1. Field of the Invention
Example embodiment(s) are related in general to systems for aligning and handling fuel rods and water rods within a nuclear fuel bundle.
2. Description of the Related Art
A reactor core of a nuclear reactor plant such as boiling water reactor (BWR) or pressurized water reactor (PWR) has several hundred individual fuel bundles of fuel rods (BWR) or groups of fuel rods (PWR). During a planned plant outage for the BWR, selected irradiated fuel bundles are removed from the reactor core at the nuclear power plant and placed in a spent fuel pool for inspection and possible reconstitution of the bundle and/or maintenance. For example, there may be leaking fuel bundle which necessitates removing the irradiated fuel bundle from the core, as it is desirable to service these bundles in the event of a broken fuel rod and/or damaged fuel spacer grid which may be causing the leak. Additionally, when the fuel bundle is removed from the core and placed in the spent fuel pool, it is desirable to manipulate the fuel bundle for inspection purposes in order to search for additional possible sources of damage or leaks, and/or to rotate the bundle for general maintenance and measurement.
A typical fuel bundle for a BWR includes a plurality of fuel rods and centrally located water rods attached between an upper tie plate and a lower tie plate. For example, in FIG. 24A, there is shown a fuel bundle 15 for a BWR which includes a plurality of fuel rods 25 and one or more water rods (water rods obscured and which may or may not be centrally located within bundle 15), connected between an upper tie plate 30 and a lower tie plate 40.
FIG. 24B shows the same fuel bundle 15 as it would look upon removal from the core and prior to removal of the channel 20 for inspection and maintenance. The bundle 15 includes a generally rectangular channel 20 which extends the length of fuel bundle 15 and surrounds the fuel rods, water rods and upper and lower tie plates 30, 40. The channel 20 is an extruded alloy which encases the bundle 15. The fuel bundle 15 is typically delivered into the spent fuel pool via a fuel handling bridge (not shown) which is permanent machinery in reactor plants. The fuel handling bridge attaches to the upper tie plate bail (handle) 35 of the fuel bundle 15 to move the fuel bundle 15 from the core to the spent fuel pool.
Typically the fuel bundle 15 shown in FIG. 24B is centered over a fuel prep machine (FPM-not shown), and a carriage of the FPM is raised to receive the fuel bundle 15. As is well known in the art, the FPM is attached to a wall of the spent fuel pool in a nuclear power plant. Once the channeled fueled bundle 15 is place within the FPM, the channel 20 and upper tie plate 30 are removed to expose the fuel rods 25 and the fuel bundle 15 upper end for inspection and/or maintenance purposes.
Of note, with continued power operations of the reactor core with the irradiated fuel bundle 15, the fuel bundle 15 can be subjected to bow or twist. Twist/bow is caused by the amount of time the fuel bundle 15 has been in-service. In other words, the more the bundle 15 is used in an operating reactor core (i.e., the greater the exposure of the bundle in megawatt-days per short time (MWD/st), the greater the twist/bow potential. Accordingly, if the bundle 15 in the FPM exhibits twist or bow, it becomes substantially more difficult to remove selected fuel rods 25 in order to service/inspect the fuel bundle 15.
A fuel bundle exhibits twist and bow due to the growth of individual fuel rods over time and exposure within the core. In an example, a fuel bundle for a BWR is typically held together with a plurality of tie rods. The lower end plugs of the fuel rod screw into the lower tie plate 40, and the upper tie plate 30 slides in place over the fuel rods 25, water rods and tie rods. The upper end plug of the tie rods are threaded and receive nuts which secures the fuel bundle 15 together.
As the fuel rods 25 grow due to irradiation, the fuel rods 25 have little room to expand as they are sandwiched between the upper tie plate (UTP) 30 and lower tie plate (LTP) 40. The fuel rods do not all grow exactly the same amount, resulting in an uneven growth; this causes portions of the fuel bundle 15 to lengthen more than other areas within the bundle 15, producing what's known as bow and twist.
Most fuel bundle designs in BWRs (such as the fuel bundle 15) and PWRs include a plurality of fuel spacers 80, also referred to as spacer grids, which are axially spaced along the length of the fuel bundle 15. A typical fuel spacer 80 or spacer grid includes a plurality of cells or openings which accommodate the fuel rods and water rods there through. These fuel spacers 80 are generally not robust in construction, and can be damaged during routine in-service fuel inspections while removing and installing full and part-length fuel rods and water rods in the bundle within the spent fuel pool. The damage caused to the fuel spacers 80 could go unnoticed, and could cause additional damage to individual fuel rods 25 if a reconstituted fuel bundle (such as fuel bundle 15) is returned to power operations within the core. Accordingly, during removal and installation of the fuel rods 25 in a given irradiated fuel bundle 15 within the spent fuel pool, there is a substantial probability for fuel bundle component damage, either to the fuel rod itself, the spacers, the water rods or end plugs of fuel rods, which can occur during the in-service maintenance of the fuel rods within the spent fuel pool.
Further, as the removed fuel bundle 15 within the spent fuel pool is completely submerged, most inspections are done remotely and maintenance or repair is done by operators standing well above the fuel bundle 15, while utilizing a remote camera system and length handling poles with implements at ends thereon. The handling poles are inserted down through the fuel bundle 15 to remove/install selected fuel rods.
With the upper tie plate 30, the channel clip (not shown) and the channel 20 removed, workers typically utilize up to a 30-foot handling pole to perform maintenance, installation and/or removal of fuel rods 25. Particularly in the case of part-length fuel rods, which in some case are substantially shorter than full-length fuel rods, only the skill and experience of the handler of the handling pole ensures that a part-length rod can be safely extracted (or installed) without causing damage to the fuel spacers 80 or adjacent fuel rods 25. This is true even with the use of remote cameras positioned down in the spent fuel pool for monitoring the maintenance procedure.
Accordingly, conventional procedures for retrieving/installing a part-length fuel rod are time consuming if not impossible, cumbersome and must rely on the experience and skill of the operator manipulating the handling pole to avoid damaging a fuel spacer 80 or adjacent fuel rod 25. As fuel bundle designs are becoming even more complex, this inadvertent damage to the fuel spacers 80 and/or fuel rods 25 is even more likely without an adequate alignment and handling system.