US3506235A - Fuel casting apparatus with collapsible core - Google Patents

Description

April 1 4, 1970 N. H. KATZ ETAL 3,506,235

FUEL CASTING APPARATUS WITH COLLAPSIBLE CORE 2 Sheets-Shet 1 Filed Sept. 12, 1967 INVENTORS NORMAN H. KATZ ROGER N. BEAM ATTORNEY FUEL CASTING APPARATUS wi'ra COLLAPSIBLE CORE Filed Sept. 12, 1967 April 14, 1970 N. H. KATZ ET AL 2 Sheets-Sheet 2 INVENTORS NORMAN H KATZ ROGER N BEAM ATTORNEY United States Patent 3,506,235 FUEL CASTIN G APPARATUS WITH COLLAPSIBLE CORE Norman H. Katz, Northridge, and Roger N. Beam,

Chatsworth, Calif., assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Filed Sept. 12, 1967, Ser. No. 667,638 Int. Cl. B29c 11/00 US. Cl. 249-82 3 Claims ABSTRACT OF THE DISCLOSURE Apparatus disclosure and method for casting annular uranium carbide and other nuclear fuel elements. The mold includes a solid graphite rod within an annular graphite core. The core is designed to collapse upon cooling of the molded fuel, while the graphite rod serves as a heat sink for the mold to prevent premature destruction of the core. The core and rod in one embodiment are separated by an air gap clearance sufi'icient to permit the molded fuel to contract when cooling without destroying the hollow core. In another embodiment, the solid rod is sectioned and includes graphite spacer rings for permitting expansion and contraction of the rod sections to thereby provide intimate thermal contact between core and rod when the fuel is molten, while per mitting contraction of the rod segments as the fuel cools.

BACKGROUND OF THE INVENTION The invention relates to casting annular shaped fissile fuel such as uranium carbide or oxide. More particularly, the invention relates to a particular method and apparatus for such casting. This invention arose in the course of, or under, Contract AT(381)430 with the United States Atomic Energy Commission.

Uranium carbide is a well known fuel element for nuclear reactors. One method for producing such fuel elements is by casting. Normally, such fuel elements are in the form of rods so that casting presents no particular difiiculty.

The casting of annular fuel elements has been difficult because of high coefiicient of thermal expansion and the inherent brittleness of uranium carbide. These characteristics limit both the amount of machining which may be performed on such cast elements and limit the obtaining of any controlled dimensions for the casting.

When such elements have been cast using solid graphite cores, substantial cracks frequently develop in the uranium carbide during the cooling process because of the differential expansion between the cast UC and solid graphite core.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide substantially crack-free cast UC.

Another object is to provide cast UC having controllable dimensions.

A further object is to provide improved annular fissile fuel elements.

The invention provides these and other objects by using 3,506,235 Patented Apr. 14, 1970 a molding process including an annular graphite core enclosing a solid core both within a mold casing, the annular core being designed to collapse when the molded metal cools and the solid core serving as a heat sink during the cooling process. 'Dwo embodiments are disclosed, one having an air gap between the solid and annular core, while the other embodiment has a collapsible solid core.

DESCRIPTION OF THE DRAWING FIGURE 1 illustrates one embodiment of the invention in an exploded view.

FIGURE 2 illustrates a second embodiment of the invention in an exploded view.

FIGURE 3 illustrates one form of a multiple mold casting apparatus.

FIGURE 4 illustrates a third embodiment of the invention in an exploded view.

DESCRIPTION OF THE INVENTION FIGURE 1 illustrates one form of the invention for making annular fuel element in diameter by A" thick and approximately 3" high.

An annular mold body or casing 1 may be of graphite or metal. The casing is closed at the bottom by a plug 2 having a circular recess 3.

The top of the casing is open and has a ridge 9 for supporting the plug 4. The top plug 4 also includes a centrally located circular recess 5. The top and bottom plugs serve as support and centering plates for the annular hollow graphite core 6 and the solid graphite heat sink 7 which fit within the recesses 3 and 5 as shown.

The top plug 4 also serves as a distributor of the liquid uranium carbide which is poured into the body of the mold casing to form the annular casting. The plug 4 is essentially a disc having four arms or salient poles 8 which extend therefrom and are supported on the ridge 9 of casing 1.

In the process of casting, molten uranium carbide is prepared in an argon atmosphere at 23 inches of mercury pressure, and is poured at 2500 C.

The molten metal is poured onto and distributed by the distributor cap 10 of the top plug 4; it flows in the voids between the arms 9 into the A" annular space 11 formed in the mold.

The thin wall elongated hollow graphite core 6 in all embodiments of the invention is designed sufiiciently strong to restrain molten UC, but low enough so that it will crack due to the contraction existing in the UC when cooled. In FIGURE 1, this tube is .010 inch thick and has a A inch diameter.

An air gap 12 exists between the hollow core 6 and solid graphite core 7. The air gap is quite small since the solid core 7 serves primarily as a heat sink to distribute the liquid metal heat by conduction to the casing members 1, 2 and 4. The air gap space ideally is equal to the contraction experienced by the fuel in cooling.

FIGURE 2 illustrates a second embodiment of the invention designed for casting 2" diameter cores having the same /1" thickness as in FIGURE 1. Essentially, FIG- URE 2 is very similar to FIGURE 1 with several exceptions. The same numerals are used as in FIGURE 1 for cor-responding parts. The air gap 12 between hollow core 6 and solid core 7 is larger-of the order of .042 inch.

Because of the larger size of all the elements, the method of alignment and centering of the solid core 7 is slightly different. The heat sink action of FIGURE 2 must be more efiicient than in FIGURE 1. Accordingly, the solid core 7 has a cap or extension 13 adapted for mating with the opening 14 in distributor plate 4. Thus the extension can radiate heat directly to the atmosphere as well as conductively through the distributor. The bottom plate 2 includes. a ridge 15 for supporting hollow core 6 and spacing it from the solid core 7 which is supported on the bottom of the plate.

FIGURE 3 illustrates a multiple mold casting method utilizing five molds of the type in FIGURES l, 2 and 4. One of the problems encountered with multiple molding is that the temperature of the liquid metal is usually higher and there is a greater possibility of destroying the hollow core during casting. 1

Accordingly, a large conical graphite distributor 16 is used to provide a laminar flow into the mold cavities. Liquid metal is poured from a ladle or other means onto this distributor. A tube 17 is shown to illustrate the direction of pouring. This distributor device has proven quite effective in providing just sufiicient chilling of the molten UC to protect the cores. The use of a flat distributor caused too much chilling and resulted in unacceptable mold.

FIGURE 4 illustrates a third embodiment of the invention which is particularly suitable for large castings and other applications where more heat is generated then can be dissipated by the embodiments shown in FIGURES 1 and 2. This embodiment resembles those previously discussed in that it includes a mold cavity 11 defined by cylindrical outer casing or body 1, hollow collapsible core 6, and base plate 2; a top plate or distributor 4; and an inner core or heat sink 7. It differs from FIGURES 1 and 2 in that there is no air gap between hollow core 6 and heat sink core 7, the two cores being in direct contact with each other, thereby providing more effective transfer of heat from the casting.

Heat sink core 7 comprises an assembly of three or more elongated pie-shaped segments 27 linked together by resilient members or springs and shown separated by air gaps 28 in the normal or hot conditions. For purposes of illustration, four segments 27 are shown in FIGURE 4. Graphite spacer springs 20 tend to expand core assembly 7 radially, such that each of its segments 27 is pressed in direct contact against the inner surface of hollow core 6. At the same time, springs 20 are flexible enough to permit core assembly 7 to collapse along with hollow core 6 as the casting cools and contracts, thereby closing the air gaps. In the drawing, springs 20 are shown as resilient C-rings operating in slots in the top and bottom of core segments 27.

The spring-loaded core assembly 7 in this embodiment tends to center itself within core 6. Hence, additional centering means such as the peg and hole arrangement, 13 and 14, in FIGURE 2 is unnecessary. In FIGURE 4, the opening 14 in top plate 2 is closed with plug 24.

Subsequently, as the liquid metal cools in annular space 11 and the annular fuel element contracts, the spacer rings 20 are compressed and contract until limited by the meeting of the four segments in the center of the mold with the elimination of air gaps 28.

The advantage of the apparatus of FIGURE 4 is that the solid core is always in direct thermal contact with the hollow core and the liquid metal, while yet permitting contraction of the fuel element. Thus this embodiment is especially adapted for large fuel element casting.

The emodiment depicted in FIGURE 4 has produced crack-free uranium carbide fuel elements as large as 6 inches long and 3 /2 inches in diameter, with a 4-inch wall thickness, thus illustrating the merit of combining a flexible core with efiicient heat transfer means.

While the invention has been described as required,

the chart below indicates the relevant factors in design for several sizes of molds:

TABLE l.DIMENSIONS OF SINGLE-OAVITY GRAPHITE MOLDS AND AS-CAST CYLINDERS Casting dimensions (in.) Shrinkage (AD) (in.)

Nominal A in. CD by Z in. ID by 3 in. long U0 casting Mold dimensions (in.)

Nominal 2.000 in. OD by 1.5 in. ID by 3 in. long UC casting Outer mold-tube: OD: D.

D, 2. Top, 1.937 Top, 0.063 ID, 2.0 Bottom, 1.035 Bottom, -0.065 Inner shell core: ID: D:

0D, 1.520 Top, 1.490 Top, 0.030

Bottom, 1.488 Solid core insert OD,

Bottom, 0.032

Nominal 3.500 in. OD by 3 in. ID by ll gin. long UO casting Outer mold tube: OD: D.

0D, 4.000 Top, 3.305 Top, 0.105 ID, 3.500 Bottom, 3.390 Bottom, 0.l10 Inner shell core: ID: I ID:

3.000 Top, 2.030 Top, 0.070 ID, 2.970 Bottom, 2.030 Bottom, 0.070

Solid core insert OD,

TABLE 2.DIAMETRICAL SIIRINKAGE OF CAST ANNULAR UC FUEL [Measured vs. calculated] Diametrieal shrinkage (in.)

Nominal as-east Measured (avg) Calculated cylinder OD (in.) O D ID 0 D ID While several embodiments of the invention have been described, it will be obvious that modification can be made thereto. Furthermore, the invention has been de scribed with reference to uranium carbide and other brittle nuclear fuel elements. However, it is anticipated that the invention may find use in other and less critical cast ing applications. Accordingly, the invention is defined in the following claims.

What is claimed is:

1. Apparatus for casting annular elongated fuel ele ments comprising: an elongated hollow casing having an open end and a closed end; a thin, elongated annular hollow graphite core and a second graphite core within and coextensive with said hollow core; and a graphite distribution plate having a plurality of salient poles posi' tioned for substantially closing the open end of said casing and for positioning both cores with their axes coaxial with said casing, whereby a cavity is formed between the inside diameter of the casing and the outer diameter of the hollow core, said second core including a plurality of elongated coextensive spaced segments of solid graphite and a resilient graphite ring-type spring adapted for permitting expansion and compression of said segments in unison in intimate contact with the inner circumference of the hollow core while the poured liquid metal cools.

2. Apparatus for casting annular elongated fuel elements comprising: an elongated hollow casing having an open end and a closed end; a thin, elongated annular hollow graphite core and a second graphite core within and coextensive with said hollow core; and a graphite distribution plate having a plurality of salient poles positioned for substantially closing the open end of said casing and for positioning both cores with their axes coaxial with said casing, whereby a cavity is formed between the inside diameter of the casing. and the outer diameter of the hollow core, said second core including a plurality of elongated coextensive spaced segments of solid graphite 5 6 and a resilient graphite ring-type spring adapted for perhollow core as metal poured into the mold cavity mitting expansion and compression of said segments in cools; and unison in intimate contact with the inner circumference (d) a distributor plate located at the open end of the of the hollow core while the poured liquid metal cools, casing and cavity for centering the cores within said segments being spaced from each other a distance the casing and for sealing oil the volume within corresponding to the contraction of liquid metal in the 5 the hollow core from the flow of metal poured into cavity when cooled. the mold cavity.

3. Apparatus for casting elongated annular fuel elez p h n h d References Cited a an eongate o ow casing aving an open en and a dog e d end; 10 UNITED STATES PATENTS (b) a thin, elongated hollow core positioned within 773,590 11/1904 Rafter 2 said casing such that an elongated annular mold 1,946,451 2/ 1934 l y 9 X cavity is formed having one open end and one 7 K 1/1957 BYOWHStBiH- closed end between the casing and the hollow core; 15 2,405,254 8/1946 H p in 2 2 X (c) a flexible heat sink core assembly within the hol- 2,583,248 1/1952 Bajfovic 249-42 X low core, comprising a plurality of elongated pieshaped segments linked together by resilient means, ROBERT D. BALDWIN, Primary Examiner for maintaining the core segments in intimate con- US Cl XR tact with the inner surface of the hollow core, and 20 for permitting the assembly to contract with the 249-107, 122, 134, 177

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