Patent Number: 042971683
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the preferred practice of this invention, the additive is associated with the fuel in any suitable manner as by mechanically blending the additive in power form with the nuclear fuel material in a similar finely-divided condition. It is also feasible, according to this invention, to apply the additive as a coating to part or all of the surface of a fuel pellet, or it may be applied as a coating on the inside surface of the cladding for contact with fuel pellets loaded therein. As indicated above, it is also contemplated that the additive in powder form can be disposed in the pellet assembly as it is loaded into cladding. In any event, it is desirable that the vanadium oxide additive be distributed in respect to the nuclear fuel material to insure that substantially all cadmium generated as a fission product in the operation of the reactor comes into contact with and is reacted with the additive during reactor operation so as to obtain the new results and advantages of this invention described above. Generally, the new and highly useful cadmium immobilization result of this invention can be achieved in accordance with relatively small amounts of V.sub.2 O.sub.4 or V.sub.2 O.sub.5. Thus, 0.16 gram of V.sub.2 O.sub.5 is sufficient for the present purpose in a reactor operation at 20,000 megawatt days per metric ton of uranium in a boiling water reactor fuel rod which generated 0.11 gram of cadmium. In the best practice presently contemplated, the vanadium oxide additive will be present in association with the fuel in one or the other of the several alternative ways described above in such stoichiometric amount. Appreciably less than such stoichiometric amount will leave the way open to some extent for cadmium embrittlement of cladding, while use of substantially more than the stoichiometric amount burdens the system with inert material, using space that should be occupied by fissile or fertile material. When the additive is incorporated in the fuel elements, they make take any desired geometric form or configuration, but it is preferred that the nuclear fuel material be in the form of right cylindrical pellets which are incorporated in a tubular cladding of a zirconium alloy. The swelling of the pellets in the cladding is accommodated by providing porosity in the fuel pellet or by forming it with dished ends or axial openings or the like to accommodate such swelling. From the foregoing description, it will be understood that this invention achieves the chemical inerting of reactive fission product cadmium through the use of V.sub.2 O.sub.4 or V.sub.2 O.sub.5 additives which react with cadmium under normal nuclear reactor operating conditions to form stable compounds so that fission product cadmium is not available or free to react with or attack fuel cladding or any other metal that it may come in contact with during reactor operation. In this manner, the additive which is effective for the purposes of this invention blocks potential cladding-fission product reaction and so increases the cladding reliability and useful life. In a test conducted for the purpose of confirming that cadmium embrittles zirconium alloy cladding material under elevated temperature conditions, a Zircaloy-2 tensile test specimen was broken in argon at 300.degree. C. after undergoing a 75 percent reduction in cross-sectional area and with a plastic strain of about 15 percent following a maximum stress of 60,000 psi. Fracture morphology was ductile. Then in a repetition of that test but for the presence of cadmium in contact with the test specimen, breakage occurred as a transgranular cleavage fracture with zero reduction in area and zero plastic strain at maximum stress of 40,000 psi before reaching the yield point of the specimen. Many incipient cracks were observed in the specimen on conclusion of the test. Similar results to those of the latter test were obtained in subsequent tests performed in the same manner but at temperatures between 250.degree. C. and 350.degree. C. involving the use of solid cadmium (below 321.degree. C.), liquid cadmium(above 321.degree. C.) and cadmium dissolved in liquid cesium at temperatures both above and below 321.degree. C. In testing the basic new cadmium inerting concept of this invention, oxides of variable-valence metals were equilibrated with cadmium at 350.degree. C. in evacuated quartz capsules in a thermal gradient furnace. Either a reaction occurred or it did not; and where the test result was positive in this sense, the compounds formed were stable up to the approximately 1000.degree. C. temperature limit of the furnace. As indicated above, V.sub.2 O.sub.4 and V.sub.2 O.sub.5 did so react under these conditions in this test with the apparent formation of cadmium oxide and a lower oxide of vanadium. No such reaction was observed in tests of this kind involving the use of either TiO.sub.2 or Nb.sub.2 O.sub.5, CeO.sub.2 or depleted UO.sub.2. In out-of-pile experiments performed with V.sub.2 O.sub.5, it was found that 0.1 gram of cadmium was immobilized or gettered by 1.6 grams of V.sub.2 O.sub.5 at temperatures between 300 and 950.degree. C. (the maximum test temperature). Actually, on visual examination it was observed that only about one-tenth of the total volume of V.sub.2 O.sub.5 was darkened (i.e., changed in color from yellow to black), indicating that a reaction of one-to-one stoichiometry had taken place. In using V.sub.2 O.sub.4 or V.sub.2 O.sub.5 or mixtures of them in accordance with this invention to fill the gap between the nuclear fuel and the cladding of a fuel rod, the vanadium oxide material in powder form may be packed lightly in place. With the volume of that gap typically being about 14.5 cc, a gap-filling load would be about 11.6 grams, which would insure inerting of the cadmium released at all locations in the fuel rod during reactor operation. When it is desired to provide the embrittlement protection of this invention in locations between fuel pellets, a 5-mil layer of V.sub.2 O.sub.5, for example, may be disposed between each pair of pellets. Thus, in a typical fuel rod assembly of 100 fuel pellets, each of 0.87 square centimeters end surface area, a total of about 0.9 gram of V.sub.2 O.sub.5 will be incorporated in the fuel rod. As indicated above, this amount will be in substantial excess of the stoichiometric equivalent of the cadmium produced in the normal useful life of the fuel rod in the typical boiling water nuclear reactor operation.