Patent Number: 
Section: description

Referring now to the accompanying drawings, the neutron shield of the invention, and the cask using the same are described in specific embodiments. It must be noted, however, that the invention is not limited to these embodiments alone. A neutron shield of the invention is described below. The neutron shield of the first embodiment is a mixture of a two-part reactive cold-setting epoxy resin consisting of main component and hardener, aluminum hydroxide, and boron carbide. The two-part reactive cold-setting epoxy resin is, as the name suggests, an epoxy resin which is cured at ordinary temperature as the main component and hardener are mixed. The aluminum hydroxide is blended in a large quantity, and is large in hydrogen content, and it has functions as refractory and neutron shielding material. The boron carbide is contained in a slight quantity, and it has functions of neutron decelerating agent and absorbing material. As the main component of the two-part reactive cold-setting epoxy resin, a long-chain aliphatic glycidyl ether epoxy resin containing reactive diluent is used. This long-chain aliphatic glycidyl ether epoxy resin containing reactive diluent has an epoxy equivalent nearly same as the epoxy equivalent of bisphenol A type (=184 to 194), but as compared with the viscosity of bisphenol A type (=120 poise), it is about 20 to 25 poise, and a low viscosity is realized. The hydrogen content of this long-chain aliphatic glycidyl ether epoxy resin containing reactive diluent is 7.6% by weight, which is larger as compared with hydrogen content of 7.1% by weight of bisphenol A type. Therefore, by using the long-chain aliphatic glycidyl ether epoxy resin containing reactive diluent as the main component of the two-part reactive cold-setting epoxy resin, the working efficiency at ordinary temperature is enhanced owing to its low viscosity. That is, by shortening the time required for kneading, the pot life may be utilized advantageously, and massive kneading is possible, the interruption time is shorter in manufacture of a large-sized neutron shield, and the time required for each pouring process is shortened owing to the fluidity, so that the overall working efficiency notably enhanced. Moreover, since the long-chain aliphatic glycidyl ether epoxy resin containing reactive diluent is high in hydrogen content, the heat resistance and neutron shielding capability are further enhanced. On the other hand, by using the long-chain aliphatic glycidyl ether epoxy resin containing reactive diluent as the main component of the two-part reactive cold-setting epoxy resin, the corresponding hardener of the two-part reactive cold-setting epoxy resin can be selected from a wide range, and materials excellent in heat resistance or curing reaction speed can be flexibly selected. Herein, a hardener mixing alicyclic polyamine, polyamide aliphatic polyamine, and epoxy adduct is used. The specific composition is 30% by weight of alicyclic polyamine, 20% by weight of polyamide aliphatic polyamine, and 50% by weight of epoxy adduct. By thus selecting the blend of the hardener, the curing reaction speed of the amine hardener can be slowed down, and a sufficient pot life is maintained. For example, by keeping the initial temperature in kneading constantly at 30xc2x0 C., the pot life can be improved to 3 to 3.5 hours. As a result, in addition to the low viscosity of the main component, the working efficiency is further enhanced. Besides, since the selected alicyclic polyamine is high in heat resistance, the refractory performance of the aluminum hydroxide can be enhanced. Moreover, the hydrogen content of the hardener of this selected blend is maintained at 12+/xe2x88x920.5% by weight, and hence together with the main component, the high hydrogen content may be assured sufficiently. The boron carbide slightly contained in the neutron shield is not particularly specified as far as it has a neutron absorbing capability, and other materials having a wide absorption sectional area for slow and thermal neutrons may be used, such as boron nitride, boric acid anhydride, boron iron, orthoboric acid, methaboric acid, and other inorganic boron compound, but boron carbide is particularly preferably. Next, a second embodiment will be explained. The neutron shield of the first embodiment is composed of a two-part reactive cold-setting epoxy resin consisting of main component and hardener, aluminum hydroxide, and boron carbide, but the aluminum hydroxide contained in a large quantity has been known to drop in the hydrogen content in high temperature environment. Decline of hydrogen content has adverse effects on the heat resistance and neutron shielding capability of the neutron shield. This drop of hydrogen content of aluminum hydroxide is caused by pyrolysis of part of moisture in the aluminum hydroxide in high temperature environment. Herein, aluminum hydroxide of high purity was blended in the neutron shield, and by lowering the content of soda (Na2O) contained in refining process of aluminum hydroxide, it was experimentally confirmed that there was a tendency of suppressing moisture release of part of aluminum hydroxide by pyrolysis up to a high temperature region. Generally, the dehydration pyrolysis temperature for inducing release of moisture of aluminum hydroxide is 245 to 320xc2x0C., and by decreasing the soda content in the refining process of aluminum hydroxide, it is estimated that the hydrogen content is maintained up to this temperature region. Enhancement of purity of aluminum hydroxide is possible by deposition of aluminum hydroxide in a sufficient time in refining from bauxite. Generally, the soda content contained in a commercial product of aluminum hydroxide is 0.2 to 0.3% by weight, and in this case the dehydration pyrolysis temperature of aluminum hydroxide is 120xc2x0 C. or more, but by controlling the soda content at 0.1% by weight, the dehydration pyrolysis temperature of aluminum hydroxide can be held up to about 150xc2x0 C. or more. In particular, by controlling the soda content contained in the aluminum hydroxide at 0.07% by weight or less, the weight loss by heat due to dehydration could be suppressed to 150 to 160xc2x0 C. Refining of aluminum hydroxide with the soda content of 0.07% by weight or less may be easily achieved by taking enough time for depositing as mentioned above, or by washing the commercial aluminum hydroxide in water. By blending the aluminum hydroxide of high purity in the neutron shield, the hydrogen content can be maintained even in high temperature environment. In particularly, by controlling at low soda content of 0.07% by weight or less, the hydrogen content may be held up to about 150 to 160xc2x0 C. This hydrogen content held at 150 to 160xc2x0 C. is enough for the neutron shield used in the cask as mentioned later. In the second embodiment, the neutron shield blended with aluminum hydroxide of high purity is explained to be used in the neutron shield described in the first embodiment, but it is commonly applied in the neutron shield blended with aluminum hydroxide. Next, a third embodiment will be explained. Since the neutron shield in the first embodiment is composed of a two-part reactive cold-setting epoxy resin consisting of main component and hardener, aluminum hydroxide, and boron carbide, generally, the dehydration pyrolysis temperature of aluminum hydroxide is 245 to 320xc2x0 C., and it is sometimes desired to hold the hydrogen content in a region below this temperature range. Herein, since the dehydration pyrolysis temperature of magnesium hydroxide is 340 to 390xc2x0 C., by using magnesium hydroxide as the refractory for composing the neutron shield, the heat resistance of the neutron shield in high temperature environment may be further enhanced. In the third embodiment, magnesium hydroxide is used in place of aluminum hydroxide to be blended in the neutron shield described in the first embodiment, but this blending of magnesium hydroxide is commonly applied in the neutron shield. Also in the third embodiment, magnesium hydroxide is used in place of aluminum hydroxide, but part of aluminum hydroxide may be replaced by magnesium hydroxide. Next, a fourth embodiment will be explained. In the fourth embodiment, the neutron shield explained in the first to third embodiments is applied as the neutron shield of the cask. The cask is a container for holding and storing the spent fuel assemblies. In the terminal stage of nuclear fuel cycle, the consumed fuel assemblies no longer usable are called spent fuels. The spent fuels contain FP and highly radioactive substances, and must be cooled thermally, and hence they are cooled for a specified period (3 to 6 months) in cooling pits at nuclear power plants. Then they are transferred into the shielded container called cask, and transported by truck or ship, and stored at reprocessing plants. FIG. 1 is a perspective view of a cask. FIG. 2 is an axial direction sectional view of the cask shown in FIG. 1. FIG. 3 is a radial direction sectional view of the cask shown in FIG. 1. A cask 100 is formed by machining the inner circumference of a cavity 102 of a shell main body 101 according to the outer circumferential shape of a basket 130. The inner surface of the cavity 102 is machined by exclusive milling machine or the like. The shell main body 101 and bottom plate 104 are carbon steel forged parts having gamma-ray shielding function. Instead of carbon steel, stainless steel may be also used. The shell main body 101 and bottom plate 104 are bonded by welding. To maintain an enclosed performance as a pressure-tight container, a metal gasket is placed between a primary lid 110 and the shell main body 101. The space between the shell main body 101 and outer tube 105 is filled with a neutron shielding resin 106, or the neutron shield mentioned above, which is a high polymer material with high hydrogen content. Plural copper inner fins 107 for heat conduction are welded between the shell main body 101 and outer tube 105, and the resin 106 is injected into the space formed by the inner fins 107 in a fluid state through a pipe not shown herein, and is cooled and solidified. The inner fins 107 should be preferably provided at high density in the area of large heat generation in order to cool uniformly. A thermal expansion allowance 108 of about several millimeters is provided between the resin 106 and outer tube 105. The thermal expansion allowance 108 is formed by disposing an extinguishing type outer tube 105 having a heater buried in hot-melt adhesive or the like at the inner side, injecting and solidifying the resin 106, and heating the heater for melting and discharging. A lid 109 is composed of a primary lid 110 and a secondary lid 111. The primary lid 110 is a disc of stainless steel or carbon steel for shielding gamma-rays. The secondary lid 111 is also a disc of stainless steel or carbon steel, but its upper surface is coated with a neutron shielding resin 112, that is, the neutron shield as mentioned above. The primary lid 110 and secondary lid 111 are fitted to the shell main body 101 by stainless steel or carbon steel bolts 113. Further, among the primary lid 110, secondary lid 111, and shell main body 101, metal gaskets are provided, and the inside is kept airtight. The lid 109 is surrounded with an auxiliary shield 115 sealed with resin 114. At both sides of the cask main body 116, trunnions 117 are provided for suspending the cask 100. In FIG. 1, the auxiliary shield 115 is provided, but when conveying the cask 100, the auxiliary shield 115 is detached, and a buffer 118 is attached instead (see FIG. 2). The buffer 118 has a structure of assembling a buffer material 119 such as redwood into an outer tube 120 formed of a stainless steel material. A basket 130 is composed of 69 square pipes 132 for forming a cell 131 for containing the spent fuel assemblies. The square pipes 132 are composed of aluminum composite material or aluminum alloy formed by adding powder of B or B compound having neutron absorbing performance to Al or Al alloy powder. As the neutron absorbing material, cadmium may be also used instead of boron. The cask 100 mentioned herein is a huge structure of 100-ton class, and by using the neutron shield explained in the first to third embodiments as the resin 106, 112, 114, the weight is reduced substantially, and a sufficient neutron shielding performance and heat resistance will be achieved, and even in locations having a complicated structure such as the inner fins 107, by the improvement of fluidity and pot life, the time and labor required in pouring of the resin 106, 112, 114 can be saved substantially. As described herein, according to the neutron shield and the cask of the invention, since the long-chain aliphatic glycidyl ether epoxy resin containing reactive diluent is used as the main component, the viscosity can be lowered to about 20 to 25 poise, and therefore, the working efficiency is enhanced. Furthermore, the hydrogen content in the main component can be also increased to about 7.5 to 8.5% by weight. By using this main component, a flexible material can be selected for the hardener, as the hardener having favorable effects on the pot life, by using alicyclic polyamine, polyamide polyamine, aliphatic polyamine, or epoxide adduct, either alone or in a mixture of two or more kinds, as the hardener, a sufficient pot life is assured, and the amount of active hydrogen in curing process is increased, and by using alicyclic polyamine, in particular, a two-part reactive cold-setting epoxy resin further enhanced in heat resistance is realized. The pot life can be specifically extended to about 3 to 3.5 hours, for example, when the temperature is about 30xc2x0 C. when kneading the neutron shielding materials containing this two-part reactive cold-setting epoxy resin, and hence the possible pouring time is increased, and massive kneading neutron shielding materials is possible, and the number of times of interruption is decreased in the process of forming a large-sized neutron shield, so that the time and labor required in forming the neutron shield may be substantially saved. Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.