Patent Number: 042971691
Section: summary

The present invention relates generally to the art of corrosion prevention in nuclear reactors and is more particularly concerned with novel boiling water reactor nuclear fuel compositions and with a new method involving the use of CuFe.sub.2 O.sub.4 or CuTiO.sub.3 or mixtures thereof to prevent embrittlement of nuclear fuel cladding by cadmium. CROSS REFERENCES This invention is related to those of my following two patent applications assigned to the assignee hereof and filed of even date herewith: Patent Application Ser. No. 700,736, filed June 29, 1976, entitled "Nuclear Fuel Assembly and Process" which discloses and claims the concept of preventing embrittlement of fuel cladding by chemically inerting fission product cadmium through the use of relatively small but effective amounts of gold, silver or palladium or mixtures thereof. Patent Application Ser. No. 700,735, filed June 29, 1976, entitled "Nuclear Fuel Assembly and Process" which discloses and claims the concept of preventing embrittlement of fuel cladding by chemically inerting fission product cadmium through the use of relatively small but effective amounts of V.sub.2 O.sub.4 or V.sub.2 O.sub.5 or mixtures thereof. BACKGROUND OF THE INVENTION Boiling water reactor nuclear fuel in suitable compacted form is usually enclosed in corrosion-resistant, non-reactive, heat-conductive containers or cladding which in assembly may take the form of rods, tubes or plates. A plurality of fuel elements of this kind are assembled in a fixed spaced relation in a coolant flow channel, and a number of these assemblies are combined to form a reactor core capable of a self-sustained fission reaction. The core is contained in a reactor vessel through which water as a coolant is run continuously. A prime necessity in the operation of a nuclear reactor is the containment of radioactive fission products. The cladding serves this purpose, preventing release of those products into the coolant and, in addition, preventing contact and chemical reaction between the nuclear fuel and the coolant. Common cladding materials include zirconium and its alloys, particularly Zircaloy-2 and Zircaloy-4. During operation of a nuclear powered reactor, a fissionable atom of U-233, U-235, Pu-239 or Pu-241 undergoes a nuclear disintegration producing an average of two fission products of lower atomic weight and great kinetic energy. Some of such fission products, including iodine and bromine, have been found or considered to have corrosive effects on the cladding. Thus, cladding failure resulting from such corrosion has been observed during operation of nuclear reactors over long periods of time. As disclosed and claimed in U.S. Pat. No. 3,826,754, assigned to the assignee hereof, certain additives can be incorporated in nuclear fuels to prevent corrosive attack on cladding by fission products. This result is achieved without offsetting disadvantage by chemical combination or association of the additives with deleterious fission products whereby those fission products are prevented from migrating in the nuclear fuel to reach the cladding. SUMMARY OF THE INVENTION This invention is based upon my discovery that cadmium, which is produced in only relatively small amounts in the fission of an atom of U-232, U-235, Pu-239, Pu-241 or the like has a markedly deleterious effect upon common nuclear fuel cladding materials. In particular, I have found that embrittlement of zirconium alloy cladding is caused by cadmium in the temperature range of 300.degree.-340.degree. C. Thus, such destructive attack occurs in the presence of solid cadmium at 300.degree. C., liquid cadmium at 340.degree. C. and cadmium dissolved in liquid cesium at any temperature in that range. Still further, the presence in nuclear fuel of the immobilizing additives of the prior art does not prevent or limit this embrittling effect of cadmium. This invention is additionally based upon my discovery that the chemical displacement compounds, copper ferrite and copper titanate, have the capability individually and in combination of reacting with cadmium under normal boiling water reactor operating conditions and thereby preventing embrittlement of nuclear fuel cladding by cadmium in liquid or solid form or in solution in liquid cesium. Further, I have found that these compounds may be admixed with a nuclear fuel as a simple additive or used as a component of a multifunctional fuel additive, or they may be applied as a coating on fuel pellets or on the cladding inside surface, or distributed as a layer between fuel pellets. However, in whatever form and manner the additive is used for this cadmium-inerting purpose, it should be proportioned to insure that there will not be a substantial amount of cadmium free to contact and embrittle the fuel cladding. Thus, 0.0025 to 0.025 weight percent CuTiO.sub.3 on the basis of the nuclear fuel material (preferably 0.0075 weight percent) and on the same basis 0.0033 to 0.033 weight percent CuFe.sub.2 O.sub.4 (preferably about 0.01 weight percent) should be used in accordance with this invention. It will be understood by those skilled in the art that this invention has both process and composition aspects, the new process comprising the step of providing in contact with nuclear fuel material an amount of CuFe.sub.2 O.sub.4 or CuTiO.sub.3 effective to prevent cadmium embrittlement of nuclear reactor structural components such as fuel cladding at reactor operating temperatures. In its composition-of-matter aspect, in general, this invention comprises an oxide composition nuclear fuel material in compacted pellet form containing an amount of CuFe.sub.2 O.sub.4, CuTiO.sub.3 or admixture thereof effective to immobilize cadmium resulting from the nuclear fission chain reactions of the fuel material by reacting with the cadmium and thereby prevent reaction of the cadmium with the metal of reactor nuclear fuel cladding under reactor operating conditions.