US2926113A - Heat treated u-mo alloy - Google Patents

Description

Temperature C.

HEAT TREATED UMo ALLOY Filed 061?. 11, 1955 I 7 7+Ma I l I 600- cr+r 9 0 E 3500 U a. s e +MO i I I I l i .i. Fi l. i a '9 lo (I 1'2 l3 l4 (5 is 1'? Weigh 7. Molybdenum Fig. 2.

Time For Beginning of Transformation of Gamma to Alpha Phase of Uranium- Molybdenum Alloys.

055 i '5 f0 5'0 I00 Log ime in Hours sbo I000 2 Sheets-Sheet l Feb. 23, 1960 R. K. McGEARY EI'AL 2,926,113

HEAT TREATED U-Mo ALLOY 2 Shee ts-Sheet 2 Filed Oct. 11, 1955 o I I I I I I I 56 Days I Day Weight Loss of Uranium-Molybdenum Alloys in 600 Water.

Fig. 3.

INVENTORS Rqbert K.McGeory and wnlliorn M.Justusson.

BY M TTOR EY United States Patent HEAT TREATED U-MO ALLOY Robert K. McGeary, Pittsburgh, Pa., and William M.

Justusson, Dearborn, Mich., assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commis- 81011 Application October 11, 1955, Serial No. 539,782

1 Claim. (Cl. 148--32) This invention relates to binary alloys composed of molybdenum and uranium which are particularly suit able for forming members for use in nuclear reactors.

Metallic uranium is a desirable fuel element for nuclear reactors including reactors moderated with water. However, the corrosion resistance of uranium metal when in contact with high temperature water is so poor that members of substantial size will be completely disintegrated in less than one day in water at 650 F. While cladding of a protective material may be applied to the uranium fuel elements, it is difficult to produce a completely impervious cladding and, consequently, there is a reasonable chance that cracks or microscopic fissures may be present in the cladding so that the high temperature water will come in contact with a portion of the uranium fuel element, and thereby cause its disintegration as well as undesirable effects such as bulging and the like.

While it has been proposed to prepare alloys comprising up to 9% molybdenum and the balance being uranium, these alloys have not proven to be satisfactory for a number of reasons. Among the disadvantages is the fact that the uranium transforms readily from the desirable gamma phase to the alpha phase in which the solubility of molybdenum in uranium does not exceed approximately 1.7% by weight. The alpha phase has low corrosion resistance and relatively poor stability when subjected to radiation. Other disadvantages are known to those skilled in the art.

The object of this invention is to provide an alloy composed of from 11% to 16% by weight of molybdenum, the balance being uranium, the alloy being free from appreciable amounts of the alpha phase structure.

Another object of the invention is to provide a member suitable for use as a fuel element in a nuclear reactor comprising an alloy composed of from 11% to 16% by weight of molybdenum, the balance being uranium, the alloy being substantially all of the gamma phase structure and having been annealed at a temperature of from 350 C. to 525 C. for at least seven days in order to impart improved corrosion resistance.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawing, in which:

Figure l is a portion of the phase diagram of molybdenum and uranium;

Fig. 2 is a graph plotting the time for the beginning of the transformation of the gamma to the alpha phase of a number of uranium-molybdenum alloys; and

Fig. 3 is a graph plotting the total weight loss of various uranium-molybdenum alloys in 600 F. water for various periods of time.

We have found that members prepared from uranium base alloys containing from 11% to 16% by weight of molybdenum and the balance being either natural or enriched uranium are characterized by a body centered cubic crystal structure (gamma phase) which is highly which have a relatively stable gamma phase.

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resistant to corrosion in high temperature water and stable to radiation under expected reactor conditions. These alloys may be readily fabricated into member's Furthermore, the gamma phase transforms relatively slowly by an order-disorder reaction to the epsilon phase, which is a slightly tetragonal modification of the body centered cubic structure of the gamma phase. Uranium-molybdenum alloy in either the gamma or epsilon phase is highly desirable for forming into reactor fuel elements, because the fuel elements having these alloy phases are not subject to an appreciable or irregular change of shape during use in reactors.

V More particularly, the members of the present invention are prepared by combining molybdenum in proportions of from 11% to 16% by weight and uranium, either natural uranium or enriched uranium. The alloy may contain small amounts of impurities such as carbon, nitrogen, silicon, nickel, etc. We have employed with success uranium having a total of from 200 to 300 parts per million of such impurities. In some instances, zirconium oxide and other metal oxides derived from the crucibles in which the metals have been melted and handled may be present in amounts of up to several hundredths of 1% without detriment.

Good results have been obtained by melting shortlength bars or pieces of the relatively pure molybdenum and uranium in a graphite crucible coated with a wash of zirconium oxide. The melting may be carried out under vacuum or in an inert atmosphere such as argon or helium gas. The melt may be cast in a graphite mold into a small ingot. Such ingot may be then arc-melted, using the ingot as a consumable electrode, to produce a larger ingot with a highly uniform composition throughout. The resulting ingots, either arc-melted or as originally cast, may be fabricated by hot metal working procedures into bars, sheets or other desired fuel elements. We have been able to extrude the alloy satisfactorily by heating billets of the alloy in a salt bath to a temperature of 1700 F. to 2100 F. and then extruding the heated billets in a conventional extrusion press. We have readily produced 0.5 inch diameter rod from 2.4 inch diameter billets. Also, the alloys of the present invention have been press-forged at 1950 F. Hot rolling also has been satisfactory in producing fiat plates.

After the molybdenum-uranium alloy members have been prepared by suitable hot-working technique to the desired shape, the members are preferably given a homogenizing anneal by reheating to a temperature in the gamma field for several hours, and quickly cooled to room temperature. A suitable process we have used comprises heating to approximately 900 C. for 12 to 24 hours and then water quenching. Water quenching has proven quite satisfactory in producing a member in which the entire structure is of the gamma phase. In some instances, air cooling the member from 900 C. to room temperature in one or two hours has resulted in the gamma phase structure being present throughout.

It has been found that improvement in corrosion resistance is obtained if the homogenized anneal member of the molybdenum-uranium alloy is further annealed within a temperature range of from 350 C. to 525 C. for at least seven days. The lower temperatures are applied to the alloys having the lower proportions of molybdenum. In particular, alloys containing from 11% to 13% molybdenum are annealed within a temperature range of 350 C. to 450 C. for a period of time of from 7 to 35 days with improvements in overall properties and a marked increase in corrosion resistance. The alloys with 14% to 16% molybdenum are preferably annealed at 500 C. and higher. We have found that the anealing at tempertaures of 500 C. and higher produces optimum benefits within to days. Annealing for materially longer times does not produce any improvement.

Referring to Fig. l of the drawing, there is illustrated a portion of the phase diagram for binary uraniummolybdenum alloys having between approximately 6% and 20% by weight of molybdenum and extending within a temperature range of approximately 400 to 620 C. It will be noted that the alloy comprising 11% of molybdenum in being cooled from a temperature in the gamma field which is above 600 C. does not transform to a structure in which the alpha phase is present until a temperature of 583 has been reached. Alloys containing, for example, 7% to 9% of molybdenum on being cooled from above about 620 C. undergo a first transformation into an alpha plus gamma phase. At 583 C. there is a further transformation to the alpha and epsilon phase. The time required to initiate transformation and the rate of transformation to the alpha phase is controlled greatly by the amount of molybdenum present. We have found that with 9% and less of molybdenum the transformation begins quite rapidly as compared to a 12% molybdenum alloy. Furthermore, the transformation itself progresses at a proportionately faster rate for the lower molybdenum content alloys.

The time at various temperature for the beginning of the transformation of the gamma to the alpha phase in a number of molybdenum-uranium alloys is illustrated by the curves of Fig. 2. It will be noted that a 7% molybdenum alloy begins to transform at 500 C. in approximately A of an hour, whereas the 12% molybdenum alloy only begins to transform from gamma to alpha phase after 30 hours at this temperature. The 12% molybdenum aloy, therefore, begins transformation ap' proximately 100 times slower than the 7% molybdenum alloy. The rate of transformation is correspondingly slower for the high molybdenum alloy. At 400 C. the rate of transformation of the 12% molybdenum alloy is approximately A that of the 7% to 9% molybdenum alloys. Alloys with more than 12% molybdenum begin transformation at progressively longer times. Consequently, there is a marked difference in the time for initiation of the transformation of gamma to alpha in the uranium-molybdenum alloys of the present invention as compared to uranium alloys having 9% and less of molybdenum.

Improvements of this order in the stability of the gamma phase as characteristics of the alloys of the present invention are highly important in fuel elements for nuclear reactors. In many cases, a fuel element need only be useful for approximately 125 days or 3,000 hours to be satisfactory for a reactor. The alloys having from 11% to 16% of molybdenum of the present invention will retain the gamma phase for this period of time in nuclear reactors which are so controlled that the temperatures in the fuel element do not exceed 450 C. for any appreciable period of time. Ordinarily, in pressurized water reactors and other reactors employing water as a moderating element as well as a means for conveying the heat of the reactor to a turbine, the temperature of the water usually will not exceed approximately 600 F. (315 C.) and the fuel elements in such reactors ordinarily will maintain their temperature below approximately 400 C. for 99% of the time.

We have tested specimens of the various molybdenumuranium alloys in high temperature water for various periods of time and determined the weight loss of such specimens for the various compositions. The results of some of these tests are summarized in Fig. 3 of the drawing wherein weight loss is plotted against molybdenum content of the alloys for a series of tests which were carried out for l, 7, 14, 28 and 56 days. It will be noted that the minimum weight loss, which is also the maximum corrosion resistance, occurs for the alloys having between 11% and 12% molybdenum. The alloys employed in the tests of Fig. 3 were all in the gamma phase. If the alloys containing 10% or less of molybdenum had been in the alpha phase, the corrosion rate would have been several orders of magnitude higher. In a nuclear reactor it would have been a considerable problem to maintain 7% or 8% molybdenum-uranium alloy member in the gamma phase. However, the alloys containing 11% to 16% molybdenum can all be maintained in the gamma phase with no difficulty for any reasonable reactor use.

The following examples illustrate the practice of the invention:

Example I In an induction furnace comprising a graphite crucible coated with a wash of zirconium oxide there was melted uranium and molybdenum in proportions providing 12 parts by weight of molybdenum, 88 parts by weight of natural uranium. The uranium included the following impurities:

Element: P.p.m. Carbon 26 to 36. Zirconium 50 maximum. Nitrogen 20 maximum. Iron 40 to 55. Silicon 36 to 46. Nickel 10 to 1. Chrome 4 to 6. Magnesium 3 to 5. Others Less than 2.

The molten alloy was then cast in a graphite crucible coated with zirconium oxide into an ingot having a diameter of 2.4 inches. The ingot was heated to 1900" F. in a salt bath and extruded into a bar. The bar was annealed at 900 C. for 24 hours and then water quenched. Metallurgical examination showed no alpha phase being present whatever and the entire structure of the alloy was gamma phase. When tested at room temperature, the alloy had a tensile strength of 166,000 pounds per square inch and an elongation of 9%. The extruded members were then given a homogenizing anneal by heating at 400 C. in a helium atmosphere for 21 days. In 650 water, specimens of a thickness of 0.25 inch of the annealed member withstood 63 days. Specimens of this same alloy which had not been so annealed, however, failed from corrosion in approximately 20 days. In these tests the specimens had corroded moderately and then cracked into a number of pieces. When tested in 600 F. water, the alloy prior to annealing had a corrosion life of approximately 70 days. Annealing at 400 C. for 21 to 35 days appears to increase the corrosion resistance so that the specimens will stand up in the water for approximately 3 times as long as they would without such an anneal. The annealing appears to homogenize the alloy members and thereby produces improved corrosion resistance properties. Hardness is also increased substantially by the annealing treatment.

Example II A member was prepared from an alloy comprising 115% molybdenum, the balance being uranium, following the procedure of Example I. After having been annealed at 350 C. for 35 days, specimens of this alloy were placed in an autoclave containing water at 650 C. The specimen had a life of approximately days.

Example III A member comprising 14% molybdenum, the balance being uranium was prepared in accordance with the procedure of Example I. After being annealed for 7 days at 500 C. the member was put into an autoclave in which water at 650 was present. The member had a corrosion life of approximately 49 days under these conditions.

It will be understood that the uranium-molybdenum alloys of the present invention are suitable for use as fuel elements Without any protective cladding material. They may be clad with the zirconium alloy set forth in application, Serial No. 416,396, filed March 15, 1954, now Patent No. 2,772,964, assigned to the assignee of the present invention. The application of such cladding will greatly improve the corrosion resistance and other properties of the alloys of the present invention even though the cladding may have present minute cracks or fissures or other flaws exposing the uranium to high temperature water or steam.

The molybdenum-uranium alloys of the present invention may comprise enriched uranium. For example, members may be prepared from 12% molybdenum and the balance, 88% by Weight, comprising natural uranium enriched with of uranium 235. Other degrees of enrichment may be employed as is required for any particular reactor application. Irradiation tests of the uranium-molybdenum alloys of the invention have shown no adverse or undesirable changes in shape of the members of such alloy as compared to marked anisotropic changes in uranium members.

It is understood that the above detailed description and drawing are only exemplary of the invention and not in limitation thereof.

We claim as our invention:

A member suitable for use as a fuel element in a nuclear reactor, the member comprising a hot worked member of an alloy composed of from 11% to 16% by weight of molybdenum, and the balance being uranium, the alloy being in the gamma phase, the member having been annealed at least 7 days at a temperature of from 350 C. to 525 C., the annealing in the temperature range of 350 C. to 450 C. being applied to the 11% to 13% molybdenum alloy and the annealing above 450 C. being applied to alloys having over 13% molybdenum, the annealing conferring improved corrosion resistance when the member is subjected to water at 500 F. and higher.

References Cited in the file of this patent UNITED STATES PATENTS Anderson Feb. 5, 1957 OTHER REFERENCES AEC Document AECD-2046, page 6, declassified June 9, 1948.

I. Inst. Metals (London), vol. 77, page 553 (1950).

I. Inst. Metals (London), vol. 77, page 561 1950).

BMI72, The Constitution Diagram of Uranium- Rich Uranium-Molybdenum Alloys, June 1, 1951, pages 4-6, 13-22.

BMI-957, Transformation Kinetics of Uranium- Molybdenum Alloys, Oct. 21, 1954, pages 7-8.

Glasstone: Principles of Nuclear Reactor Engineering, D. Van Nostrand Co., Inc., N.Y. (July 1955), page 459.

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