Patent Number: 
Section: description

Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. FIG. 5 is a perspective view of a side-slotted nozzle type double sheet spacer grid for nuclear fuel assemblies having a 5xc3x975 array in accordance with a primary embodiment of the present invention. FIG. 6 is a perspective view of the spacer grid of FIG. 5, with the perimeter strips removed from the periphery of the grid. In the spacer grid of FIGS. 5 and 6, one fuel rod is shown for ease of description. As shown in the drawings, the side-slotted nozzle type double sheet spacer grid 310 according to the primary embodiment of the present invention receives and supports a plurality of elongated nuclear fuel rods 325 in a nuclear fuel assembly, and comprises a plurality of double sheet inner strips 316 and four double sheet perimeter strips 320. The inner strips 316 intersect each other at right angles in accordance with a designed array, and are encircled with the four perimeter strips 320, thus forming an egg-crate pattern with a plurality of four-walled cells for receiving and supporting the elongated nuclear fuel rods 325. Each of the inner strips 316 comprises a plurality of unit strip parts, each being fabricated by integrating two different unit sheet parts, that is, first and second unit sheet parts 361 and 362, together into a single structure, such that the two unit sheet parts 361 and 362 face each other and a nozzle type coolant channel with one inlet and at least one outlet is defined between the two unit sheet parts 361 and 362. Each of the four perimeter strips 320 is fabricated by integrating an inner thin sheet comprising the unit sheet parts 362 with a flat outer thin sheet 410 having a width corresponding to the width of the inner thin sheet into a single structure. The inner strips 316 each have a plurality of notches (not shown) at the junctions of the unit strip parts such that each notch vertically extends downward or upward from upper or lower edge of each inner strip 316. The inner strips 316 are assembled with each other by intersecting together at right angles at the notches, and are welded together at intersections thereof. In addition, the junctions of the intersecting inner strips 316 and the four perimeter strips 320 are welded at the outside of the perimeter strips 320 through a seaming process, thus producing a desired double sheet spacer grid with the four-walled cells for the nuclear fuel rods 325. In the present invention, the welding process for the intersections of the inner strips 316 may be performed through a conventional welding process, such as a TIG welding process, an electron beam welding process or a laser beam welding process. The thin sheets of each of the inner and perimeter strips 316 and 320 are continuously welded together at the junctions thereof, thus enhancing the buckling strength of the spacer grid and allowing the spacer grid to more effectively resist laterally directed force acting on the spacer grid. When two unit sheet parts 361 and 362 are integrated into a unit strip part, a nozzle type coolant channel with one inlet and at least one outlet is formed between the two unit sheet parts 361 and 362. In such a case, the two unit sheet parts 361 and 362 have predetermined shapes, each defining a half of the cross-sectional area of the coolant channel. The two unit sheet parts 361 and 362 of each unit strip part are designed such that a coolant channel inlet having a predetermined cross-sectional area is formed at an upstream end of the channel, that is, at the lower end of the unit strip part. The cross-sectional area of the coolant channel gradually increases to be maximized at a position around a spring at which the unit strip part comes into contact with a fuel rod. The cross-sectional area is gradually reduced after the spring to become zero at a downstream end of the channel, that is, at the upper end of the unit strip part. Therefore, the upper edges of the two unit sheet parts 361 and 362 are in contact with each other when the two sheet parts 361 and 362 are integrated into a unit strip part. The coolant channel of each unit strip part has one or more outlets 355 formed by cutting an upper portion of the second part 362 of the two unit sheet parts 361 and 362. For example, in the spacer grid 310 of FIGS. 5 and 6, each second unit sheet part 362 has two symmetric coolant channel outlets 355, while each first unit sheet part 361 does not have any coolant channel outlet. In the spacer grid 310, each of the inner strips 316 is fabricated by integrating two thin sheets into a single structure. Each of the two thin sheets has an alternating arrangement of a plurality of first unit sheet parts 361 not having any coolant channel outlet and a plurality of second unit sheet parts 362 each having two coolant channel outlets 355. When fabricating each inner strip 316, the two thin sheets are preferably arranged such that one first unit sheet part 361 and one second unit sheet part 362 form one unit strip part. The outlets 355 of each inner strip 316 are thus alternately arranged at left and right sides as shown in FIGS. 5 and 6. Each of the two unit sheet parts 361 and 362 is provided with a slot 315 longitudinally formed on each side surface of a spring which is projected from each unit sheet part to support a fuel rod 325 within a four-walled cell. FIGS. 7 and 8 are perspective views respectively showing a first unit sheet part 361 not having any coolant channel outlet, and a second unit sheet part 362 having two symmetric coolant channel outlets 355. The shapes of the slots 315 as well as the shapes of the two unit sheet parts 361 and 362 may be easily appreciated from the above drawings. In the side-slotted nozzle type double sheet spacer grid 310 of FIGS. 5 and 6, the sheets of each double sheet inner strip 316 are fabricated by an alternating arrangement of a plurality of first unit sheet parts 361 of FIG. 7 not having any coolant channel outlet and a plurality of second unit sheet parts 362 of FIG. 8 each having the two coolant channel outlets 355. Different from conventional nozzle type double sheet spacer grids, the side-slotted nozzle type double sheet spacer grid 310 of the present invention enhances its spring performance due to the slot 315 longitudinally formed on each side surface of the springs of the unit sheet parts 361 and 362, such that the spring performance of the present spacer grid 310 almost approaches a level which is expected from equal-arm springs of a conventional single sheet spacer grid. The spacer grid 310 of the present invention thus has a high buckling strength, effectively resisting a lateral load acting on the spacer grid 310, since the spacer grid 310 supports the fuel rods 325 while surrounding them when the lateral load is applied from the fuel rods 325 to the spacer grid 310. The slot 315 of the spacer grid 310 has a width in the range from 0.35 mm to 0.8 mm, preferably in the range from 0.3 mm to 0.6 mm, and a length in the range from 12 mm to 16 mm. When the slot 315 is designed to have the above-mentioned size, it is possible for the spacer grid 310 to have the desired spring performance. FIG. 9 is a perspective view of a side-slotted nozzle type double sheet spacer grid for nuclear fuel assemblies having a 5xc3x975 array in accordance with a second embodiment of the present invention. Different from the spacer grid of FIGS. 5 and 6, the spacer grid of FIG. 9 further includes at least one mixing blade at each unit sheet part. FIGS. 10 and 11 are perspective views showing unit sheet parts 365 and 366 that constitute a double sheet strip used in the spacer grid of FIG. 9. The unit sheet part 365 of FIG. 10 is fabricated by forming both a swirl flow blade 330 and a lateral flow blade 335 at the upper end of the unit sheet part 361 of FIG. 7 not having any coolant channel outlet. The unit sheet part 366 of FIG. 11 is fabricated by forming a swirl flow blade 330 at the upper end of the unit sheet part 361 of FIG. 7. As shown in FIG. 9, the spacer grid according to the second embodiment of the present invention is designed to enhance the coolant mixing efficiency. In order to accomplish the above object, each of the double sheet strips of the spacer grid includes unit sheet parts 365 and 366 each having one or more mixing blades 330 and/or 335 at the upper end thereof. The mixing blades of the spacer grid of FIG. 9 may be selected from the swirl flow blades 330 creating a swirl flow of coolants at the intersections of the strips in the spacer grid, the lateral flow blades 335 creating a lateral flow of coolants between neighboring four-walled cells, and a combination of the swirl flow blades 330 and the lateral flow blades 335. That is, the unit sheet part of the double sheet strips according to the second embodiment may have both a swirl flow blade and a lateral flow blade as shown in FIG. 10, or have a swirl flow blade or a lateral flow blade as shown in FIG. 11, or have two swirl flow blades or two lateral flow blades (not shown). The spacer grid according to the second embodiment of the present invention may be fabricated by a combination of the unit sheet parts 365 each having a swirl flow blade 330 and a lateral flow blade 335 as shown in FIG. 10, the unit sheet parts 366 each having a swirl flow blade 330 as shown in FIG. 11, and the unit sheet parts 362 each having two coolant channel outlets 355 as shown in FIG. 8. FIGS. 12 and 13 are perspective views of perimeter strips 320 which may be used in the spacer grids according to the primary and second embodiments of the present invention. That is, the perimeter strips 320 may encircle the intersecting inner strips 310 of FIG. 6 having the unit sheet parts 361 and 362 of FIGS. 7 and 8, or encircle the intersecting inner strips 318 of FIG. 9 having the unit sheet parts 362, 365 and 366 of FIGS. 8, 10 and 11, thus producing a spacer grid of the present invention. Due to the perimeter strips 320, it is possible to fabricate a spacer grid without any difference in the fuel rod support performance between the inside four-walled cells positioned at the inner area of the spacer grid and the outside four-walled cells positioned at the outer area of the spacer grid. The perimeter strips 320 are fabricated by integrating an inner thin sheet, comprising a plurality of unit sheet parts 421 or 422 each having at least one coolant channel outlet 355 or 356, with a flat outer thin sheet 410 into a single structure. That is, the perimeter strips 320 may be fabricated using the unit sheet parts 421 each having two coolant channel outlets 355 as shown in FIG. 12, or using the unit sheet parts 422 each having one deformed coolant channel outlet 356 as shown in FIG. 13. In the spacer grid of the present invention, the cross-sectional area of each coolant channel of the perimeter strips 320 is a half of the cross-sectional area of each coolant channel of the inner strips 316 and 318, so desired symmetry of the spring performance and coolant mixing efficiency in the spacer grid is accomplished regardless of the positions of the inside and outside four-walled cells. In a side-slotted nozzle type double sheet spacer grid 310 of the present invention, the inner strips 316 or 318 and the perimeter strips 320 are each fabricated by integrating two stamped thin sheets together into a single structure. Each of the thin sheets of the inner and perimeter strips is preferably made of zircaloy, which is the alloy of tin, iron, chrome and zirconium. It is also possible to make the thin sheets of the strips using inconel, which has been typically used as a material of the grid strips in the prior art. In the spacer grid of the present invention, each of the inner and perimeter strips 316, 318 and 320 preferably has a thickness in the range from 0.25 mm to 0.40 mm, and each of the springs of the unit sheet parts 361, 362, 365 and 366 preferably has a width in the range from 7 mm to 10 mm between both side edges thereof. FIG. 14 is a graph showing displacements as a function of load for side-slotted unit sheet parts of FIGS. 7, 8, 10 and 11 according to the present invention and a unit sheet part of a conventional slotless nozzle type double sheet spacer grid not having any slot, the displacements being measured by a universal testing machine to measure the spring performances of the unit sheet parts. In the graph, the first and second slotted unit sheet parts are the side-slotted unit sheet parts of the present invention, having slots with different lengths. That is, the first slotted unit sheet part has a shorter slot, and the second slotted unit sheet part has a longer slot. In the graph, the spring performance, that is, the spring strength of each unit sheet part, is represented by the gradient of an associated displacement-load curve. The graph shows that the side-slotted unit sheet parts of the present invention have improved spring performances regardless of the shapes thereof, and the spring strength of the side-slotted unit sheet parts is lower than that of the conventional slotless unit sheet part. In accordance with the test, the spring strength of each side-slotted unit sheet part represented by the gradient of an associated displacement-load curve is linearly changed, and the elastic range of the side-slotted unit sheet part, wherein the displacement-load curve is shown as a linear curve with the same gradient, is enlarged by appropriately designing the slots 315. Therefore, it is noted that the side-slotted nozzle type double sheet spacer grid 310 of the present invention having the longitudinal slots 315 has enhanced spring performance. As described above, the present invention provides a side-slotted nozzle type double sheet spacer grid for nuclear fuel assemblies. In the spacer grid of the present invention, a plurality of inner strips intersect each other at a predetermined angle to form a plurality of four-walled cells to receive and support a plurality of fuel rods. Each of the inner strips comprises a plurality of unit strip parts, and each of the unit strip parts is fabricated by integrating two unit sheet parts together into a single structure, such that the two unit sheet parts face each other and a nozzle type coolant channel is defined between the two unit sheet parts. Due to the coolant channels of the unit strip parts, the spacer grid of the present invention effectively deflects and mixes coolants together so as to enhance the heat transfer effect between the fuel rods and the coolants. The cross-sectional area of each coolant channel gradually increases, and a portion of each unit sheet part at which the coolant channel has the maximum cross-sectional area acts as a spring at which the unit sheet part comes into contact with a fuel rod. Therefore, the present spacer grid supports a fuel rod at the four sides within a four-walled cell, so that the spacer grid more stably supports the fuel rods in comparison with conventional single sheet spacer grids which support a fuel rod by one spring within a four-walled cell and may fail to stably support a unidirectional displacement of fuel rods caused by flow-induced vibration in a fuel assembly. In the spacer grid of the present invention, each of the unit sheet parts is provided with a slot longitudinally formed on each side surface of the spring. Due to the slots, the spacer grid desirably reduces its spring strength while maintaining advantages of conventional nozzle type double sheet spacer grids. The spacer grid of the present invention thus accomplishes desired soundness of a nuclear fuel assembly. The present spacer grid desirably reduces the strength of the springs that directly support the fuel rods, and enlarges the elastic range wherein the spacer grid effectively and elastically supports the fuel rods in the fuel assembly, so that the spacer grid more stably supports the fuel rods. The spacer grid also prevents its springs from being damaged during a process of installing fuel rods in the spacer grid while producing a nuclear fuel assembly, thus effectively protecting the fuel rods from fretting wear caused by flow-induced vibrations within the fuel assembly. FIG. 14 is a graph showing the spring performance of the side-slotted nozzle type double sheet spacer grid according to the present invention, measured by a universal testing machine. In the graph, the spring performance, that is, the spring strength of the spacer grid, is represented by the gradient of a displacement-load curve. As shown in the graph, a conventional slotless nozzle type double sheet spacer grid has high spring strength, and its displacement-load curve varies with an refraction point when the displacement exceeds a reference level. This means that the spring strength of the slotless spacer grid, represented by the gradient of the displacement-load curve, is high. However, the side-slotted nozzle type double sheet spacer grid of the present invention is reduced in its spring strength and enlarges its elastic range, wherein the displacement-load curve is shown as a linear curve with the same gradient. It is thus noted from the test that the spacer grid of the present invention has improved spring performance. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.