Nuclear fuel assembly support feature

Abstract

A nuclear fuel assembly having lateral support provided by a bimetallic spring that extends from a side of the fuel assembly under certain core conditions to pressure against an adjacent component and withdraws under other core conditions, such as shutdown, to enable the nuclear fuel assembly to be aligned or withdrawn from the core and repositioned.

Claims

1. A nuclear fuel assembly having an elongated dimension and comprising: a plurality of interconnected components wherein at least some of the interconnected components comprise: a top nozzle; a bottom nozzle; a plurality of guide thimbles extending between the top nozzle and the bottom nozzle; a plurality of fuel rods extending between the top nozzle and the bottom nozzle; and a plurality of grids arranged in a tandem spaced relationship that extends between the top nozzle and the bottom nozzle along the elongated dimension, with each of the grids having a plurality of cells some of which support fuel rods and others through which the guide thimbles respectively pass and attach to the plurality of grids; wherein at least one grid of the plurality of grids comprises a peripheral surface area that extends in a plane a distance along the elongated dimension; and wherein the at least one grid of the plurality of grids comprises a bimetallic spring that moves between a first and second position relative to the plane as the fuel assembly transitions from a reactor core shutdown temperature to a reactor core operating temperature, with the second position placing the bimetallic spring in contact with a corresponding grid of an adjacent nuclear fuel assembly located in a reactor core.

2. The nuclear fuel assembly of claim 1 wherein the bimetallic spring has an elongated dimension and the elongated dimension of the bimetallic spring extends transverse to the elongated dimension of the nuclear fuel assembly.

3. The nuclear fuel assembly of claim 2 wherein the bimetallic spring is continuous and extends across several of the grid cells.

4. The nuclear fuel assembly of claim 1 wherein the at least one grid of the plurality of grids is a mid-grid.

5. The nuclear fuel assembly of claim 1 wherein the at least one grid of the plurality of grids is one of an upper grid or a lower grid or both an upper grid and a lower grid.

6. The nuclear fuel assembly of claim 1 wherein the bimetallic spring is configured in a rectangular shape having an elongated dimension.

7. The nuclear fuel assembly of claim 6 wherein the elongated dimension of the bimetallic spring extends substantially parallel to the elongated dimension of the nuclear fuel assembly.

8. The nuclear fuel assembly of claim 1 wherein the bimetallic spring comprises stainless steel and either FeNi36 or 64FeNi.

9. The nuclear fuel assembly of claim 8 wherein the bimetallic spring is formed on each corner of the at least one grid of the plurality of grids.

10. The nuclear fuel assembly of claim 1 wherein the peripheral surface area of the at least one grid of the plurality of grids comprises a base metal, and wherein the bimetallic spring is formed by coating the base metal with a material having a lower coefficient of thermal expansion than the base metal.

11. The nuclear fuel assembly of claim 1 wherein the bimetallic spring is formed on a corner of the at least one grid of the plurality of grids.

12. The nuclear fuel assembly of claim 1 wherein the bimetallic spring does not protrude outwardly from the plane in the first position and protrudes outwardly from the plane in the second position to contact the corresponding grid of the adjacent nuclear fuel assembly.

13. The nuclear fuel assembly of claim 1 wherein the bimetallic spring is configured in a round disc shape.

14. The nuclear fuel assembly of claim 13 wherein the bimetallic spring has a dome when exposed to the reactor core operating temperature.

15. The nuclear fuel assembly of claim 13 further comprising relief holes spaced around the circumference of the bimetallic spring.

16. The nuclear fuel assembly of claim 1 wherein the bimetallic spring comprises a base material coated with a material having a different coefficient of thermal expansion than the base material.

17. The nuclear fuel assembly of claim 1 wherein the bimetallic spring comprises a lamination of a first material and a second material, wherein the first material has a different coefficient of thermal expansion than the second material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 is a simplified schematic of a nuclear reactor system to which this invention can be applied;

(3) FIG. 2 is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity;

(4) FIG. 3 is a plan view of a corner portion of a border grid strap with one embodiment of the bimetallic spring of this invention shown protruding outward away from the interior of the fuel assembly as the spring would protrude as the reactor is brought up to power;

(5) FIG. 4 is a side view of the outside of the portion of the border grid strap shown in FIG. 3;

(6) FIG. 5 is a side view of the inside of the portion of the border grid strap shown in FIG. 3;

(7) FIG. 6 is a perspective view of FIG. 4;

(8) FIG. 7 is a perspective view of a second embodiment of the bimetallic spring of this invention formed in a circular disc configuration;

(9) FIG. 8 is a side view of FIG. 7;

(10) FIG. 9 is a plan view of four adjacent fuel assemblies at operating temperatures with their opposing springs in contact;

(11) FIG. 10 is a perspective view of a portion of a top fuel assembly nozzle with a disc metallic spring shown in one corner of each side with a recess in the other corner to receive the forward portion of an opposing spring extending from an adjacent nozzle;

(12) FIG. 11 is a perspective view of a disc bimetallic spring with optional relief openings;

(13) FIG. 12 is a perspective view of a portion of a top nozzle employing the embodiment of the bimetallic spring shown in FIG. 11;

(14) FIG. 13 is a plan view of three adjacent top nozzles; and

(15) FIG. 14 is an enlarged plan view of FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(16) The attached figures in FIGS. 3-14 show varying configurations of the bimetallic concept to attain different benefits. One embodiment of this invention is a fuel assembly 22 with a mid-grid with a bimetal protrusion spring 26 arrangement formed in an outer grid strap 24 shown in FIGS. 3-6. Any arrangement of these springs 26 could be present on each outer strap 24 (such as one in each corner as shown in the FIG. 9). A rectangular shaped spring/protrusion 28 will likely be oriented vertically to minimize concerns for fuel handling should the spring not return fully to the flat original shape or return beyond the outer strap envelope. However, the rectangular shaped spring may also be mounted horizontally and fall within this concept. The spring may also be of a circular shape 30 such as a “pop-out” disc or other configuration that result in the desired deflection and load capability as shown in FIGS. 7 and 8. The bimetal area may be a lamination attached mechanically or a coating on the base strap material. This grid spring 26 will provide the benefit of reducing or eliminating the fuel assembly gaps during operation to reduce seismic/LOCA (Loss Of Coolant Accident) impact loads, and to provide energy absorption during such accident conditions to prevent grid damage.

(17) Another embodiment disclosed herein is a bimetallic protrusion spring attached to a fuel assembly top or bottom nozzle 62 and 58, or Inconel top or bottom grid. This feature would provide alignment benefits for conditions such as damaged upper core plate fuel assembly alignment pins that have been removed. The top nozzle 62 or other host component would support itself upon all adjacent nozzles 62 (or other like-adjacent components) with this feature, ensuring alignment with the intent of reducing or eliminating penalties for removed pins. This second feature may be on all the fuel assemblies or just the ones with damaged alignment. However, preferably it is on the fuel assemblies with damaged pins and the adjacent fuel assemblies have recesses 32 in which the springs can seat. Sample calculations for many of the arrangements are also available showing significant load capability can be attained depending on the feature geometry. Thus, this invention employs bimetallic features to provide either fuel assembly alignment benefits or improved fuel assembly response during seismic/LOCA accident conditions.

(18) More specifically, one such feature is the top nozzle alignment spring shown in FIGS. 10-14. This bimetallic spring attached to a fuel assembly top nozzle would provide alignment benefits for conditions such as upper core plate fuel assembly alignment pins that have been damaged and removed. The top nozzle would support itself upon all adjacent nozzles with this feature, ensuring alignment with the intent of reducing or eliminating cut pin penalties that such plants must impose. This spring may be of varying shapes such as a rectangular beam 28 or circular disc 30 as shown in the figures. It may be attached with varying methods such as brazing, riveting, or welding. The spring may be fabricated from various materials or alloys, but will most likely be an INVAR™/Stainless combination to attain the desired deflection and load capability while facilitating attachment to the nozzle or other host component. INVAR™ is generally known as FeNi36 or 64FeNi.

(19) Another concept feature is a typical outer grid strap 24 containing a bimetal material spring feature 26 as shown in FIGS. 3-11. The outer strap bimetal features would extend outside the nominal grid envelope at operating temperature due to the high expansion side of the material to reduce or eliminate the gap between fuel assemblies, allowing for support between adjacent assemblies. This support between adjacent fuel assemblies during operation would result in lower fuel assembly loads during Seismic/LOCA events. The features may be of long rectangular beam designs similar to grid springs, “pop out” disc shaped designs shown in FIGS. 7 and 8, or some other unique design shape needed to attain the desired deflection and load capability. The bimetallic spring features may only be needed at one or two mid-grid locations near the axial center of the fuel assembly. This would reduce any neutronic penalty due to the material used. The bimetal combination could consist of many materials, but for the purposes of doing sample calculations to determine the feasibility of the concepts, a bimetal laminate of INVAR™ as the low expansion material and a stainless steel variant as the high expansion material was considered. The bimetal feature may also be obtained by coating the base strap material with a low-to negative coefficient of thermal expansion material. The basic feature could also be joined to the grid similar to the top nozzle feature described above.

(20) Significant load capability can be designed into the features for Seismic/LOCA load absorption. This feature can also be used in reverse, i.e., wherein the spring retracts at operating temperature and is in an expanded state below operating temperature to facilitate alignment of the fuel assemblies when the upper core plate is being installed.

(21) 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.