PROCESS FOR COMPACTING RADIOACTIVE METAL WASTES

Abstract: A process for compacting radioactive waste (3) by precompressing the waste (3) in each of capsules (2) while filling the waste into the capsule (2) to thereby compress the waste to the desired bulk/true density ratio under a small total pressing load and diminish damage to the inner surface of the die used, each of the capsules having a cross sectional form corresponding to a divided segment of the cross section of an HIP treatment container (5), then placing the capsules (2) into the HIP treatment container (5), subsequently filling the HIP treatment container (5) with a metal powder as a filler, thereafter degassing and sealing the HIP treatment container and subjecting the contaier to an HIP treatment in its entirety. The process precludes scattering of the radioactive substance from the waste and release of waste fragments to assure improved safety.

Description: The present invention relates to a compacting process suitable for storing with safety over a prolonged period of time radioactive metal wastes such as spent nuclear fuel cladding (hereinafter referred to as "hulls") which are sheared into short lengths for nuclear fuel reprocessing. Document DE-A-3 129 852 discloses precompressing the waste material in capsules, subsequently applying high pressure to said capsules and placing them in an overall container which finally is sealed. For compacting and stabilizing such radioactive metal waste for storage of the waste, attention has been directed in recent years to a hot isostatic pressing (hereinafter referred to as "HIP") treatment. For example, Examined Japanese Patent Publication SHO 57-959 discloses a process comprising precompressing radioactive metal waste in a die to obtain a block, loading the block into an HIP treatment container, degassing or non-degassing and sealing the HIP treatment container, and subjecting the sealed HIP treatment container to HIP treatment in its entirety. This process includes a precompressing step in order to increase the bulk density (before the HIP treatment) of the radioactive metal waste to be loaded into the HIP treatment container. The increased bulk density serves to minimize the deformation of the HIP treatment container by the HIP treatment and avoid breakage of the HIP treatment container that might occur if the HIP treatment container is deformed greatly. From this viewpoint, it is desired that the precompressed (press-formed) block have a bulk density of at least 60% of its true density (waste metallic density). With the process disclosed in the above publication, the compressed block to be loaded into the HIP treatment container is compressed to an outside diameter slightly smaller than the inside diameter of the HIP treatment container and is thus approximately equal to the HIP treatment container in cross sectional area. Thus, the waste is precompressed in a die which when closed is approximately equal in cross sectional area to the HIP treatment container. Because the die when closed is generally equal in cross section to the HIP treatment container a very great total pressing load is required to give the desired bulk density/true density ratio. For example, for a die 300 mm in outside diameter to achieve a bulk/true density ratio of 65%, the total pressing load needed is as great as 1400 t. The precompressing step of the above process therefore requires the use of a heavy pressing machine which can provide a large pressing load and which requires a large space for installation and is very expensive. Further with the above process, the compressed block is loaded directly into the HIP treatment container without the use of another container or the like, with the result that the radioactive substance adhering to the waste is likely to scatter about or the waste will release small fragments during the filling procedure. Another problem in that since the waste is loaded into the die directly for precompression, the waste comes into direct frictional contact with the die which can mark and damage the innter surface of the die. We will describe a process for compacting radioactive metal wastes free of the foregoing problems. The present invention provides a process for compacting radioactive metal waste comprising: loading the waste into a number of capsules; pre-compressing the waste in each said capsule to ensure that the capsule is filled with said waste in a compressed state; loading the thus filled capsules together with a metal powder into an HIP treatment container and sealing the HIP container; and subjecting the sealed HIP container with the capsules therein to an HIP treatment; characterised in that said capsules co-operate as an assembly in the HIP container to correspond to and substantially fill the internal cross-sectional area of the HIP container, that the cross-sectional area of each said capsule is a fraction of the internal cross-sectional area of the HIP container, and that said metal powder fills any voids within the HIP container between the capsules and between the capsules and the HIP container. In this way, the waste is precompressed as it is loaded in each of the capsules which has a cross section corresponding to a divided segment of the cross section of the HIP treatment container, whereby the waste can be pressed to the desired bulk/true density ratio under a great pressure by a smaller pressing load than in the prior art without the likelihood of causing damage to the die used for the precompression. Furthermore, the waste whilst accommodated within the capsules is placed into the HIP treatment container. This prevents scattering of the radioactive substance or release of small waste fragments to assure improved safety. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a process of compacting radioactive metal waste according to an embodiment of the present invention; Fig. 2 (a) is a side elevation showing a compressed block formed by the process; Fig. 2 (b) is a sectional view taken along the line B-B in Fig. 2 (a); Figs. 3 to 6 are sectional views showing HIP treatment containers and capsules according to other embodiments of the present invention; and Fig. 7 is a perspective view of a HIP treatment container carrying capsules piled up in a multiplicity of layers, with a part broken away and in section. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to Figs. 1, 2 (a), 2 (b), and 7. A die 1 is formed with a cavity having a sectorial cross sectional form corresponding to a quarter of a circle. A hollow capsule 2 approximately identical with the cavity in cross sectional form is inserted into the die 1 in step P1. The cross sectional form is identical with a quarter of the cross section of an HIP treatment container 5 to be described later. Hulls (radioactive metal waste) 3 are loaded into the capsule 2 and pressed within the capsule 2 by a plunger 4, whereby the hulls 3 are precompressed. The plunger 4 is slightly smaller than the hollow space of the capsule 2 in cross sectional area so as not to collapse the upper end of the capsule 2. The plunger 4 is then withdrawn. By repeating the pressing operation and repeatedly replenishing the capsule 2 with hulls 3, the capsule 2 is filled with hulls 3 in a compressed state and then withdrawn in its entirety from the die 1 (step P2). Four capsules 2 thus filled with hulls 3 are loaded without any clearance into the HIP treatment container 5 which is in the form of a hollow cylinder having an open upper end (step P3). The capsules 2 may be placed into the HIP treatment container 5 in a single layer with respect to the vertical direction as illustrated in Fig. 1. Such capsules 2 may be arranged in a multiplicity of layers with respect to the vertical direction as illustrated in Fig. 7. When the HIP treatment container 5 is thus filled, a clearance or voids occur at the upper ends of the capsules 2 or inside the HIP treatment container 5. Accordingly, to fill such voids, stainless steel or like metal powder is filled into the HIP treatment container 5. This minimizes the deformation of the HIP treatment container 5 due to HIP treatment to prevent a break of the HIP treatment container 5 due to great deformation. With the HIP treatment container 5 thus completely filled up, the opening of the HIP treatment container 5 is closed with a closure 7 having a degassing tube 6, and the closure 7 is welded along its periphery to the HIP treatment container 5 and thereby secured to the HIP treatment container (step P4). Any slight clearance, if occurring between the closure 7 and the capsules 2, is preferably filled up with stainless steel powder or the like as described above. Subsequently, a vacuum pump 8 is connected to the degassing tube 6 and operated to remove air from the inside of the HIP treatment container 5, and the degassing tube 6 is collapsed by a sealer 9 to seal off the HIP treatment container 5 (step P5). The sealed HIP treatment container 5 is then subjected in its entirety to HIP treatment at a high temperature and high pressure (step P6). Consequently, a compacted block of waste can be obtained which has a density approximately equal to the true density as illustrated in Figs. 2 (a) and (b). Thus, the radioactive metal waste can be compacted and stabilized. As an example, a multiplicity of small pieces of Zircaloy hulls, 10 mm in diameter, 30 mm in length and 0.8 mm in wall thickness, were compressed under a surface pressure of 2000 to 2500 kgf/cm² within a sectorial capsule measuring 70 mm in radius, 240 mm in height and 2.5 mm in wall thickness. Four such capsules were placed into an HIP treatment container 145 mm in diameter and 280 mm in height, and the HIP treatment container was then degassed with a closure welded thereto, sealed off and subjected to HIP treatment. The compressed block obtained was found to have a density almost equal to the true density. With the present process, hulls 3 are precompressed within the capsule 2 which has 1/4 the cross sectional area of the HIP treatment container 5, so that the pressure to compress the radioactive metal waste to a desired bulk/true density ratio can be obtained with a smaller total pressing load than in the prior art. This permits the use of smaller equipment including the die 1, etc. and serves to diminish the damage to the inner surface of the die 1. Moreover, hulls 3 can be transported as accommodated in the capsules 2 in the precompression step through the plant into the HIP treatment container 5. This precludes scattering of the radioactive substance from the hulls and release of waste fragments to assure improved safety. Furthermore, hulls can be compressed in small amounts within the capsule 2 to provide a precompressed block of uniform density, with the result that the block accommodated in the HIP treatment container can be subjected to HIP treatment with reduced local deformation. According to the present embodiment, the HIP treatment container 5 in the form of a hollow cylinder is filled with capsules 2 without any clearance and thereby prevented from deformation during the HIP treatment to the greatest possible extent. The HIP treatment container 5 can then be easily checked by a smear test a method of for contamination by the radioactive substance) by placing the HIP treatment container 5 on a rotary table or the like and holding a blade or the like in contact with the HIP treatment container during rotation. This achives an improved work efficiency. According to the present invention, the capsule 2 is not limited specifically in its cross sectional form. As seen in Fig. 3, the capsule may have a cross section identical with one of the six equal divided segments of the cross section of the HIP treatment container 5. Alternatively as seen in Fig. 4, capsules 2 may be so arranged that a cylindrical capsule 2a is positioned centrally of the HIP treatment container 5 and surrounded by a plurality of capsules 2b. These modifications achieve the same result as the above embodiment. Further according to the present invention, the entire cross section of the HIP treatment container 5 need not be completely divided by the capsules. For example as seen in Fig. 5, a plurality of cylindrical capsules 2 may be filled in the cylindrical container 5 with some clearances left inside the HIP treatment container 5. In this case, however, the clearances are likely to permit the HIP treatment to locally deform the HIP treatment container 5 greatly to result in damage or breakage, so that it is desirable to fill up the clearance with stainless steel powder 10 or the like as shown in Fig. 5. The HIP treatment container 5 is not limited to a circular cross section. The same result as already described can be achieved, for example, by filling without clearance an HIP treatment container 5′ having a quadrilateral cross section with capsules 2′ each having a cross section identical with one of four equally divided segments of the guadrilateral as seen in Fig. 6.

Claims: 1. A process for compacting radioactive metal waste comprising: loading the waste (3) into a number of capsules (2); pre-compressing the waste in each said capsule to ensure that the capsule is filled with said waste in acompressed state; loading the thus filled capsules together with a metal powder into an HIP -hot isostatic pressing- treatment container (5) and sealing the HIP container; and subjecting the sealed HIP container with the capsules therein to an HIP treatment; characterised in that said capsules co-operate as an assembly in the HIP container to correspond to and substantially fill the internal cross-sectional area of the HIP container, that the cross-sectional area of each said capsule is a fraction of the internal cross-sectional area of the HIP container, and that said metal powder fills any voids within the HIP container between the capsules and between the capsules and the HIP container. 2. A process as claimed in Claim 1, characterised in that said metal powder is loaded into any said voids within the HIP treatment container (5) after the capsules (2) are placed therein. 3. A process as claimed in Claim 1 or 2, characterised in that said metal powder is stainless steel powder. 4. A process as claimed in any one of Claims 1 to 3, characterised in that the HIP treatment container (5) is in the form of a hollow cylinder. 5. A process as claimed in Claim 4, characterised in that there are four said capsules (2) each having cross-sectional corresponding to sector of the cylinder of the HIP container. 6. A process as claimed in Claim 4, characterised in that there are six said capsules each having a cross sectional correspnding to a sector of the cylinder of the HIP container. 7. A process as claimed in Claim 4, characterised in that one said capsule which is cylindrical (2a) is positioned centrally in the HIP treatment container and it is surrounded by a plurality of said capsules (2b). 8. A process as claimed in any one of Claims 1 to 3, characterised in that the HIP treatment container (5) is quadrilateral in cross section. 9. A process as claimed in Claim 8, characterised in that there are four said capsules (2) each having a quadrilaterial cross sectional form identical with a quarter of the cross-section of the HIP treatment container. 10. A process as claimed in any of Claims 1 to 9, characterised in that a single vertical layer of said capsules (2) are placed into the HIP treatment container (5). 11. A process as claimed in any of Claims 1 to 9, characterised in that a number of vertical layers of said capsules (2) are placed in the HIP treatment container (5).

Inventors: INADA, EIICHI 3B-14, DONEN-HYAKUZUKAHARA-DANCHI, KITAGAWA, KAZUO, KOMATSU, FUMIAKI, MASAKI, TAKAYOSHI, SAWADA, YOSHIHISA, SHIOTSUKI, MASAO
Applicants: DORYOKURO KAKUNENRYO KAIHATSU JIGYODAN, KABUSHIKI KAISHA KOBE SEIKO SHO
Application year: 1989
Main classification: G21F9
IPC codes: G21F9