Patent Number: 043127070
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT When a nuclear fuel rod embodying this invention is constructed, an adsorbent carrying a tag gas is sealed in the cladding tube of said nuclear fuel rod. Where a nuclear fuel rod constructed as described above is set in a nuclear reactor, a tag gas is released from the adsorbent when an operating nuclear reactor produces a progressively increasing output and the fuel rod temperature rises accordingly. The released tag gas is diffused through the inner space of the cladding tube, thus providing a nuclear fuel rod containing a freed tag gas. The adsorbent used in this invention may be of the type which is applied in the customary adsorption process. Namely, the adsorbent includes, for example, active carbon, Molecular Sieves (trademark), silica gel, and alumina. Particularly preferred is active carbon. The adsorbent is separated from the fuel pellet pile by being held in a perforated container or a container open at the top which is made of stainless steel, copper or alumina. The tag gas is formed of a mixture of isotopes of noble gases such as neon, xenon and krypton blended in various ratios. With a fuel rod used with, for example, a liquid metal-cooled fast breeder, it is generally necessary to provide 10 ml (N.T.P.) of a tag gas for detection of a fuel rod failure. Where a tag gas is formed of xenon alone, a 1 g active carbon adsorbs about 500 ml (N.T.P.)/g of xenon at a temperature of 50.degree. C. (FIG. 2). Accordingly, 20 mg of active carbon is required to adsorb 10 ml (N.T.P.) of xenon. As seen from FIG. 2, an active carbon heated to a higher temperature than 300.degree. C. releases more than 99% of the xenon which was initially adsorbed at a temperature of 50.degree. C. Part of the released xenon gas is sometimes again adsorbed to the adsorbent when a nuclear reactor is put out of operation. When, however, the nuclear reactor is restarted, the xenon gas is released once more. The aforesaid temperature of 300.degree. C. of the active carbon is reached when a nuclear reactor generates 10% output. A tag gas can be adsorbed to an adsorbent in a required amount at normal temperature, for example, 0.degree. to 50.degree. C. However, a tag gas may be adsorbed at a very low temperature as -180.degree. C. An adsorbent can adsorb a sufficient amount of a tag gas when placed for 10 minutes in a tag gas flowing at a flow rate of 10 cc/min. FIG. 3 is a sectional view of a nuclear fuel rod embodying this invention. The parts of FIG. 3 the same as those of FIG. 1 are denoted by the same numerals. With the fuel rod of FIG. 3, a disc 7 drilled at the center is set above the plenum section 5 to support a perforated adsorbent container 8. This adsorbent container 8 is filled with an adsorbent such as active carbon adsorbing a tag gas, and is placed in a chamber defined between the support disc 7 and plug 3. A plenum spring 6 is provided between the support disc 7 and the top of the nuclear fuel pellet pile 2. With the nuclear fuel rod of FIG. 3, the adsorbent container 8 was disposed between the plenum spring 6 and plug 3. However, this invention is not limited to this arrangement. But it is possible to set the adsorbent container 8 between the plenum spring 6 and the top of the nuclear fuel pellet pile 2. In this case, the adsorbent container 8 lying below the plenum section 5 is advised to have an opening at the top. Where active carbon is used as a tag gas adsorbent, it is preferred to form the adsorbent container 8 of copper in order to prevent the carborization of the cladding tube 1. With the nuclear fuel rod of this invention, a tag gas is sealed therein simply by placing a container filled with a tag gas-carrying adsorbent in the inner space of the nuclear fuel rod. Therefore, the tag gas-sealing process of this invention has its cost reduced to about one thousandth part of that which has hitherto been required to mechanically break a tag gas-sealed capsule, and also to about one five hundredth part of that which has formerly been required to melt away a tag gas-sealed capsule. Considering that one nuclear reactor contains scores of thousands of nuclear fuel rods, this invention offers prominent advantage is reducing the manufacturing cost of a nuclear reactor. In other words, the invention provides an very advantageous nuclear fuel rod, wherein a tag gas can be sealed in the cladding tube and released therefrom at a far lower cost than has been possible in the past.