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(11) | EP 0 030 467 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method and apparatus for treatment of radioactive waste |
| (57) Radioactive wastes from a nuclear reactor plant are dried into powder and compacted
into pellets. The resultant pellets are conveyed to a sealed vessel (54) by a belt
conveyor system (48, 50, 52). To control the spacing of the pellets on the belt conveyors
and thereby control the radioactive dose around the belt conveyors, two vibratory
feeders (42, 44) are arranged in series and operated so that the rate of loading of
the pellets onto the belt conveyors is always kept lower than a predetermined level.
By the use of the vibratory feeders, adhesion of radioactive powder removed from the
pellets onto the belt conveyors is also prevented. |
[0001] The present invention relates to a method and apparatus for treatment of radioactive waste, e.g. that produced from a nuclear reactor, and more particularly to the treatment of radioactive waste which is compacted into pellets and stored for a period of time sufficient to allow decay of its radioactivity to a desired level.
[0002] It has recently been proposed, in Japanese Laid Open Patent Specification No. 52-34200 (1977), that - radioactive waste such as filtering agents and ion exchange resins is dried into powder and compacted into pellets to achieve high reduction of waste volume. Radioactive waste in a tank is supplied to a concentrator in which it is heated to vaporize the water content of the waste.
[0003] The solid matter obtained is compacted into pellets of desired shape by a pellet-forming machine. The pellets are then conveyed and stored in a sealed vessel for a predetermined period of time for radioactive decay.
[0004] In this type of treatment system, care must be taken that the surface dose of radioactivity of the transported pellets is maintained at a predetermined allowable level so that maintenance workers are able to approach the conveying devices.
[0005] An object of the present invention is to provide a method and system for treatment of radioactive waste which is able to convey radioactive pellets into a sealed vessel while maintaining the surface dose of radioactivity of the pellets below a predetermined allowable level.
[0007] By using vibratory feeders it is possible to achieve a generally uniform spacing of the pellets on the subsequent conveyors, which are preferably belt conveyors. In this way, the surface dose of radioactivity at the belt conveyors where maintenance is most frequently required is kept below a predetermined allowable level.
[0008] Also, the use of a vibratory feeder advantageously reduces malfunctions and maintenance because the feeder can be mechanically so simple in construction.
[0009] The use of the vibratory feeder advantageously also prevents adhesion of radioactive powder removed from the pellets onto the conveying system, which would cause contamination of the conveying system, because the feeder is-capable of selectively conveying the pellets as they have a rather larger mass than the radioactive powder removed. The radioactive:powder is preferably separated by suitable powder separators and returned to the pellet-forming machine.
[0010] According to a preferred embodiment of the present invention, two vibratory feeders are connected in cascade, with the first supplying a large number of pellets periodically to the second. The second feeder continuously conveys pellets at a constant rate towards the storage vessel. This arrangement advantageously prevents overload of the second vibratory feeder, so that the rate of discharge of pellets from it is maintained at below the predetermined allowable level.
[0011] A preferred embodiment of the present invention will now be described by way of example with reference to the drawings, in which:-
Figure 1 is a shematic diagram of a radioactive waste treatment system of the present invention,
Figure 2 is an enlarged side view showing the details of vibrating feeders employed in the system of Figure 1, and
Figure 3 is a vertical cross-section view taken along line III-III of the vibrating feeder of Fig. 2.
[0012] A centrifugal drying machine 14 is connected to a waste liquid supply tank 10 via a supply pump 12. The centrifugal drying machine 14 has a container a rotating shaft and movable vanes. A jacket is mounted around the container which is connected with a steam pipe through which steam is supplied into the jacket to heat the container. The outlet of the centrifugal drying machine 14 is connected to a hopper 16. A three-way valve 20 is disposed downstream of the hopper 16 between the hopper 16 and a hopper 22 of a pellet-forming machine 24. A screw feeder (not shown) is disposed in the hopper 22. In the pellet-forming machine 24 there are a pair of opposed rolls 26 each having a plurality of recesses on the roll surfaces. The pellets formed pass to a pellet shaping machine 28 in which a rotating cylindrical wire net 30 is installed. The pellet shaping machine 28 has two outlets 32,34. The outlet hopper 34 leading to a powder collecting machine 38 is for discharge of powder and the outlet hopper 32 is for discharge of pellets. The upper end of the powder collecting machine 38 is connected to the hopper 22. The lower end of the hopper 32 via a hopper 40 opens into vibratory feeders 42 and 44.
[0013] The vibratory feeders 42 and 44 have paths for the pellets along which the pellets are conveyed in a desired manner by means of vibration into a powder separator 46. Details of the vibrating feeders will be given later with reference to Figs. 2 and 3. The powder separator 46 functions to separate powder from the pellets, and the pellets are then loaded on a belt conveyor system 48,50 and 52. The belt conveyor system has a horizontal belt conveyor 48, a vertical belt conveyor 50 and horizontal conveyor 52. The pellets loaded on the belt conveyor system are conveyed into a tank 54 for storing the pellets for a predetermined period of time to allow their radioactivity to attenuate.
[0014] As the waste passing through the treatment system described above is radioactive, the whole system must be hermetically sealed from the outside atmosphere, and preferably the pressure inside the treatment system is kept below atmospheric pressure. Also, vent gas discharged from the dryer 14 is treated in a mist separator 13 and sent to a vent gas treatment system (not shown) before being discharged into atmosphere.
[0015] Details of the vibratory feeders 42,44 and powder separator 46 will now be given. The vibratory feeder 42 has a trough 106 (Fig. 2) and a drive unit 110. The trough 106 is a square cylinder with a pellet inlet connected by a flexible joint 104 to a discharge pipe 102 of the hopper 40 and a pellet outlet 108 connected by a flexible joint 120 to a pellet inlet 122 of the feeder 44. The trough 106 is connected by a reinforcement plate 112 to the drive unit 110. The plate 112 and the drive unit 110 are supported by springs 106 and 118. The details of the drive unit 110 are the same as the drive unit of the feeder 44 to be described below. The trough has a flat bottom plate which may be inclined upwardly towards the outlet 108.
[0016] The feeder 44 also has cylindrical trough 124 in which is installed a spiral passage formed by a spiral bottom plate and spiral partition wall 126. The inlet 122 opens at the centre of the trough 124 so that the pellets enter at the beginning of the spiral passage. An exhaust nozzle 128 is connected at the end of the spiral passage. The bottom plate of the spiral passage may be upwardly inclined towards the discharge nozzle 128.
[0017] Disposed under the trough 124 is a drive unit (as in the feeder 42) which has a stationary core 134 with a coil 136 wound on it and a movable core 138. The stationary core 134 is rigidly mounted on a casing 132 enclosing it. The upper end of the casing 132 is closed by a cover 130, which is supported by a plurality of leaf springs 140 installed in the casing 132. The casing 132 is not in contact with the cover 130. The movable core 138 is mounted to the underside of the cover 130 so as to face the stationary core 134 with a predetermined gap between the core 134 and the:core 138. The casing 132 is mounted on the floor by springs 142. The underside of the trough 124 is rigidly secured to the cover 130. A vibration amplitude detector (not shown) is fitted to the trough 124.
[0018] The powder separator 46 is disposed downstream of the vibrating feeder 44 and has a slit tube 150 which is formed of a number of circumferentially arranged fine rods. The slit tube 150 connects the pellet inlet 146 and the pellet outlet pipe 152 which are both mounted on a hopper 148. The pellet inlet 146 is connected to the discharge nozzle 128 of the feeder 44 by a flexible joint 144. The powder discharge nozzle 154 provided at the lower end of the hopper 148 is connected to the powder collecting machine 38. The pellet outlet pipe 152 opens to the horizontal belt conveyor 48.
[0019] Operations of this treatment system will now be explained. Radioactive waste produced in a nuclear reactor mainly composed of sodium sulfate, is concentrated and collected in the tank 10. The concentrated waste is then supplied to the centrifugal drying machine 14 by the pump 12, the wall of which is heated by steam supplied into the jacket. The movable vanes rotating within the machine centrifugally force the liquid waste against the inner wall of the machine so that the liquid waste is pressed into a thin film which, as it sinks, is heated by the container wall. As the water evaporates and sodium sulfate begins to be deposited, the waste liquid changes into slurry. As the waste dries further, powder is formed between the movable vanes and the wall.
[0020] This powder is introduced to the hopper 16 where its water content is detected by the water content meter 18. Waste powder whose water content is below a certain value is supplied to the pellet forming machine 24. Powder with water content higher than a predetermined value is recycled to the drying machine 14 using the three-way valve 20. The powder led to the pellet forming machine 24 is supplied to between thepair of rolls 26, where it is compacted to form pellets within the recesses on the roll surfaces.
[0021] The pellets thus formed are supplied to the wire net cylinder 30 rotating in the pellet shaping machine 28. Flashes on the pellet surfaces are removed from the pellets by the rotating wire net cylinder 30 and at the same time the powder dust passes through the wire net. This flash and powder are then led to the powder collecting machine 38 which recirculates the powder to the hopper 22 by means of the pump 36.- Defective pellets are also sorted out by the cylindrical wire net 30 and delivered to the powder collecting machine 38. The pellets 100 of predetermined size delivered from the outlet 32 of the pellet shaping machine 28 are led through the hopper 40 into the trough 106 of the vibratory feeder 42.
[0022] When electric current is supplied to the coil wound around the stationary core of the feeder 42, the stationary core becomes magnetized and attracts the movable core. When the movable core contacts the stationary core, the current in the coil is interruped and the stationary core is de-energized, releasing the movable core. In this way, the movable core is caused to vibrate. Its vibration is transmitted to the trough 106 by the reinforcement member 112. Since, as shown in Figure 2, the drive unit 110 is located at the lower left hand side of the feeder 42, the trough 106 vibrates in the direction from lower left to upper right of Figure 2. This vibration forces the pellets 100 to move in the direction of the arrow 107 so that they are discharged from the outlet 108.
[0023] The pellets thus discharged are then introduced, through the inlet nozzle 122, into the central region of the trough 124, of the vibratory feeder 44. When the current is supplied to the coil 136 on the core 134, the core 134 become magnetized and attracts the movable core 138. When the core 138 contacts the core 134, the current to the coil 136 is interrupted, de-energizing the core 134 and releasing the movable core 138. In this way, the movable core 138 is caused to vibrate.
[0024] Its vibration is transmitted to the trough 124 through the cover 130. Since the cover 130 is supported by the springs 140, the vibration of the trough 124 forces the pellets 100 circumferentially as indicated by the arrow 123 in Figure 3, so that the pellets in the central region are forced to move along the spiral passage in the direction of the arrow 123 until they reach the discharge nozzle 128.
[0025] Before the pellets are introduced into the vibratory feeders, most of flash and powder are removed from them by the machine 28, but some powder may be produced in the feeders. This problem can be overcome because a vibratory feeder by its nature functions to separate particles depending on the mass. In the present case, only the pellets which have a much larger mass than the powder particles can be forced to move forward by the vibration, and the powder remains in the paths in the feeders.
[0026] In order to prevent overload of the feeder 44, it is preferable to operate the feeder 42 such that the amount of the pellets in the feeder 44 is constant or below a predetermined maximum load. Preferably, the feeder 42 is operated intermittently so that the load on the feeder 44 will not exceed the predetermined maximum load. Also, the feeder 44 may be operated constantly to feed pellets at a desired rate to the belt conveyors.
[0027] In the above embodiment, two types of vibratory feeders are connected in cascade, but it is possible to employ a single feeder 42 or 44 instead of the two feeders. Also, it is possible to employ more than two vibrator feeders as feeding means for conveying the pellets.
[0028] In order to enhance the durability of the treatment system, the powder separator 46 is provided. The pellets discharged from the discharge nozzle 128 as indicated by arrow 125 are introduced through the pellet inlet 146 into the slit tube 150. Powder adhering to the surface of the pellets cannot be removed completely by the pellet shaping machine 28 and is therefore sent to the feeders 42 and 44. While being vibrated in the feeders, this powder comes off the pellet surfaces, and is separated by the slit tube 150, i.e. the powder falls through the slits of the slit tube 150 into the powder discharge nozzle 154, from which it is returned to the powder collecting machine 38.
[0029] From the slit tube 150, the pellets are supplied via the outlet pipe 152 onto the horizontal belt conveyor 48, and are carried by the conveyor 50 and the conveyor 52 to be thrown into the tank 54.
[0030] The conveyor 50 has buckets to carry the pellets upwardly. The pellets are stored in the tank 54 until their radiation level attenuates to a certain level, after which they are taken out of the tank 54 to be loaded in drums and solidified with a solidifying agent such as asphalt.
[0031] The feeder 42 is operated intermittently for a rough adjustment of the amount of pellets loaded into the feeder 44. The feeder 44 is operated continuously for fine adjustment of the amount of pellets supplied to the belt conveyor. In other words, the feeders 42 and 44 act as a valve to adjust the rate of supply of pellets to the conveyor 48.
[0032] Since pellets of the same size are discharged continuously from the feeder 44, they are distributed over the horizontal belt conveyor 48 at the generally constant intervals. Thus they may be transported on the conveyor 48 spaced apart by a distance greater than their own dimensions, e.g. more than five times their diameter. Therefore, the surface doses of radioactivity from the pellets on the conveyors are kept low and uniform over the entire surface of the conveyor 48. This also applies to the conveyor 50 as well as the conveyor 52. This reduces the risk of maintenance workers being exposed to an excess amount of radiation, allowing access to the belt conveyors for maintenance. If a large quantity of pellets was accumulated at some location on the belt conveyors, the radiation level at that location would become high, making it dangerous for maintenance workers to approach the conveyors.
[0033] Whether the pellets are being supplied at the required constant rate onto the horizontal belt conveyors can be checked by the photosensor fitted to the pellet discharge pipe 152 which consists of a light source, a light receiver and a plurality of light reflectors. The light reflectors are mounted on the inner surface of the cylindrical wall of the discharge pipe 152. The light emitted from the light source is reflected by the reflectors before reaching the light receiver. The pellets that are passing through the photosensor intercept the light emitted from the light source. The light receiver counts the number of pellets by recognizing the change of light intensity.
[0034] Because the powder is removed by the powder separator 46, the belt conveyors can be prevented from being fouled and therefore powder does not accumulate on the bottom of the casing near the ends of the belt conveyors.
[0035] Since the drive units of the vibratory feeders 42 and 44 are provided under the troughs 106 and 124 through which the pellets of radioactive substance pass, the drive units are not polluted by the radiation. This makes their maintenance easier. The troughs 106 and 124 may be provided with a suitable cleaning liquid supply means, so that when defective pellets molded with insufficient strength aresupplied into the vibrating feeders and cause trouble, they can easily be removed by cleaning the troughs using the cleaning means.
1. A method for the treatment of radioactive waste which comprises the steps of:
(a) drying and powdering a radioactive waste derived from a source of wet radioactive waste material,
(b) pelletizing the resulting powder,
(c) transporting the resulting pellets into a storage container ,
(d) temporarily storing the pellets in the storage container for attenuation of their radioactivity, characterized in that:
said transporting step comprises subjecting the pellets to movement by vibration in such a manner that the pellets are conveyed into the storage container along a path while keeping a predetermined distance from each other so that the surface radioactivity dose of the transported pellets in said path is maintained at or below a predetermined allowable level.
2. A method according to claim 1 wherein said movement of the pellets by vibration
comprises a first stage of intermittent transport of the pellets and a second stage
of continuously transport of the pellets delivered from the first stage.:
3. Apparatus for the treatment of radioactive waste comprising:
(a) drying means (14) for drying and powdering a radioactive waste derived from a source of wet radioactive waste material,
(b) means (24,28) for pelletizing the resulting powder of radioactive waste,
(c) feeding means (42,44,48,50,52) for conveying the resultant pellets,
(d) storage means (54) for storing the pellets fed by said feeding means for attenuation
of their radioactivity,
characterized in that:
said feeding means includes one or more vibratory feeders (42,44) for the pellets and a conveying path (48,50,52) from the vibratory feeder(s) to the storage means (54), the vibratory feeders being adapted and arranged so that the pellets are loaded onto said conveying path and conveyed thereon with a predetermined distance from each other so that the surface radioactivity dose of the pellets is maintained at a predetermined allowable level.
4. Apparatus according to claim 3, wherein said vibratory feeder(s) comprise at least
a first vibratory feeder (42) and a second vibratory feeder (44) connected in cascade,
said first feeder (42) being adapted to be operated intermittently to feed the pellets
to said second feeder (44) and said second feeder (44) being adapted to be operated
continuously to arrange the pellets on the conveying path at a predetermined distance
from each other.
5. Apparatus according to claim 4, wherein said first and second feeders (42,44) respectively
having an inlet and an outlet for the pellets and means for vibrating the feeder so
that the pellets loaded on the feeding path in the feeder are conveyed from the inlet
to the outlet.
6. Apparatus according to claim 4 or claim 5, wherein said second feeder (44) has
a feeding path extending spirally from the inlet toward the outlet of the feeder.