Patent Number: 048790907
Section: summary

BACKGROUND OF THE INVENNTION 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 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". 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 When the control rods are withdrawn, these channels are filled with moderator which increases the local moderating capacity and thereby increases the power generated in the fuel.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 structures 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. To maximize the benefit of the vanes, the size, shape, bend angle, and location of the vanes must be optimized. The vanes are especially beneficial adjacent to the aforementioned hot channels. The remaining components of the grid which include the strips, rod support features and welds must be streamlined to reduce the turbulence generated in the vicinity of the vanes. Further constraints on designing the grids include minimizing grid pressure drop and maximizing grid load carrying strength. Grids are generally of 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 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 with flow deflector vanes 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. Also, the joining of the interlocking zircaloy strips has always been by welding which requires the melting of some grid material to form a weld nugget. The increases strip thickness and weld nuggets for zircaloy grids of the prior art increase turbulence and grid pressure drop and reduce the effectiveness of the vanes. Therefore, the DNB performance of a zircaloy grid containing flow vanes of the prior art will be degraded relative to an Inconel grid design. In U.S. Pat. No. 4,089,741, a split vaned grid is disclosed in which first and second welding tabs are disposed in intersecting relation. Fusing of the protruding tabs at the intersection points down into the intersection joints occurs such that the protruding tabs are consumed whereupon there is formed in said vanes an opening at the base thereof, but within the bent and flow exposed vanes and not the vertical sections supporting them. The openings have a shape of the same general configuration as that of said first protruding tab, whereby flow is through the opening and in that patent, it is alleged the flow mixing capability of said spacer is improved. FIG. 1 is a prior art view showing what happens to create flow separation when a vane such as that of U.S. Pat. No. 4,089,741 has a nugget weld "unshielded" from the flow and an opening in the vane itself. U.S. Pat. Application Ser. No. 856,888 of Donald W. Krawiec, now U.S. Pat. No. 4,725,402 assigned to the assignee of the instant invention teaches "shielding" the weld nugget from the flow path within the confines of the strips in openings along their lines of intersection, to minimize pressure drop. This application does not specifically disclose integral vanes of the type in U.S. Pat. No. 4,089,741, however, which have openings which increase pressure drop by flow separation during flow therethrough. Water table tests were performed to visualize how the weld nugget and the welding hole cutout in the prior art vane for a nugget affects the flow passing by and through the vane. FIG. 1 illustrates the prior art flow pattern with a nugget and its weld hole in the vane. It can be seen that the weld nugget/weld access hole generates a very large wake, which, in turn, promotes decay of the vane effectiveness downstream of the grid. Velocity measurements downstream of the grid, both in water table tests and in an air model, using Laser-Doppler Anemometry, support the claim that the vane of the invention is more effective in directing the flow into the fuel rod gap because the weld nugget/weld access hole is not present in the vane. 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 a novel and improved perimeter and interior strip. The novel perimeter strip is characterized by small stiffening ribs and round dimple stiffening features, both of which have been located differently in the prior art, as seen, for example, in U.S. Pat. Nos. 4,224,107 and 3,751,335. The ribs of the invention extend around the perimeter strips at two elevations and are ridged inwardly. The dimples also extend inwardly into some or all of the fuel rod receiving grid sectors serving to rigidize the perimeter strip and functioning as either arches for fuel rod support or backup arches for the integral fuel rod positioning springs which extend inwardly from the perimeter strip. The junction of the internal strip to the perimeter strip, in accordance with the present invention, is characterized by a weld seam of substantially greater length than has previously been employed. The increased weld seam length also enhances the strength of the grid. Interior orthogonal strips are designed to limit cutouts in the unslotted section of the strip. This is accomplished by the use of small 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 section of the interior grid strip. The design maintains a load path through the unslotted 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. In addition, 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 zircaloy grid design indicate these unique features significantly increase coolant mixing and DNB performance by comparing the test results to an Inconel grid design with similar flow vanes. These comparisons verified that the zircaloy grid design produced 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 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. Intermediate welds of the type taught in U.S. Application Ser. No. 856,888, now U.S. Pat. No. 4,725,402 were also provided to improve the strength of the grid. The slots were tapered at their ends to facilitate welding at intermediate locations, thus improving grid strength. Tests have also been performed which support this claim and have also shown an increase in strength of approximately 15% over grids of the prior art. This increase would be additive to that described in the paragraph above. The outer strips of the grid have also been optimized with respect to strength, handling, turbulence generation, and pressure drop. To obtain additional strength, small ribbed and round dimple stiffeners were employed along the strip's entire length. These stiffeners did not only increase the buckling resistance of the grid but improve the strip's resistance to interact, i.e catch or hang-up with adjacent fuel assemblies which reduces the potential for handling damage. In addition, the optimized outer strip design more effectively spreads accident loadings throughout the grid interior strips thus increasing strength. The outer strip also diverts just enough flow to the interior of the fuel assembly to match the thermal power distribution of the fuel array and eliminate any corrosion concerns on peripheral fuel rods. The novel features described herein summarize the improvements made to a straight strip zircaloy grid containing flow vanes to improve upon reactor performance, load carrying strength, pressure drop, and handling performance relative to zircaloy grid designs of the prior art. To improve reactor performance, the integral flow deflector vanes were optimized with respect to size, shape, and bend angle in order to maximize coolant mixing and fuel rod heat transfer downstream of the vanes. The weld nuggets were optimized with respect to size, strength, corrosion resistance, and location, i.e., the nugget is positioned upstream in the grid strip and is shielded by the integral flow deflector vanes. By recessing the nugget into the grid with no cutout in the vane, the turbulent wake produced by the nugget and cutout has less impact on vane performance. The location of the nuggets, upstream and shielded by the integral flow deflector vanes, leads to less turbulent wakes and better vane mixing and fuel rod heat transfer performance than the prior art. The novel staggering of grid support features also increased vane performance and lowered grid pressure drop as compared to 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 outer 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.