NUCLEAR FUEL ROD

Abstract

A Gland Seal End Plug closure for a nuclear fuel rod cladding composed of silicon carbide or other materials that cannot be welded. The sealant is, preferably, made from one or more forms of pure graphite and the ram, seat and other components of the Gland Seal End Plug are formed from high temperature metallic or ceramic materials.

Claims

1. A nuclear fuel rod comprising: a tubular cladding constructed from a material that cannot be welded or brazed, that surrounds an interior volume; a first closure sealing off one end of the tubular cladding; a nuclear fuel occupying a portion of the interior volume; and a gland seal end plug sealing off a second end of the tubular cladding.

2. The nuclear fuel rod of claim 1 wherein the gland seal end plug comprises: a flexible material supported between a wall of the tubular cladding; and a force generator configured to exert a radially outward force on the flexible material to pressure the flexible material against a wall of an interior of the tubular cladding and seal off the second end of the tubular cladding.

3. The nuclear fuel rod of claim 2 wherein the force generator comprises a ram that extends into an interior of the tubular cladding from an end stopper that substantially closes off the second end of the tubular cladding, the ram being configured to support and exert the radially outward force on the flexible material.

4. The nuclear fuel rod of claim 3 wherein the ram expands the flexible material against the wall of the interior of the tubular cladding and a seat on an interior of the end stopper.

5. The nuclear fuel rod of claim 4 wherein the ram includes a stem that extends through the end stopper from an interior of the tubular cladding to an exterior thereof with an exterior of the stem being threaded, including a nut coupled to an exterior portion of the thread and configured to draw the stem through the end stopper to pressure the flexible material against the wall of the interior of the tubular cladding.

6. The nuclear fuel rod of claim 5 wherein the flexible material is pressured between an interior end of the ram, the stem, the interior wall of the tubular cladding and the seat.

7. The nuclear fuel rod of claim 6 including a spring washer that is configured to be compressed long with the flexible material and maintains a force on the flexible material even if there is thermal expansion of the stem or relaxation of the flexible material.

8. The nuclear fuel rod of claim 7 wherein the spring washer comprises an alloy such as X-750 or 718.

9. The nuclear fuel rod of claim 7 wherein the spring washer comprises a high temperature ceramic such as silicon carbide or a partially stabilized Zirconium Oxide.

10. The nuclear fuel rod of claim 7 wherein the spring washer comprises a ceramic material and includes a stack of ceramic spring washers.

11. The nuclear fuel rod of claim 3 wherein the end stopper is configured to prevent the flexible material from expanding through an interface between the interior of the tubular cladding and the end stopper,

12. The nuclear fuel rod of claim 1 wherein the tubular cladding is silicon carbide.

13. The nuclear fuel rod of claim 9 wherein the flexible material comprises graphite.

14. The nuclear fuel rod of claim 13 wherein the flexible material consists of pure graphite.

15. The nuclear fuel rod of claim 13 wherein the flexible material comprises a mixture graphite fiber and expanded graphite.

16. The nuclear fuel rod of claim 3 wherein the end stopper and/or the ram comprises Zircaloy 4.

17. The nuclear fuel rod of claim 5 wherein one or more of the seat, ram and nut comprises silicon carbide.

18. The nuclear fuel rod of claim 5 wherein the end stopper is coated with silicon carbide.

19. The nuclear fuel rod of claim 5 wherein the seat is constructed in two layers including a gasket between the two layers of the seat that form a seal between the seat and the stem and the seat and the wall of the tubular cladding.

20. The nuclear fuel rod of claim 19, wherein the gasket comprises a plurality of O-ring gaskets.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:

[0018] FIG. 1 is a simplified schematic of a nuclear reactor system to which this invention can be applied;

[0019] FIG. 2 is an elevational view, partially is section, of a nuclear reactor vessel and internal components to which this invention can be applied;

[0020] FIG. 3 is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form, parts broken away for clarity;

[0021] FIG. 4 is a schematic representation of a cross section of one embodiment of this invention;

[0022] FIG. 5 is a schematic representation of a cross section of a second embodiment of this invention;

[0023] FIG. 6 is a schematic representation of a cross section of a third embodiment of this invention; and

[0024] FIG. 7 is a schematic representation of a cross section of a fourth embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] This invention overcomes the foregoing difficulties by providing a Gland Seal End Plug for Accident Tolerant Fuel (ATF) that uses a soft graphite packing material to achieve a hermetic seal for SiC cladding that can function at pressures expected in a PWR or BWR, even in accident situations. The design is capable of maintaining a seal under high rod internal pressures and high reactor coolant system pressures.

[0026] A conceptual drawing of the Gland Seal End Plug for ATF is shown in FIG. 4. The plug (76) is composed of five basic components. The sealant (78) is a flexible material that expands against the inside diameter of the silicon carbide tube (80) as it is compressed between the gland seal ram (82) and the gland seal seat (84). The sealant (78) engages with the rough surface of the silicon carbide tube (80) inside diameter, such that once the sealant (78) is fully expanded, the plug (76) cannot be pulled from the tube (80) or forced further into the tube. A nut (86) that is threaded onto the gland seal ram stem or shaft (88) is used to draw the gland seal ram (82) toward the gland seal seat (84) to compress the sealant (78). A spring washer (90) is compressed along with the sealant (78), and maintains the force on the sealant even if there is thermal expansion of the gland seal ram (82), or a long term relaxation of the sealant (78). The Allen Socket (92) allows the nut (86) to be tightened without spinning the ram (82).

[0027] The plug (76) is designed with dimensions such that it can be pushed into the silicon carbide tube (80) with little or no force before it is expanded. The outside diameter of the plug (76) is sized such that neither the ram (82) nor the seat (84) will apply force to the inside diameter of the silicon carbide tube due to radiation induced swelling and/or thermal expansion.

[0028] The preferred material for the sealant (78) is pure graphite. Graphite packing has been used for high temperature valves and is known to be able to withstand temperatures up to 3000° F. (1649° C.). The ideal packing would be a mix of graphite fiber and expanded graphite. The fiber holds the expanded graphite flakes in place and prevents the sealant from being extruded through gaps such as between the tube wall inside diameter and the ram. Expanded graphite sealants maintain their resilience for long periods and at high temperatures, so the spring washer does not need to exert force over a large distance.

[0029] A variety of different materials could be used for the other plug components. Zirconium alloys, such as Zircaloy 4, would make a suitable ram, seal seat, and nut material due to proven performance in water cooled reactors, low coefficient of thermal expansion, and high melting point. Zircaloy 4 can be easily welded which would allow several other features to be added to the plug. A hole could be located in the ram for filling the rod with helium and for pressure testing, and afterwards the hole could be welded closed. A tack weld between the nut and the ram could be used to assure that the nut did not loosen during operation.

[0030] Another favorable material for the seat, ram, and nut would be silicon carbide. This would give the plug the same thermal expansion as the cladding and the same high temperature performance. Being the same material as the cladding would allow bonding schemes that would otherwise not be possible. For instance, the plug region could be coated with a thin layer of Chemical Vapor Deposition (CVD) silicon carbide which would serve to provide an additional gas barrier, but the layer would not have to have mechanical strength.

[0031] The spring washer could be a high temperature metal such as Alloy X-750 or Alloy 718. These materials are known to be corrosion resistant and resistant to stress relaxation during normal operation. The fuel end plugs would not heat up to the same extent as more central core locations during an accident, so these alloys would likely not limit ATF performance. However, if it is determined that for a particular fuel design and core that the spring washer would limit performance, they could also be manufactured from a tough, high temperature ceramic such as silicon carbide or a partially stabilized Zirconium Oxide. If a ceramic material is used for the spring washer, a stack of ceramic spring washers would be required to compensate for growth of the ram and sealant relaxation.

[0032] A second embodiment of the invention is shown in FIG. 5. A set of high temperature O-rings (94) has been added to provide a back-up sealing capability. The inside of the tube has been machined to produce a smooth surface for sealing. The top of the seat (84) is now constructed of two segments (96, 98), with one of the two segments (96) containing O-ring grooves. The grooves are constructed such that the O-ring expands to contact both the ram stem (88) and the cladding (80) when the Gland Seal End Plug assembly (76) is fully compressed with the nut (86). Besides providing a sealing function themselves, the O-rings (94) also transmit pressure to the graphite sealant (78), assisting the spring washer (90) in compensating for ram (82) growth or sealant (78) compression during operation.

[0033] In a preferred embodiment of the secondary O-ring seal, the O-rings (94) are composed of a high-temperature alloy such as X-750 or 718, coated with a soft metallic coating such as platinum or nickel. The invention does not limit the shape of the sealing ring to a simple circular design, but it also includes C-ring, U-rings, V-rings and W-ring, as well as designs that have a more nearly square cross section.

[0034] Multiple sealing ring segments may be stacked to afford additional reliability. This is shown in FIG. 6. This embodiment, where two sets of C-rings (94) are stacked with the C-ring facing in opposite directions, is particularly favorable due to the fact that the sealing force will increase with pressure, and sealing is equally effective for excluding coolant from the interior of the fuel rod and preventing gasses within the fuel rod from escaping.

[0035] More than one sealing segment can be used as well as more than one sealing ring. This is shown in FIG. 7, where two different packing segments (100, 102) are connected by a packing spacer (104). Note that in this embodiment of the invention, the seat (84), the ram (82), and the spacer (104) have all been shaped to transmit more of the pressure from sealant compression toward the sealing surfaces along the tube (80) and the ram stem (88). Such shaping can be employed whether or not multiple flexible sealants are used. Note that in this embodiment, the end of the tube does not contact the seat, so that system pressurization can more effectively apply sealing pressure to the flexible sealant.

[0036] Accordingly, this invention provides a nuclear fuel rod cladding (80) formed from a material such as a ceramic that cannot be welded with a flexible gland seal closure. Preferably, the gland sealant (78) is formed from a material such as pure graphite. A screw ram (82) and a wedge-shaped seat (84) are used to compact and radially expand the sealant (78) and the seat (84), ram (82), and nut (86) are preferably composed of high temperature metallic materials such as Zircaloy 4. In an alternated embodiment of the device all components, except the graphite sealant, are composed of tough ceramic materials such as silicon carbide or partially stabilized zirconium oxide. A spring washer (90) or stack of spring washers are used to maintain compressive force on the sealant to compensate for sealant shrinkage or ram thermal or radiation induced expansion. In another alternate embodiment of the device, resilient metal sealing O-rings (94) provide a secondary seal and also apply force to the sealant to compensate for sealant shrinkage or ram thermal or radiation induced expansion. Multiple sets of sealing rings may be used. The resilient metal seal rings may be O, C, V, U or W shaped and the rings may be coated with a soft metallic material such as nickel or platinum so the soft metal and the resilient base metal do not corrode or otherwise degrade below 1200° F. (649° C.) in a primary water or steam environment. In still another embodiment, the ram (82) has a sealable passage through which helium can be added to the rod and the seal pressure tested. Where seal rings are employed, the end of the fuel rod inside diameter being sealed is machined to a flatness and ovality such that sealing rings will function. The ram and seat are shaped to optimally transmit compressive forces in a radial direction to the sealing surfaces. Multiple sealing rings and/or flexible sealants may be used.

[0037] While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.