited excellent durability since the container 3 and the cover 5 had been made of a concrete, the alkali silicate composition that was a filler 1 exhibited good adhesiveness to the cover 5 made of a concrete, and further since the container 3, the cover 5 and the filler 1 were composed of inorganic materials. Example 2: This example is the same as example 1 with the exception that a steel drum is used instead of the container made of a concrete. The same effects are obtained as in example 1. However, the adhesiveness between the container and the concrete cover, and durability of the container, are slightly inferior to those of example 1. Example 3: This example is the same as example 1 with the exception of using, as the cover 5, a porous plate composed of lead having a thickness of 15 mm and many small holes 8 of a diameter of 10 mm as shown in Fig. 8. The same effects are obtained as those of example 1. The defect of this example may be an increased manufacturing cost. However, lead has a large specific gravity, and the cover 5 needs have a thickness smaller than that of the concrete cover which contains wire gauze. This helps increase the charging capacity of the radioactive waste pellets. Example 4: According to this example as shown in Fig. 9, a wire gauze 6 is used as a portion of the cover, and steel masses 9 are placed as weights thereon. The filler is poured up to the upper portion of the weights 9. The same effects as those of example 1 are obtained. In this case, meshes of the wire gauze correspond to ports for injecting the filler. Example 5: In example 1, many small holes 8 were uniformly distributed in the cover. In example 5, however, use is made of the cover having two holes; i.e., a small hole 8 for injecting the filler and a hole 10 for discharging the air as shown in Fig. 10. This also makes it possible to obtain the same effects as those of example 1. Fig. 11 shows a system for charging the radioactive waste pellets, that is adapted to example 5. A stirrer 12 is installed above a kneading vessel 11 for kneading the filler. The kneading vessel 11 contains stirrer vanes 13 and further has a port 14 for introducing the water. A rotary valve 15 is installed under the kneading vessel 11, and a slide rack 16 is provided by the kneading vessel 11. The cover 5 is placed on the drum 3. An air vent pipe 19 equipped with an ultrasonic water gauge 17 is attached to an air vent 10. A hole 8 for injecting the filler is equipped with a filler injection pipe 18 that is connected to the rotary valve 15. A PEPA-filter 20 is provided at one end of the air vent pipe 19, and a ventilation duct 21 is connected to the PEPA-filter 20. The drum 3 is secured on a rack 22, and a temporarily working cover 23 is provided on the upper side to inject the filler. The drum 3 containing the radioactive waste pellets which include at least light waste pellets is placed on the rack 22 which has a stopper to secure the drum, and the slide-type cover for injection is set to the drum. The filler and water are poured into the kneading vessel 11, and are kneaded by the kneader 12. Simultaneously with the completion of the kneading, the rotary valve 15 is operated to inject the filler from the injection pipe 18 into the drum 3. The air is discharged through the air vent pipe 19, the concentration of radioactivity is decreased through the PEPA-filter 20, and the air is ventilated through the ventilation duct 21. The filler which is injected in sufficient amounts is then detected by the water gauge 17, and the rotary valve 15 is closed. According to the above-mentioned system which employs the rotary valve to inject the filler, the time required for injecting the filler can be reduced. Example 6: In this example 6, the cover 5 has a hole at the center thereof, and a filler injection pipe 24 having a diameter of about 10 mm is inserted in the hole to inject the filler as shown Fig. 12. A clearance 25 of a width of about 10 mm is maintained between the cover and the container 3. The clearance 25 is selected to such a size that the radioactive waste pellets 4 will not flow out. According to this example, the injection of filler starts from the lower portion of the container 3 through the lower portion of the injection pipe 24 penetrating through the hole of the cover 5. Therefore, the filler can be injected even when it has a slightly large viscosity. Example 7: Described below is another example for preparing the cover having a filler injection port. First, silica or a material having excellent resistance against alkali and having a spherical shape or nearly a spherical shape, is arranged in a cylindrical frame to a predetermined thickness. Then, the silica or the like material is adhered together with a cement and a binder such as a solution of sodium silicate, to prepare the cover of the shape of a disc. In the thus prepared cover are automatically formed paths, i.e., filler injection ports through which the filler will infiltrate into gaps among silica stones. This method of preparing the cover is highly practicable since the filler injection ports can be easily formed. In this case, the filler flows down to the lower portion of the cover from the upper portion of the cover passing through a plurality of small amorphous filler passing through openings that work as filler injection ports, and then fall onto the container so as to be charged therein. Described below are the radioactive light waste pellets that are solidified according to the present invention. In this example, pellets of a mixture consisting of sodium sulfate and used ion-exchange resin are treated as the radioactive waste. Other examples of the radioactive waste may include resin pellets obtained by drying and granulating slurry wastes such as ion-exchange resin and the like, sludge pellets obtained by drying and granulating slurry waste of sludge, as well as various solid materials such as PEPA-filter, cloths made of vinyl sheets, wood pieces, and the like, or pulverized products thereof. Mixture pellets such as those obtained by drying and granulating at least one of resin pellets, sludge pellets or various solid pellets, or pellets of pulverized products thereof, and a concentrated waste liquid such as sodium sulfate, sodium borate, and the like may also be treated. Or, the mixture pellets may further be composed of a mixture of resins and concentrated waste liquids such as sodium sulfate, sodium borate and the like. The shape of the radioactive light waste pellets need not be limited to the almond shape but may be cylindrical shapes, granular shapes, or may be in a pulverized form. In addition to the solution of alkali silicate composition, the filler may be a thermosetting plastic material, a plastic material which melts upon the heating, asphalt, mortar, cement, or the like having mobility. When the filler is selected from the above-mentioned examples, the light waste pellets having specific gravities smaller than that of the filler and included in the radioactive waste pellets should, for instance, be resin pellets, sludge pellets, various solid pellets, or mixture pellets consisting of a mixture of resin and concentrated waste liquid. Further, when the cover is made of a concrete or lead, the filler injection port may be formed as numerous small holes, a single small hole, or as a clearance between the outer periphery of the cover and the container. When the wire gauze with weights is used as the cover, mesh of the wire gauze or the clearance between the outer periphery of the wire gauze and the container serves as the filler injection ports. When the cover is prepared by using, for instance, spherical silica stones and a binder, amorphous paths formed among the silica stones will serve as filler injection ports. In this case, the filler permeates through amorphous paths to enter into the container.

Claims: Radioactive waste pellets in solidified form comprising: a container (3) which contains radioactive waste pellets (4) including at least light waste pellets having specific gravities (ρ p ) smaller than that (ρ K ) of a filler (1) and the filler (1) for solidifying said radioactive waste pellets (4) and a cover (5) which is disposed in an opening portion of said container (3) to generally cover said radioactive waste pellets (4) and said filler (1), characterized in that the cover has a weight greater than the buoyancy which said light waste pellets (4) receive in said filler (1) and has filler injection ports (8) that do not permit the passage of said light waste pellets. Radioactive waste pellets in solidified form according to claim 1, characterized in that the light waste pellets (4) contain resin or sludge, and the filler (1) contains a solution of a sodium silicate composition. Radioactive waste pellets in solidified form according to claim 2, wherein said light waste pellets are resin pellets, sludge pellets, mixture pellets of at least one of said pellets and pellets made from a concentrated waste liquid, or mixture pellets made of a mixture of a resin and a concentrated waste liquid. Radioactive waste pellets in solidified form according to claim 2, wherein said filler injection ports (8) are small pores having an equal distance or nearly an equal distance from the periphery of the injection port to the center thereof, said small pores having sectional areas of from about 10 mm² to about 80 mm². Radioactive waste pellets in solidified form according to claim 2, wherein said cover (5) is made of silica stones, a cement and a binder, and the filler injection ports (8) of said cover is formed by a plurality of small openings. A method for solidifying radioactive waste pellets including the following steps: introducing light weight pellets (4) that include pellets having specific gravities (ρ p ) smaller than that (ρ K ) of a filler (1) into a container (3); and solidifying the pellets (4) and the filler (1), characterized in that a cover (5) with a weight greater than the buoyancy which said light waste pellets (4) receive in said filter (1) is disposed in an opening portion of said container (3) to cover said radioactive waste pellets (4), said cover (5) having filler injection ports (8) that do not permit the passage of said light waste pellets; and the filler (1) is injected into said container (3) through said filler injection ports (8) formed in said cover (5). A method according to claim 6, wherein said radioactive waste pellets (4) include light waste pellets that consist of resin or sludge, and said filler (1) contains a solution of sodium silicate. A method according to claim 6, wherein in the step of injecting the filler (1) into said container (3) through the filler injection ports (8) of said cover (5), the filler (1) is injected through small pores (8) or a plurality of small openings that are formed in said cover (5). A method according to claim 7, wherein said light waste pellets are resin pellets, sludge pellets, mixture pellets of at least one of said pellets and pellets made of a concentrated waste liquid, or mixture pellets made of a mixture of a resin and a concentrated waste liquid.

Inventors: HORIUCHI, SUSUMU, KIKUCHI, MAKOTO, MIZUNO, HIROKO, TAMATA, SHIN
Applicants: HITACHI, LTD.
Application year: 1984
Main classification: G21F9
IPC codes: G21F9