Corrosion Resistant Zirconium Alloys Containing Copper, Nickel and Iron

1. Field of the Invention

The present invention relates to zirconium based alloys suitable for use in nuclear reactor service, and in particular for use in claddings of fuel elements.

2. Description of Related Art

Nuclear fuel element cladding serves several purposes and two primary purposes are: first, to prevent contact and chemical reactions between the nuclear fuel and the coolant or the moderator if a moderator is present; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant or the moderator. The failure of the cladding, i.e., a loss of the leak-proof seal, can contaminate the coolant or moderator and the associated systems with radioactive long-lived products to a degree which interferes with plant operation.

Zirconium-based alloys have long been used in the cladding of fuel elements in nuclear reactors. A desirable combination is found in zirconium by virtue of its low thermal neutron cross-section and its generally acceptable level of resistance to corrosion in a boiling water reactor environment. Zircaloy 2, a zirconium alloy consisting of about 1.2 to 1.7 percent tin, 0.07 to 0.2 percent iron, 0.05 to 0.15 percent chromium, 0.03 to 0.08 percent nickel, up to 0.15 percent oxygen, and the balance zirconium, has enjoyed performance in reactor service, but also possesses some deficiencies that have prompted further research to find materials providing improved performance. For example, Zircaloy 2 cladding on fuel elements in nuclear reactors absorbs hydrogen while the reactor is operating. When the reactor is shut down and the cladding cools the Zircaloy 2 is embrittled by the absorbed hydrogen. Zircaloy 4 was one alloy developed as a result of further research to improve Zircaloy 2. Zircaloy 4 is similar to Zircaloy 2 but contains less nickel (0.007% max. wt. percent) and slightly more iron. Zircaloy 4 was developed as an improvement over Zircaloy 2 to reduce absorption of hydrogen in Zircaloy 2. Zircaloy 2 and Zircaloy 4 are herein referred to as the Zircaloy alloys or Zircaloy.

The Zircaloy alloys are among the best corrosion resistant materials when tested in water at reactor operating temperatures, typically about 290.degree. C., but in the absence of radiation from the nuclear fission reaction. The corrosion rate in water at 290.degree. C. is very low and the corrosion product is a uniform, tightly adherent, black ZrO.sub.2 film. In actual service, however, the Zircaloy is irradiated and is also exposed to radiolysis products present in reactor water. The corrosion resistance properties of Zircaloy deteriorate under these conditions and the corrosion rate thereof is accelerated.

Research efforts directed at improving the corrosion properties of the zirconium-based alloys have yielded some advances. Corrosion resistance has been enhanced in some instances through carefully controlled heat treatments of the alloys either prior to or subsequent to material fabrication. Added heat treatment cycles, however, generally increase the expense of making finished products, and in those instances where an installation requires welding to be performed, the area affected by the heat of the welding operation may not possess the same corrosion resistance characteristics as the remainder of the article. Variations in the alloying elements employed and the percentages of the alloying elements have also been propounded in an effort to address the deterioration in the corrosion-resistance of these alloys when they are irradiated.

The deterioration under actual reactor conditions of the corrosion resistance properties of Zircaloy is not manifested in merely an increased uniform rate of corrosion. Rather, in addition to the black ZrO.sub.2 layer formed, a localized, or nodular corrosion phenomenon has been observed in some instances on Zircaloy tubing in boiling water reactors. In addition to producing an accelerated rate of corrosion, the corrosion product of the nodular corrosion reaction is a highly undesirable white ZrO.sub.2 bloom which is less adherent and lower in density than the black ZrO.sub.2 layer.

The increased rate of corrosion caused by the nodular corrosion reaction will be likely to shorten the service life of the tube cladding, and also this nodular corrosion will have a detrimental effect on the efficient operation of the reactor. The white ZrO.sub.2, being less adherent, may be prone to spalling or flaking away from the tube into the reactor water. On the other hand, if the nodular corrosion product does not spall away, a decrease in heat transfer efficiency through the tube into the water is created when the nodular corrosion proliferates and the less dense white ZrO.sub.2 covers all or a large portion of a tube.

Actual reactor conditions cannot be readily duplicated for normal laboratory research due to the impracticality of employing a radiation source to simulate the irradiation experienced in a reactor. Additionally, gaining data from actual use in reactor service is an extremely time consuming process. For this reason, there is no conclusory evidence in the prior art which explains the exact corrosion mechanism which produces the nodular corrosion. This limits, to some degree, the capability to ascertain whether other alloys will be susceptible to nodular corrosion before actually placing samples made from these alloys into reactors.

Laboratory tests conducted under the conditions normally experienced in a reactor at approximately 300.degree. C. and 1000 psig in water, but absent radiation, will not produce a nodular corrosion product on Zircaloy alloys like that found in some instances on Zircaloy alloys which have been used in reactor service. However, if steam is used, with the temperature increased to over 500.degree. C. and the pressure raised to 1500 psig, a nodular corrosion product like that occasionally found on Zircaloy in reactor service can be produced on Zircaloy alloys in laboratory tests. Specimens of Zircaloy alloys which are annealed at 750.degree. C. for 48 hours are particularly susceptible to nodular corrosion under these test conditions. These annealed Zircaloy specimens will produce, in tests run for relatively short times, i.e. 24 hours, a degree of nodular corrosion comparable to that of Zircaloy tube cladding in actual reactor service that has been found to have nodular corrosion. At this higher temperature and pressure, a simulated nuclear reactor environment is provided which will allow researchers to determine the susceptibility of new alloys to nodular corrosion. With this test, a comparison between samples from new alloys and Zircaloy specimens tested under the same conditions can be made.

To be considered as a suitable alternate or replacement for the Zircaloy alloys, any new alloy must not only be less susceptible than the Zircaloy alloys to nodular corrosion, but must maintain acceptable uniform corrosion rates, comparable to those of the Zircaloy alloys, to ensure sufficient service life. Zircaloy alloys have been used extensively as fuel rod cladding and are known to contain many desirable properties that alternate or replacement alloys must also contain. Zircaloy alloys have the desirable properties of a low neutron absorption cross section and at temperatures below 750.degree. F. are strong, ductile, extremely stable and as mentioned previously have excellent uniform corrosion resistance in water at reactor operating temperatures.

Fuel element performance has revealed another problem with brittle splitting of nuclear fuel element cladding due to the combined interactions between the nuclear fuel, the cladding and the fission products produced during nuclear fission reactions. It has been discovered that this undesirable performance is due to localized mechanical stresses on the fuel cladding resulting from differential expansion and friction between the fuel and the cladding. Fission products are created in the nuclear fuel by the fission chain reaction during operation of a nuclear reactor, and these fission products are released from the nuclear fuel and are present at the cladding surface. These localized stresses and strains in the presence of specific fission products, such as iodine and cadmium, are capable of producing cladding failures by phenomena known as stress corrosion cracking or liquid metal embrittlement.

The present invention relates to corrosion resistant zirconium-based alloys and corrosion resistant nuclear fuel elements encased with cladding container tubing made from such corrosion resistant zirconium alloys. In one embodiment, a corrosion resistant first alloy consists essentially of by weight percent about 0.5 to 2.0 percent tin, about 0.24 to 0.40 percent of a solute composed of copper, nickel and iron wherein the copper is at least 0.05 percent, and the balance zirconium.

In another embodiment, a corrosion resistant second alloy consists essentially of in weight percent about 0.5 to 2.0 percent tin, a solute composed of copper, iron and nickel so that each solute element is present in an amount from 0.05 to 0.20 percent, and the balance zirconium.

In another embodiment, a corrosion resistant third alloy consists essentially of in weight percent about 0.5 to 2.0 percent tin, about 0.25 to 0.35 percent of a solute composed of copper and nickel wherein the copper is at least 0.05 percent, and the balance zirconium.

These alloys provide increased resistance to nodular corrosion in high pressure and temperature steam testing, and will maintain acceptable uniform corrosion rates in water and steam tests.

Corrosion-resistant nuclear fuel elements are provided by making elongated cladding containers from the first, second or third zirconium alloys described above.

Improved corrosion resistant nuclear fuel elements are also made from composite cladding container tubing having a Zircaloy alloy tube with a surface layer metallurgically bonded on the outside of the Zircaloy tube. The surface layer being about 5 to 20 percent of the thickness of the Zircaloy tube and consisting essentially of the first, second, or third zirconium alloy described above. The surface layer is a protective shield thick enough to prevent nodular corrosive attack on the Zircaloy tube.

Another nuclear fuel element is made from a composite cladding container that is resistant to nodular corrosion, stress corrosion cracking and liquid metal embrittlement. An elongated composite cladding container is made from a Zircaloy alloy tube having a corrosion resistant surface layer metallurgically bonded to the outside surface and an inner barrier layer of zirconium metallurgically bonded on the inside of the alloy tube. The inner barrier being about 1 to 30 percent of the thickness of the Zircaloy tube and comprised of moderate purity zirconium such as sponge zirconium. The outer surface layer being about 5 to 20 percent of the thickness of the Zircaloy tube, and consisting essentially of the first, second or third zirconium alloy described above.

Cladding container tubing is manufactured by heating an extrusion billet of the first, second or third zirconium alloy described above to about 590.degree. to 650.degree. C., extruding the billet into tube shell followed by standard tube reduction and subsequent heat treatments at about 570.degree. to 590.degree. C. to achieve desired tube dimensions and mechanical properties. The standard tube reduction process of zirconium alloy tubing used in nuclear fuel elements is pilger-rolling. Pilger-rolling is a tube reduction process using traveling, rotating dies on the outer tube surface to forge the tube over a stationary mandrel die inside the tube.

Composite cladding containers are manufactured by starting with a tube blank made from a Zircaloy alloy, and an outer tube placed on this tube blank. The outer tube is composed of the first, second or third zirconium alloys described above. This composite tube is then heated to a temperature in the range of 590.degree. to 650.degree. C. and is extruded. In the process, a metallurgical bond between the two zirconium alloys results. Subsequent tube reduction and heat treatments between 570.degree. to 590.degree. C. are performed to achieve the desired tube dimensions and mechanical properties. The outer tube is of at least a thickness so that after tube reduction it is about 5 to 20 percent of the thickness of the Zircaloy tube.

Another composite cladding container is manufactured by starting with a tube blank of a Zircaloy alloy and an outer tube is placed on the tube blank. The outer tube is made from the first, second or third zirconium alloys described above. A hollow collar of a metal barrier is placed inside the tube blank. The metal barrier is comprised of moderate purity zirconium such as sponge zirconium. The composite tubing is heated to 590.degree. to 650.degree. C. and extruded to form a metallurgical bond between the outside surface layer and the tube blank, and between the inner metal barrier and the tube blank. Extrusion is followed by tube reduction and subsequent heat treatments between 570.degree. C. and 590.degree. C. to develop the desired tube dimensions and mechanical properties. The outer tube and hollow collar are of at least a thickness so that after tube reduction the outer layer is about 5 to 20% of the thickness of the Zircaloy tube and the inner barrier is about 1 to 30% of the Zircaloy tube thickness.

The cladding containers and the composite cladding containers enclose a nuclear fuel material, leaving a gap between the fuel and the cladding. In the composite cladding containers having a barrier layer, the barrier layer shields the alloy tube from the nuclear fuel material held in the cladding as well as shielding the alloy tube from fission products and gases. Because of its purity the liner remains soft during irradiation and minimizes localized strain inside the nuclear fuel element, thus serving to protect the alloy tube from stress corrosion cracking or liquid metal embrittlement.

The first, second or third zirconium alloys and the barrier layer do not introduce any significant neutron capture penalites, heat transfer penalites, or material incompatibility problems for the nuclear fuel elements of this invention.

The alloys of the present invention have demonstrated adequate resistance to uniform corrosion to be considered for nuclear reactor service, and the corrosion resistance is generally comparable to the excellent uniform corrosion resistance possessed by the Zircaloy alloys. The alloys of the present invention also demonstrate improved resistance to nodular corrosion.

The addition of tin to zirconium has been practiced in the art prior to this invention, as evidenced by Zircaloy and other known zirconium-based alloys. The presence of tin, which stabilizes the alpha-form of zirconium, primarily contributes to the strength of the alloy, although there is some improvement in uniform corrosion resistance attributable to tin. It has been determined that concentration levels below about 0.5% by weight of tin will yield an alloy in which the uniform corrosion rate in water is unacceptably high. Additionally, concentration levels in excess of about 2.0% by weight of tin will yield an alloy which demonstrates an unacceptable level of accelerated corrosion in laboratory steam tests. The alloys of the present invention have a tin content within the range of from about 0.5 to about 2.0 percent by weight, preferably from 1.0 to about 1.5 percent by weight, and most preferably contain about 1.5% by weight of tin. The alloys of the present invention further contain certain additional alloying elements, termed collectively herein as a solute portion of the alloy. The solute portion of the alloys of the present invention differs from the additional alloying elements found in the Zircaloy alloys, and is primarily responsible for the comparative increase in resistance to nodular corrosion. Conventional impurities will also be present in these alloys.

It should be noted that the alloys of the present invention will also optionally contain from about 0.09 to 0.16 weight percent of oxygen. Most commercial grade sponge zirconium which would be used in making alloys such as the ones in the present invention will contain small amounts of oxygen, roughly on the order of about 800-1300 parts per million. In some instances, it will be desirable to increase the concentration of oxygen in the alloy. Adding oxygen is one way to increase room temperature yield strength. Thus, the alloys of the present invention may be produced with or without the additional oxygen, as this will have little or no effect on the corrosion resistance of the alloys.

There are several parameters which should be considered in choosing alloying elements for the candidate zirconium based alloys to be used for fuel cladding in boiling water reactor service. The thermal neutron cross-section of the element should be relatively low to permit products of the fission reaction to easily pass through the fuel cladding, thereby allowing the boiling water reactor to operate as efficiently as possible. The cost of the material should be taken into account, and must not be prohibitively high. The ease or difficulty with which an alloy containing the element or elements and zirconium can be produced must also be considered. It is further desired that the element or elements will enhance the corrosion resistance properties of the zirconium under actual or simulated boiling water reactor conditions.

The thermal neutron cross-section of an element is generally a known property of the element if it has ever come under consideration for use in a nuclear reactor. The costs of the materials can be ascertained from historic price data, with extrapolation if required. The alloying process of the alloys of the present invention is similar to conventional methods for alloying zirconium and thus ease of making alloy additions is fairly predictable. The alloying is accomplished preferably by arc melting a zirconium billet having a suitable amount of the alloying metals encased in a hollow portion of the billet. This molten metal is then cast as an alloy billet, which will then be subjected to finishing processes to produce final shapes.

Generally, of the parameters discussed above, the most difficult to predict is whether the alloying element will contribute to the enhancement of corrosion resistance.

In the present invention, zirconium.-based alloys have been discovered that perform substantially better than Zircaloy 2 in tests conducted to determine resistance to nodular corrosion. These alloys also perform well in tests for determining resistance to uniform corrosion. A first alloy consists essentially of by weight percent 0.5 to 2.0 percent tin, about 0.24 to 0.40 percent of a solute composed of copper, nickel and iron, wherein the copper is at least 0.05 percent, and the balance zirconium. A second alloy consists essentially of by weight percent about 0.5 to 2.0 percent tin, a solute composed of copper, iron and nickel so that each solute element is present in an amount from 0.05 to 0.20 percent, and the balance zirconium. A third alloy consists essentially of by weight percent about 0.5 to 2.0 percent tin, about 0.25 to 0.35 weight percent of a solute composed of copper and nickel wherein the copper is at least 0.05 percent, and the balance zirconium.

Solute elements copper, nickel and iron possess the low thermal neutron cross-section, low cost, ease of alloying, and corrosion resistance properties that are desirable in zirconium based alloys.

Tests for both uniform corrosion resistance and nodular corrosion have been conducted on alloys of the present invention. These tests have shown that a dramatic decrease in susceptibility to nodular corrosion can be attained in an alloy which is relatively insensitive to heat treatment while retaining essentially the same uniform corrosion resistance of a Zircaloy 2 alloy. Solute concentrations ranging from as low as 0.24 weight percent to as high as 0.40 weight percent have been tested and have been shown to exhibit superior resistance to nodular corrosion, compared to the performance of Zircaloy 2.

Alloys having copper and nickel as solutes had greatly improved resistance to nodular corrosion when they were given the 750.degree. C./48 hour anneal that sensitizes Zircaloy 2 to nodular corrosion. Zirconium alloy tubing is heat treated several times during tube production; therefore, the zirconium alloys containing solutes copper and nickel will provide improved nodular corrosion resistance when properly heat treated during tube production.

The corrosion-resistant fuel elements of this invention are shown by referring now more particularly to FIG. 1, where there is shown a partially cutaway sectional view of a nuclear fuel assembly 10. This fuel assembly consists of a tubular flow channel 11 of generally square cross section provided at its upper end with lifting bale 12 and at its lower end with a nose piece (not shown due to the lower portion of assembly 10 being omitted). The upper end of channel 11 is open at 13 and the lower end of the nose piece is provided with coolant flow openings. An array of fuel elements or rods 14 is enclosed in a channel 11 and supported therein by means of upper end plate 15 and a lower end plate (not shown due to the lower portion being omitted). The liquid coolant ordinarily enters through the openings in the lower end of the nose piece, passes upwardly around fuel elements 14, and discharges at upper outlet 13 in a partially vaporized condition for boiling reactors or in an unvaporized condition for pressurized reactors at an elevated temperature.

The nuclear fuel elements or rods 14 are sealed at their ends by means of end plugs 18 welded to the cladding 17, which may include studs 19 to facilitate the mounting of the fuel rod in the assembly. A void space or plenum 20 is provided at one end of the element to permit longitudinal expansion of the fuel material and accumulation of gases released from the fuel material. A nuclear fuel material retainer means 24 in the form of a helical member is positioned within space 20 to provide restraint against the axial movement of the pellet column, especially during handling and transportation of the fuel element.

The fuel element is designed to provide an excellent thermal contact between the cladding and the fuel material, a minimum of parasitic neutron absorption, and resistance to bowing and vibration which is occasionally caused by flow of the coolant at high velocity.

A nuclear fuel element or rod 14 is shown in a partial section in FIG. 1 constructed according to the teachings of this invention. The fuel element includes a core or central cylindrical portion of nuclear fuel material 16, here shown as a plurality of fuel pellets of fissionable or fertile material positioned within a structural cladding or container 17. In some cases, the fuel pellets may be of various shapes, such as cylindrical pellets or spheres, and in other cases, different fuel forms such as particulate fuel may be used. The physical form of the fuel is immaterial to this invention. Various nuclear fuel materials may be used, including uranium compounds, plutonium compounds, thorium compounds and mixtures thereof. A preferred fuel is uranium dioxide or a mixture comprising uranium dioxide and plutonium dioxide.

Referring now to FIG. 2, the nuclear fuel material 16 forming the central core of the fuel element 14 is surrounded by a cladding 17. The cladding container encloses the core so as to leave a gap between the core and the cladding container during use in a nuclear reactor. The cladding is comprised of a corrosion-resistant zirconium alloy tube 21. The alloy tube 21 is made from either the first, second or third zirconium alloys described above.

It should be noted that the first, second or third zirconium alloys described herein will also optionally contain from about 0.09 to 0.16 weight percent of oxygen. Most commercial grade sponge zirconium which would be used in making alloys such as the ones in the present invention will contain small amounts of oxygen, roughly on the order of about 800-1300 parts per million. In some instances, it will be desirable to increase the concentration of oxygen in the alloy. Adding oxygen is one way to increase room temperature yield strength. Thus, the alloys of the present invention may be produced with or without the additional oxygen, as this will have little or no effect on the corrosion resistance of the alloys.

Another embodiment of this invention is shown by referring to FIG. 3. The nuclear fuel material 16 forming the central core of the fuel element 14 is surrounded by a composite cladding 17. The composite cladding container encloses the core so as to leave a gap 23 between the core and the cladding container during use in a nuclear reactor. The composite cladding is comprised of a zirconium alloy tube 21 made from either the first, second or third zirconium alloy described above. The alloy tube has bonded on the inside surface thereof a metal barrier 22 so that the metal barrier forms a shield between the alloy tube 21 and the nuclear fuel material held in the cladding The metal barrier forms about 1 to about 30 percent of the thickness of the cladding, and is comprised of a low neutron absorption material, namely, moderate purity zirconium. One moderate purity zirconium is sponge zirconium. The metal barrier 22 protects the alloy tube portion of the cladding from contact and reaction with gasses and fission products from the nuclear fuel, and prevents the occurrence of localized stress and strain.

The content of the metal barrier of moderate purity zirconium is important and serves to impart special properties to the metal barrier Generally, there is at least about 1,000 parts per million (ppm) by weight and less than about 5,000 ppm impurities in the material of the metal barrier and preferably less than about 4,200 ppm. Of these, oxygen is kept within the range of about 200 to about 1,200 ppm. All other impurities are within the normal range for commercial, reactor-grade sponge zirconium.

In another embodiment of this invention, a corrosion-resistant nuclear fuel element is shown by referring to FIG. 4. The nuclear fuel material 16 forming the central core of fuel element 14 is surrounded by a composite cladding 17. The composite cladding container encloses the core so as to leave a gap 23 between the core and the cladding container during use in a nuclear reactor. The composite cladding is comprised of a zirconium alloy tube 30 made from a Zircaloy alloy. The alloy tube has bonded on the outside surface thereof a metal layer 32 so that the metal layer forms a corrosion protective shield over the alloy tube. The outer metal layer is about 5 to 20 percent of the thickness of the alloy tube and is comprised of either the first, second or third zirconium alloy described above. The outer metal layer protects the Zircaloy alloy tube portion of the cladding from nodular corrosion.

Another improved nuclear fuel element is shown by referring to FIG. 5. The nuclear fuel material 16 forming the central core of fuel element 14 is surrounded by a composite cladding 17. The composite cladding container encloses the core so as to leave a gap 23 between the core and the cladding container during use in a nuclear reactor. The composite cladding is comprised of a Zircaloy alloy tube 30. The alloy tube has bonded on the inside surface thereof a metal barrier 22 so that the metal barrier forms a shield between the alloy tube 30 and the nuclear fuel material held in the cladding. The metal barrier is about 1 to about 30 percent of the thickness of the alloy tube and is comprised of a low neutron absorption material, namely, moderate purity zirconium as described above. The metal barrier 22 protects the alloy tube portion of the cladding from contact and reaction with gases and fission products from the nuclear fuel, and prevents the occurrence of localized stress and strain. An outer surface layer is bonded on the outside surface of the alloy tube 30. The outer metal layer is about 5 to 20 percent of the thickness of the alloy tube and is comprised of either the first, second or third zirconium alloy described above. The outer metal layer protects the Zircaloy alloy tube portion of the cladding from nodular corrosion.

Sponge zirconium metal forming the metal barrier in the composite cladding is highly resistant to radiation hardening, and this enables the metal barrier after prolonged irradiation to maintain desirable structural properties such as yield strength and hardness at levels considerably lower than those of conventional zirconium alloys. In effect, the metal barrier does not harden as much as conventional zirconium alloys when subjected to irradiation, and this together with its initially low yield strength enables the metal barrier to deform plastically and relieve pellet-induced stresses in the fuel element during power transients. Pellet induced stresses in the fuel element can be brought about, for example, by swelling of the pellets of nuclear fuel at reactor operating temperatures (300.degree. C. to 350.degree. C.) so that the pellet comes into contact with the cladding.

It has further been discovered that a metal barrier of sponge zirconium of the order preferably about 5 to 15 percent of the thickness of the cladding and a particularly preferred thickness of 10 percent of the cladding bonded to the alloy tube of a zirconium alloy provides stress reduction and a barrier effect sufficient to prevent failures in the composite cladding.

The corrosion resistant nuclear fuel rod cladding used in the nuclear fuel elements of this invention can be fabricated from a billet comprised of a zirconium alloy made from either the first, second or third zirconium alloy described above. The billet is heated to 590.degree. to 650.degree. C. and extruded. The extruded tubing is then subjected to a process involving conventional tube reduction until the desired size of tubing is achieved.

In another method, a hollow collar of the sponge zirconium selected to be the metal barrier is inserted into a hollow billet of the first, second or third zirconium alloy described above. The assembly is heated to 590.degree. to 650.degree. C. and extruded. The extruded tubing is then subjected to a process involving conventional tube reduction until the desired size of cladding is achieved.

In another method, a tube blank is made from a Zircaloy alloy and an outer tube is placed on this tube blank. The outer tube is composed of the first, second or third zirconium alloy described above. This assembly is then heated to a temperature in the range of 590.degree. to 650.degree. C. and is extruded. The extruded tubing is then subjected to a process involving conventional tube reduction until the desired size of tubing is achieved.

In another method, a tube blank is made from a Zircaloy alloy and an outer tube is placed on this tube blank. The outer tube is composed of the first, second or third zirconium alloy described above. A hollow collar of the sponge zirconium selected to be the metal barrier is inserted into the tube blank. The assembly is heated to a temperature in the range of 590.degree. to 650.degree. C. and is extruded. The extruded tubing is then subjected to a process involving conventional tube reduction until the desired size of tubing is achieved.

Intermediate and final anneals are used during the tube reduction processes described above. Anneals range between 570.degree. to 590.degree. C.

The invention includes a method of producing a nuclear fuel element comprising making a cladding or a composite cladding container comprised of a zirconium alloy, or a zirconium alloy and a barrier layer, or a Zircaloy alloy and a surface layer, or a Zircaloy alloy and an outer surface layer and an inner barrier layer. The container is open at one end and filled with a core of nuclear fuel material leaving a gap between the core and the container and leaving a cavity at the open end. A nuclear fuel material retaining means is inserted into the cavity and an enclosure is applied to the open end of the container, leaving the cavity in communication with the nuclear fuel. The end of the clad container is then bonded to the enclosure to form a tight seal therebetween.

The present invention offers several advantages promoting a long operating life for a nuclear fuel element. A greater resistance to nodular corrosion protects the strength and integrity of the cladding. On cladding having a barrier layer, the reduction of chemical interaction on the cladding, the minimization of localized stress on the zirconium alloy tube portion of the cladding, and the minimization of stress corrosion and strain corrosion on the zirconium alloy tube portion of the cladding, all reduce the probability of a splitting failure occurring in the zirconium alloy tube. The invention further reduces expansion or swelling of the nuclear fuel into direct contact with the zirconium alloy tube, and this reduces the occurrence of localized stress on the zirconium alloy tube, initiation or acceleration of stress corrosion of the alloy tube and bonding of the nuclear fuel to the alloy tube.

An important property of the composite cladding of this invention is that the foregoing improvements are achieved with no substantial additional neutron absorption penalty. Further, the composite cladding has a very small heat transfer penalty in that there is no thermal barrier to transfer of heat, such as results in a situation where a separate foil or a liner is inserted in a fuel element. Also, the composite cladding of this invention is inspectable by conventional non-destructive testing methods during various stages of fabrication and operation.

The following examples are offered to further illustrate the improved nodular corrosion resistance of the alloys used in this invention.

Table I lists several examples of alloys of the present invention, along with three entries at the bottom of the table which are Zircaloy 2 alloys in three different cold-rolled and heat treated states. These alloys were tested in water containing 8 ppm oxygen at 288.degree. C. and 1500 psig, conditions similar to a reactor operating temperature and pressure but absent the radiation, to evaluate the resistance to uniform corrosion.

It can be seen from the results in Table I that the tested alloys of the present invention exhibit excellent resistance to uniform corrosion. Although tested for much longer periods of time, the tested alloys of this invention exhibited corrosion rates comparable to those of the Zircaloy 2 specimens. None of the specimens tested under these conditions exhibited any sign of the formation of nodular corrosion products.

Table II reports the results of tests conducted to determine the susceptibility of the alloys of the present invention to nodular corrosion. The tests were performed using steam at 510.degree. C. and 1500 psig. In the laboratory, these same test conditions induce the formation of the nodular corrosion product on Zircaloy alloys which have been given a 750.degree. C./48 hour anneal, and is also identical to the nodular corrosion found on sometimes Zircaloy after being used in reactor service. For purposes of comparison, the weight gains of annealed Zircaloy samples under the same test condition were on the order of several thousand milligrams per square decimeter.

The tests in Table II were also performed on samples in various cold-reduced and heat treated states. The results in Table II provide an indication that the corrosion resistant properties of these alloys are relatively insensitive to the heat treatment state of the specimen. However, alloys having the solute composed of copper and nickel preferably are heat treated. Some compositions were tested using specimens in cold-rolled plate form, both with and without a subsequent anneal. Two compositions were tested only after having been cold-reduced and annealed. The 750.degree. C. anneal for 48 hours, which all of the tested samples were subjected to, is the heat treatment which strips the Zircaloy 2 alloy of its resistance to nodular corrosion under the laboratory steam tests.

All of the weight gains recorded in Table II are far superior to the results obtained when sensitized Zircaloy 2 annealed at 750.degree. C. for 48 hours is tested. Most of the tested alloys of the present invention produced weight gains of less than 100 milligrams per square decimeter, and one produced a weight gain of 107 mg/dm.sup.2. As previously mentioned, weight gains reported in tests of sensitized Zircaloy specimens under the same test times and conditions are in the order of several thousand milligrams per square decimeter.

In addition to the reduced weight gains evidenced in the alloys of the present invention, none of these alloys showed any sign of formation of nodular corrosion products. Under the test conditions, these alloys clearly provide improvement in resistance to nodular corrosion.