Radiant energy – Radiation controlling means – Shielded receptacles for radioactive sources
Reexamination Certificate
2002-09-06
2004-09-28
Lee, John R. (Department: 2881)
Radiant energy
Radiation controlling means
Shielded receptacles for radioactive sources
C252S478000, C523S445000, C264S109000
Reexamination Certificate
active
06797972
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a neutron shielding material and more particularly to a neutron shielding material which is preferably applicable to radiation shielding parts such as reactor vessels, radioactive material treating facilities such as a nuclear fuel reprocessing facility, a spent fuel storing facility, and an accelerator facility, a cask for transporting radioactive materials, and a cask for storing radioactive materials.
2. Description of the Prior Art
Spent fuel assembly are taken out from an atomic reactor, stored in water-cooled pools at the atomic power plant site for a preset time period to attenuate radiation dose and calorific power, and then transported to a processing facility such as a fuel reprocessing factory and the like. Recently in countries outside Japan, the spent nuclear fuel assembly are transported to a centralized storage facility (dry storage facility) and stored there. A radioactive storing shell called a metal cask is used to carry the spent nuclear fuel assembly from the atomic power plant site to such a facility and store there.
A metal cask consists of an outer shell which forms the container, an inner shell having heat-transferable fins made of high heat-conductivity metal plates such as copper or aluminum spaced on the outer periphery of the inner shell, and a metallic basket placed inside the inner shell. The space between the outer and inner shells is filled with a hardened resin which works as a neutron shielding material. The inner shell having an opening on the top is made of carbon steel and can shield gamma rays. The metallic basket has a plurality of cells each of which is designed to store a spent fuel aggregate. One metallic basket can store a total of 30 to 70 spent fuel assembly. The opening of the inner shell is closed with a primary lid to prevent leakage of radioactive materials and a secondary lid which is placed over the primary lid.
The resin working as a neutron shielding material is a material containing a lot of hydrogen atoms, that is a material having a high hydrogen number density. Among various kinds of high polymer compounds, the metal casks usually employ epoxy resins because the relationship of heat resistance and hydrogen number density is well balanced. In this case, the resin is a homogeneous mixture of base liquid epoxy resin, amine type hardener, aluminum hydroxide which gives flame resistance to the resin, and boron carbide which works as a neutron absorbing material. This liquid resin is poured into the space surrounded by the inner shell, the outer shell and the heat-transferable fins and hardened there at room temperature.
Below will be explained neutron shielding materials using thermo-setting resins such as epoxy resin which are applied to other than metal casks. Japanese Application Patent Laid-open Publications No. 06-148388 discloses a neutron shielding material which is obtained by mixing a multifunctional epoxy resin, a poly-amine mixture, and an imidazole compound, and reacting thereof to harden at room temperature. Japanese Application Patent Laid-open Publications No. Hei 06-180388 discloses a neutron shielding material which is hardened under pressure and heating with a phenol resin as a binder.
It has been discussed whether dry storage of spent fuel in and outside the nuclear power plant site is available for loose storage of spent fuel assembly in the water-cooled pools. In future, dry storage will be available for spent fuel assembly which are not stored so long in the water-cooled pools and further for high-burnup fuel assembly (45 GWd/ton). Such fuel assembly have great calorific power due to decays of fission-produced nuclides and transuranic elements. When the number of spent fuel assembly to be stored in such a metal cask increases, the neutron shielding material will have a greater thermal load as its thermal conductivity is smaller than that of metals.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a neutron shielding material which can be available at a higher temperature.
The above object of the present invention can be attained by constructing a neutron shielding material with the hardened material prepared by mixing a base resin which contains a compound including two or more epoxy groups in the molecule as at least one component with a hardener for opening said epoxy rings and polymerizing thereof at a temperature higher than room temperature.
Even when the neutron shielding material of the present invention is kept at 150° C. to 200° C., its neutron shielding performance will not go down as the hydrogen number density of the neutron shielding material has a very little rate of reduction at such a high temperature. The spent fuel storing shell employing the neutron shielding material of the present invention can store more spent fuel assembly which are stored for a short period in a water-cooled pool or high-burnup fuel assembly.
Another object of the present invention can be attained by constructing a neutron shielding material with the hardened material prepared by mixing a base resin which is prepared by mixing a base resin which contains a compound including two or more epoxy groups in the molecule as at least one component with a hardener for opening said epoxy rings and polymerizing thereof, wherein the setting temperature is higher than room temperature.
We inventors cleared up the problems involved in storing spent fuel assembly which are stored for a short period in a water-cooled pool or high-burnup fuel assembly in metal casks and discussed measures to solve the problems. The result of the discussion will be explained in detail below.
The spent fuel assembly which are stored for a short period in a water-cooled pool or high-burnup spent fuel assembly generate high calorific power due to decays of fission products and transuranic elements. We found “the temperature of the neutron shielding material in the metal cask goes up to 150° C. to 200° C. when a lot of assembly are stored in a single metal cask.”
When heated up, the neutron shielding material having a high-polymer compound as its main component becomes oxidized and deteriorated by heat and oxygen or gradually decomposed by radioactive rays such as gamma rays and neutrons and loses hydrogen atoms. As the result, the neutron shielding material gradually loses its neutron shielding performance. The rate of losing hydrogen atoms goes greater as the temperature becomes higher. For long-term storage of spent fuel assembly which are stored for a short period in a water-cooled pool and high-burnup spent fuel assembly (which are also called high exothermic spent fuel assembly) at a high temperature densely in a metal cask, a neutron shielding material must be developed which loses hydrogen atoms so slowly and does not lose the neutron shielding performance for a preset time period at high temperature. It is possible to suppress the radiation dose low on the surface of the metal cask if the hydrogen atom losing rate of the neutron shielding material is below the decaying rate of the neutron-emitting nuclides in the spent fuel assembly. Judging from these, one of measures in densely storing high exothermic spent fuel assembly is to produce neutron shielding material by using a high-polymer compound which has a high hydrogen number density and is reluctant to lose hydrogen atoms under high-temperature conditions.
We inventors have made various discussions, studies and researches to realize neutron shielding materials which are slow to lose hydrogen atoms at 150° C. to 200° C. and developed by mainly using epoxy resins because the epoxy resins have good heat resistance, neutron-shielding performance, and dimensional stability of molded products. The term “epoxy resin” here mainly means so-called 2-component hardening type epoxy resin. The 2-component hardening type epoxy resin comprises a base epoxy resin having two or more epoxy groups in the molecule and a hardening agent which is used together to harden. The epoxy resins are class
Iga Kiminori
Kamoshida Mamoru
Nishi Takashi
Oda Masashi
Shimizu Masashi
Dickstein , Shapiro, Morin & Oshinsky, LLP
Hashmi Zia R.
Hitachi , Ltd.
Lee John R.
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