Patent Number: 047088451
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIGS. 1 to 3, there is shown a nuclear fuel assembly, generally designated 10 for a boiling water nuclear power reactor (BWR), in which the improvement of the present invention is incorporated. The fuel assembly 10 includes an elongated outer tubular flow channel 12 that extends along substantially the entire length of the fuel assembly 10 and interconnects an upper support fixture or top nozzle 14 with a lower base or bottom nozzle 16. The bottom nozzle 16 which serves as an inlet for coolant flow into the outer channel 12 of the fuel assembly 10 includes a plurality of legs 18 for guiding the bottom nozzle 16 and the fuel assembly 10 into a reactor core support plate (not shown) or into fuel storage racks, for example in a spent fuel pool. The outer flow channel 12 generally of rectangular cross-section is made up of four interconnected vertical walls 20 each being displaced about ninety degrees one from the next. Formed in a spaced apart relationship in, and extending in a vertical row at a central location along, the inner surface of each wall 20 of the outer flow channel 12, is a plurality of structural ribs 22. The outer flow channel 12, and thus the ribs 22 formed therein, are preferably formed from a metal material, such as an alloy of zirconium, commonly referred to as Zircaloy. Above the upper ends of the structural ribs 22, a plurality of upwardly-extending attachment studs 24 fixed on the walls 20 of the outer flow channel 12 are used to interconnect the top nozzle 14 to the channel 12. For improving neutron moderation and economy, a hollow water cross, as seen in FIGS. 1, 2 and 4 and generally designated 26, extends axially through the outer channel 12 so as to provide an open inner channel 28 for subcooled moderator flow through the fuel assembly 10 and to divide the fuel assembly into four, separate, elongated compartments 30. The water cross 26 has a plurality of four radial panels 32 composed by a plurality of four, elongated, generally L-shaped, metal angles or sheet members 34 that extend generally along the entire length of the channel 12. The sheet members 34 of each panel 32 are interconnected and spaced apart by a series of elements in the form of dimples 36 formed therein and extending therebetween. The dimples 36 are provided in opposing pairs that contact each other along the lengths of the sheet members 34 to maintain the facing portions of the members in a proper spaced-apart relationship. The pairs of contacting dimples 36 are connected together such as by welding to ensure that the spacing between the sheet members 34 forming the panels 32 of the central water cross 26 is accurately maintained. The hollow water cross 26 is mounted to the angularly-displaced walls 20 of the outer channel 12. Preferably, the outer, elongated lateral ends of the panels 32 of the water cross 26 are connected such as by welding to the structural ribs 22 along the lengths thereof in order to securely retain the water cross 26 in its desired central position within the fuel assembly 10. Further, the inner ends of the panels together with the outer ends thereof define the inner central cruciform channel 28 which extends the axial length of the hollow water cross 26. Also, the water cross 26 has a lower flow inlet end 38 and an opposite upper flow outlet end 39 which each communicate with the inner channel 28 for providing subcoolant flow therethrough. Disposed within the channel 12 is a bundle of fuel rods 40 which, in the illustrated embodiment, number sixty-four and form an 8.times.8 array. The fuel rod bundle is, in turn, separated into four mini-bundles thereof by the water cross 26. The fuel rods 40 of each mini-bundle, such being sixteen in number in a 4.times.4 array, extend in laterally spaced apart relationship between an upper tie plate 42 and a lower tie plate 44. The fuel rods in each mini-bundle are connected to the upper and lower tie plates 42,44 and together therewith comprise a separate fuel rod subassembly 46 within each of the compartments 30 of the channel 12. A plurality of grids or spacers 48 axially spaced along the fuel rods 40 of each fuel rod subassembly 46 are composed of interleaved inner straps 49 and an outer strap 51 which maintain the fuel rods in their laterally spaced relationships. The lower and upper tie plates 42,44 of the respective fuel rod subasselmblies 46 have flow openings 50 defined therethrough for allowing the flow of the coolant fluid into and from the separate fuel rod subassemblies. Also, coolant flow paths provide flow communication between the fuel rod subassemblies 46 in the respective separate compartments 30 of the fuel assembly 10 through a plurality of openings 52 formed between each of the structural ribs 22 along the lengths thereof. Coolant flow through the openings 52 serves to equalize the hydraulic pressure between the four separate compartments 30, thereby minimizing the possibility of thermal hydrodynamic instability between the separate fuel rod subassemblies 46. The above-described basic components of the BWR fuel assembly 10 are known in the prior art, being disclosed particularly in the Doshi application cross-referenced above, and have been discussed in sufficient detail herein to enable one skilled in the art to understand the improved feature of the present invention presented hereinafter. For a more detailed description of the construction of the BWR fuel assembly, attention is directed to both of the above cross-referenced Barry et al and Doshi patent applications. Spacer and Fuel Bundle Modifications of Present Invention The improvements of the present invention relate to modifications made in the spacer and fuel bundle design described above which significantly improve the CHF characteristics of the BWR fuel assembly 10. The mechanical design modifications provided by the present invention for improved cooling performance are the following two. First, fuel rods 40a are provided at the corners of the spacers 48, as seen in FIGS. 4 to 7, which have a diameter less than that of the remaining interior and side fuel rods 40b,40c. The reduction in the diametric size of the corner fuel rods 40a requires an increase in the distance through which the protrusions formed on the inner and outer straps 49,51 of the spacer, e.g. the spacer dimples 54a and springs 56a, extend into the corner spacer cells 58a as compared to the extension of the remaining dimples 54b and springs 56b into the interior and side spacer cells 58b,58c. Such reduction in the diametric size of the corner fuel rods 40a results in an increase in the flow area per rod (i.e., equal to that for side and interior locations) for the four corner locations. An optimized diameter for the corner rod fuel rods 40a would result in providing a flow area per rod equal to the interior spacer cells 58b. In general, this is obtained from the following formula: EQU A.sub.I =A.sub.c.sbsb.1 +.pi./4(d.sub.c.sbsb.1.sup.2 -d.sub.c.sbsb.2.sup.2) or EQU d.sub.c.sbsb.2 =4/.pi.(A.sub.c.sbsb.1 -A.sub.I +.pi./4(d.sub.c.sbsb.1.sup.2))].sup.1/2 where, d.sub.c.sbsb.1 =corner rod diameter in present design PA1 d.sub.c.sbsb.2 =reduced corner diameter in proposed design PA1 A.sub.I =Flow Area per rod in interior spacer locations A.sub.c.sbsb.1 =Flow Area per rod for corner rod in present design Substituting the dimension values of a given spacer and rod design into the above formula, a reduced rod diameter of 0.431 inch for the corner rods were obtained, which is smaller than the diameter of 0.458 inch for the other non-corner rods. Second, openings or perforations 60 are made in the outer strap 51 of each spacer 48 at the corner and side spacer cells 58a,58c, as seen in FIG. 7. These perforations 60 reduce the amount of wall area of the spacer sides 62 adjacent to the corner and side fuel rods 40a,40c and increases access of coolant to these spacer locations. To summarize, the above-described first and second modifications respectively increase the flow area around the heated fuel rod and decrease the amount of unheated wall surface area adjacent to the heated fuel rod. Thus, correspondingly, coolant flow is increased in the areas of the spacer, the corners thereof, at which coolant normally encounters the highest resistance and pressure drop, and the clinging of coolant onto the spacer wall which lessens the amount of coolant provided to the heated fuel rod surface is reduced. While the first and second modifications ordinarily improve the CHF characteristics of the fuel assembly 10, a third modification of the present invention is desirable from a strategic standpoint. This is due to the fact that the mechanical design modifications outlined above can only reduce the detrimental impact of limiting spacer locations, but not eliminate them. Thus, a good nuclear design for the fuel bundle to avoid excessive rod peaking in the limiting locations becomes essential. In the third modification, the nuclear design of the fuel bundle (i.e., enrichment, burnable poison distribution, etc.) is changed in a manner which assures that rod peaking does not occur in the corner/limiting locations. The third modification is directed to the use of a uniform poison, such as boron, in the form of coatings on the inside of the majority of the fuel rods 40. More particularly, each of the fuel rods 40 includes an outer cladding tube 62 having an inner clad surface 64 and a plurality of fuel pellets 66 contained within the tube. As seen in FIG. 6, the uniform poison coating 68 is applied to either the outer surface of the fuel pellets 66 or to the inner clad surface 64 of each of the tubes 62 of the fuel rods 40 in the majority thereof. Typically, the cladding tube 62 in a BWR fuel assembly is composed of Zircaloy-2, whereas, in a PWR fuel assembly, the cladding tube 62 is composed of Zircaloy-4. It is anticipated that the technique for applying the boron coating on the inside of clad tube 62, or clad surface 64, would be accomplished in the same manner as applying the coating to the outside surface of the pellets. Basically, zirc di-boride is sputtered onto the pellets by hitting a zirc di-boride target in a vacuum chamber wherein the boron is sputtered onto the surface of the pellet. It should suffice to say that the boron coating technology is known by those skilled in the art and is not a part of this invention. A practical boron coating thickness would range from 0.0005 inch to 0.0015 inch, even 0.002 inch may be practical. A 0.0005 inch thick boron coating represents a 1.9 mg/inch of rod length B-10 poison loading on the pellet. Example Preliminary lattice transport theory calculations, using the PHOENIX code, were carried out to determine whether a fuel assembly can be neutronically designed in such a manner as to always assure that both the corner and side rods 40a,40c have pin powers below the lead rod in the bundle. The PHOENIX code is a two-dimensional X-4 transport theory code developed by ASEA-ATOM and licensed in the United States by the NRC to perform single or four-bundle physics calculations. These calculations include the calculation of the pin-wise power distribution, depletion of the fuel pins and burnable absorber pins and the generation of cross-sections homogenized over the assembly. The reference bundle design chosen as the starting point was a QUAD+ BWR bundle design for a C lattice (equal outer water gaps) plant with 2.912 w/o U-235 bundle-average enrichment and eight gadolinia rods of 3.2 w/o Gd203. The enrichment pattern and the pin-wise power distribution verses exposure are shown in Table IV. The bundle reactivity (k-inf) is tabulated verses exposure in Table I. The bundle reactivities and pin power distribution correspond to a 50% void depletion. A change to the U-235 enrichment pattern was made to assure that the four corner rods in each of the four QUAD+ mini-bundles comprising the assembly would never lead the bundle in pin power. The bundle average enrichment of 2.912 w/o U-235 was maintained. The gadolinium pattern and loading remained unchanged. The revised U-235 enrichment pattern is shown in Table V along with the pin-wise power distribution at various exposures. Table II compares the maximum pin powers for the lead pin in the bundle verses the highest-powered corner pin. The corner pins stay substantially below the lead pin in the bundle. In the 7 22 GWD/MTU burnup range where the bundle will be critical power ratio limited, the maximum corner pin power is 12-14% below the lead pin in the bundle. The corner pins never lead the bundle in pin power at any burnup as demonstrated in Table II. Thus, it is clearly demonstrated that if a fairly homogeneous bundle absorber is used and applied to the majority of the fuel rods, then it is possible to develop a U-235 enrichment pattern that always assures that neither side nor corner pins lead the bundle in pin power. Such a U-235 enrichment pattern corresponding to rod locations shown in FIG. 8, along with pin-wise power distribution in the bundle at various exposures, is given in Table II. Should the corner rods be found limiting from a critical power ratio viewpoint as a result of the spacer design, a bundle enrichment design can be developed that decreases the corner rod powers and correspondingly increases the pin powers of the remaining rods so as to balance out the critical power ratio performance of all the rods. Table I compares the bundle reactivity of this reduced corner rod power case against the reference case. In the 22-26 GWD/MTU exposure range, typical of the end-of-cycle core average exposure for reload cycles, Table I shows that there is only a 30-60 pcm difference in reactivity which is a trivial difference. Table III compares the maximum pin power of any side or corner rod verses the lead pin and confirms that neither the corner nor side fuel rods are ever the peak rod in the bundle until 38-42 GWD/MTU. Typically, the maximum corner/side rod is three to four percent lower in pin-power than the lead rod. This is estimated to be possible only with a uniform poison design, such as mentioned above. The design shown in FIG. 8 and Table VI maintains the corner/side rods at least three to four percent in rod power below the lead (interior) rod in the bundle. This difference in side/corner verses interior rod power can be adjusted through proper selection of the U-235 enrichment pattern to balance out the critical power performance of all the fuel rods. TABLE I ______________________________________ COMPARISON OF BUNDLE K-inf Difference Burnup (Reference-Corner Rod) (MWD/MTU) Case ______________________________________ 0 +622 pcm 2000 +255 4000 +61 6000 +134 8000 +176 10000 +146 14000 +118 18000 +90 22000 +62 26000 +34 30000 +6 34000 -25 38000 -43 42000 -58 ______________________________________ TABLE II ______________________________________ COMPARISON OF ROD-WISE LOCAL PEAKING FACTORS-CASE WITH CORNER RODS NEVER LIMITING FOR PIN POWER Burnup Peak RodPower ForRelative Peak Corner ##STR1## (MWD/MIU) In Bundle Rod Power 100% ______________________________________ 0 (no Xenon) 1.135 1.032 9.1% 0 (Equil. Xenon) 1.133 1.034 8.7% 500 1.126 1.032 8.3% 1000 1.116 1.028 7.9% 2000 1.097 1.014 7.6% 3000 1.081 0.996 7.9% 4000 1.067 0.979 8.2% 5000 1.056 0.962 8.9% 6000 1.047 0.949 9.4% 7000 1.068 0.941 11.9% 8000 1.087 0.938 13.7% 9000 1.093 0.938 14.2% 10000 1.094 0.939 14.2% 14000 1.089 0.943 13.4% 18000 1.083 0.946 12.7% 22000 1.076 0.950 11.7% 26000 1.067 0.955 10.5% 30000 1.056 0.963 8.8% 34000 1.050 0.974 7.2% 38000 1.041 0.984 5.5% 42000 1.028 1.000 2.7% ______________________________________ TABLE III __________________________________________________________________________ COMPARISON OF ROD-WISE LOCAL PEAKING FACTORS BOTH CORNER AND SIDE RODS NEVER LIMITING FOR PIN POWER Relative Power Max. Pin Power Difference (MWD/MIU)Burnup In BundleFor Peak Rod Side RodsFor Corner or ##STR2## __________________________________________________________________________ 0 (No Xenon) 1.106 1.026 7.2% 0 (Equil. Xenon) 1.103 1.029 6.7% 500 1.099 1.031 6.2% 1000 1.095 1.032 5.8% 2000 1.090 1.034 5.1% 3000 1.086 1.034 4.8% 4000 1.082 1.035 4.3% 5000 1.080 1.034 4.3% 6000 1.077 1.034 4.0% 7000 1.075 1.033 3.9% 8000 1.073 1.032 3.8% 9000 1.072 1.031 3.8% 10000 1.070 1.030 3.7% 14000 1.065 1.025 3.8% 18000 1.060 1.019 3.9% 22000 1.053 1.014 3.7% 26000 1.045 1.013 3.1% 30000 1.036 1.016 1.9% 34000 1.024 1.018 0.6% 38000 1.021 1.021 0.0% 42000 1.022 1.022 0.0% __________________________________________________________________________ TABLE IV ______________________________________ REFERENCE CASE (*IDENTIFIES LEAD INTERIOR PIN) ______________________________________ 0 MWD/MTU (NO XENON) RELATIVE POWER (W/CM.sup.2) WT % U-235 PER PIN 1.052 1.898 AVE: 2.9120 1.107 1.092 2.474 3.378 1.123 0.368 0.938 2.774 3.378 3.378 1.108 1.124* 1.095 1.066 2.474 3.253 3.253 2.774 8000 MWD/MTU O MWD/MTU (EQUIL. XENON) RELATIVE POWER RELATIVE POWER (W/CM.sup.2) (W/CM.sup.2) 1.054 0.938 1.107 1.089 0.982 1.052* 1.123* 0.374 0.936 1.020 1.003 0.967 1.109 1.122 1.093 1.065 0.994 1.042 1.002 0.959 14000 MWD/MTU 1000 MWD/MTU RELATIVE POWER RELATIVE POWER (W/CM.sup.2) (W/CM.sup.2) 1.054 0.935 1.095 1.076 0.975 1.045* 1.109* 0.455 0.932 1.012 1.012 0.978 1.099 1.107 1.077 1.053 0.991 1.040 1.007 0.968 22000 MWD/MTU 4000 MWD/MTU RELATIVE POWER RELATIVE POWER (W/CM.sup.2) (W/CM.sup.2) 0.996 0.938 1.033 1.056 0.968 1.036 1.059 0.768 0.943 1.003 1.010 0.994 1.043 1.067* 1.032 1.000 0.988 1.039* 1.017 0.982 ______________________________________ TABLE V ______________________________________ CORNER RODS NEVER LIMITING LOCATIONS (*IDENTIFIES LEAD INTERIOR PIN) ______________________________________ 0 MWD/MTU (NO XENON) RELATIVE POWER (W/CM.sup.2) WT % U-235 PER PIN 1.007 1.769 AVE: 2.9120 1.133 1.067 2.493 3.219 1.125 0.410 1.049 2.719 3.819 3.819 1.032 1.135* 1.105 0.995 2.219 3.219 3.219 2.493 8000 MWD/MTU MWD/MTU (EQUIL. XENON) RELATIVE POWER RELATIVE POWER (W/CM.sup.2) (W/CM.sup.2) 1.009 0.907 1.133* 1.065 0.990 1.020 1.125 0.416 1.045 1.011 1.087* 1.046 1.034 1.133 1.102 0.995 0.938 1.038 0.998 0.904 14000 MWD/MTU 1000 MWD/MTU RELATIVE POWER RELATIVE POWER (W/CM.sup.2) (W/CM.sup.2) 1.011 0.911 1.116* 1.051 0.980 1.016 1.109 0.507 1.034 1.003 1.089* 1.050 1.028 1.115 1.084 0.986 0.943 1.035 1.002 0.920 22000 MWD/MTU 4000 MWD/MTU RELATIVE POWER RELATIVE POWER (W/CM.sup.2) (W/CM.sup.2) 0.959 0.920 1.046 1.027 0.971 1.010 1.052 0.845 1.032 0.994 1.076* 1.057 0.979 1.067* 1.032 0.939 0.950 1.033 1.012 0.941 ______________________________________ TABLE VI ______________________________________ BOTH CORNER AND SIDE RODS NEVER LIMITING FOR PIN POWER (*IDENTIFIES LEAD INTERIOR PIN) ______________________________________ O MWD/MTU (NO XENON) RELATIVE POWER (W/CM.sup.2) WT % U-235 PER PIN 0.940 2.052 AVE: 2.9120 1.015 1.106* 2.628 3.532 0.974 1.057 1.010 2.628 3.532 3.532 1.026 0.972 0.932 0.991 2.628 2.928 2.928 2.928 8000 MWD/MTU O MWD/MTU (EQUIL. XENON) RELATIVE POWER RELATIVE POWER (W/CM.sup.2) (W/CM.sup.2) 0.946 0.976 1.017 1.103* 1.015 1.073* 0.975 1.053 1.005 0.983 1.033 0.993 1.029 0.971 0.931 0.991 1.032 0.976 0.942 0.997 14000 MWD/MTU 1000 MWD/MTU RELATIVE POWER RELATIVE POWER (W/CM.sup.2) (W/CM.sup.2) 0.957 0.971 1.019 1.095* 1.006 1.065* 0.978 1.047 1.000 0.983 1.032 0.999 1.032 0.971 0.931 0.992 1.025 0.981 0.954 1.002 22000 MWD/MTU 4000 MWD/MTU RELATIVE POWER RELATIVE POWER (W/CM.sup.2) (W/CM.sup.2) 0.972 0.965 1.019 1.082* 0.994 1.053* 0.981 1.037 0.993 0.981 1.033 1.010 1.035 0.972 0.935 0.994 1.014 0.989 0.971 1.009 ______________________________________ It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.