Nuclear fuel assembly grid

Abstract: Integral vanes 4 with optimized size, shape and bend angles maximize coolant mixing and fuel rod 12 heat transfer downstream. Recessed weld nuggets 5 with no vane cutout are optimized for size, strength and corrosion resistance. Staggered arches 20′,22′and springs 20,22 minimize turbulence and reduce grid pressure drop and promote coolant mixing. Crowned arches 20′,22′and springs 20,22 decrease scoring of fuel rods 12 and are sized to minimize turbulence and pressure drop. Minimum cutouts in unslotted section of grid strip 46,46′ give 15-20% strength increase. Intermediate weld and tapered end slots 48 give 15% strength increase. Ribbed and round dimple stiffeners 38,39 on outer strips increase buckling resistance, reduce handling damage and spreads accidental loading. Outer strips 32 are optimized for strength, handling, turbulence generation and pressure drop. They also divert enough flow to interior of fuel assembly to match thermal power distribution and eliminate fuel rod corrosion concerns.

Description: BACKGROUND OF THE INVENTION The invention described herein relates to nuclear reactor fuel assemblies and more particularly to a zircaloy fuel assembly grid designed to improve strength and reactor performance, and to be manufactured at a cost less than conventional grids. It is well known that the fuel or fissionable material for heterogeneous nuclear reactors is conventionally in the form of fuel elements or rods which are grouped together. These groupings or fuel assemblies also include rods comprising burnable poisons and hollow tubes through which control element assemblies are arranged to pass. The liquid moderator-coolant, normally water, flows upwardly through the reactor core in channels or longitudinal passageways formed between the members, including support grids that comprise the core. One of the operating limitations on current reactors is established by the 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 and coolant velocity. 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". Grids to maintain fuel element spacing are generally of "egg-crate" configuration and are spaced along the fuel assembly to provide support for the fuel rods, 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 components of the grid, which include the strips, rod support features and welds, must be streamlined to reduce grid pressure drop and maximize grid load carrying strength. 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 planes; 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 of the prior art have usually been fabricated substantially or entirely of Inconel or a zirconium-tin alloy, i.e., zircaloy. An Inconel grid has the advantage of greater strength because of better material characteristics and because the brazing process bonds the intersection of the strips along its entire length. Brazing also has the advantage of providing little or no obstruction to flow. Due to the increased strength, the strip thickness of an Inconel grid can be reduced relative to the zircaloy grid to reduce pressure drop and turbulence in the vicinity of the vanes. The use of annealed zircaloy has been directed by its desirable combination of mechanical strength, workability, and low neutron capture cross-section. The most important of these characteristics is its low neutron capture cross-section which makes the nuclear fission more efficient, thus making the nuclear reactor operate more economically. However, to achieve a strength equivalent to that of an Inconel grid, the strip thickness for a zircaloy grid must be increased, thus creating more turbulence and higher pressure drop. The increased strip thickness for zircaloy grids of the prior art increase grid pressure drop and reduce the effectiveness of the DNB performance of a zircaloy grid relative to an Inconel grid design. SUMMARY OF THE INVENTION The present invention overcomes the above briefly discussed deficiencies and disadvantages of the prior art by enhancing the strength and mixing ability of the grid. Grids employing the present invention have a measurable beneficial effect on reactor performance, operating cost and efficiency when compared to the prior art. In accordance with the invention, the crush strength of a zircaloy reactor fuel assembly spacer grid is increased. This increase is principally attributable to novel grid strips. Interior orthogonal strips are designed to limit cutouts in the unslotted section of the strip. This is accomplished by the use of small integral cantilevered springs, designed to laterally impress a controlled resistive force on each fuel rod. The spring's size allows it to be located in the slotted half-section of the interior grid strip. The design maintains a load path through the unslotted half-section of the interior strip which is much larger than in the prior art and thus leads to a much higher strength grid as compared to grids of equal size of the prior art. The load path in the unslotted half-section of the interior strips of the novel grid is in contrast to the load path of FR-A-2 377,077 (Figs. 6 and 7). See also a more typical strip in EP-A-0 146,896 (Fig. 7) where each strip is symmetrical with respect to the cutouts and springs and therefore not possessing a strip with a load-path along a half-section unbroken by slots and integral spring cutouts. FR-A-2 302,570 is an example of the use of separate, non-integral, lantern spring assemblies (20) which accordingly, do not present the same weakened strip area problem due to integral spring cut-outs. Such separate springs, which attach to strip tabs (18) present other problems, however, because they are not integral with the strip as in the present invention. In the present invention, integral grid support features, i.e. support arches and springs, have been positioned in a staggered manner so that turbulence produced is minimized, thus obtaining even better performance from the integral flow deflector vanes. The staggered positioning of grid support features also reduces grid pressure drop and promotes coolant mixing by staggering the flow through a grid cell. DNB tests performed for the stronger and therefore thinner zircaloy grid design indicate these unique features significantly increase DNB performance. These comparisons verified that the zircaloy grid design permitted improvements in performance relative to the Inconel design. As described previously, grids are formed of "egg-crate" construction by zircaloy strips which form multiple cells or sectors, each sector having springs on two adjacent walls and a pair of projections or arches on each of the other two walls forming a sector. The springs laterally impress controlled resistive forces on each fuel rod in the assembly. Although this fuel assembly design performs exceptionally well in a nuclear reactor, one disadvantage inherent in the design is that the inwardly projecting springs and arches occasionally mark or score the surface of fuel rods during the time they are being pulled or pushed into the fuel assembly grids. In carrying out this fuel rod loading operation, the grids are held immovably in position while a longitudinal steel rod attached to the end of a fuel rod push or pull it axially through the aligned openings or sectors in the grids. As the rod engages the springs and arches in the grid sectors, their edges engage the exposed surface of the moving fuel rod and, in some cases, score its surface sufficiently deep so as to cause the rod to fall outside established fuel rod surface specifications. To eliminate this problem, arches and springs have been designed with a crown. The crown's size has again been optimized with respect to flow blockage to minimize turbulence and pressure drop and the scorability of the rod. The number of spacer grids employed in a single fuel assembly will be minimized, to an extent commensurate with structural requirements, in the interest of enhancing reactor operating efficiency. While possessing adequate resistance to buckling under normal operating conditions, laboratory tests have shown that prior art zircaloy spacer grids may not have the mechanical strength required to absorb severe lateral stresses as might be encountered as a result of high seismic loading. Higher strength grids are required in plants whose locations are in areas of high seismic activity. While the strength of reactor fuel assembly spacer grids could be increased by the use therein of metals having a greater stiffness than annealed zircaloy, most of such higher strength materials are also characterized by higher neutron capture cross-section when compared to zircaloy and a principal objective in the design of a fuel assembly for a nuclear reactor is to maximize operating efficiency by minimizing neutron capture. To maintain a zircaloy grid strip material while obtaining the required strength, the unslotted section of the grid strip was provided with minimum cutouts. This was accomplished by the use of a small cantilevered spring, designed to laterally impress a controlled resistive force on each fuel rod. The spring's size allowed it to be located in the slotted section of the grid strip. The design maintains a continuous load path of unslotted material which is much larger than load paths of previous art and thus has a much higher strength as compared to grids of equal thickness and height. Tests have been performed which support this claim and have shown an increase in strength of 15-20% over grids of the prior art. It will occur to those skilled in the art that the grid strip thickness can be reduced by virtue of the increased strength provided by the staggered support configuration and the stronger interior strip design. This, in turn, reduces the pressure loss experienced by the coolant in flowing through the reactor core. Having in mind the above and other objects that will be evident from an understanding of this disclosure, the present invention comprises the combinations and arrangements and method as illustrated and disclosed in the presently preferred embodiment of the invention which is hereinafter set forth. BRIEF DESCRIPTION OF THE DRAWINGS While the foregoing discussion identifies problems presently existing in the prior art together with a general description of how they may be overcome, it is believed the following disclosure of the preferred embodiment of the invention will be better understood by those skilled in the art by reference to the accompanying drawings wherein: 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 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 in a set position.; Fig. 3 is a fragmentary side-elevational view of upper grid interior strips which, when assembled with lower interior strips form the interior of the grid of this invention; Fig. 4 is a fragmentary side-elevational view of lower grid interior strips which when assembled with upper interior strips of Fig. 3 form the interior of the grid of this invention; and Fig. 5 is a partial isometric view of an internal portion of an improved spacer grid for the fuel assembly of Fig. 1, which also illustrates the staggering of the fuel rod support features. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows a nuclear reactor fuel assembly 10 comprising an array of fuel rods 12 held in spaced relationship with each other by 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 serving 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 melting through of the cladding. In the 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 improved design depicted in Figs. 2-5. It is to be noted that due to differential expansions which will not be described herein, when the reactor is running hot, the spacing between the fuel assemblies will be larger than under cold conditions. The aforementioned spacing includes the clearance left between fuel assemblies to accommodate thermal and irradiation induced growth. Under a seismic load, the fuel assemblies could, with this spacing, move about and impact against each other and against the walls of the core shroud. Such impacts could, if sufficiently strong, cause the permanent distortion of the fuel assembly spacer grids of the prior art, could also cause bending of the guide tubes and could result in damage to the cladding of individual fuel assemblies through varying the coolant flow characteristics of the fuel assembly or otherwise. Additionally, seismically induced stresses could exceed the elastic limit of the integral grid assembly springs of the prior art fuel assemblies. With reference now jointly to Figs. 2-5, each of the zircaloy spacer grids 14 support and align the fuel rods 12 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 20' and 22' in each sector or cell of the grid. The stop members 20' and 22' will customarily be provided in pairs with the individual stops 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 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 in perimeter strip 32 or are butt welded to the perimeter strip. The perimeter strip 32 is also stamped so as to form, in alternate sectors 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 member to support and align a fuel rod of the outer row of the fuel assembly. Interior strips 46,46' are provided with large unslotted sections which are kept free of large windows or cutouts 33 and only contain one arch 20' or 22' in the section as can best be seen in Figs. 3 and 4. The wide unslotted section with a minimum of windows or cutouts 33 provides a larger load path than grids of prior art, which increases the resistance to bending of the strip and thus the strength of the grid. 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 bending the strip and thus the strength of the grid. Tests have shown that the spacer grid of Figs. 2-5 exhibits an improvement in impact strength and this increase in impact strength has been achieved with essentially no degradation in performance, perturbation in enrichment, or added resistance to coolant flow, i.e., no increase in pressure drop across the fuel assembly and with little change in the cost of fabrication of the grid. 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 their grids 14 during the time the fuel assembly is being moved or transferred from one location to another. The arches are impressed in the strips 46,46' and stiffening dimples 38,39 may be impressed in the peripheral strip 32 during the strip punching and stamping operation. After the appropriate grid strips 46,46' and 32 are assembled into the form of a grid 14, the arches project into each sector, except the sectors having control rod guide tubes 16, from two adjacent walls as shown in Figs. 2 and 5. As shown in Fig. 5, 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 coolant mixing vane is desired, there is provided a solid mixing vane 28 containing a longitudinally disposed slot 48. These mixing vanes are disposed so as to provide the desired directional flow of the fluid coolant. The placement of the weld 26, shielded by the vane 28, substantially eliminates flow separation on the downstream side of the mixing vane. This results in an improvement of the vane's fluid mixing capability between subchannels and rod heat transfer ability downstream of the vanes. The intersecting joints are formed in the usual manner by providing strips 46,46' with complementary slots 48 which are oriented as shown in Figs. 3 and 4 for engagement. Slots 48 may be tapered at their ends if intermediate welds are required for additional grid strength. It will be appreciated from the foregoing description that a novel and improved nuclear fuel grid spacer 14 for a nuclear fuel reactor has been disclosed and enjoys significant advantages over conventional spacers as discussed heretofore.

Claims: A fuel element grid (14) including integral springs (20,22) for supporting a plurality of nuclear fuel elements (12) intermediate their ends in spaced relation for fluid flow therebetween, said grid including a polygonal perimeter strip (32) and a first and second plurality of half-slotted (48) interlocking interior divider strip members (46,46') arranged in a transverse intersecting egg-crate fashion to define a plurality of fuel element cells, said grid characterized in that: the fuel element support grid (14) has each integral element support spring (20,22) and defining cutout (33) located in the slotted (48) half of the divider strip members thereby making the material that is parallel and makes up the other half of the divider strip member unbroken by said half-slots (48) and integral spring cutouts (33) to provide greater strength. The grid of claim 1 in which the fuel element support grid (14) includes integral support arches (20',22') or dimples located adjacent to the support springs (20,22) in the slotted half of each divider strip member. The grid of claim 1 in which the transverse slotted fuel element support grid interior divider strip members of the second plurality (46') intersecting the first plurality (46) of half-slotted divider strip members and extending into a given cell do not have their support springs (22) co-planar with the support springs (20) of the first plurality of slotted strips. The grid of claim 2 in which the transverse slotted fuel element support grid interior divider strip members of the second plurality (46') intersecting the first plurality (46) of slotted divider strip members and extending into a given cell do not have their support arches (22') co-planar with the support arches (20') of the first plurality of slotted strips.

Inventors: BRYAN WILLIAM JAYE, CORSETTI LAWRENCE VICTOR, HATFIELD STEPHEN CHARLES, JOFFRE PAUL FELIX, KAROUTAS ZESES EVANGELOS, PEROTTI PATRICK ANDREW, BRYAN, WILLIAM JAYE, CORSETTI, LAWRENCE VICTOR, HATFIELD, STEPHEN CHARLES, JOFFRE, PAUL FELIX, KAROUTAS, ZESES EVANGELOS, PEROTTI, PATRICK ANDREW
Applicants: COMBUSTION ENG, COMBUSTION ENGINEERING, INC.
Application year: 1988
Main classification: G21C3
IPC codes: G21C3