CA1104336A - Nuclear fuel assembly - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Rupture of nuclear fuel cladding resulting from embrittlement by fission product cadmium is prevented by adding the stoichiometrically equivalent amount of material selected from the group consisting of gold, silver, palladium and mixtures thereof; CuFe2O4 and CuTiO3 and mixtures thereof and V2O4 and V2O5 and mixtures thereof, sufficient to inert the cadmium generated during the life of the fuel.

Description

The present invention relates generally to the art of corrosion prevention in nuclear reactors and is more particularly concerned with novel nuclear fuel compositions and with a new fuel system incorporating material selected from the group consisting of V2O4 or V2O5 or mixtures thereof; gold, silver, palladium and mixtures thereof, and CuFe2O4 and CuTio3 and mixtures thereof, to prevent embrittlement of nuclear fuel cladding by cadmium.
Nuclear fuel in compacted form suitable for power reactors is usually enclosed in corrosion-resistant, non-reactive, heat-conductive containers or cladding which in assembly may take the form of rods or tubes, or in the case of Boiling Water reactors even 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 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 ~LD 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 1~J4336 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 i~ U. S. Patent 3,826,754 - dated July 30, 1974 - L. N. Grossman et al, assigned to the assignee hereof, certain additives can be incorporated in nuclear fuels to prevent corxosive attack on cladding by fission produets. 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.
This invention is based upon my diseovery that cadmium, which is produced in only relatively small amounts in the fission of an atom of U-233, 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 300-340C. Thus, sueh destructive attac~ occurs in the presence of solid eadmium at 300C, liquid cadmium at 340C 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 diseovery that CuFe2O4 and CuTio3 individually and in ~ 336 RD 8041 V2O4 and V2O5 individually and in combination and gold, silver and palladium, individually and in admixture, all have the capability 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 they 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 so proportioned to insure that there will not be a substantial amount of cadmium free to contact and embrittle the fuel cladding. Thus, in the case of CuTiO3, and V2O5 and their admixtures, these materials should be used in amount between 0.0025 and 0.025 weight per cent on the basis of the nuclear fuel material, and preferably in amount about 0.0075 weight per cent on that same basis.
In the case of CuFe2O4 an amount in the range of 0.0033 to 0.033 weight per cent (preferably about 0.01 weight per cent) should be used.
In the case of gold, silver and palladium, on the basis of 4000 grams of uranium, contained as a fcr instance, in a typical Boiling Water reactor fuel rod charge of uranium oxide fuel, which in 20,000 megawatt days will generate about 0.1 gram of cadmium, there should be used between 0.3 and 2.0 grams of yold, or between about 0.1 and 0.6 gram of silver, or between 0.1 and 0.8 gram of palladium, in accordance with this invention, to nullify or make inert the cadmium.
This invention comprises recognition of the role played by cadmium in embrittlement, and the step of providing in contact with nuclear fuel material an amount of cadmium-immobilizing additive effective to prevent such cadmium embrittlement of nuclear reactor structural components such as fuel cladding at reactor operating temperatures.
Thus, the invention comprises an oxide composi-tion nuclear fuel material in compacted pellet form containing an amount of additive material selected from the group of additives, effective to immobilize cadmium resulting from the nuclear fission chain reactions of the fuel material, by reacting with the cadmium and thereby preven reaction of the cadmium with the metal of nuclear fuel cladding under reactor operating conditions.
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 powder 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 selected additive material 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 the use of relatively small amounts of the selected additive material. Thus, in order to immobilize the cadmium generated on a basis of uranium oxide fuel in a reactor operation at 20/000 megawatt days per metric ton of uranium, for a reactor fuel rod in a boiling water reactor, in which Q.ll gram of cadmium would be generated, the respective quantities of additive would be:
0.16 gram CuTiO3;
or 0.21 gram CuFe2O4;
or 0.16 gram V2O5.
In the best practice presently contemplated, the additive will be present in association with the fuel in one or the other of the several alternative ways described above in such stoichiometric a~ount. 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 ~1~433~;;
axial openings or the like to accommodate such swelling.
In the case of the metallic additives those skilled in the art will understand that when part or all of the metal additive is to be provided in the form of a coating on nuclear fuel powder particles, or on fuel pellets, or on the inside surface of the fue~ cladding, one has a choice of methods of providing that coating.
Thus, the vacuum deposition technique widely employed in semiconductor production may be used, the gold, silver or palladium being vaporized in a vacuum of 10 3 to 10 5 torr to condense on the surface of the powder, pellet or cladding in the vacuum chamber. Alternatively, to avoid the necessity for a vacuum operation, the desired coating can be applied by the firing-on technique involving the application by brushing or spraying of a metal-organic solution of the selected metal to the material or surface to be coated, followed by heating in air to decompose the compound and volatilize the organic consti-tuents leaving the metal film in place. Whatever method is employed for this coating purpose, it will further be understood that it is not necessary to consistently obtain the new results and advantages of this invention that the films produced be continuous or nonporous. Also, as indicated above, it is likewise not necessary that the proportion or the absolute amount of metal additive incorporated in the nuclear fuel system be closely controlled, the principal consideration being that substantially all cadmium generated as a fission product in the operation of the reactor comes into contact with and is reacted with the metallic additive during reactor operation so as to obtain the new results and advantages of this invention described above.

11(~4~36 Generally, the new and highly useful cadmium immobilization result of this invention can be achieved in accordance with relatively small amounts of gold, silver or palladium. Thus, as indicated above, 0.3 gram of gold, 0.25 gram of silver and 0.1 gram of palladium 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 generates 0.11 gram of cadmium. In the best practice presently contemplated, the metallic additive will be used indi-vidually rather than in admixture with another metallic additive and will be present in association with the fuel in one or the other of the several alternative ways described above in amounts between 0.6 and 1.0 gram of gold, between 0.5 and 0.7 gram of silver and between 0.1 and 0.6 gram of palladium.
From the foregoing description, it will be understood that this invention achieves the chemical inerting of reactive fission product cadmium through the use of selected 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 B that cadmium embrittles zirconium alloy cladding material r~
under elevated temperature conditions, a Zircaloy-2 tensile test speciman was broken in argon at 300C after under-going a 75 per cent reduction in cross-sectional area and with a plastic strain of about 15 per cent 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 oeeurred as a transgranular cleavage fracture with zero reduction in area and zero plastic strain at maximum stress of 40,000 psi before reaehing the yield point of the speeimen. Many incipient eraeks were observed in the speeimen on eonelusion 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 250C and 350C involving the use of solid eadmium (below 321~C), liquid cadmium (above 321C) and cadmium dissolved in liquid eesium at temperatures both above and below 321C.
In testing the basie new eadmium inerting coneept of this invention, oxides of variable-valence metals were equilibrated with cadmium at 350C in evaeuated quartz eapsules in a thermal gradient furnaee. Either a reaetion oeeurred or it did not; and where the test result was positive in this sense, the eompounas formed were stable up to the approximately 1000C temperature limit of the furnaee. As indieated above, all of the seleeted group of materials did so reaet under these conditions in this test with the apparent formation formation in the case of the oxide additives of cadmium oxide and a non-active substanee, such as elemented eopper or a lower oxide of vanadium respectively. No such reaction was observed in tests of this kind involving the use o~ either TiO2 or Nb2O5, CeO2 or depleted UO2.
In the ca~e of the metal additives there was apparently formed a stable cadmium intermetallic compound.
In testing the basic new cadmium inerting concept of this invention, copper, gold, silver, palladium, silicon, chromium, iron, nickel, aluminu, yttrium and niobium were each equilibrated with cadmium at 350C in evacuated quartz capsules in a thermal gradient furnace in a series of tests. Either a reaction occurred or it did not; and where the test result was positive in this sense, the compounds formed were stable to relative high temperature. As indicated above, only gold, silver and palladium did so react under these conditions in this test with the apparent formation of cadmium intermetallic compounds which were stable up to 550C in the case of palladium, 650aC in the case of silver and 1000C (the temperature limit of the furnace) in the case of gold.
No such reaction was observed in these tests of the other metals listed above.
In out-of-pile experiments performed with gold, it was found that 0.1 gram of cadmium was immobilized or gettered by 2.0 grams of gold at temperatures between 300C and 950ac (upper limit of the test). Actually, on visual examination, it was observed that only about one-tenth of the total volume of gold powder was affected, indicating that a stoichiometric reaction of one-to~one stoichiometry had taken place.
In using gold, silver or palladium 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 metallic material in powder form may be R~ 8041 11~4~36 packed lightly in place. With the volume of that gap typically being about 14.5 cc, a gap-filling load would be about 35.0 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 coating of gold, for example, may be applied to one of each pair of opposed pellet end surfaces. Thus, in a typical fuel rod assembly of 100 fuel pellets, each of 0.87 square centimeters end surface area, a total of about 1.2 grams of gold 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, but will not constitute a significant displacement of fissile or fertile material.
In out-of-pile experiments performed with V2O5, - it was found that 0.1 gram of cadmium was immobilized or gettered by 1.6 grams of V2O5 at temperatures between 300 and 950C (the maximum test temperature). Actually, on visual examination it was observed that only about one-tenth of the total volume of V2O5 was darkened (i.e.
changed in color from yellow to black), indicating that a reaction of one-to-one stoichiometry had taken place.
To test the copper oxide additives a variety of compounds were equilibriated with cadmium at 350C
in evacuated quartz capsules in a thermal gradient furnace. These compounds included TiO2, 13 per cent il~433t; RD 8041 A12O3 in sio2, 25 per cent A12O3 in SiO2, copper chromate, copper tungstate, copper molybdate, nickel molybdate and nickel titanate. 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 1000C temperature limit of the furnace.
As indicated above, CuFe2O4 and CuTiO3 did so react under these conditions in this test with the apparent release of copper in metallic or elemental form through displace-ment by cadmium in the ferrite and titanate compounds. No such reaction was observed in tests of this kind involving the use of any of the other compounds listed above.
In out-of-pile experiments performed with CuYe2O4, it was found that 0.1 gram of cadmium was immobilized or gettered by 2.9 grams of CuFe2O4 at temperatures between 300 and 950C. Actually, on visual examination it was observed that copper was formed. Thus, only about one-tenth of the total volume of CuFe2O4 powder would have reacted by the stoichiometry o~ the cadmium-copper displacement reaction.
In using CuTio3 or V2O4 or V2O5 or mixtures of them in accordance with this invention to fill the gap between the nuclear fuel and the cladding of a uel rod, the vanadium oxide material in powder form may be packed lightly in place. ~Jith the volume of that gap typically being about 14.5 cc, a gap-filling load would be about 11 grams, which would insure inerting of the cadmium xeleased at all locations in the fule rod during reactor operation~ The load of CuFe2O4 would be about 32 grams.

When it is desired to provide the embrittlement protection of this invention in locations between fuel ~1~4~3~ RD 8041 pellets, a 5-mil layer of CuFe2O4 or V2O5, for example, may be disposed between each pair of pellets. Thus, in a typical fuel rod assembly of 100 fuel pel.lets, each of 0.87 square centimeters end surface area, a total of about 2.5 grams of CuFe2O4 or 0.9 gram of V2O5 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 cperation.

Claims (16)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. For use in a nuclear reactor, nuclear fuel having an oxide composition nuclear material in compacted form containing at least one fissionable isotope and an amount of material selected from the group consisting of gold, silver, palladium and mixtures thereof, CuFe2O4 and CuTiO3 and mixtures thereof, and V2O4 and V2O5 and mixtures thereof, said selected material being effective to immobilize cadmium resulting from nuclear fission chain reactions of the nuclear material through a reaction between said cadmium and said selected material and thereby prevent cadmium embrittlement of nuclear fuel cladding at reactor operating temperatures.

2. The fuel of claim 1, in which the nuclear material comprises compounds selected from the group consisting of uranium oxide compounds, plutonium oxide compounds, thorium oxide compounds and mixtures thereof.

3. The fuel of claim 1, in which the nuclear material comprises uranium oxide compounds.

4. The fuel of claim 1, in which said selected material is V2O5.

5. The fuel of claim 1, in which said selected material is V2O4.

6. The fuel of claim 4 or 5, in which said selected material is present in the nuclear fuel in an amount between about 0.0025 and 0.025 weight per cent on the basis of the nuclear material.

7. The fuel of claim 4 or 5, in which said selected material is present in the nuclear fuel in an amount of about 0.0075 weight per cent on the basis of the nuclear material.

8. The fuel of claim 1, in which said selected material is gold.

9. The fuel of claim 1, in which said selected material is silver.

10. The fuel of claim 1, in which said selected material is gold in an amount between about 0.3 and 2.0 grams per 4000 grams of nuclear material.

11. The fuel of claim 1, in which said selected material is silver in an amount between about 0.1 and 0.6 gram per 4000 grams of nuclear material.

12. The fuel of claim 1, in which said selected material is palladium in an amount between about 0.1 and 0.8 gram per 4000 grams of nuclear material.

13. The fuel of claim 1, in which said selected material is CuFe2O4.

14. The fuel of claim 1, in which said selected material is CuTiO3.

15. The fuel of claim 1, in which said selected material is CuTiO3 in an amount between about 0.0025 and 0.025 weight per cent on the basis of the nuclear material.

16. The fuel of claim 1, in which said selected material is CuFe2O4 in an amount between about 0.0033 and 0.033 weight per cent on the basis of the nuclear material.