Patent Number: 044951418
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 illustrating one embodiment of the tagging gas releasing element of the present invention, the tagging gas releasing element 1 comprises a number of circular thin discs 2 laminated closely to one another, and each circular thin disc comprises an injection substrate 3 and a tagging gas-injected layer 4 formed on the surface of the substrate 3. For formation of the injection substrate 3, there may be employed metals such as titanium, aluminum, zirconium, nickel and stainless steel; alloys of these metals; metal films formed by depositing these metals or alloys on surfaces of same or different metals or alloys or other solid carriers by using plating, sputtering, ion-plating or the like; and amorphous metals or heat-resistant solid having a high activity of adsorbing xenon, krypton or the like, such as zeolite, activated carbon and boron nitride (BN). The tagging gas may be composed of at least two isotopes of a rare gas such as xenon, krypton, neon, helium or the like. One rare gas may be used singly, and a mixture comprising two or more rare gases at a predetermined mixing ratio may also be used. A mixture of stable isotopes of xenon and/or krypton is preferred. Injection of the tagging gas into the substrate may be accomplished by using ion injection, high pressure induced diffusion, thermal diffusion, physical adsorption or combination of two or more of these methods. When a thin metal film is used as the injection substrate, there can be attained an advantage that the film thickness can optionally be determined. When sputtering or ion-plating is adopted for formation of such thin metal film, a vacuum device or ion generating device may also be used for ion injection at the step of injecting the tagging gas into the thin metal film. It is therefore preferred to adopt the the sputtering method or ion-plating method for formation of the thin metal film. When a thin metal film is formed by using the plating method or ion-plating method, an amorphous metal film is resulted. If the operation of forming a thin metal film and the operation of injecting a tagging gas into the thin metal film are alternately repeated several times, a structure in which a plurality of gas-injected metal films are laminated as shown in FIG. 1 is effectively prepared. The ion injection method is most preferred as the method for injecting the tagging gas. According to this method, the tagging gas is ionized by, for example, low pressure gaseous discharge, the resulted ions are accelerated by an electric field, and thus the ions are held in an inorganic solid material by injection and/or adsorption. It is possible by this method to inject ions at such a degree as up to about 50% to the metal atoms, and the amount of the injected ions is much larger than that in other injection methods. In case of physical adsorption, there is a fear that the tagging gas once adsorbed is substituted by other gas during manufacture or storage of the element. Accordingly, a care must be taken so that such substitution of tagging gas is not caused. More definitely, when krypton-84 is accelerated at 50 KeV and is ion-injected into aluminum, the ions can be injected at a surface density of up to 1.times.10.sup.17 ions per cm.sup.2. Accordingly, when an aluminum foil having a diameter of 5 mm and a thickness of 6.mu. is employed as a thin metal disc, 5.4.times.10.sup.19 ions of krypton are held in 2470 layers of aluminum foil to form a tagging gas releasing element having a height of about 16 mm. That is, 2 ml of krypton at 0.degree. C. and 1 atmosphere are held. The thus held krypton is released at a temperature higher than about 340.degree. C. Therefore, as shown in FIG. 2, when this gas releasing element 1 is disposed in a gas plenum 11 of a nuclear fuel rod 10, the tagging gas is released at an operation temperature of a fast reactor and is filled in the interior of the nuclear fuel rod 10. In FIG. 2, reference numerals 12, 13, 14, 15 and 16 represent a lower end plug, a cladding tube, an upper end plug, a nuclear fuel pellet and a plenum spring, respectively. Release of the once injected or adsorbed tagging gas is caused by thermal diffusion releasing, high temperature desorption and melting of the metallic substrate under operation conditions of the unclear reactor, or by action of radiation, or by combination of these releasing mechanisms. FIG. 3 illustrates another embodiment of the present invention. In the case where the injected tagging gas is released by melting of the thin metal disc 2 of the injection substrate, a capsule is constructed by a cylinder 5 composed of a hardly fusible metal and porous walls 6 of the same hardly fusible metal formed on both ends of the cylinder 5, and a laminate of the thin discs 2 as shown in FIG. 1 is inserted into the capsule. If the compatibility between the thin metal disc 2 and the nuclear fuel pellet is poor, it is required to construct a capsule of a metal having a good compatibility, into which a laminate of the thin discs is inserted. Since the amount of the tagging gas held in the injection substrate is generally in proportion to the surface area of the injection substrate, a large amount of the tagging gas can be effectively held by adopting a multi-layer structure as described in the foregoing embodiments. However, the present invention is not limited to such multi-layer structure, and the shape and size may optionally be changed according to the function of the gas releasing element. For example, as shown in FIG. 4, there may be adopted a structure in which a metal film 17 is applied on the surface of a nuclear fuel pellet 15 and a tagging gas is injected in the outer surface of the metal film 17. Further, as shown in FIG. 5, there may be adopted another structure in which a metal foil 18, in the outer surface of which a tagging gas has been previously injected, is helically wrapped around the peripheral surface of a nuclear fuel pellet 15. Moreover, as shown in FIG. 6, it is possible to charge a porous granular material 19, in which a tagging gas has been previously adsorbed, in a capsule same as that in FIG. 3. As shown in foregoing, since the tagging gas is held by a solid material and release of the tagging gas depends mainly on the temperature determined by the holding solid material, the manufacture of the tagging gas releasing element of the present invention can be remarkably facilitated and the structure thereof can be remarkably simplified. Moreover, since the tagging gas is present in the substantially pressurized state in the solid, the size of the entire element can be remarkably diminished. Accordingly, limitations on designing of nuclear fuel rods can be moderated. For example, the dead space of the gas plenum can be remarkably reduced. Furthermore, the element is tough and strong, and has a good adaptability to operation, and release of the tagging gas can be accomplished with high reliability. Still in addition, a variety of tagging gases can optionally be combined by using unit solids for respective gases, and discrimination of tags can be performed very easily. Furthermore, by using the element of the present invention, the application range of the tagging method can be remarkably broadened. Thus, various effects and advantages can be attained according to the present invention. While the present invention is described by exemplifying preferred embodiments, various modifications can be made without departing from the spirit and scope of the present invention.