Patent Number: 
Section: description

(Embodiment 1) Among combinations of bisphenol A epoxy resin and various hardeners, the resin product prepared by heat-hardening bisphenol A epoxy resin in combination with acid anhydride and aromatic amine or the like features a low thermal weight reduction rate, excellent heat resistance, and a low hydrogen number density, which is already explained referring to FIG. 1. Alicyclic di-glycidyl ether type epoxy resin is used as the base resin to increase the hydrogen number density without reducing the heat resistance thereof. We inventors prepared base resins by mixing bisphenol A epoxy resin and alicyclic di-glycidyl ether epoxy resin at various ratios, heat-hardened them by acid anhydride or aromatic amine, and evaluated their weight reduction rates at 200xc2x0 C. FIG. 3 shows the result of the experiment. In this experiment, we used bisphenol A epoxy resin having epoxy equivalent of 180 to 190 grams/equivalent and viscosity of about 100 dPa.s at room temperature as the bisphenol A epoxy resin, and commercially-available hydrogenated bisphenol A epoxy resin having epoxy equivalent of about 240 grams/equivalent and viscosity of about 35 dPa.s at room temperature as the alicyclic di-glycidyl ether type epoxy resin. Further we used a mixture of methylcyclopentadiene to which maleic anhydride is added and a small amount of imidazole as an acid anhydride type hardener, and methylene di-aniline as an aromatic amine type hardener. When the aromatic amine hardener is used to harden the epoxy resin, the thermal weight reduction rate becomes greater as the hydrogenated bisphenol A epoxy resin occupies more in the base resin. Contrarily, when the acid anhydride hardener is used, the thermal weight reduction rate of the hardened resin is low even when the base resin is all hydrogenated bisphenol A epoxy resin (100%). Further as the content of the hydrogenated bisphenol A epoxy resin increases in the base resin, the hardened resin has higher hydrogen number density and higher neutron shielding performance. On the basis of the above experimental knowledge, this embodiment explains a neutron shielding material comprising a hardened resin prepared by hardening the hydrogenated bisphenol A epoxy resin by acid anhydride. The neutron shielding material of this embodiment is prepared with the base resin, a hardener, and a hardening promoter which are already described above. The neutron shielding material of this embodiment is prepared in the procedure below. This embodiment uses hydrogenated bisphenol A epoxy resin whose epoxy equivalent is about 240 grams/equivalent as the base resin. We prepared a mixture of 100 parts by weight of base resin, about 65 parts by weight of acid anhydride hardener such as methylcyclopentadiene to which maleic anhydride is added, 0.1 to 2 parts by weight of 2-ethyl 4-methyl imidazole as a hardening promoter, 130 to 200 parts by weight of magnesium hydroxide whose mean grain size (of primary particles) is 1 to 2 xcexcm as a fire retardant, and about 3 parts by weight of boron carbide powder whose mean grain size is 100 xcexcm. This mixture was fully mixed up at a constant temperature of 70 to 100xc2x0 C. and poured into a preheated die. Initially, the mixture was heated at about 80 to 130xc2x0 C. for 2 to 4 hours for primary hardening, then at about 140 to 170xc2x0 C. for 4 to 12 hours for secondary hardening, at about 200xc2x0 C. for a short time period if necessary, and cooled the mixture gradually. We used this hardened resin as a neutron shielding material. The above neutron shielding material of this embodiment will not reduce the neutron shielding performance even when the neutron shielding material is exposed to high temperature of 150 to 200xc2x0 C. for a long time period. Referring to FIG. 4, a metal cask will be explained below which employs the neutron shielding material of this embodiment. A metal cask 1 consists of an outer shell (outer shell) which forms the container, an inner shell 2 having heat-conductive aluminum fins 4 spaced on the outer periphery of the inner shell 2 (inner shell), and a grid-like metallic basket 6 placed inside the inner shell. The neutron absorbing material 5 prepared by this embodiment is filled in the space between the outer shell 3 and the inner shell 2 which is partitioned by the heat-conductive fins 4. The inner shell having an opening on the top is made of carbon steel and can shield gamma rays. The metallic basket 6 has a plurality of cells each of which is designed to store a spent fuel aggregate. The opening of the inner shell 2 is closed with a primary lid 7 to prevent leakage of radioactive materials and a secondary lid 8 which is placed over the primary lid. The inner space of the primary lid 7 is also filled with the neutron absorbing material 5. In case the metallic basket 6 in the metal cask 1 stores 70 spent fuel assembly which are stored for a short period in a water-cooled pool or high-burnup fuel assembly, the temperature of the neutron shielding material 5 goes up to 150 to 200xc2x0 C. due to the heat emitted from the fuel assembly. However, in this case, the neutron shielding performance of the metal cask 1 does not go down because the neutron shielding material 5 keeps the neutron absorbing performance even when it is exposed to a high temperature of 150 to 200xc2x0 C. for a long time period. As explained above, the metal cask 1 can store about 60 or more spent fuel assembly which are short-stored in a water-cooled pool or high-burnup fuel assembly. The neutron shielding material of this embodiment is also applicable to shield the high temperature areas of 150 to 200xc2x0 C. in the radioactive material treating facilities such as reactor vessels, nuclear fuel reprocessing facilities, spent fuel storing facilities, and accelerator facilities. The embodiments 2 to 5 below are also applicable to such radioactive material treating facilities. The base epoxy resin can be hydrogenated bisphenol A epoxy resin singly or together with bisphenol A epoxy resin. Further the epoxy resin to be mixed therewith can be bisphenol A epoxy resin, bisphenol F epoxy resin, or novolac type epoxy resin such as phenol novolac epoxy resin and cresol novolac epoxy resin. The glycidyl ether type epoxy resin can be substituted by glycidyl ester type epoxy resin, glycidyl amine type epoxy resin, biphenyl type epoxy resin, or naphthalene type epoxy resin. Further, the hydrogenated bisphenol A type epoxy resin can be substituted by any epoxy compound such as alicyclic epoxy compound having more hydrogen atoms in the molecule. Any combination of base resin and hardener is selectable as long as the base resin and the hardener are fit for heat-hardening and the hardened resin contains hydrogen atoms of 5xc3x971022 atoms/cm3 or more. For explanation, this embodiment uses methylcyclopentadiene to which maleic anhydride is added as a hardener, but it can be substituted by any known acid anhydride hardener selected from the group of phthalic anhydride, maleic anhydride, methyl nadic anhydride, succinic anhydride, pyromellitic anhydride, chlorendic anhydride, and modification thereof, or a mixture thereof. If it is possible to take much time for hardening, the hardening promoter such as imidazole type hardener or the like need not be added. Although this embodiment uses magnesium hydroxide as a fire retardant, the fire retardant need not be added if the neutron shielding material is applied to what does not require flame resistance. To prepare a neutron shielding material which does not want the reduction in the hydrogen number density during heat-hardening aluminum hydroxide can be used instead of magnesium hydroxide. Calcium hydroxide, hydro garnet and the like can be used as a fire retardant. In the above description, the quantity of magnesium hydroxide to be added is determined according to the viscosity, the mixing time, and effect of flame resistance assuming that the resin mixture is mixed up at about 80xc2x0 C. However unless the viscosity of the mixture exceeds a maximum of 200 grams/eq.s, the quantity of magnesium hydroxide to be added can be changed according to the viscosity and the mixing temperature. Similarly, it is also possible to determine the quantity of magnesium hydroxide to be added from a point of view that the viscosity is 200 grams/eq.s or less for at least one hour or longer. Further, it is possible to determine the quantity of a fire retardant to be added from a point of view that the oxygen index of the hardened resin exceeds 20. Any other boron compounds such as boron nitride than boron carbide can be added to the neutron absorbing material. Further, the neutron absorbing material can be omitted for some special applications. The boron compounds can be substituted by cadmium oxide, gadolinium oxide, and samarium oxide. (Embodiment 2) This embodiment uses, as a neutron shielding material, a heat-setting epoxy resin prepared by hardening alicyclic di-glycidyl ether type epoxy resin as the base resin by a mixture of acid anhydride and amine hardeners. This embodiment as well as Embodiment 1 uses hydrogenated bisphenol A epoxy resin as the alicyclic di-glycidyl ether type epoxy resin. Methylcyclopentadiene to which maleic anhydride is added is used as an acid anhydride hardener as well as in Embodiment 1. This embodiment uses a mixture of alicyclic polyamine and methylcyclopentadiene to which maleic anhydride is added as a hardener and an imidazole compound as the hardening promoter. As well as Embodiment 1, this embodiment uses magnesium hydroxide as the fire retardant and boron carbide as the neutron absorbing material. When acid anhydride is used singly as the hardener, the ratio of acid anhydride to the base resin is determined by a stoichiometric relationship between the equivalent of base epoxy resin and the equivalent of acid anhydride. The content of hydrogen atoms in the acid anhydride is comparatively small. Therefore, it is assumed that the acid anhydride, when used singly as the hardener, works to dilute the hydrogen atoms in the base resin. Accordingly, this embodiment reduces the quantity of acid anhydride to be added to the base resin and add an amine type hardener to make up for it. The quantity of the amine hardener to harden a predetermined quantity of the base resin is generally half to one third of the quantity of the acid anhydride hardener to harden the base resin. Therefore, we can increase the ratio of the base resin relative to the whole resin by substituting part of the acid anhydride hardener by the amine type hardener. This can also increase the hydrogen number density of the hardened resin. This embodiment describes an example of compounding ratio which reacts about 30% of the whole epoxy resin groups in the base resin with the amine type hardener and the remainder with the acid anhydride hardener. The compounding ratio is 100 parts by weight of hydrogenated bisphenol A epoxy resin as the base resin, 45 parts by weight of acid anhydride as part of the hardener, about 8 parts by weight of alicyclic polyamine, about 150 parts by weight of magnesium hydroxide as the fir retardant, and about 3 parts by weight of boron carbide as the neutron absorbing material. This mixture was fully mixed up at about 80xc2x0 C. and poured into a die. The mixture was heated and hardened in the same manner as Embodiment 1. We used this hardened resin as a neutron shielding material. The neutron shielding material of this embodiment which has been prepared as explained above does not lose the neutron shielding performance even when it is exposed to a high temperature of 150 to 200xc2x0 C. for a long time period. The neutron shielding material of this embodiment can have higher hydrogen number density than that of Embodiment 1. The metal cask employing the neutron shielding material of this embodiment uses the neutron absorbing material 5 (of FIG. 4) of the neutron shielding material of this embodiment. The metal cask 1 employing the neutron shielding material of this embodiment can store about 60 or more spent fuel assembly which are short-stored in a water-cooled pool or high-burnup fuel assembly. This embodiment as well as neutron shielding material 1 can substitute the base resin, the anhydride, the fire retardant, and the neutron shielding material by the other materials. Any publicly-known alicyclic polyamine compound can be used as a hardener as long as it can be used for heat-setting. (Embodiment 3) This embodiment uses, as a neutron shielding material, a heat-setting epoxy resin prepared by hardening a base resin by acid anhydride or the like and adding hydrogenated titanium thereto. This embodiment uses bisphenol A epoxy resin as the base resin and acid anhydride as a hardener. To complete the hardening reaction within about one day, a hardening promoter such as imidazole is added to the resin mixture. Further, to this mixture are added magnesium hydroxide as the fire retardant, boron carbide as the neutron absorbing material, and halogenated titanium. Their ratios by weight in the mixture are about 30% of magnesium hydroxide, about 3% or less boron carbide, 20% to 30% of halogenated titanium, and the remainder of bisphenol A epoxy resin. This mixture was fully mixed up at 80xc2x0 C. and poured into a die. The mixture was heated and hardened in the same manner as Embodiment 1. We used this hardened resin as a neutron shielding material. The base resin can be bisphenol A epoxy resin, its modification, and various novolac epoxy resins such as glycidyl ether type epoxy resin, glycidyl ester type epoxy resin, glycidyl amine type epoxy resin, and biphenol type epoxy resin or a mixture thereof with alicyclic di-glycidyl ether type epoxy resin. Besides acid anhydride, the hardener can be any kind of publicly-known amine type hardener for heat-setting resins. The fire retardant and the neutron absorbing material can be substituted as well as in Embodiment 1. Hydrogenated titanium can be substituted by a hydrogen-absorbing alloy such as magnesium and nickel alloy. The neutron shielding material of this embodiment which has been prepared as explained above does not lose the neutron shielding performance even when it is exposed to a high temperature of 150 to 200xc2x0 C. for a long time period. The neutron shielding material of this embodiment can have higher hydrogen number density than that of Embodiment 1. In other words, this embodiment can provide a neutron shielding material which does not lose the neutron shielding performance even when it is exposed to a high temperature for a long time. Further, as this embodiment uses a metal hydride to increase the hydrogen number density of the neutron shielding material, the neutron shielding material can have excellent heat resistance and neutron shielding performance. The metal cask employing the neutron shielding material of this embodiment uses the neutron absorbing material 5 (of FIG. 4) of the neutron shielding material of this embodiment. The metal cask 1 employing the neutron shielding material of this embodiment can store about 60 or more spent fuel assembly which are short-stored in a water-cooled pool or high-burnup fuel assembly. (Embodiment 4) As seen from FIG. 3, for the use of an aromatic amine hardener, when the rate of hydrogenated bisphenol A epoxy resin increases in the base resin (which is a mixture of bisphenol A epoxy resin and hydrogenated bisphenol A epoxy resin), the hydrogen number density of the hardened resin increases but the thermal weight reduction rate becomes greater. However, as long as hydrogenated bisphenol A epoxy resin is up to 50% by weight of the whole base resin, the hardened resin is fully available under a high temperature condition of 150xc2x0 C. or higher. This embodiment explains an example of using, as a neutron shielding material, a heat-setting epoxy resin prepared by hardening a mixture of bisphenol A epoxy resin and hydrogenated bisphenol A epoxy resin as the base resin by an aromatic amine hardener This embodiment uses the same materials as those used for FIG. 3 tests. The materials are bisphenol A epoxy resin having epoxy equivalent of about 180 to 190 grams/equivalent and hydrogenated bisphenol A epoxy resin having epoxy equivalent of about 240 grams/equivalent as the base resin, and methylene di-aniline compound as the aromatic amine hardener. The base resin of this embodiment is a mixture of 50 parts by weight of bisphenol A epoxy resin and 50 parts by weight of hydrogenated bisphenol A epoxy resin. To this base resin are added about 30 parts by weight of aromatic amine, 100 to 160 parts by weight of magnesium hydroxide as a fire retardant, and about 3 parts by weight of boron carbide. This mixture was fully mixed up at a constant temperature in the range of 70 to 100xc2x0 C. and poured the homogeneous mixture liquid into a die. The mixture in the die was heated at 80 to 120xc2x0 C. for about 2 hours for primary hardening, then heated at 120 to 180xc2x0 C. for about 4 to 12 hours for secondary hardening, heated to about 200xc2x0 C. for a comparatively short time for final hardening if necessary, and then cooled. We used this hardened resin as a neutron shielding material. The neutron shielding material of this embodiment which has been prepared as explained above does not lose the neutron shielding performance even when it is exposed to a high temperature of 150 to 200xc2x0 C. for a long time period. The metal cask employing the neutron shielding material of this embodiment uses the neutron absorbing material 5 (of FIG. 4) of the neutron shielding material of this embodiment. The metal cask 1 employing the neutron shielding material of this embodiment can store about 60 or more spent fuel assembly which are short-stored in a water-cooled pool or high-burnup fuel assembly. Epoxy resins to be combined with hydrogenated bisphenol A epoxy resin as the base resin can be bisphenol A epoxy resin and other epoxy resins such as novolac epoxy resin listed in Embodiment 1. The hydrogenated bisphenol A epoxy resin can be substituted by any epoxy compound such as alicyclic epoxy which contains a lot of hydrogen atoms in the molecule. Base resins and hardeners can be determined from a point of view that the hydrogen number density of the hardened resin is 5xc3x971022 atoms/cm3. Any publicly-known alicyclic polyamine compound can be used as a hardener as long as it can be used for heat-setting. Further the fire retardant and the neutron absorbing material can be substituted by other substances as well as in Embodiment 1. (Embodiment 5) This embodiment explains a neutron shielding material prepared by hardening bisphenol A epoxy resin singly by an alicyclic polyamine hardener. The epoxy resin mixture comprises 100 parts by weight of bisphenol A epoxy resin as the base resin, about 30 parts by weight of alicyclic polyamine, 150 to 200 parts by weight of alumina trihydrate, and 3 parts by weight of boron carbide powder. This mixture was fully mixed up at room temperature to make it uniform. This liquid resin mixture was poured into a die, left at room temperature for one day or longer or preferably about 7 days to harden it, heated up to 180 to 200xc2x0 C. for secondary hardening, heated to 180 to 200xc2x0 C. for a comparatively short time for final hardening, and then cooled. It is also possible to gradually increase the primary hardening temperature from about 40xc2x0 C. to about 90xc2x0 C. and proceed to secondary hardening under the above condition. The above hardened resin is placed outside the inner shell of the metal cask. The neutron shielding material of this embodiment which has been prepared as explained above does not lose the neutron shielding performance even when it is exposed to a high temperature of 150 to 200xc2x0 C. for a long time period. The metal cask employing the neutron shielding material of this embodiment uses the neutron absorbing material 5 (of FIG. 4) of the neutron shielding material of this embodiment. The metal cask 1 employing the neutron shielding material of this embodiment can increase the number of spent fuel assembly which are short-stored in a water-cooled pool or high-burnup fuel assembly. In summary, this embodiment can provide a neutron shielding material whose shielding performance does not go down even when it is exposed to a high temperature for a long time. The primary hardening of the mixture at room temperature can lessen the thermal load in application. Further this embodiment enables secondary hardening in the execution of a heat transfer test. The base resin can be substituted by any of glycidyl ether type epoxy resin, glycidyl ester type epoxy resin, glycidyl amine type epoxy resin, and biphenyl type epoxy resin. For easier handling, the bisphenol A epoxy resin can be made less viscous by reducing the degree of cross-linking, by a proper diluting agent, or by a type modified to reduce the viscosity. It is possible to obtain a hardened resin of high hydrogen number density by using a hydrogen-rich epoxy compound such as alicyclic di-glycidyl ether type epoxy resin singly or in combination with various epoxy resin such as bisphenol A epoxy resin. In any case, the epoxy resin can be heat-hardened into a neutron shielding material whose neutron shielding performance doe not go down for a long time period. The fire retardant and the neutron absorbing material of this embodiment can be changed as well as in Embodiment 1.