Cask for a radioactive material and radiation shield

Abstract: A cask for a radioactive material which has a single gamma ray and neutron shielding layer disposed on the outside of a vessel body, wherein the shielding layer is formed of the compact of a mixture of lead and a metal hydride dispersed therein. This cask can exhibit an excellent shielding effect according to the balance of radiation source intensity between gamma rays and neutrons, an can be made more compact than conventional casks.

Description: BACKGROUND OF THE INVENTION Field of the Invention: This invention relates to a transport and/or storage cask for a radioactive material such as spent fuel or the like, and a radiation shield. Description of the Related Art: A cask for a radioactive material such as spent fuel from a nuclear power plant or the like must shield gamma rays and neutrons emitted from the radioactive material and to effectively dissipate heat generated through the decay of a radioactive material such as spent fuel or the like contained therein. Examples of such a cask are disclosed, for example, in Japanese Patent Application Laid-Open No. 7-27896 (kokai) and Japanese Patent Application Publication No. 5. 39520 (kokoku). A cask disclosed in Japanese Patent Application Laid-Open No. 7-27986 (kokai) has a gamma ray shielding lead layer interposed between an inner shell made of a steel plate and an outer shell made of a steel plate and a neutron shield disposed on the outer surface of the outer shell, so that gamma rays and neutrons emitted from a radioactive material are shielded by these two layers, respectively. This cask is further provided with cooling fins disposed on the outer side of the neutron shield. The lead layer is closely brought into contact with the outer surface of the inner shell via a thin film of a lead-tin material, thereby efficiently dissipating outward heat generated within the inner shell, such as that resulting from decay of a radioactive material, so that a radioactive material such as spent fuel is transported safely in the cask. A cask disclosed in Japanese Patent Application Publication No. 5-39520 (kokoku) is also based on the technical thought of shielding gamma rays and neutrons separately, wherein gamma rays emitted from a radioactive material are shielded by carbon steel, and neutrons by a neutron shield. More specifically, it is composed of a cylindrical vessel made of carbon steel by which gamma rays are shielded, a plurality of metallic heat conductive members which are disposed adjacent to each other around the vessel and between the vessel and the outer shell, and a neutron shield material filling in each of closed spaces formed by the heat-conductive members and the outer shell. Each of the heat conductive members has an L-shaped cross-section and is composed of a portion which extends in the longitudinal direction of the vessel so as to contact the outer surface of the vessel and a portion which extends in the radial direction of the vessel and whose end is attached to the inner surface of the outer shell. The cask disclosed in Japanese Patent Application Laid-Open No. 7-27896 (kokai) has an advantage that the inner shell can be made thin because the lead layer having an excellent shielding capability against gamma rays is disposed between the inner and outer shells, and an advantage that heat generated within the inner shell, such as that resulting from decay of a radioactive material, can be efficiently dissipated outward because the lead layer closely contacts the outer surface of the inner shell via the thin film of a lead-tin material. However, in order to attain a close contact between the lead layer and the outer surface of the inner shell, the lead layer is formed employing a so-called homogenizing treatment comprising the steps of applying flux containing zinc chloride, stannous chloride, and the like to the outer surface of the inner shell; coating the outer surface with molten lead-tin material; assembling the inner and outer shells together; and casting lead between the inner and outer shells. As a result, the fabrication of the cask takes a longer period of time and involves higher costs. Further, lead must be carefully cast between the inner and outer shells so as to not introduce defects such as voids, and after casting, the cask must undergo an ultrasonic inspection for such defects. Moreover, heat generated during casting causes the inner and outer shells to deform, resulting in a nonuniform clearance between the inner and outer shells and thus forming a thinner portion in the thus-cast lead layer. It is therefore necessary to cast more lead than a required quantity corresponding to a required shielding thickness. The cask disclosed in Japanese Patent Application Publication No. 5-39520 (kokoku) uses a vessel which is made of only carbon steel, thereby shielding gamma rays. When the vessel is made of only carbon steel, the thickness thereof must be considerably large to shield gamma rays because carbon steel is inferior to lead in terms of gamma ray shielding capability. Even though the vessel is relatively thick, the heat-conductive performance thereof is relatively good, and thus no problem arises with respect to heat; however, the vessel's capacity for containing a radioactive material reduces accordingly, resulting in a reduced storage efficiency. The present inventors proposed, in Japanese Patent Application No. 7-199594, a cask for a radioactive material having a gamma ray shielding layer and a neutron shielding layer disposed on the outer surface of an inner shell, as well as heat conductive members penetrating through the gamma ray shielding layer and neutron shielding layer, as a vessel having a high efficiency of storing a radioactive material and an excellent heat-conductive performance. This transport/storage cask for a radioactive material, a transverse cross-section of which is typically shown in FIG. 8, is composed of a vessel inner shell 1 having a basket 5 for containing a radioactive material, an outer shell 2, and a gamma ray shield layer 3 and a neutron shield layer 4 successively disposed between the vessel inner shell 1 and the outer shell 2. According to this invention, the heat-conductive performance of the vessel is particularly excellent, compared with the above-mentioned prior arts, by the presence of the heat-conductive members. When it is required to shield both gamma rays and neutrons, as in a cask for spent fuel, radiation source intensities of gamma rays and neutrons are differed depending on spent fuel, and it is important to effectively shield gamma rays and neutrons according to the balance of radiation source intensity between the both or the degree of intensity. In the above-mentioned structures of the prior arts, since gamma rays and neutrons are shielded independently by a gamma ray shielding layer and a neutron shielding layer, respectively, it is difficult to design the material or thickness of each shielding layer according to the balance of radiation source intensity. Therefore, a conservative design such as thickening of the respective shielding layers is needed, resulting in an increase in size of the vessel, and construction of the shielding layers and the heat conductive members is obliged to be complicated. SUMMARY OF THE INVENTION This invention has been achieved to solve the above-mentioned problems. An object of this invention is to provide a cask for a radioactive material which can exhibit an excellent shielding effect according to the balance of radiation source intensity between gamma rays and neutrons or the degree of intensity, and can be made more compact or contain a greater quantity of a radioactive material with the same size. In order to attain the above objective, the cask for a radioactive material according to this invention comprises a gamma ray and neutron shielding layer disposed around a vessel body, the shielding layer being composed of a single shielding layer formed of a mixture of lead and titanium hydride dispersed therein. The single shielding layer referred to herein means that it has a simplified function for shielding both gamma rays and neutrons, and can be discriminated from a conventional shielding layer having a plurality of shielding layers, each layer bearing part of the function or role of shielding gamma rays and neutrons, respectively. In other words, when the shielding layer of this invention is provided on the vessel body, or used as a radiation shield, pluralization (pluralization of a shielding layer having the same function) in the sense of providing or using a block formed of a mixture of lead and powder of titanium hydride mixed and dispersed therein in the form divided into a plurality of pieces in the longitudinal direction or thickness direction thereof is included within the range of this invention. It is known that lead and titanium hydride are effective for shielding gamma rays and neutrons, respectively. Titanium hydride is an effective material as a neutron shield with a high hydrogen content, in theory, per unit volume. However, titanium hydride is, in general, inevitably provided in a powdery state because of its manufacturing process. Therefore, it has an extremely low bulk density of about 30 % of the theoretical density, and it can not be molded to a density that could be applicable for a neutron shield by general press molding. Although it was also attempted to mold titanium hydride powder into a plate having a density of 90 % or more of the theoretical density by applying a pressure of several thousands tons by a large-size press, this method was not practically employed because of an extremely high fabrication cost including the pressing itself, and pretreatment and post-treatment thereof This invention has solved the above-mentioned problem by dispersing titanium hydride into lead. To be concrete, the dispersion of titanium hydride into lead can be executed by (1) mixing the powder of titanium hydride into molten lead to disperse it into lead, (2) mixing powdery lead and the powder of titanium hydride together and dispersing the powder of titanium hydride into the lead powder followed by compression molding. The compact of a mixture of lead and titanium hydride dispersed therein means that the mixture is molded (blocked) into the application form within a cask of the gamma ray and neutron shielding layer. In the above-mentioned method (1), lead may be molten in a die having the application form of the gamma ray and neutron shielding layer, to which titanium hydride is mixed followed by cooling and coagulation, or the mixture may be cooled and coagulated followed by molding into the application form of the gamma ray and neutron shielding layer. When titanium hydride is added to molten lead, it is necessary to add the titanium hydride while sufficiently stirring molten lead or lay the titanium hydride to be added into the form of mixture in order to uniformly disperse the titanium hydride into the lead, because both lead and titanium hydride have different specific gravity. While titanium hydride generally has the characteristic of decomposing under conditions of a temperature of 400 °C and a high vacuum, lead has a low melting temperature of 300-400 °C. Therefore, even when the powder of titanium hydride is mixed into molten lead in the method (1), a mixture having the titanium hydride uniformly dispersed in lead can be provided without decomposing the titanium hydride. In the method (2), the mixture of lead and titanium hydride may be molded into the application form of the gamma ray and neutron shielding layer in compression molding, or it may be worked into the application form by cutting after compression molding. In compression molding, powders of lead and titanium hydride are mixed together to penetrate the lead powder into cavities formed between the mutual titanium hydride powders followed by compression molding of the mixture. According to this method, a high pressure as in press molding of titanium hydride alone is dispensed with by the action of the lead so that the mixture is soft and easy to mold. The bulk density of the mixture compact can be also increased. The thus-obtained mixture of lead and titanium hydride powder dispersed therein can form a practical compact in respect of density and strength, and also can provide a single shielding material having the function of shielding both gamma rays and neutrons. This invention, therefore, satisfies both the fabrication of a shield that can be used practically in respect of its density and strength, and improves the function of a shield in shielding both gamma rays and neutrons. Compared with the thus-obtained shield according to this invention, a compact obtained by mixing powdery lead with a powdery titanium hydride followed by kneading and molding by use of a binder such as resin or rubber has a problem of deterioration in use as a shield because of the low heat resisting temperature and durability of the inevitably used binder. This method is still not realistic with respect to the fact that the bulk density of the compact can not be increased to a density resistant to use as the above-mentioned lead by general press molding even by the use of resin or rubber. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view illustrating the relation between the mixing ratio of titanium hydride to lead and the shielding effect to gamma rays and neutrons in the application of a radiation shield according to an embodiment of this invention, which is a transport cask for a radioactive material. Fig. 2 is a view illustrating the relation between the mixing ratio of titanium hydride to lead and the shielding effect to gamma rays and neutrons in the application of the radiation shield according to an embodiment of the invention, which is a storage cask for a radioactive material. FIG. 3 is a transverse cross-section of a cask for a radioactive material according to another embodiment of this invention. FIG. 4 is a longitudinal cross-section of the cask for a radioactive material according to an embodiment. FIG. 5 is a transverse cross-section of the cask of FIG. 4. FIG. 6 is an enlarged view of portion X of FIG. 5. FIG. 7 is a view illustrating a block of a gamma ray shielding layer according to the embodiment, wherein FIG. 7a is a view illustrating a block having slant ends for joint, and FIGS. 7b and 7c are views illustrating a block having rabbeted ends for joints. FIG. 8 is a transverse cross-section of a conventional transport/storage cask for a radioactive material. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the gamma ray and neutron shielding layer in this invention, titanium hydride is preferably mixed to lead within a range of 15-100 % in order to make lead and the titanium hydride exhibit respective gamma ray and neutron shielding effects to a maximum. The meaning of this mixing ratio of lead to titanium hydride will now be illustrated. The simulation results of the relation between mixing ratio of titanium hydride to lead and shielding effect to gamma rays and neutrons are shown in Fig. 1 for a transport cask for a radioactive material and in Fig. 2 for a storage cask for a radioactive material, respectively. The simulation was performed under the following conditions; (1) vessel: those having the vessel structure shown in FIG. 4 with a height (length) of 5200 cm and a diameter of 2450 cm for both the transport cask and the storage cask, (2) radioactive material to be contained: spent fuel of a pressurized water reactor (PWR) with a cooling period of 2 years (transport cask) and 10 years (storage cask), (3) calculation code: one-dimensional shield calculation code (ANISN) used in designing and safety analysis of cask, (4) shielding effect to gamma rays and neutrons: measured by the radiation dose rate at 1 m from the surface of the vessel side surface center part. In FIG. 1, reference marks ▪, □ and ◆ denote dose rates of gamma rays, neutrons, and the total of gamma rays and neutrons, respectively. As is apparent from FIG. 1, neutron dose rate is sharply increased with a mixing ratio of titanium hydride to lead of less than 15%, while gamma ray dose rate is sharply increased with a mixing ratio above 50%. It is found from the same figure that the optimum mixing range of titanium hydride to lead where the dose rates of gamma rays and neutrons are less than 100 µ Sv/h is 20-40%. In FIG. 2, reference marks ▪, □, and ◆ denote dose rates of gamma rays, neutrons, and the total of gamma rays and neutrons, respectively. As is apparent from FIG. 2, neutron dose rate is sharply increased with a mixing ratio of titanium hydride to lead of less than 20%, while gamma ray dose rate is not so much sharply increased up to 100%. It is found from the same figure that the optimum mixing ratio range of titanium hydride to lead where the dose rates of gamma rays and neutrons are less than 100 µ Sv/h is 30-60 %. Considering two typical cases of a transport cask for a radioactive material and a storage cask for a radioactive material, the mixing ratio of titanium hydride to lead within a range of 15 %-100 % is adaptable, and the optimum range is 20-60%. Since the cost of titanium hydride to lead is high, titanium hydride is used more economically with a lower mixing ratio within this optimum range. Further, other metal hydrides such as zirconium hydride also have the same tendency. Of the metal hydrides, titanium hydride is used in consideration of both its excellent neutron shielding effect and its availability. From FIGS. 1 and 2, it is found that the optimum mixing ratio of titanium hydride to lead is varied between a transport cask for a radioactive material and a storage cask for a radioactive material, because the radioactive material to be contained and the balance in the quantity of gamma rays and neutrons, differ between the two. This means that it is important to shield gamma rays and neutrons according to the balance of radiation source intensity between the two when it is needed to shield both gamma rays and neutrons as in a cask for spent fuel. Each of the prior arts having a shielding layer divided into a gamma ray shielding layer and a neutron shielding layer substantiates the statement of the present inventors that the material or thickness of each shielding layer is difficult to design according to the balance of radiation source intensity. Furthermore, the shield according to this invention is suitably used as a shield for gamma rays and neutrons, particularly, generated from nuclear power facilitates, radiation generating devices, and equipments having radiation sources. Although a mixture of silicone rubber and powder of lead or tungsten was employed in the past as a shield of this kind, (1) it had a problem that it was not usable under a high temperature because silicone rubber has a low heat resisting temperature, and (2) an absorbing margin for thermal expansion taking the high temperature of the shield in a cask into consideration was required in construction because of its high thermal expansion coefficient, and this absorbing margin had the possibility of causing a radiation leak (streaming). The above-mentioned inconveniences are solved by the use of the shield according to this invention. A preferred embodiment of a transport/storage cask for a radioactive material according to this invention is illustated in FIGS. 3-6. FIG. 3 typically shows a transverse cross-section of a cask for a radioactive material according to the embodiment of this invention, which corresponds to FIG. 8 showing the above-mentioned prior art. FIG. 4 is a longitudinal cross-section of the cask, FIG. 5 is a transverse section of the cask of FIG. 4, and Fig. 6 is an enlarged sectional view of part X of FIG. 5. In the figures, reference numeral 1 denotes an inner shell, reference numeral 2 denotes an outer shell, and reference numeral 6 denotes a single shielding layer of gamma rays and neutrons. The inner shell 1 and the outer shell 2 are made of steel and are cylindrical, and the inner diameter of the outer shell 2 is greater by a predetermined value than the outer diameter of the inner shell 1. The inner shell 1 has a minimum thickness required to function as a hermetically sealed vesseL By adapting such a minimum required thickness, the efficiency of storing a radioactive material is improved, and the weight of the whole cask can be reduced. In this embodiment, the same heat conductive members 7 as in Japanese Patent Application No. 7-199594 described above are provided in order to further improve heat conductive characteristic. Each of the heat conductive members 7 is a relatively lengthy member formed by bending a metallic sheet, such as that of copper or aluminum, having good heat conductivity into a relatively elongated shape having an L-shaped cross-section. The heat-conductive member 7 is disposed around the inner shell 1 in the following manner: side portions 8 of the L-shaped cross-section are arranged at a predetermined pitch along the outer circumference of the inner shell 1; a surface extending longitudinal from each side portion 8 contacts the outer surface of the inner shell 1 under pressure; and the end of another side portion 8 is welded to the inner surface of the outer shell 2. By mounting the heat conductive members 7 in this way, a space 9 defined by the side portions 8 is formed between the inner shell 1 and the outer shell 2. The heat generated within the inner shell 1 is transferred efficiently to the outer shell 2 via the heat-conductive members 7, and dissipated outwardly from the outer shell 2. Instead of being contacted to the outer surface of the inner shell 1 under pressure, the surface extending longitudinally from the side portion 10 may be mounted closely to the outer surface by bolting or brazing. The gamma ray and neutron shielding layer 6 is formed of blocks, each having a thickness required to shield gamma rays. Each block has a cross-sectional shape to fit into a corresponding portion, located adjacent to the outer surface of the inner shell 1, of the space 9 with a length substantially equal to the length of the space 9. The blocks are inserted into the space 9. When the necessary thickness of the shielding layer in this embodiment is compared with that of the shielding layer divided into two gamma ray and neutron shielding layers as is available in conventional shields, a total thickness of 27 cm is necessary in the conventional example of FIG. 8 consisting of a gamma ray shielding layer of 15 cm and a neutron shielding layer of 12 cm while the single shielding layer 6 of this embodiment has a thickness of 22 cm, resulting in a reduction in the weight of the cask according to this invention. The reduction in weight of the cask reversely leads to an increase in the storage capacity of a radioactive material, the number of fuel assemblies can be increased to 37 in this embodiment against 32 in the conventional example, and the storage capacity can be increased by about 20 %. At the bottom opening of a cylindrical vessel body 11 having the above-mentioned structure, an inner bottom 12 made of the same material as that of the inner shell 1 is welded to the inner shell 1, and an outer bottom (protective bottom) 13 is mounted so as to cover the inner bottom 12. At the top opening of the cylindrical vessel body 11, an inner lid 14 made of the same material as that of the inner shell 1 or of stainless steel is mounted, and an outer lid (protective cover) 15 is mounted so as to cover the inner lid 14. In the transport/storage cask for a radioactive material having the above-mentioned structure, gamma rays and neutrons emitted from a radioactive material contained within the vessel are shielded by a single layer which is the gamma ray and neutron shielding layer 6 disposed outside the inner shell 1. Thus, the inner shell 1 may have a minimum thickness required to function as a pressure vessel, thereby improving the efficiency of storage of a radioactive material. Since the heat-conductive members 7 penetrate through the gamma ray and neutron shielding layer and are disposed between the inner shell 1 and the outer shell 2, heat resulting from the decay of a radioactive material contained within the vessel is transferred efficiently via the heat-conductive members 7 from the inner shell 1 to the outer shell 2. Thus, it is not necessary to improve the heat-conductive performance of the gamma ray and neutron shielding layer 6 by a special treatment such as the homogenizing treatment, thereby facilitating the fabrication of the cask and reducing fabrication cost. The gamma ray and neutron shielding layer 6 can be preliminarily formed as blocks, as described above, by (1) mixing a powder of titanium hydride into molten lead to disperse it into the lead, or (2) mixing powdery lead and a powder of titanium hydride together followed by compression molding. The shielding layer 6 can be thus constructed by a simple method of inserting the blocks into the space 9. Therefore, it is not necessary to cast, at a factory, the materials of the gamma ray and neutron shielding layer 6, but the blocks can be prior produced at a dedicated casting factory. This makes it suitable for mass production and reduces the work needed for forming the gamma ray and neutron shielding layer 6, thereby advantageously reducing fabrication cost. Each block of the gamma ray and neutron shielding layer 6 may be divided in the longitudinal direction thereof into sub-blocks, each having a predetermined length. In this case, since the length of the sub-blocks is shorter than that of the blocks, the sub-blocks are more readily produced at the above-mentioned dedicated casting factory or plant. In order to prevent the streaming of radiation, a longitudinal end of each sub-block has a slant surface 16 as shown in FIG. 7a or a rabbeted surface 17 as shown in FIGS. 7b and 7c. According to the embodiment described above, the vessel body 11 is cylindrical. The vessel body 11 may have a rectangular or polygonal shape. According to the embodiment described above, the gamma ray and neutron shielding layer has a uniform thickness in the longitudinal direction of the vessel. Upper and lower end blocks may be thicker than intermediate blocks. In other words, when the gamma ray and neutron shielding layer 6 is formed of blocks, the thickness can be easily varied in the longitudinal or circumferential direction of the vessel according to the distribution of radiation sources of a radioactive material contained within the vessel. As described so far, the transport/storage cask for a radioactive material according to this invention can be relatively easily manufactured, thereby suppressing fabrication cost, and it is also capable of containing a radioactive material at an enhanced efficiency, it exhibits excellent heat conductive performance, and effectively shields gamma rays and neutrons.

Claims: A cask for a radioactive material having a single shielding layer to gamma rays and neutrons disposed on the outside of a vessel body, the shielding layer being formed of a compact of a mixture of lead and titanium hydride dispersed therein. The cask as defined in claim 1, wherein the titanium hydride is mixed in a ratio of 15%-100% to lead. The cask as defined in claim 1 or 2, wherein the titanium hydride is mixed in a ratio of 20%-60% to lead. A radiation shield to gamma rays and neutrons formed of a compact of a mixture of lead and a titanium hydride dispersed therein, the titanium hydride being mixed in a ratio of 15%-100% to lead. The radiation shield as defined in claim 4, wherein the titanium hydride is mixed in a ratio of 20%-60% to lead.

Inventors: AKAMATSU HIROSHI, TANIUCHI HIROAKI, AKAMATSU, HIROSHI, TANIUCHI, HIROAKI, Akamatsu, Hiroshi, c/o Kobe Steel, Ltd., Taniuchi, Hiroaki, c/o Kobe Steel, Ltd.
Applicants: KOBE STEEL LTD, KABUSHIKI KAISHA KOBE SEIKO SHO
Application year: 1997
Main classification: G21F1
IPC codes: G21F1, G21F5, G21F9