US3633033A

US3633033A - Controlled vent for isotope fuel capsule

Info

Publication number: US3633033A
Application number: US825779A
Authority: US
Inventors: Alfred E Cottam
Original assignee: Westinghouse Electric Corp
Current assignee: Westinghouse Electric Corp
Priority date: 1969-05-19
Filing date: 1969-05-19
Publication date: 1972-01-04
Anticipated expiration: 1989-01-04
Also published as: JPS4824996B1

Abstract

A controlled vent for an isotope fuel capsule utilizes physical passages for gas flow which are large enough to permit flow to take place be viscous or free molecular flow and are small enough or sufficiently tortuous to prevent passage of fuel particles. A plurality of punctured thin metal discs or foils are stacked together with the holes misaligned to form a continuous flow channel, thereby permitting gas generated within the capsule to escape through the vent. Thus, the weight and volume of the fuel capsule can be reduced since the internal pressure is reduced.

Description

United States Patent lnventor Alfred E. Cottam Bethel Park, Pa.

Appl. No. 825,779

Filed May 19, 1969 Patented Jan. 4, 1972 Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

CONTROLLED VENT FOR ISOTOPE FUEL CAPSULE 9 Claims, 4 Drawing Figs.

US. Cl 250/ 106 S, 176/67 Int. Cl G211: 5/00 Field of Search 250/106 S; 176/66, 67

[56] References Cited UNITED STATES PATENTS 2,830,190 4/1958 Karp 250/106 S 3,133,196 5/1964 Rochlin 250/106 S 3,488,502 1/1970 Dukes 250/106 S Primary Examiner-Archie R. Borchelt AttomeysA. T. Stratton and Z. L. Dermer ABSTRACT; A controlled vent for an isotope fuel capsule utilizes physical passages for gas flow which are large enough to permit flow to take place be viscous or free molecular flow and are small enough or sufficiently tortuous to prevent passage of fuel particles. A plurality of punctured thin metal discs or foils are stacked together with the holes misaligned to form a continuous flow channel, thereby permitting gas generated within the capsule to escape through the vent. Thus, the weight and volume of the fuel capsule can be reduced since the internal pressure is reduced.

mmmm 4m l 3l SINGLE ORFICE 3: PER use 3 (I) (1) LL] D: D. 5 A 3 TWO ORFICES a PER DISC 24 0 U1 9 FOUR ORFICES 2 PER DISC FIGS.

EQUIVALENT LEAK PASSAGE LENGTH INVENTOR WITNESSES 7 Alfred E. Cottom BY I I I 27mm ATTORNEY CONTROLLED VENT FOR ISOTOPE FUEL CAPSULE BACKGROUND OF THE INVENTION This invention relates, generally, to radioisotopes and more particularly, to fuel capsules for radioisotopes utilized as a supplementary source of electrical power for space vehicles.

When a fuel, such as plutonium-238 which generates helium gas, is utilized in a radioisotope capsule, the safety aspects of the capsule are important. Conventional capsules relay on a heavy structural refractory metal shell to contain the high internal pressures generated by the fuel and to ensure capsule survival and fuel containment after impact.

An object of this invention is to provide for venting gas generated by fuel in a radioisotope capsule while containing the fuel particles within the capsule.

Another object of the invention is to reduce dependence on the long term rupture characteristics of the structural shell of the fuel capsule.

A further object of the invention is to enhance impact survival by substantially reducing the fuel capsule internal pressure.

Still another object of the invention is to prevent oxidation of the refractory metal shell through oxygen diffusion.

Other objects of the invention will be explained hereinafter or will be apparent to those skilled in the art.

SUMMARY OF THE INVENTION In accordance with one embodiment of the invention, a controlled vent for a radioisotope fuel capsule comprises a plurality of thin punctured metal discs which are stacked one upon another with the holes through the discs misaligned. After stacking, the discs are welded together at the periphery to form a solid cylinder which is secured in a wall of the cap sule. Thus, physical passages are provided for gas flow which are large enough to permit flow to take place by viscous or free molecular flow and are small enough or sufficiently tortuous to prevent passage of fuel particles. The leakage can be controlled as a function of hole diameter, number of layers and number of holes in each disc.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the nature and objects of the invention, reference may be had to the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic view, partly in section, of a radioisotope generator with a fuel capsule having a controlled vent embodying principal features of the invention;

FIG. 2 is an exploded view, showing the manner of stacking the discs utilized in the vent;

FIG. 3 is an enlarged view, in section, of the discs in assembled relation, and

FIG. 4 is a graphical representation showing the relation between inside capsule pressure and equivalent passage length for different numbers of orifices in the discs utilized in the vent.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, and particularly to FIG. 1, the radioisotope generator shown therein may be of a type well known in the art suitable for use in a space vehicle. As shown, the generator 10 is generally cylindrical in shape with closed ends. The generator may comprise an outer metal shell 11, a shield 12, a fuel capsule 13 containing a suitable fuel 14, such as plutonium-238, and a plurality of thermoelectric converters 15 disposed between the shield 12 and the capsule l3 and surrounded by thermal insulation 16. The thermoelectric converters l5 transform part of the isotope decay heat into electricity in a manner well known in the art. The remaining heat energy produced by the fuel flows into the outer shell from which it is radiated or conducted to the surrounding air or space.

As explained hereinbefore, when plutonium-238 is utilized as a fuel, helium gas is generated within the capsule 13. In order to prevent an excessive pressure within capsule, a controlled vent 21 is provided for the capsule. The vent 21 provides physical passages for gas flow which are large enough to permit flow to take place by viscous or free molecular flow and are small enough or sufficiently tortuous to prevent passage of fuel particles.

As shown more clearly in FIGS. 2 and 3, the vent 21 comprises a plurality of thin punctured metal discs or foils 22 which are stacked one upon another with the holes in the disc misaligned. Each disc 22 has one or more holes 23 therethrough which are of a small diameter, for example 0.0005 inches. By way of example, each disc 22 may be 0.005 inches thick. The holes may be drilled in the discs by an electron beam or laser, or other suitable means.

As shown in FIG. 2, each disc contains two pairs of diametrically opposed holes 23 and holes 23 are spaced substantially apart. As shown, the discs are stacked with the holes in alternate discs rotated substantially 45. After being stacked, the discs are welded together at the periphery, as shown at 24 in FIG. 3, to form a solid cylinder. The assembled vent 21 is then secured in a wall of the capsule 13, as by welding.

The discs 22 may be composed of platinum alloys with rhodium, thereby providing a stable vent, free from oxidation and compatibility problems after reentry and impact. The permeation of gas through the controlled vent depends on the flow regime, which, in turn, is determined by the pore size, pressure, and leak rate. If calculated as a labyrinth seal, leakage can be controlled as a function of hole diameter and number of layers.

Each curve in FIG. 4 shown internal capsule pressure versus leak passage length in which the effective path length resulting from a geometrical arrangement has an effect similar to a labyrinth seal. The evaluation of the seal depends on the clearance between the layers and the complicated path involved. An estimation of the pressure drop due to this effect is made using the equation for a labyrinth seal.

The different curves in FIG. 4 show the results obtained with a single orifice per disc, two orifices per disc and four orifices per disc. In this manner, the leakage of gas from inside the capsule 13 may be controlled by means of the vent 21. A vent 25, which may be of a conventional type, may be provided in the shield 12 and outer shell 11 to permit the gas to escape from within the shield 12.

From the foregoing description, it is apparent that the invention provides a controlled vent for reducing the pressure within an isotope fuel capsule, thereby permitting a reduction in the size and weight of the capsule without reducing the safety features of the capsule. The controlled vent permits leakage of a gas without permitting passage of fuel particles through the vent. Desired leakage rates may be obtained by varying the number of discs, the number of holes and the size of the holes in the discs utilized in the vent structure.

Since numerous changes may be made in the abovedescribed structure, and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all subject matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In a radioisotope generator, in combination, a capsule containing a radioisotope fuel, a controlled vent for the capsule, said vent comprising a plurality of punctured metal discs stacked one upon another, and means retaining said discs in a wall of the capsule.

2. The combination defined in claim 1, wherein each disc has at least one hole therethrough and the discs are assembled with the holes in misalignment.

3. The combination defined in claim 2, including means retaining the discs in said assembled relationship.

4. The combination defined in claim 2, wherein the discs are secured in assembled relationship by being welded together at their periphery.

5. The combination defined in claim ll, wherein each disc has at least two diametrically opposed holes therethrough and the discs are assembled with the holes in alternate discs angularly rotated. I

6. The combination defined in claim ll, wherein each disc has two pairs of diametrically opposed holes therethrough spaced substantially 90 apart.

7. The combination defined in claim 6, wherein the discs are assembled with the holes in alternate discs angularly rotated substantially 45.

8. A controlled vent for a capsule containing a radioisotope fuel, said vent comprising a plurality of puncturcdmetal discs stacked one upon another with the holes through the discs in misalignment, and means retaining said discs in a wall of the capsule.

9. The vent defined in claim 8, including means retaining the discs in said assembled relationship. 1

Claims

5. The combination defined in claim 1, wherein each disc has at least two diametrically opposed holes therethrough and the discs are assembled with the holes in alternate discs angularly rotated.

6. The combination defined in claim 1, wherein each disc has two pairs of diametrically opposed holes therethrough spaced substantially 90* apart.

7. The combination defined in claim 6, wherein the discs are assembled with the holes in alternate discs angularly rotated substantially 45*.

8. A controlled vent for a capsule containing a radioisotope fuel, said vent comprising a plurality of punctured metal discs stacked one upon another with the holes through the discs in misalignment, and means retaining said discs in a wall of the capsule.

9. The vent defined in claim 8, including means retaining the discs in said assembled relationship.