WO1994008341A1 - Fuel assembly which will increase its pressure drop over its operating life - Google Patents

Abstract

A nuclear reactor fuel assembly having flow modulators in the form of mixing vanes (28) and/or flow deflectors. The flow modulators are formed in substantially planar intersecting grid strips and have portions (28a) which have been cold-worked by shot peening on one side. Under neutron flux, the cold-worked portion (28a) preferentially expands forcing a bending of the flow modulator with increased reactor life. This concurrently causes an increase in pressure drop across the fuel assembly over the fuel assembly life.

Description

  
 



   FUEL   ASSEMBLY    WHICH WILL INCREASE   IT8      PRESSURE    DROP
 OVER   IT8    OPERATING LIFE
FIELD OF THE INVENTION
 The invention described herein relates to nuclear reactor fuel assemblies and more particularly to zirconium alloy fuel assembly grids making use of mixing vanes or other flow deflecting devices that display an increase in their ability to block flow over the lifetime of the fuel assembly, causing an increase in fuel assembly pressure drop.



  BACKGROUND OF THE INVENTION
 It is well known that the fuel or fissionable material for heterogeneous nuclear reactors is conventionally provided in the form of fuel elements or rods which are grouped together. These groupings, or fuel assembles, also include rods comprising burnable poisons and hollow tubes through which control element assemblies are arranged to pass. In nuclear reactors energy is transferred from the fuel by use of liquid coolant. As the liquid flows through the core, its temperature increases. In the case of boiling water reactors ("BWRs") it leaves the reactor as steam. In the case of pressurized water reactors ("PWRs") it leaves as a liquid approximately 600F hotter than it entered. The liquid coolant, normally water, flows upwardly through the reactor core in channels or longitudinal passageways formed between the members that comprise the core.

  One of the operating limitations on current reactors is established by onset of film boiling on the surfaces of the fuel elements. The phenomenon is commonly referred to as departure from nucleate boiling ("DNB") and is affected by the fuel element spacing, system pressure, heat flux, coolant enthalpy, coolant velocity, and so forth. When DNB occurs, there is a rapid rise in the  temperature of the fuel element due to the reduced heat transfer which can ultimately result in failure of the element. Therefore, in order to maintain a factor of safety, nuclear reactors must be operated at a heat flux level somewhat lower than that at which DNB occurs. This margin is commonly referred to as the "thermal margin".



   Nuclear reactors normally have some regions in the core which have a higher neutron flux and power density than other regions. This situation may be caused by a number of factors, one of which is the presence of control rod channels in the core. When the control rods are withdrawn, these channels are filled with moderator which increases the local moderating capacity and thereby increases the power generated in the fuel. In these regions of high power density, known as "hot channels", there is a higher rate of enthalpy rise than in other channels. It is such hot channels that set the maximum operating conditions for the reactor and limit the amount of power that can be generated, since it is in these channels that the critical thermal margin is first reached.



   Attempts have been made in the past to solve these problems and increase DNB performance by providing the support grid structure, which is employed to contain the members of the fuel assembly, with integral flow deflector vanes. These vanes can improve performance by increasing coolant mixing and rod heat transfer ability downstream of the vanes. These attempts to improve performance have met with varying success, depending on the vane design and the design of other grid components which can impact the effectiveness of vanes. For example, to maximize the benefit of the vanes, the size, shape, bend angle, and location of the vanes are generally optimized. The vanes are especially beneficial adjacent to the aforementioned hot channels.

  Moreover, the remaining components of the grid which include the  strips, rod support features and welds, should be streamlined to reduce the turbulence generated in the vicinity of the vanes. Further constraints on designing the grids include considerations such as grid pressure drop and grid load carrying strength.



   Grids generally comprise a first and second plurality of half-slotted strips in "egg-crate" configuration and are spaced along the fuel assembly to provide support for the fuel rods, maintain fuel rod spacing, promote mixing of coolant, provide lateral support and positioning for control assembly guide tubes, and provide lateral support and positioning for an instrumentation tube. The grid assembly usually consists of individual strips that interlock to form a lattice.



  The resulting square cells provide support for the fuel rods in two perpendicular plains. In general, each plane has three support points: two support arches and one spring. The springs and arches are stamped and formed in the grid strip and thus are integral parts of the grid assembly. The springs exert a controlled force, preset so as to optimally maintain the spring force on the fuel rod over the operating life of the fuel assembly.



   Fuel assemblies employing spacer grids with flow deflector vanes are conventionally fabricated substantially or entirely of Inconel or a zirconium-tin alloy, i.e., zircaloy. Problems associated with conventional zircaloy grids have been eliminated or reduced by the nuclear fuel assembly grid proposed in
U.S. Patent No. 4,879,090 to Patrick et al., the disclosure of which is incorporated herein by reference.



   Most commercial reactors operate using a 3 cycle method, whereby a third of the fuel assemblies in the reactor core are replaced every one to two years.



  The newer fuel assemblies generate more neutrons thus producing more heat and greater energy. Presently,  burnable poisons are used to moderate the production of neutrons in the newer fuel assemblies. This is done to more evenly burn the fuel through its 3 cycles of operation. However, using the technique, the higher production of neutrons from the new fuel assemblies is not used to provide additional power output. Thus, for optimal fuel efficiency coolant flow should initially be provided at maximum levels to allow more energy to be transferred in the early stages of the fuel assembly life, but the levels of coolant flow required should diminish as the production of neutrons decreases with fuel assembly age.



     SUMMARY    OF THE INVENTION
 In view of the foregoing, it is apparent that prior art fuel assemblies are not designed to provide a change in coolant flow with fuel assembly age. One advantage of the present invention is the ability to allow greater coolant flow to pass through new fuel assemblies, resulting in greater energy transfer and increased reactor efficiency.



   These and other advantages have been achieved by the construction of a fuel assembly having mixing vanes or other flow deflecting devices which, upon neutron irradiation, bend to a new configuration, resulting in increased fuel assembly pressure drop and thus decreased coolant flow. For example, a support grid for a nuclear reactor fuel assembly is proposed which comprises a plurality of substantially planar intersecting straps forming openings for receiving, supporting and spacing a plurality of nuclear fuel and rods. A plurality of substantially planar flow modulators are formed in the straps.

  Each of the modulator faces have a cold-worked portion covering a preselected area of the flow modulator such that each flow modulator becomes increasingly biased out of  alignment with the strap and into the opening with increased fuel assembly burn-up, due to the preferential growth of the cold-worked portion under neutral flux.



  This results in an increase in pressure drop across the fuel assembly. The flow modulators can comprise, for example, mixing vanes and/or flow deflectors.



   Upon study of the specification and appended claims, further advantages of this invention will become apparent to those skilled in the art.



  BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is an elevational view, partly in section, of a fuel assembly illustrating the relationship of the grids of the invention to other components in the fuel full assembly;
 FIG. 2 is a fragmentary plan view of an improved spacer grid for the fuel assembly of FIG. 1, illustrating the relationship of springs, dimples, arches, and strips used for holding fuel rods and guide thimbles   inua    set position (alternate bend configurations are used for vanes adjacent to a guide thimbles);
 FIG. 3 is a partial side elevation view of the perimeter strip of the spacer grid of FIG. 2;
 FIG. 4 is a cross-sectional side-elevational view taken along line 4-4 of FIG. 3;

  ;
 FIG. 5 is a fragmentary side-elevational view of one of the upper grid interior strips which, when assembled with lower interior strips FIG. 6 form the interior of the spacer grid of FIG.2;
 FIG. 6 is a fragmentary side-elevational view of one of the lower grid interior strips which when assembled with upper interior strips of FIG. 5 form the interior of the spacer grid FIG.2;
 FIG. 7 is a partial isometric view of an interior section of the spacer grid of FIG. 2, which also  illustrates the staggering of the fuel rod support features of FIGS. 5 and 6;
 FIG. 8 is a cross-sectional view of a mixing vane of the present invention.



   FIG. 9 is a fragmentary side-elevational view of one of the lower grid interior strips which is similar to that shown in FIG. 6 and which contains flow deflectors according to an embodiment of the present invention.



   FIG. 10 is a fragmentary plan view of the lower grid interior strip of FIG. 9.



   FIG. 11 is a fragmentary side-elevational view of one of the upper grid interior strips which is similar to that shown in FIG. 5 and which contains flow deflectors according to an embodiment of the present invention.



   FIG. 12 is a fragmentary side-elevational view of a lower grid interior strip according to an embodiment of the present invention which is similar to that shown in FIG. 6 and which has both conventional mixing vanes and flow deflectors with a cold-work portion.



   FIG. 13 is a fragmentary plan view of the lower grid interior strip of FIG. 12 at the beginning of fuel assembly life.



   FIG. 14 is a fragmentary plan view of the lower grid interior strip of FIG. 12 in the middle of fuel assembly life.



   FIG. 15 is a fragmentary plan view of the lower grid interior strip of FIG. 12 at the end of fuel assembly life.



  DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 shows a nuclear reactor fuel assembly 10 comprising an array of fuel rods 12 held in spaced relationship with each other by means of grids 14 spaced along the fuel assembly length.  



   Fuel assembly 10 includes, extending longitudinally therethrough, guide tubes 16. Control rods 50, in the form of neutron absorber elements, move within guide tubes 16; such control rods serve as a means for regulating the thermal output power of the reactor.



  The fuel assembly also includes a plurality of fuel rods 12. Each fuel rod 12 comprises a hermetically sealed elongated tube, known in the art as the cladding, which contains a fissionable fuel material, such as uranium, in the form of pellets. As may best be seen from FIG. 1, the individual fuel rods 12 are supported in the fuel assembly by means of a plurality of spacer grids 14, such that an upwardly flowing liquid coolant may pass along the fuel rods, thus preventing overheating and possible failure of the cladding. In a manner well-known in the art, the coolant, after passing through the reactor core and being heated through contact with the fuel rods, will be delivered to a heat exchanger and the heat extracted from the circulating coolant will be employed to generate steam for driving a turbine.



   As noted, and as may be seen from FIG. 1, the positioning and retention of the fuel rods in fuel assembly 10 is accomplished through use of a plurality of the spacer grids 14. All or several of the spacer grids 14 may be of the design depicted in FIGS. 2-7.



   With reference now jointly to FIGS. 2-7, each of the zircaloy spacer grids 14 support and space the fuel rods 12 within openings 25 through the establishment of six points of contact therewith. Thus, as depicted in
FIG. 2, each of the fuel rods 12 is contacted by a pair of generally transversely oriented springs 20, 22 which respectively urge the fuel rod against oppositely disposed stop members or arches 20' and 22' in each sector or cell of the grid. The arches 20' and 22' will customarily be provided in pairs with the individuals of  each pair being respectively vertically above and below a plane through the point of contact of the springs 20, 22 with the cladding of fuel rod 12. Thus, considering fuel rod 12, this element is urged by means of springs 20 and 22 against pairs of arches 20' and 22' formed respectively on upper strip members 46 and lower strip members 46'.



   The spacer grid 14 is assembled by interweaving of the internal strip members 46 and 46'. The ends of the strip members 46, 46' may be engaged in slots 30 provided in the spacer grid perimeter strip 32. Welds are formed at all points of the intersection within the spacer grid and the ends of the strip members 46, 46' are either welded into the slots 30 in perimeter strip 32 or are butt welded to the perimeter strip 32.



   Referring now to FIG. 3, the perimeter strip 32 is provided with cutouts or "windows" 34 in regions corresponding to alternate sectors of the outer row of the spacer grid. These "windows" enhance grid strength while maintaining coolant flow to the outer row of fuel rods. The perimeter strip 32 is also stamped so as to form, in sectors which alternate with the sectors provided with "windows" 34, inwardly extending integral springs 36. As may be seen from FIG. 2 by reference to fuel rod 12, each of the springs 36 cooperates with an internal spring 36' on a grid internal strip to support and align a fuel rod 12 of the outer row of the fuel assembly 10.



   The perimeter strip 32 may also be provided, above and below each of the windows 34, with inwardly extending dimples 38. Dimples 38, in the manner known in the art, enhance the rigidity of perimeter strip 32.



  Restated, the presence of dimples 38 increases the resistance of perimeter strip 32 to bending in response to a force component directed along its length.



  Additionally, as may be seen in the case of fuel rod 12,  dimples 38 function as stops or arches against which the fuel rod 12   will bye    urged by the internal springs 20 and 22, integral with the strip members 46 and 46'. The dimples 37 must be provided above and below each of the perimeter strip "windows" 34. Additional parts of dimples 39 may also be provided in sectors of the perimeter strip 32 which have the integral springs 36 formed therein. In the interest of facilitating understanding of the drawing, the dimples 39 have not been shown in FIG. 1. When employed, dimples 39 will not extend into the fuel assembly sector as far as the fuel rod contacting springs 36.

  The dimples 39 will thus function as backup arches to prevent the elastic limit of springs 36 from being exceeded should the fuel assembly be subjected to vibration in excess of that encountered during normal operation. The pairs of dimples 39, if provided, will also enhance the rigidity of perimeter strip 32.



   The perimeter strip 32 is also provided, above and below each of the windows 34, with an inwardly ridged horizontal rib 40, as may best been seen from joint consideration of FIGS 3 and 4. Ribs 40, enhance the rigidity of the perimeter strip 32. The presence of the ribs 40 increases the section modulus of the perimeter strip 32 and results in increased resistance to bending compared to a flat non-ribbed perimeter strip or a strip provided with a plurality of irregularities.



   The valleys or slots defined by tabs 44 of the serrated upper and lower edges of perimeter strip 32 function as partial lead-in tabs for the fuel rods 12 which facilitate their insertion; the bases of the slots are aligned with the center of the windows 34 and springs 36 in the perimeter strip 32. The tabs 44 function as anti-hangup devices, preventing the hanging or interference between adjacent fuel assemblies during refueling.  



   Interior strips 46, 46' are provided with large unslotted sections which are kept free of large windows 34 or cutouts 33 and only contain one arch 20' or 22' in the section as can best be seen in FIGS 5 and 6.



  The wide unslotted section with a minimum of windows 34 or cutouts 33 provides a larger load path than conventional grids, which increases the resistance to bending of the strip and thus the strength of the grid 14. Strip slots 48 may also be tapered at the ends to facilitate the welding at the intermediate locations.



  The presence of intermediate welds increases the resistance to strip bending and thus the strength of the grid 14.



   As best seen in FIG. 1, in assembling a fuel assembly 10, an array of control rod guide tubes 16 having control rods 50 adapted for slidable longitudinal movement therein, are positioned to extend axially through selected sectors in the grids 14 and are thereupon welded to grid tabs or strip walls to form the fuel assembly skeleton structure. Opposite ends of the guide tubes 16 are attached to top and bottom end fittings 52 and 54 using a threaded fastener.



   Reference to the plan view of FIG. 2 illustrates the relative disposition of fuel rods 12 and guide tubes 16 and, particularly, how the fuel rods are held in a relatively immovable position in each grid.



  Each fuel rod 12 is biased by a spring 20 and 22 into engagement with arches 20' and 22' formed on the grid strip walls and, as shown, project inwardly into each sector or cell 24. This construction serves to preclude axial movement of the fuel rods 12 in the grid 14 during the time the fuel assembly is being moved or transferred from one location to another. The arches 20' and 22' are impressed in internal strips 46 and 46' and dimples may be impressed in the peripheral strip during the strip punching and stamping operation. After the appropriate  internal and peripheral grid strips 46, 46' and 32 are assembled into the form of a grid 14, the arches 20' and 22' project into each sector, except the sectors having control rod guide tubes 16, from two adjacent walls as shown in FIGS. 2 and 7.



   As shown in FIGS. 5-7 the intersecting strips 46' 46' are welded together at each junction with the weld nugget being designated 26. At each intersecting joint of the strips where a mixing vane is desired, there are provided mixing vanes 28 having a longitudinal slot 48 disposed therebetween. These mixing vanes 28 are disposed so as to provide the desired directional flow of the fluid coolant as explained heretofore. Each vane shields a small opening or window 35 which, according to the shown embodiment, is formed under the bottom end in each vane 28 and directly above and adjacent to the junction of the intersecting strips 46, 46'. While the windows 35 are shown as oval, other shapes and configurations, such as rectangular, semi-circular, square, and so forth can be employed. It is also possible to locate the window 35 and weld nugget 26 at different elevations within the grid 4.

  The function of the window 35 in each case is to create material and provide clearance for welding the strips 46, 46' together. The placement of the weld 26, shielded by the vane 28, substantially eliminates flow separation on the downstream side of the mixing vane 28. This results in an improvement of the vane's 28 fluid mixing capability between subchannels and rod heat transfer ability downstream of the vanes 28. The intersecting joints are formed in the usual manner by providing strips 46, 46' with complementary slots 48 for engagement. Slots 48 may be tapered at their ends if intermediate welds are required for additional grid strength.



   The spacer strips 46, 46' are provided with integrally formed vanes 28. The spacer strips 46' have  formed therewith in the area where the window 35 is to be provided, consumable weld tabs 60' complementary to the shape of the window. The complementary spacer strip 46 is provided at its edge with a consumable weld tab 60 similar to consumable weld tab 60', only being unslotted which comprises a continuation of the slot 48 continuation line by virtue of the orientation of the consumable weld tab directly over the slot 48. These spacer strips 46, 46', when intersected have their tabs 60, 60' similarly intersected. These tabs 60, 60' are made of a material such as zircaloy or Inconel which is consumed during the welding of the joints. The consumable weld tabs 60,60', 62,62' are dissolved to form the weld nuggets 26.

  The consumable weld tabs are integral with the intersecting strips 46, 46' which are made of the same material.



   Referring now to FIG. 8, a cross-sectioned view of a preferred embodiment of a substantially planar mixing vane 28 of the present invention is illustrated.



  As can also be seen in FIG. 7, mixing vane 28 is substantially bent at a predetermined position along its length. A portion of one face of the mixing vane 28 has been selectively subjected to a shot-peening process to provide a cold-worked vane portion 28a.



   When compared to conventional designs, the grid spacer strips 46, 46' of the present invention would be formed in the usual manner except that the vanes 28 would not be bent as far out of the plane of the spacer strips 46, 46'. Prior to assembly, the strips 46, 46' would be masked so as to expose only those portions of the strip surfaces associated with the convex face of the mixing vane 28 which is formed when the mixing vane 28 is bent.



  The strips 46, 46' would then be peened to a sufficient intensity, using an appropriately sized shot, so as to cold work the material down to approximately one-half of its thickness. This will cause the curvature at the base  of each vane 28 to increase so'that the vanes 28 protrude from the plane of the strips 46, 46'. This final protrusion will verify that the intensity and coverage of the peening was correct. After removal of the mask, the strips 46, 46' would be assembled into spacer grids 14 in the usual manner.



   During reactor operation, the irradiationinduced growth of this cold-worked layer will be greater than that of the underlying material. Because the base of the vane 28 has been peened on its convex side, the differential growth will increase the curvature of the vane. This will cause the vane 28 to move toward the fuel rod 12 increasing the pressure drop across the fuel assembly 10. The amount that the vane 28 moves toward the fuel rod 12 will be proportional to the amount of neutron fluence it has received. In other words, fuel assembly pressure drop will increase with assembly burnup.



   Calculations performed assuming that half of the strip thickness has been cold worked and that the cold-worked material would grow an additional one percent during its life suggest that a 17x17 grid vane 28 like that shown in FIG. 7 will deflect toward the fuel 12 rod by approximately 0.010" during the fuel life time.



   By increasing the pressure drop in fuel assemblies 10 that have been subjected to neutron flux, increased coolant flow will be diverted to newer, less burned assemblies 10, thus increasing the reactor's efficiency.



   Increased pressure drop across the fuel assembly 10 with assembly burn-up could also be established using other mechanisms. For example, FIGS. 9 and 11 illustrate upper and lower grid interior strips 46, 46' wherein flow deflectors 29 have been provided according to an embodiment of the present invention. All numerals in FIGS. 9 and 11 which are the same as those in  
FIGS. 5 and 6 correspond to substantially similar features. As with the vanes 28 discussed above, the flow deflectors 29 can be provided with a cold-work portion in the region where a convex surface is formed when the flow deflector 29 is bent to the desired configuration.

  Such a feature will result in a flow deflector 29 which will increase in deflection with an increase in fuel assembly burn-up, causing the flow deflector 29 to bend further out of the plane of the upper and lower grid interior strips 46, 46' and thus bend toward the fuel rod 12.



  FIG. 10 is a plan view of the lower grid interior strip 46' shown in FIG. 9 and illustrates the manner in which the arch is 22' and the flow deflectors 29 deviate from the overall plane of the lower interior grid strip 46' at a given instant in time in the life of fuel assembly 10.



   FIG. 12 illustrates a portion of a lower interior grid strip 46' with both conventional mixing vanes 28 and with flow deflectors 29 having cold-work portions according to an embodiment of the present invention. FIGS. 13-15 represent plan views of the lower interior grid strip 46' shown in FIG. 12 at different stages in the life of fuel assembly 10. As shown in FIG.



  13, at the beginning of the life of fuel assembly 10, the arches 22' and mixing vanes 28 are interposed in respective positions out of the plane of the lower interior grid strip 46'. The flow deflectors 29, however, cannot be seen at this stage because they remain substantially within the overall plane of the lower interior grid strip 46'. Referring now to FIGS. 14 and 15, at the middle and end of the life of fuel assembly 10, the arches 22' and mixing vanes 28 are in essentially the same position as they were at the beginning of the life of fuel assembly 10. The flow deflectors 29, however, extend further and further from the overall plane of the lower interior grid strip 46' as the fuel  assembly 10 ages, resulting in a concurrent increase in pressure drop across the fuel assembly 10 with time.



   Flow deflectors 29 like those described above would also be useful for wavy strip grids where vanes are not part of the grid design. Specially designed flow deflectors would thus be pierced into the wavy grid strip as seen above. Also as discussed above, the base of the flow deflectors 29 would be shot-peened in a flat state.



   The differential growth with reactor operation would cause curvature at the base of deflector, thus increasing the fuel assembly pressure drop.



   In addition to cold-working the zircaloy strip, the texture of the strip can be changed on a side by melting the strip through half of its thickness. This in turn will result in growth differences in the strip.



  Also, alloying differences can be produced by heating one side of the strip. Different alloys will grow at different rates and will also result in bending of the unrestrained strip.



   From the forgoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and without departing from the spirit and scope thereof can make various changes and modifications of the invention to adapt at the various usages and conditions. 

Claims

IN THE CLAIMS

1. A spacer grid for use in a nuclear reactor fuel assembly comprising: openings for receiving, supporting and spacing a plurality of nuclear fuel rods; and a plurality of substantially planar flow modulators, one face of each of said flow modulators having a cold-worked portion covering a preselected portion of said face such that said flow moderators are increasingly biased into said openings with increasing fuel assembly burn-up which results in a concurrent increase in pressure drop across said fuel assembly due to the preferential growth of said cold-worked portion under neutron flux.

2. The spacer grid of claim 1 wherein said flow modulator comprise mixing vanes.

3. The spacer grid of claim 1 wherein said flow modulators comprise flow deflectors.

4. The spacer grid of claim 1 wherein said flow modulators comprise both mixing vanes and flow deflectors.

5. The spacer grid of claim 1 wherein said cold-worked portion of said flow modulators extend to a depth which is approximately one-half the overall thickness of said flow modulators.

6. The spacer grid of claim 1 wherein said cold-worked portion is formed by masking said flow modulators so as to expose only said preselected portion and subjecting remaining unmasked portions to shot peening.

7. A support grid for use in a nuclear reactor fuel assembly comprising: a plurality of substantially planar intersecting straps forming openings for receiving, supporting and spacing a plurality of nuclear fuel and rods; a plurality of substantially planar flow modulators formed in said straps, one face of each of said modulators having a cold-worked portion covering a preselected portion of said face such that said flow modulators becomes increasingly biased out of alignment with said straps and into said openings with increased fuel assembly burn-up which results in an increase in pressure drop across said fuel assembly due to the preferential growth of said cold-worked portions under neutral flux.

8. The spacer grid of claim 7 wherein said flow odulators comprise mixing vanes.

9. The spacer grid of claim 7 wherein said flow modulators comprise flow deflectors.

10. The spacer grid of claim 7 wherein said flow modulators comprise both mixing vanes and flow deflectors.

11. The spacer grid of claim 7 wherein said cold-worked portion of said flow modulators extend to a depth which is approximately one-half the overall thickness of said flow modulators.

12. The spacer grid of claim 7 wherein said cold-worked portion is formed by masking said flow modulators so as to expose only said preselected portion and subjecting remaining unmasked portions to shot peening.