Patent Number: 050158635
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT In this invention, composite particles A are used as the material for a shield that is required to provide the heat release function; they are obtained by coating minute core particles with an excellent radiation-shielding property of organic or inorganic materials, various kind of metals, and so on. It is about 20 to 100 .mu.m, for example, in diameter and a thickness of the coating metal with high thermal conductivity is between 0.5 and 10 .mu.m for example, as shown in FIG. 1. Methods of applying the composite particles A to a radiation shield include (a) a method that involves filling a shield container of prescribed shape with composite particles A, (b) a method that involves fabricating a shield by closely packing the space in a container containing radioactive wastes, and (c) a method that involves forming composite particles A into a prescribed shape by hot-press forming (press forming at elevated temperature) or other forming processes. Using these methods makes it possible to provide an excellent radiation shield with excellent heat transferring property for a container containing exothermic radioactive wastes. The two examples in which these methods are applied to a cask for transporting and storing spent nuclear fuels are described in the following with reference to FIGS. 2 and 3. FIG. 2 is a sectional view of the cask in which the cylindrical cask body 2 contains the spent nuclear fuel assemblies 1. The container body 2 is covered with a neutron shield 9 made of composite particles A according to this invention and this neutron shield is surrounded by neutron shield core 4. In the example shown in FIG. 3, a neutron and gamma (.gamma.) ray shield 10 composed of composite particles A is formed on the basis of this invention between an internal cylinder 6 and an external cylinder 8 of the cask body. In these shields, coated core particles a have the function of shielding radiations, such as neutron and gamma (.gamma.) rays, and the coating metal b has the function of heat transfer and heat release; thus composite particles A serve as a shielding material with the function of heat transfer and heat release. Concerning combinations of a core particle a and a coating metal b that compose a composite particle A, materials as shown below are selected depending on the service conditions. Materials for the core particle a include: polyethylene, polystyrene, polypropylene, bakelite, graphite, beryllium, oxides of beryllium, boron, compounds of boron, aluminum, oxides of aluminum, iron, ferroalloys, lead, lead alloys, gadolinium, oxides of gadolinium, cadmium, cadmium alloys, indium, indium alloys, hafnium, hanium alloys, depleted uranium, and so on. Materials for the coating metal b include: aluminum, aluminum alloys, beryllium, beryllium alloys, copper, copper alloys, iron, ferroalloys, silver, silver alloys, magnesium, magnesium alloys, molybdenum, molybdenum alloys, zinc, zinc alloys, tin, tin alloys, tungsten, tungsten alloys, iridium, irridium alloys, gold, and so on. Examples of typical combination of these materials for composite particles A and particle sizes are shown in the following. Incidentally, particles are coated according to the electroplating process, spattering process, and so on (1) In the cafe of neutron shielding materials: Polyethylene (including super-high-molecular polyethylene) or boron carbide (B.sub.4 C) is used for core particles a, and copper or aluminum is used for the coating metal b. (2) In the case of gamma-ray-shielding materials: Lead or depleted uranium is used for core particles a, and copper or depleted uranium is used for the coating metal b. (3) In terms of the balance between the shielding performance and the heat release function, preferable diameters of core particle a are 20 to 100 .mu.m and preferable thicknesses of coating metal b are about 0.5 to 10 .mu.m. The composite particles in accordance with this invention car also be applied to the neutron-shielding and blanket material of nuclear fusion reactors, neutron absorber for nuclear criticality safety control or neutron reflector of reactors in addition to the above application. To sum up this invention, composite particles obtained by coating particles of a substance having an excellent radiation-shielding property with a metal of high thermal conductivity are used as a radiation-shielding material with an excellent heat-transferring property. As a result, it has become possible to obtain a high-performance shielding material that combines the radiation-shielding performance and an excellent heat-transferring property. As will be apparent from the above, it has become possible to save the time and labor hitherto required for installing radiating fins in a shield and to obtain an excellent radioactive-substance-shielding material of good decontamination property without the problem of neutron streaming from the fins. In addition, it has become possible to eliminate the difficulty which has so far been encountered in uniformly mixing metal powder of high thermal conductivity into a shield and to achieve the high thermal conductivity which has not so far been obtained.