Method and Apparatus for Treating Radioactive Waste

The following references are cited to show the state of the art:

Japanese Laid-Open Patent Application Specification No. 64699/75

U.S. Pat. No. 3,679,595

This invention relates to a method and apparatus for treating radioactive waste, and more particularly to a method and an apparatus for treating aqueous solutions and suspensions of radioactive waste through a step of drying and pulverizing the aqueous solutions and suspensions, and a successive step of compressing and solidifying the resulting powders of radioactive waste.

It is desirable to convert aqueous solutions and suspensions of radioactive waste discharged from atomic power stations into a chemically and physically stable solid state convenient for storage.

So far available methods for solidifying the aqueous solutions and suspensions of radioactive waste are based on cement solidification, asphalt solidification, etc. The cement solidification is carried out by mixing the aqueous solutions and suspensions of radioactive waste with cement to solidify the solutions and suspensions into a solid state, but has such a disadvantage as an increase in volume of solidified body, because usually the aqueous solutions and suspensions of radioactive waste are mixed, at a low concentration of the radioactive waste, with the cement. The asphalt solidification includes a step of placing the aqueous solutions and suspensions of radioactive waste into hot asphalt, which offers a danger of fire by the hot asphalt.

A method for treating the aqueous solutions and suspensions of radioactive waste by drying and solidification has been proposed to overcome the foregoing disadvantages encountered in the solidification treatment. That is, U.S. Pat. No. 3,679,595 discloses an art of solidification by adding a polyelectrolyte and bone glue to the aqueous solutions and suspensions or radioactive waste, heating and drying the resulting mixture on rollers, and compressing the rolled product.

The present invention provides a similar treatment of aqueous solutions and suspensions of radioactive waste by drying and pulverizing the aqueous solutions and suspensions of radioactive waste, and then compressing the resulting powders, thereby making solidified bodies of small volume.

A step of drying and pulverization of the aqueous solutions and suspensions of radioactive waste is carried out principally by a centrifugal film drier, and a step of compressing the resulting powders to make solidified bodies is carried out by a pelletizer. The pelletizer usually has two counter-disposed rollers, and powders are put into between the rollers by force, and compressed by revolution of the rollers to make solidified bodies. The rollers have recesses or indents on their surfaces and the solidified bodies are formed in the shapes of such recesses or indents. Usually, solidified bodies in a pellet state usually having sizes of a few centimeters are formed.

The solidified bodies in the pellet state are stored in drums, or, if necessary, filled with asphalt, etc., and thrown away into the ocean.

The treatment of the aqueous solutions and suspensions of radioactive waste by drying and solidification has a problem in obtaining good solidified bodies. That is, no good solidified bodies are obtained only by compressing the powders to make the solidified bodies, unless the powders having a water content below a definite limit content are employed. The limit content is called "limit water content", which is usually a few percents, but is determined on the basis of strength, etc. required for the solidified bodies.

The aqueous solutions and suspensions of radioactive waste are continuously fed to the drier, and powders are formed. The resulting powders have not always a constant water content, depending upon the properties of the aqueous solutions and suspensions of radioactive waste and operating conditions of the drier. Sometimes, powders having a water content for above the limit water content are formed. No good solidified bodies are obtained from such poor powders only by direct compression, and thus a further step is necessary to take the resulting powders having the undesired water content, from the drier.

The present invention provides a method and an apparatus which comprises measuring a water content of powders between a step of drying and pulverizing aqueous solutions and suspensions of radioactive waste and a step of compressing and solidifying the resulting powders, and eliminating the powders, when the measured water content of the powders is above a limit water content, from a system of the treatment by washing without passing through the step of compressing and solidifying.

An object of the present invention is to form good solidified bodies, that is, solidified bodies of high strength.

The present invention further provides a method which comprises eliminating the poor powders having a water content above the limit water content rapidly by washing with hot water at a temperature of not lower than 32.degree. C. as a washing solution, when a main solid component of the radioactive waste is Na.sub.2 SO.sub.4.

The present invention will be described in detail below, referring to the accompanying drawings.

FIG. 1 is a schematic flow diagram showing one embodiment of the present invention.

FIG. 2 is a diagram showing a solubility curve of sodium sulfate.

The present method for treating aqueous solutions and suspensions of radioactive waste comprises a step of drying and pulverizing aqueous solutions and suspensions of radioactive waste, a step of transferring powders formed in the step of drying and pulverizing to a hopper, a step of measuring a water content of the powders, and a step of pelletizing the powders, wherein a further step of introducing a washing solution into the hopper at the bottom when the measured water content of the powders fails to satisfy a predetermined condition, thereby dissolving the powders, and then discharging the washing solution is provided. The predetermined condition means a water content below the limit water content.

As a means for removing the powders from the hopper, an ordinary powder transportation means such as pneumatic transportation, etc. would be available, for example, in the case of dry powders, but is not applicable to powders of high water content. Thus, when the water content of powders is above the limit water content, a means of dissolution and washing is employed in the present invention. However, in that case, it is not suitable merely to introduce the washing solution to the hopper at the top and wash out the powders, because the washing solution cannot permeate into the powders. That is, dissolution of the powders can be readily carried out by introducing the washing solution to the hopper at the bottom, causing natural stirring in the hopper.

To realize the present method, the present invention provide an apparatus for treating aqueous solutions and suspensions of radioactive waste, which comprises a drier, a pelletizer provided at downstream of the drier, and a hopper and a water content-measuring device each provided in a conduit from the drier to the pelletizer, wherein an inlet for introducing a washing solution is provided at the bottom of the hopper. The term "the bottom of the hopper" means not only a bottom end of the hopper, but also a lower part near the bottom end, because sometimes the inlet for introducing the washing solution cannot be provided at the bottom end of the hopper owing to a design problem. The term "the bottom of the hopper" covers the bottom end and also part near the bottom end and means a region where a stirring action is attained by introducing the washing solution.

Furthermore, a gas pipe can be connected to the inlet for introducing the washing solution to the hopper at the same time to inject air, etc. to the hopper at the washing to intensify the stirring action, or to inject dry air, etc. after the washing to dry the system.

Further feature of the present invention relates to a treatment of a waste regenerating solution for desalters and resins, which takes most of the aqueous solutions and suspensions of radioactive waste generated in the atomic power stations. More than 80-90% of the powders formed by drying the waste regenerating solution for desalters and resins is comprised of sodium sulfate, Na.sub.2 SO.sub.4. It is known as a physical property of sodium sulfate that sodium sulfate becomes very hard when deposited from its solution or when its powders are admixed with cold water.

As shown in FIG. 2, sodium sulfate has a maximum solubility 33% at about 32.degree. C. The solubility is slightly decreased in a temperature range above 32.degree. C., but considerably decreased at a temperature range below 32.degree. C. It is also known that precipitates are formed above 32.degree. C. take a form of Na.sub.2 SO.sub.4, whereas those formed below 32.degree. C. have water of crystallization in the form of Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O. For example, the present inventors found by experiments that, when two-fold volume (based on volume of Na.sub.2 SO.sub.4 powders) of city water at about 15.degree. C. was poured onto the Na.sub.2 SO.sub.4 powders to dissolve the powders, very hard crystals were formed and dissolution of the powders was difficult to make, whereas, when the same volume of water at a temperature above 32.degree. C., for example, hot water at 60.degree. C., was poured onto the Na.sub.2 SO.sub.4 powders, no crystals were formed, and the powders were converted to a slurry state.

That is, the present invention is further characterized by using water above 32.degree. C., for example, hot water at 60.degree. C. or higher, as the washing solution, when most of solid matters of the aqueous solutions and suspensions of radioactive waste is comprised of Na.sub.2 SO.sub.4.

One embodiment of the present invention will be described referring to FIG. 1.

As shown in FIG. 1, one embodiment of the present apparatus for treating aqueous solutions and suspensions of radioactive waste comprises a centrifugal film drier 1, a drier outlet valve 10, a hopper 2, a water content-measuring device 3 fixed to the hopper, a three-way switch valve 12, a pelletizer 4, washing pipings 18, 16, 19 open to the bottom of the hopper, washing solution-discharging pipings 17, 21, 29, drier-washing pipings 23, 24, a feed piping 13 for aqueous solutions and suspensions of radioactive waste, a washing tank 6, a pump 7, a washing sump 8, a sump pump 9, a sump pump piping 22, drying air pipings 25-28, a condenser 5, a negative pressure suction piping 15, and a condensate outlet piping 14.

Aqueous solutions and suspensions of radioactive waste are fed to centrifugal film drier 1 through feed piping 13, and pulverized. The resulting powders are retained once in hopper 2, and their water content is measured in a batch by water content-measuring device 3. In the present embodiment, the powders are led to pelletizer 4 through valves 11, 12, if their water content is less than 3% by weight, and pelletized. The resulting pellets are filled in drums (not shown in the drawing) and reserved in a storing place. If the water content of the powders is higher than 3% by weight, washing water is introduced to the hopper through an inlet (not shown in the drawing) provided at the bottom of the hopper through washing piping 18 under a pressure of 3-5 kg/cm.sup.2 gage at a flow rate of 10-15 m.sup.3 /hr. When, for example, about 40 l of powders consisting mainly of Na.sub.2 SO.sub.4 in the hopper is redissolved, the weight of the powders will be 36 kg, assuming that the density of the powders is 0.9 g/cm.sup.3. Suppose hot water at 60.degree. C. is used to dissolve the powders up to a concentration of 15% by weight, a volume of the resulting solution will be 240 l. In the present embodiment, washing water is introduced up to the inside of the drier through washing piping 18. The volume of the drier up to its top is about 500 l, and thus the dissolution can be satisfactorily carried out. To promote the dissolution and washing, it is desirable to introduce compressed air to the hopper through piping 25 and rotate the rotor of the centrifugal film drier at the same time. After the completion of dissolution and washing, hopper outlet valve 11 is opened. Three-way switch valve 12 is switched to be connected to washing piping 16, and valve 30 is opened. The solution flows down to washing sump 8.

The foregoing washing procedures can be conducted in combination or independently according to the following system.

Hot water is filled in washing tank 6, and introduced to hopper 2 at the bottom through three-way switch valve 12 by pump 7 to dissolve the powders. The resulting solution is returned to the washing tank through piping 20. Such recycling is carried out for a duration of 10-20 minutes to wash valves 12, 11, and hopper 2. After the completion of recycling operation, the washing solution is discharged to washing sump 8 through pipings 16 and 21. The washing solution is returned to a system inlet by sump pump 9 and retreated.

After the washing, drying is carried out with drying air. Drying air is injected through pipings 25, 26, 27, 28 to effect drying after the washing. The drying is carried out with drying air having a dew point of -10.degree. to 30.degree. C. at a flow rate of about 50 Nm.sup.3 /hr, and the drying is completed for a duration of about 20-30 minutes. Restarting of the operation of the system is made ready thereby.

The foregoing procedure relates to the washing of the system, when the water content of the powders in the hopper is above the limit water content.

When only centrifugal film drier 1 is washed separately, valve 10 is closed, and washing water is introduced to the drier through pipings 23, 24 to fill it in drier 1. Then, the rotor is rotated to improve the washing effect. The washing water is discharged to washing sump 8 through piping 17. Piping 17 is elevated to a level over valve 10, and discharge is carried out by utilizing a siphon effect after the washing. This is to prevent wetting of the route valve 10, hopper 2, valves 11, 12, and pipings 16, 29 and the resulting requirement for drying, if such route is employed to discharge the washing water from the drier.

As the kind of valve 10 at the outlet of centrifugal film drier, valve 11 at the outlet of hopper, and three-way switch valve 12 used in the foregoing system, ball valves are structurally most suitable in view of handling both powders and water.

According to the present method and apparatus, powders failing to satisfy the predetermined water content can be readily eliminated from the system, resulting in easy maintenance of a facility for treating radioactive wastes.