Patent Number: 056087683
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION With reference to FIG. 1, a conventional fuel assembly 10 includes a plurality of elongated, full length fuel rods (FLR's) 12 supported between a lower tie plate 14 and an upper tie plate 16. Fuel rods 12 pass through a plurality of fuel rod spacers 18 which provide intermediate support to retain the elongated rods in spaced relation and to restrain them from lateral vibration. Each fuel rod 12 comprises an elongated tube containing the fissile fuel (such as uranium or plutonium dioxide) in the form of pellets, particles, powder or the like, sealed in the tube by upper and lower end plugs 20 and 22. Lower end plugs 22 are formed with a taper for registration and support in support cavities 24 formed in the lower tie plate 14. Upper end plugs 20 are formed with extensions 26 which register with support cavities 28 in the upper tie plate 16. A channel 30 encloses the bundle in the usual manner. Several of the support cavities 24 in the lower tie plate 14 are formed with threads to receive tie fuel rods or PLR's (one shown at 32 in FIG. 1). As already mentioned hereinabove, there are typically eight tie rods and as many as fourteen PLR's in current fuel bundle designs which are attached to the lower tie plate by threaded holes and threaded end plug shanks. These rods typically terminate adjacent one of the spacers 18 as shown in FIG. 1. FIG. 2 illustrates a conventional end plug 34 of the type used with, for example, the PLR 32 shown in FIG. 1. The end plug 34, is formed of Zircaloy and includes a generally cylindrical upper body portion 36, including an annular fuel rod support shoulder 38 (the first rod is typically welded to the end plug), and a lower threaded portion 40 including a tapered lower end 42. This one piece end plug is adapted to be received in a threaded hole 44 formed in the lower tie plate 14. This conventional arrangement is illustrated in FIG. 3. It has been experienced that, after irradiation, many of the rods threaded into the lower tie plate 14 are difficult to remove. It has been determined that the cause of the binding or sticking of fuel rods within the lower tie plate is related to the corrosion which occurs on the male threads at the lower end 40 of the Zircaloy end plug 34. More specifically, the corrosion process creates a zirconium oxide with a net volume increase relative to the original volume of Zircaloy, and this excess material creates locking forces between the male end plug threads on the lower end 40 of the end plug, and the female threads in the hole 44 in the lower tie plate 14. Since the lower tie plate 14 is made of stainless steel (a harder and more corrosion resistant material than Zircaloy which does not experience any significant corrosion in the BWR environment), it has also been determined that the lower tie plate per se does not contribute to the locking problem. The invention here, and as best seen in FIG. 4, relates to a new Zircaloy end plug 46 which includes an upper body portion 48 which is similar to the upper body portion 36 of the conventional end plug 34, and a lower portion which includes a removable stainless steel connecting shank 50. The shank 50 is threaded at an upper section 52 and at a lower section 54 with a smooth intermediate portion 56 connecting the threaded sections. The upper threaded section 52 is receivable within a tapped hole 58 formed in the now axially shortened Zircaloy end plug upper body portion 48, and the lower threaded section 54 of the connecting shank 50 is adapted to be threaded directly into the correspondingly threaded hole 44 in the lower tie plate 14 as best seen in FIG. 5. In the preferred embodiment, the sections 52 and 54 of the stainless steel connecting shank 50 are threaded in opposite directions. At the same time, the tapped holes 44 and 58 in both the lower tie plate 14 and the Zircaloy end plug upper body portion 48, respectively, are correspondingly threaded. With this arrangement, when the fuel rod 32 (for example) is removed from the lower tie plate 14, both the end plug upper body portion 48 and end plug lower portion, i.e., the connecting shank 50, can be removed with the fuel rod 30, or the connecting shaft 50 can be left within the lower tie plate 14, depending on the direction of rotation of the fuel rod 32 relative to the end plug 46. In other words, if it is desired to maintain the stainless steel connecting shank 50 in the lower tie plate 14, then the fuel rod may be turned in, for example, a clockwise direction, thereby separating the fuel rod 30 and the end plug upper body portion 48 from the upper threaded section 52 of the shank 50, such that shank 50 remains in the lower tie plate 14. On the other hand, if it is desired to maintain the connecting shank 50 within the end plug upper body portion 48, then the fuel rod 30 is rotated in an opposite direction so that the fuel rod 32 and the entire end plug 46 including the upper connecting shank 50 are removed together from the lower tie plate 14. The above arrangement also provides an additional option in the event of any sticking problem which may be experienced when attempting to remove the fuel rod. In other words, if the threads on shank 50 are frozen in one direction of rotation of fuel rod 32, an option remains by virtue of being able to rotate the fuel rod in the opposite direction so that the fuel rod can be removed, with or without the shank 50. It is also a feature of this invention that if a concern exists relative to debris or dirt clogging an open threaded hole (e.g., hole 44) in the lower tie plate 14 during fuel bundle reconstitution work, then the stainless steel connector shank 50 can be left in the lower tie plate 14 rather than in the end plug body 48, thereby precluding dirt or debris from entering the lower tie plate 14. In the preferred embodiment, and as already noted above, the connecting shank 50 is preferably constructed of stainless steel but in any event, in order to minimize if not completely eliminate the corrosion and sticking problem discussed herein, the connecting shank material should be harder, stronger and more corrosion resistant than the Zircaloy end plug body material. Under certain conditions, it might be advantageous to favor one fuel rod removal technique over another and this can be facilitated by incorporating differential torque characteristics at opposite threaded ends of the connector shank 50. In addition, while the invention has been described above in connection with partial length fuel rods, it will be appreciated that the utilization of a connector shank 50 of the type disclosed herein may also be used with fuel bundle tie rods as well as full length fuel rods where appropriate. It will also be appreciated that the threaded end plugs 46 of this invention may be utilized not only with PLR's but with tie rods and FLR's as well. By utilizing threaded end plugs as disclosed herein for the FLR's, it will be appreciated that the biasing springs conventionally used to bias FLR's into the lower tie plate holes (utilizing non-threaded fuel rod end plugs) can be eliminated. Utilizing threaded end plugs to secure fuel rods within the lower tie plate also eliminates the need for conventional end plugs which protrude through the lower tie plate, and thus removes or eliminates concern for coolant flow impact resulting from such protruding end plugs. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.