Nuclear fuel element

#This invention relates to a hydrogen getter capable of preventing a cladding tube from becoming brittle and being destroyed by hydrogen gas and also to a nuclear fuel element provided with said hydrogen getter.

#A nuclear fuel element used with various types of nuclear reactor generally consists of fuel material sealed in a container or cladding tube made of corrosion-resistant, nonreactive and heat-conducting material. The cladding tube is customarily formed of stainless steel, aluminium or alloys thereof, or zirconium or alloys thereof. However, a cladding tube constructed of, for example, zirconium or alloys thereof sometimes tends to get locally hydrogenated and brittle by hydrogen gas evolving from various sources in the cladding tube during the run of a nuclear reactor and be destroyed by the resultant increase in a pressure difference originally occuring inside and outside of the cladding tube.

#A nuclear fuel element used with, for example, a light-water type power reactor is constructed by packing a zircalloy cladding tube with pellets of uranium dioxide as fuel material and sealing helium gas in said cladding tube. The uranium dioxide pellets generally contain hydrogen and remnants of hydrogen atom-carrying organic compounds, for example, metal salts of stearic acid used as a binder. All these contaminants are generally liable to be brought into said pellets while they are prepared. Further, water particles tend to be adsorbed to the surface of the uranium dioxide pellets or the inner wall of the cladding tube. Consequently, hydrogen gas, water vapor and hydrocarbon gas are generated in the tightly closed nuclear fuel element while a nuclear reactor is operated. Water vapor and hydrocarbon gas often release hydrogen by reaction with the cladding tube and uranium dioxide pellets. No serious problems arise, as long as atmosphere in the nuclear fuel element containing the above-mentioned gasseous mixture is of the oxidizing type. Where, however, said atmosphere is turned into the reducing type due to increased amounts of released hydrogen, then the zircalloy cladding tube is rapidly hydrogenated and becomes brittle. FIG. 1 sets forth the measured hydrogen-absorbing property of zircalloy in an atmosphere (1 atmospheric pressure) consisting of a water vapor-hydrogen mixture with the proportions of both gases varied during experiments. In FIG. 1, a ratio of the partial pressure of hydrogen to that of water vapor is plotted on the abscissa, and an amount of hydrogen absorbed in 6 hours after the start of the experiment is plotted on the ordinate, both plottings being given in terms of log graduations. As apparent from FIG. 1, the more prominent the reducing tendency of the atomsphere, and the higher the atmosphere temperature, the larger the amount of hydrogen absorbed by zircalloy, resulting in the more accelerated brittleness thereof. It will therefore be easily understood that a nuclear fuel element is quickly hydrogenated in a high temperature reducing atmosphere easily produced in the cladding tube during the run of a nuclear reactor.

#FIG. 1 shows the hydrogen-absorbing property of zircalloy in a water vapor-hydrogen atmosphere maintained at 1 atompsheric pressure;

#FIG. 2 is a fractional longitudinal sectional view of a concrete nuclear fuel element according to this invention;

#FIG. 3 is a longitudinal sectional view of a concrete hydrogen getter according to the invention;

#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.