Patent Number: 048062790
Section: description

DETAILED DESCRIPTION OF THE INVENTION The apparatus shown in FIG. 1 comprises three main stages: A. High level waste vibratory impregnator PA1 B. Vibratory calciner and PA1 C. Vibratory powder mixer The waste impregnator A. comprises a downwardly inclined trough 1 having flexible mountings 2 and a vibrator 3 at its upstream end, a hood structure 4 and a series of liquid sprays 5 connected to a high level waste supply tube 6. The hood structure 4 has, at its upstream end, an inlet hopper 7 through which synthetic rock precursor material in powder or preferably in granulated form is poured. This powder is formed outside the active cell and is not radioactive. By operation of the vibrator 3, the powder continuously and steadily moves down the trough ready for discharge at the open downstream end into a discharge hopper 8. As the precursor moves down the trough it is impregnated through the sprays 5 with a solution of high level waste, the spray rate being controlled so that the powder remains sufficiently dry to remain in a fluidised and pourable state. A radiant heating unit 9 is located beneath the trough, as schemically shown, and causes evaporation of the aqueous solvent from the radioactive waste at a steady rate. The impregnated precursor discharges through the hopper 8 into a discharge tube 10 and into the upstream end of the closed tube 12 of the vibratory calciner B. The tube 12 is downwardly inclined and is connected through a downstream flexible coupling 13 to a discharge tube 14. Discharge tube 14 has an inlet pipe 15 for reducing gas (typically N.sub.2 /3 Volume % H.sub.2 or H.sub.2 alone). The reducing gas passes upwardly through the tube to a gas discharge take-off tube 16 near the upstream end. In this way volatile radioactive components produced during the calcining can be taken up and filtered out. A furnace 13 surrounds the central region of the tube for causing the synthetic rock precursor to undergo partial mineral transformations and the nitrates associated with the high level waste are decomposed. Minor amounts of volatile radioactive components may be evolved. The furnace raises the temperature of the particulate material to about 750.degree. C. At its upstream end, a flexible mounting 17 supports the tube 12 and at its downstream end a variable frequency vibrator unit 18 is provided together with a flexible mounting 19. The vibratory actuator 18 is tuned to provide the desired flow rate by varying frequency and ampitude. The calcined discharged powder falls downwardly into a vibratory mixer C, having a vibratory actuator 20 and flexible mountings 21. A secondary inlet 23 is provided for titanium powder which is intimately mixed as the powders pass downwardly through the inclined tube to be discharged to a discharge hopper 24 from which bellows canisters 25 may be filled. Reference will now be made to FIGS. 2 and 3 which illustrate how the filled canisters can be uniaxially pressed. Referring to the drawings a hydraulic press comprises a fixed base 31, an open, upwardly extending framework 32, a fixed top press frame 33, a refractory top pad 34 and just below the top pad a heating unit comprising an electrical induction coil 35 with a cylindrical metal sleeve 36 functioning as a susceptor sleeve. Furthermore, the press has an upwardly acting hydraulic ram 37 with a piston 38 on the top of which a refractory top pad 39 is mounted. For the purpose of cold pre-compaction of the canisters 25, the hydraulic press incorporates a retractable plate-like platen 40 which is horizontaly slidably displacable in guides (not shown) by actuation of a secondary ram 41. FIG. 2 shows the first stage in which a bellows canister 25 has been placed on the refractory bottom pad 39. The canister is of a heat resistant alloy or steel such as INCONEL 601. As filled through hopper 24 (FIG. 1), the calcined impregnated synthetic rock will have a typical density of 19% of the maximum theoretical density of the final synthetic rock. A cold precompaction is applied by first actuating the ram 41 to displace horizontally the platen 40 to adopt the position shown in FIG. 3 and then the hydraulic ram 47 is actuated to place the bellows canister 25 into abutment with the platen 40. pressure is maintained until the density of the synthetic rock powder approaches the maximum which can be achieved at ambient temperatures. e.g. about 35% theoretical maximum density. Typically the press will be operating at the order of 20 Mpa and the time for this pressing step will be the order of 3 minutes. The ram 37 is then lowered slightly, the ram 41 actuated to retract the platen 40. and (unless an optional separate pre-heating furnace is used) the ram 37 is raised to place the bellows container within the heating zone and to occupY the position shown in dotted lines and referenced 42'. It is necessary to heat he bellows container and its contents to a typical temperature in the range 1050.degree. to 1260.degree. C. and this will take typically 510 minutes for a 40 cm diameter bellows canister. Subsequently, pressure can be applied through the ram so that the bellows canister is in abutment with the top pad 34 and pressures of about 14 Mpa or higher are applied for several hours until full compression of the bellows canister occures and a density of about 99% theoretical density is achieved. It will be appreciated that during normal operations the induction coil is continuously operated and appropriate insulation material surrounds the upper part of the press to reduce heat losses. However, the bottom pad 39 is itself raised to very high temperatures and as soon as the canister 25 is placed on top of the pad there will be a heat flow into the metal forming the walls of the canister. It has, interestingly, been found that nevertheless, an effective precompaction can occur in the manner described above and the shape of the bellows container achieved during the final hot uniaxial pressing stage is highly predictable and repeatable.