Patent Number: 040653528
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hydrogen-absorbing metal material used in a state sealed in a nuclear fuel element according to this invention includes zirconium and alloys thereof such as zircalloy, and titanium and alloys thereof, or preferably zirconium, zircalloy and the aforesaid Ni-Ti-Zr getter alloy. As used herein, "absorption of hydrogen" is defined broadly to mean reaction with hydrogen to produce hydrogenated material. A hydrogen-permeable metal member used to enclose the above-mentioned hydrogen-absorbing metal is formed of palladium, palladium alloys such as silver-palladium alloy, rhenium or preferably palladium or alloys thereof. Said hydrogen-permeable metal member allows the passage of hydrogen alone but not water vapor, oxygen and other gases. FIG. 2 presents a preferred concrete embodiment of a nuclear fuel element according to this invention. A nuclear fuel element 1 is constructed by packing a zircalloy cladding tube 2 with a large number of pellets 3 of uranium dioxide as fuel material and further loading said cladding tube 2 with a hydrogen getter 4 consisting of zirconium particles 5 enclosed in an evacuated palladium tube 6. FIG. 2 only shown the upper portion of the nuclear fuel element 1, wherein the cladding tube 2 is tightly sealed with an end plug 7, and a spring 9 is inserted into a plenum 8 securely to set the uranium dioxide pellets 3 in place. FIG. 3 is a hydrogen getter 10 according to another embodiment of this invention, wherein zircalloy particles 12 are received in a blind metal tube 11 and the opening 13 of said tube is sealed in vacuum with a palladium plug 14. The process of enclosing hydrogen-absorbing metal material in a hydrogen-permeable metal member may be carried out either by sealing a certain amount of the hydrogen-absorbing metal material in the hydrogen-permeable metal member in vacuum by means of, for example, electron beams or by vacuum evaporating or electroplating a hydrogen-permeable metal film on individual hydrogen-absorbing metal particles. The hydrogene absorbing metal material is preferred to be formed of particles about 1 to 2 mm in diameter for quick absorption of hydrogen, that is, to provide a large contact area between the metal mateial and hydrogen. Hydrogen-absorbing metal material such as zirconium, titanium or alloys thereof has the surface coated with a protective film when exposed to air or water vapor and ceases to absorb hydrogen at lower temperature than 400.degree. C. According to this invention, therefore, particles of hydrogen-absorbing metal material such as zirconium are enclosed in a hydrogen-permeable metal member of, for example, palladium. Thereafter the metal assembly is baked at high temperature before fitted into a nuclear fuel element. It is preferred that said baking be carried out in vacuum at higher temperature than 600.degree. C for about 1 to 3 hours or more preferably at about 700.degree. C for about 1 hour. This baking causes a protective film possibly formed up to this point on the hydrogen-absorbing metal particles due to reaction with water vapor to disappear quickly. Since hydrogen-absorbing metal particles are enclosed in a hydrogen-permeable metal member, any other gas than hydrogen does not enter said hydrogen-permeable metal member, eliminating the possibility of the above-mentioned protective film being again formed on the hydrogen-absorbing metal material. Thus the hydrogen getter of this invention displays a more prominent capacity of absorbing hydrogen than the prior art hydrogen getter formed of a single element. The following experiments were carried out to compare the effect of the hydrogen getter of this invention and that of the conventional hydrogen getter. Measurement was made of an amount (mg) of hydrogen absorbed per gram of zircalloy perticles enclosed in a hydrogen-permeable palladium member and these not enclosed therein an atmosphere consisting of hydrogen having a partial pressure of 760 mm Hg and water vapor having a partial pressure of 14 mmHg at 300.degree. C. Throughout the experiments, the zircalloy had a composition of 98.357% of zirconium (Zr), 1.500% of tin (Sn), 0.112% of iron (Fe), 0.001% of nickel (Ni) and 0.030% of chronium (Cr). As apparent from FIG. 4 showing the results of experiments, zircalloy particles not enclosed in a hydrogen-permeable metal member substantially failed to absorb hydrogen due to a protective film being formed on said particles. In contrast, zircalloy particles of this invention which are covered with said hydrogen-permeable metal element efficiently absorbed hydrogen until the zirconium is converted into zirconium dihydride. Further, measurement was made of an amount (mg) of hydrogen absorbed per gram of a zircalloy hydrogen getter of this invention enclosed in a hydrogen-permeable palladium member and the Ni-Ti-Zr getter alloy not enclosed in any hydrogen-permeable metal member in an atmosphere at 300.degree. C consisting of hydrogen having a partial pressure of 15 mmHg and water vapor having a partial pressure of 15 mmHg. The Ni-Ti-Zr getter alloy used had a composition of 84.42% of zirconium (Zr), 8.70% of Titanium (Ti) and 6.88% of nickel (Ni). FIG. 5 giving the results of experiments proves that the hydrogen getter of this invention exclusively absorbed hydrogen even in the above-mentioned oxidizing atmosphere, whereas the Ni-Ti-Zr getter alloy not enclosed in any hydrogen-permeable metal member absorbed only an extremely small amount of hydrogen. This undesirable event is supposed to result from a protective film deposited on the surface of the Ni-Ti-Zr getter alloy not enclosed in a hydrogen-permeable metal member. A nuclear fuel element according to this invention provided with the above-mentioned hydrogen getter enclosed in a hydrogen-permeable metal member has been found little liable to be hydrogenated and become brittle with the possibility of being eventually destroyed during the run of a nuclear reactor.