Patent Number: 046631186
Section: description

DESCRIPTION As shown in the longitudinal cross section view of FIG. 1, a fuel assembly 11 includes a plurality of elongated fuel rods 12 supported between a lower tie plate 13 and an upper tie plate 14. Although not shown herein, ordinarily a plurality of fuel rod spacers are positioned intermediate the lower and upper tie plates for lateral support of the fuel rods 12. Each of the fuel rods 12 comprises an elongated tube containing the fissile fuel, usually in the form of pellets, sealed in the tube by lower and upper end plugs 16 and 17. Lower end plugs 16 are formed with a taper for registration and support in support cavities 18 formed in the lower tie plate 13. Upper end plugs 17 are formed with shanks 19 which register with support cavities 21 in the upper tie plate 14. Several of the support cavities 18 (for example selected ones of the edge or peripheral cavities, such as a cavity 18') in the lower tie plate 13 are formed with threads to receive fuel rods having threaded lower end plugs, such as an end plug 16'. The shanks 19' of the upper end plugs of these same fuel rods are elongated to pass through their respective cavities 21 in the upper tie plate 14 and are formed with threads to receive threaded retaining nuts 22. Springs 23 mounted on the shanks 19 urge the upper tie plate 14 upward with respect to the fuel rods 12. In this manner the lower and upper tie plates and the fuel rods are formed into a unitary structure or fuel bundle, the upper tie plate 14 being formed with an upwardly extending handle or bail 20 for handling of the fuel assembly. The fuel assembly is surrounded by a thin-walled tubular flow channel 24 of substantially square cross section which is open at its upper end. The fuel assembly 11 is a sliding fit in the flow channel 24 so that it readily can be inserted and removed. At its upper end the channel 24 may be formed with holes 25 or the like for handling. At its bottom end the flow channel 24 is secured, as described in detail hereinafter, to a tapered nose piece 26 adapted to fit into a socket of the lower core support structure (not shown). The lower part (not shown) of nose piece 26 is formed with openings to receive pressurized coolant which is directed by the nose piece 26 and the flow channel 24 upward past the fuel rods 12 (see U.S. Pat. No. 3,697,376). The nose piece 26 is formed with a shoulder 27 upon which the lower tie plate 13 rests for support of the fuel assembly 11. An upstanding rim 28 surrounds and provides lateral location of the lower tie plate 13. Typically the flow channel 24 is formed of a material having a low neutron absorption cross section such as an alloy of zirconium while the nose piece 26 is formed of a corrosion resistant iron alloy such as stainless steel. As a practical matter such different materials cannot be welded together. Previous channel-to-nozzle attachments include attachment of the channel directly to the nozzle with screws or rivets as shown in U.S. Pat. No. 3,697,375. Although simple, the drawback of this arrangement is the possibility that the screws or rivets may loosen or be overstressed and fail because of differential thermal expansion of the flow channel and nozzle due to their different material. The drawbacks of prior arrangements are avoided by the channel-to-nozzle attachment of the present invention which, as shown in FIGS. 1-3, includes tapered attachment bars 29 secured to the inside lower edges of the channel 24 and fitted into similarly tapered grooves 31 formed in the outside surfaces of the nozzle 26. The material of the attachment bars 29 is selected to have the same or a very similar thermal coefficient of expansion as the material of the channel 24. Typically, the channel 24 and the attachment bars 29 are formed of an alloy of zirconium having a thermal coefficient of expansion of about 3.2.times.10.sup.-6 inch per inch per degree F. while the stainless steel of the nozzle 26 has a substantially greater thermal coefficient of expansion of about 9.45.times.10.sup.-6 inch per inch per degree F. The temperature range experienced by these parts varies from room temperature to an operating temperature in the reactor core of 600 degrees F. or greater. As the temperature increases it is evident that the channel 24 expands, i.e. the distance D.sub.c from the center line CL.sub.v to the inside surface of the channel 24 increases. Also the width W of the attachment bars 29 increases. At the same time, the nozzle 26 expands outward a greater amount and the width of the tapered grooves 31 increases a greater amount. If the channel 24 was firmly attached to the nozzle 26, as in the prior art arrangement, the greater expansion of the nozzle 26 would cause the lower end of the channel 24 to be bent outward thereby stressing this lower end and the attachment screws or rivets. However, with the illustrated attachment arrangement of the present invention, the tapered attachment bars 29 simply move further into the more rapidly expanding tapered grooves 31, a clearance space C being provided to allow this inward movement. With proper selection of the angle of taper A the bars 29 can move more or less deeply into the grooves 31 as differential thermal expansion occurs, without any bending of the lower end of the channel 24 and with the bars 29 remaining tightly fitted in the grooves 31. The optimum angle of taper is illustrated graphically in FIG. 1. A part (the nozzle 26) under thermal expansion will change shape along lines (such as lines 32 and 33) radiating from a center point 34. Thus the grooves 31 change size along the lines 32 and 33 and, therefore, the lines 32 and 33 define the optimum angle of taper A, i.e. the angle between the opposite tapered surfaces of the grooves 31. In other words, the angle of the taper A is selected such that the tapered upper surface of the groove 31 on one side is in the same plane as the tapered lower surface of the groove 31 on the opposite side of the nozzle 26. While in the usual case the nozzle 26 has a greater thermal coefficient of expansion than the channel 24 and attachment bars 29, the invention is not so limited and can be used with any combination of different materials. Mathematically, the optimum angle of taper A can be determined as follows with reference to FIG. 3. The change in part size due to temperature change dD is given by the following relationship: EQU dD=a dT D (1) where: a is the thermal coefficient of expansion. PA0 dT is the temperature range. PA0 D is the length of part. PA0 Y is the axial (vertical from CL.sub.r) thermal growth of bar 29. PA0 X' is the radial thermal growth of nozzle 26. PA0 Y' is the axial thermal growth of groove 31. PA0 a.sub.z is the termal coefficient of expansion of the material of the channel 24 and bars 29. PA0 a.sub.s is the thermal coefficient of expansion of the material of the nozzle 26. PA0 D.sub.c is the distance from the vertical centerline CL.sub.v to the inside surface of channel 24. PA0 D.sub.n is the distance from the centerline CL.sub.v to the outside surface of nozzle 26. PA0 W is the distance from radial centerline CL.sub.r to point P. PA0 D.sub.n approximates D.sub.c =D. For any point P on the interface line between the bar 29 and groove 31. ##EQU1## where: X is the radial (lateral from CL.sub.v) thermal growth of bar 29. From relationship (1): EQU X=a.sub.z dT D.sub.c EQU X'=a.sub.s dT D.sub.n EQU Y=a.sub.z dT W EQU Y'=a.sub.s dT W where: Substituting in relationship (2): ##EQU2## Assuming that D.sub.n and D.sub.c are insignificantly different then: Thus simplifying: ##EQU3## In a practical example of the invention for use in a typical BWR (boiling water reactor), W=0.3 inches (7.62 mm) and D=2.63 inches (66.8 mm). Therefore ##EQU4## As shown in FIGS. 1 and 2, the attachment bars 29 may be secured to the lower end of the flow channel 24 by flat head screws 36. Alternatively, rivets or welding may be used for this purpose. As illustrated in FIG. 2, the ends of the grooves 31 are shown rounded since this configuration results from the tapered rotary cutting tool normally used to make the grooves 31. Although not shown, the ends of the attachment bars 29 may be similarly rounded but, in any event, the length L of the bars 29 is selected to be slightly less (depending on manufacturing tolerances) than the length of the grooves 31. This is to allow the bars 29 to find their natural and laterally unrestrained position in the grooves 31. With reference to FIG. 2, the channel 24 is assembled to the nozzle 26 by placing the attachment bars 29 in the grooves 31, slipping the lower end of the channel 24 over the bars 29 with screw holes in alignment, and then inserting and tightening the screws 36. Thus what has been described is a flow channel-to-nozzle attachment which remains tightly fitted with temperature changes without stressing the parts. An additional benefit of this arrangement is excellent control of bypass leakage flow (discussed in U.S. Pat. No. 3,697,376). The only open or leakage flow area is between the nozzle and channel at the four corners and this area tends to remain constant throughout design life.