Patent Number: 060404914
Section: description

DESCRIPTION OF AN EMBODIMENT FIG. 1, to which reference is now made, illustrates an embodiment of an apparatus according to the invention for dewatering and containing low-level or intermediate-level waste, which for exemplifying purposes is here assumed to consist of spent ion-exchange material as above. Thus, the ion-exchange material may originate from an ion-exchange filter for cleaning the aqueous phase at the secondary side of steam generators in nuclear power plants. The material may have an activity in the order of 5,000-300,000 Bq/kg, thus exceeding the limit value for direct deposition, consequently, the material is dewatered, contained and tested as to its contents of nuclides before being deposited, preferably in the ground. The apparatus shown in FIG. 1 comprises a storage tank 10, a bigbag-type inner sack 12 provided with a straining-cloth bottom 13 and being, with the aid of four lifting eyes 14, suspended from a lifting yoke 16, which in turn is suspended from a travelling (P1) trolley 18 via 17. The inner sack 12 is vertically adjustable (P2) in an upwardly open, rust-proof collecting container 20, which has a lower outlet 21 which, via a valve 22, a pump 24 and a valve 26, is connected to an inlet 27 of a buffer tank 28. An outlet 29 provided at the bottom of the buffer tank 28 is, via a valve 30 and a pump 32, connected to a sprinkler 34 which is disposed above the opening of the inner sack 12 and which comprises a number of downwardly-directed spray nozzles 36, Furthermore, FIG. 1 schematically illustrates a vacuum suction nozzle 38 which is applied against the outside of the straining-cloth bottom 13 of the inner sack 12 and which, via a vacuum pump 40, is connected to a second inlet 41 of the buffer tank 28. A valve 42 connected between the pump 24 and the valve 26 enables filtration water 17 to be drawn off from the collecting container 20 to an outlet 43. Reference numeral 44 designates a retaining wall intended to intercept any spillage resulting from the process. The Inner Sack 20 In order to illustrate the function of the apparatus of FIG. 1, a preferred embodiment of the inner sack 12 shown in FIG. 1 will now be described in more detail with reference to FIG. 2. As indicated in the foregoing, the inner sack 12 is of bigbag type having a volume in the order of 1 m.sup.3. In this example, the inner sack 12 measures 90.times.90.times.115 cm. The sides and the top of the inner sack 12 can be made of polypropylene fabric which on the inside is coated with polyethylene, whereas the bottom 13 of the inner sack is made of a straining cloth having a suitable mesh in view of the filtration. In practical tests, the mesh of the straining cloth 13 has been 125 .mu.m. As illustrated in FIG. 2, the sack 12 is in addition provided with two crossed reinforcement bands 15, which are arranged on the outside of the straining cloth 13 and at the ends are connected to the sides of the sack 12. Furthermore, the sack 12 is at the top provided with four lifting eyes 14 of a type known per se. According to the invention, the water permeability of the inner sack 12 is generally much higher at the straining-cloth bottom 13 than at the sides or the top. The scope of the invention encompasses the alternative that the sides and the top have a certain water permeability, however very restricted in comparison with that of the straining-cloth bottom 13, as well as the alternative that the sides and the top are essentially perfectly watertight. At any rate, the dewatering is completely or at least substantially carried out through the straining-cloth bottom 13. Practical Test The method according to the invention will now be described in more detail with reference to a test implemented with the aid of an apparatus of the type shown in FIG. 1. Step 1: Filling of the Storage Tank 10 With the aid of a pumping device (not shown), the storage tank 10 was filled with aqueous, particulate and low-level ion-exchange material to a suitable level 11. The material in the storage tank 10 was then subjected to batchwise dewatering and containment in accordance with the following steps. Step 2: Filling of the Inner Sack 12 An empty inner sack 12 of the type shown in FIG. 2 was suspended with the aid of its lifting eyes 14 from the lifting yoke 16 and was lowered into the collecting container 20, as illustrated in FIG. 1. By means of a submersible sludge pump (not shown) immersed in the storage tank 10, about 1 m.sup.3 of aqueous ion-exchange material was then pumped from the storage tank 10 down into the suspended inner sack 12. In the test, the sack 12 initially rested on the bottom of the collecting container 20 and was subsequently raised to a level a certain distance above the bottom of the container 20 during the final filling of the straining-cloth sack 12, as illustrated in FIG. 1. Step 3: Drainage and Recirculation During the filling of the inner sack 12, void water (reference number 17) was drawn off from the material via the straining-cloth bottom 13 to the collecting container 20 under the action of gravity. If the filtration water 17 in the collecting container 20 rose above a suitable level (30-40 cm), the excess water was pumped to the buffer tank 28 by the pump 24. The collected filtration water 17 was heavily contaminated (visual observation) and could thus not be directly deposited without previous treatment. It took approximately 5 min to fill up the straining-cloth sack 12, and the drawn-off volume of water was about 400 l. In order to gradually clean the filtration water 17, the latter was recirculated via the valve 22, the pump. 24, the valve 26, the buffer tank 28, the valve 30, the pump 32 and the sprinkler 34, by means of which the filtration water 17 was evenly sprinkled over the material in the sack 12 in order to pass through this material once again and thus be further cleaned. The recirculation involved a flow rate of approximately 10 l/min and went on (about 40 min) until the filtration water 17 was sufficiently clean (in this embodiment until the filtration water had a clear, uncoloured appearance), Measurements showed that the filtration water had a high degree of purity and essentially no activity, for which reason it could be discharged at the outlet 43. Step 4: Drip-dewatering and Vacuum Suction Thereafter, the sack 12 was left suspended for purposes of drip-dewatering. After about 2 h, an additional amount of about 30 l of water had been drawn off, and after 13 h, an additional amount of about 8 l had been drawn off. After 15 h, a minor amount of void water still remained at the bottom of the sack 12. In order to speed up the drawing-off of water, a movable vacuum-suction means 38, 40 was, after the drip-dewatering, applied to the outside of the straining-cloth bottom 13 at different places, resulting in powerful drawing-off of the void water remaining at the bottom of the sack 12. Step 5: Final Packaging in Plastic and Outer Sack After the dewatering operation had been completed, the inner sack 12 was finally packed, as illustrated in FIG. 5, in which the waste is indicated by reference number 44. The top of the inner sack 12 was first sealed in suitable fashion, as schematically indicated at reference number 46, whereupon the sack was placed in a watertight plastic sack 48 which in turn was placed in a carrying outer sack 50 of essentially the same design and size as the inner sack 12, the straining-cloth bottom being, however, replaced with a bottom of the same or similar material as that of which the sides and top of the outer sack 50 are made. The resulting disposable container structure 12, 48, 50 may then be deposited below ground. The total volume was about 1 m.sup.3, the weight was about 700 kg, and the largest outer dimensions were about 110.times.110.times.110 cm. Modification with a Suction Box In a further development of the apparatus shown in FIG. 1, the movable vacuum-suction nozzle 38 was replaced with a special suction box 52 of the type illustrated in FIGS. 3 and 4. This suction box 52 was disposed outside the collecting container 20, such that drip-dewatering and vacuum suction could be performed separately from the collecting container 20, which was advantageous in that a first sack could be drained with recirculation in the collecting container 20 while at the same time a second sack underwent drip-dewatering and vacuum suction adjacent to the suction box 52. The suction box 52 has the advantage of obviating the need of moving the nozzle 38 over the straining-cloth bottom 13, since the suction box 52 produces simultaneous vacuum suction over the whole bottom 13 of the sack 12. In this further development of the invention, the inner sack 12 was thus lifted by means of the travelling trolley 18 from the collecting container 20 after the drawing-off operation had been completed, whereupon it was moved in the lateral direction to a position above the suction box 52 and lowered onto it. The suction box 52 comprises four sloping bottom elements 54, which together form an upper supporting surface for the sack 12 and which end in a central outlet opening 56. The outlet opening 56 was connected to the vacuum pump 40 in FIG. 1 in order to transfer void water collected in the suction box to the buffer tank 28. However, the void water collected in the suction box 52 need not necessarily be transferred to the buffer tank 28 but may optionally be treated and cleaned separately. Between the bottom elements 54, there is provided a total of four channels 58 for conducting the collected void water to the outlet opening 56 and distribute the negative pressure over the supporting surface of the suction box 52. Naturally, the required durations of the recirculation, the drip-dewatering and the vacuum suction may deviate from the values given in the example above. Furthermore, it is preferred, albeit not absolutely necessary, to make use of the plastic sack 48 and the outer sack 50. It will also be conceivable to embed the inner sack 12, although this is a more expensive alternative, and hence of less interest. As to the inner sack 12, the straining-cloth material 13 need not necessarily cover the entire sack bottom, or may alternatively extend a certain distance up on the sides of the sack.