Patent Number: 050733052
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a process for compacting radioactive wastes wherein the method of the invention is practiced. First in step P1, a die 1 is filled with radioactive wastes, i.e., hulls (fuel claddings as sheared after use) 2, which are then pressed (precompressed) by plungers 3. The precompressed hulls 2 are placed into a treating container 5 along with other blocks of waste 4, if any (step P2). The amount of waste 6 thus charged in is such that a clearance of predetermined thickness will be left inside the container 5 at its upper end. A metal powder, ceramic powder or like particulate material is filled into the clearance to form a filter layer 7 (step P3). The filter layer 7 is so formed as to fulfill the requirements represented by the hatched area of the graph of FIG. 2. More specifically stated, the mean particle size of the particulate material forming the filter layer 7 and the thickness of the layer 7 need to fulfill one of the following requirements. (1) The mean particle size is not smaller than 40 .mu.m to less than 105 .mu.m, and the thickness is at least 5 mm. PA0 (2) The mean particle size is not smaller than 105 .mu.m to not greater than 210 .mu.m, and assuming that the mean particle size is d .mu.m and the thickness of the layer is D mm, the layer 7 has the following relationship between this size and the thickness. EQU D.gtoreq.(20/105).times.d-15 The reason for determining these requirements will be described later. The clearance inside the container 5 around the waste 6 to be treated is also filled up with the metal powder or like particulate material. Next, the opening of the treating container 5 is closed with a closure 9 provided with an evacuating pipe 8, and the closure 9 is joined to the container 5 by welding the outer periphery of the closure to the container (step P4). The evacuating pipe 8 is then connected to a vacuum pump 10, which in turn is operated to evacuate the interior of the container 5 (step P5). At this time, the gas inside the treating container 5 is drawn out of the container through the interstices between the particles forming the filter layer 7, whereas the radioactive substance separating off the waste 6 is blocked by the filter layer 7 which fulfills the foregoing requirement, and is prevented from being led out of the container. After the container has been evacuated completely in this way, the evacuating pipe 8 is collapsed by a sealing device 11 to seal off the container 5 (step P6), which is then checked for leaks (step P7). The container 5 is compressed hot in its entirety by HIP (step P8) or hot press (step P9), whereby the radioactive waste 6 accommodated in the container 5 is compacted and further made stabilized through diffusion and bonding actions between the blocks of waste treated. FIG. 3 shows the result of a simulation test conducted for determining the requirements for the filter layer 7 using as a simulated radioactive powder a commercial clay powder (trade name: Arizona Roaddust) which is widely used for filter trapping tests. A 5 g quantity of the clay powder was passed through a glass tube, 30 mm in diameter, at a flow rate of 22.5 liters/min. The glass tube was provided at an intermediate portion thereof with a filter layer having a predetermined thickness and formed of globular stainless steel particles with a predetermined size. The clay powder passing through the filter layer was trapped with a membrane filter, 0.8 .mu.m in pore size, to measure the amount thereof. Table 1 below shows the particle size distribution of the clay powder. Table 2 shows the particles sizes of stainless steel powders used for forming different filter layers, and the thicknesses of the layers. With reference to FIG. 3, the simulated radioactive powder can be collected 100% when the layer is made of particles of up to 105 .mu.m in size and has a thickness of 5 mm. Further with particles of 210 .mu.m in size, a collection efficiency of 100% can be achieved if the layer is 25 mm in thickness. However, if the particle size exceeds 210 .mu.m, the improvement in the collection efficiency is small even when the layer has a thickness of larger than 25 mm, and it is substantially impossible to achieve a collection efficiency of 100%. When the particle size is less than 40 .mu.m, the interstices between the particles are too small, with the result that the layer causes an exceedingly great pressure loss and offers great resistance, hence a reduced evacuation efficiency. Because of such limitations of particle size, the thickness of the layer must be at least 5 mm at all times. Consequently, the contamination due to the aspiration of radioactive substance can be completely prevented with use of filters fulfilling the requirements represented by the hatched area of FIG. 2. TABLE 1 ______________________________________ Particle Size Distribution of Clay Powder ______________________________________ Particle size (.mu.m) &lt;1 1.5 2 3 4 6 8 12 Proportion (%) 4.4 2.1 6.6 6.7 4.3 6.5 6.3 7.7 Particle size (.mu.m) 16 24 32 48 64 96 128 192 Proportion (%) 7.0 9.8 8.3 14.4 7.4 6.7 1.3 0.5 ______________________________________ TABLE 2 ______________________________________ Requirements for Filter Layer ______________________________________ Particle size of stainless 53 105 210 297 420 steel powder (.mu.m) Thickness of layer of 5 10 15 20 25 stainless steel powder (mm) ______________________________________ The particulate materials usable for forming the filter layer 7 according to the invention include, besides metal powders and stainless steel powder as mentioned above, ceramic powders such as ZrO.sub.2 and SiO.sub.2. Further the treating container 5 is not specifically limited in shape. The same advantage as above can be obtained, for example, by stretchable or contractable containers of the bellows type. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the invention, they could be construed as being included therein.