Self locking nut for fuel assembly tie rods

A method and an apparatus for locking the tie rods to block rotation relative to either the upper or lower tie plates utilizing the required fuel bundle channel. A conventional lock nut is provided for threading to the upper threaded end of the tie rods. This lock nut has attached extending wings, these wings when rotated interfering with a placed fuel bundle channel. In installation to the fuel bundle, and assuming that the tie rod has been fully threaded to the lower tie plate, the winged lock nut is rotated overlying the upper tie plate to cause full engagement of the upper tie plate to the tie rods. In such fastening, the winged lock nuts are rotated to an angle that provides no interference with the subsequently placed channel. Thereafter, and when the channel is placed, the channel interference with the rotational path of the wings of the lock nut and thus prevents relative rotation between the tie rod and winged lock nut. The fastening of the wings to the lock nut leaves portions of the hex surface of the lock nut exposed. Thus when bundle disassembly is required, only removal of the channel followed by conventional counter or unthreading rotation of the winged lock nut with sufficient force to overcome the locking characteristic of the nut is required. Consequently, disassembly for inspection of an irradiated fuel bundle assembly is facilitated.

This invention relates to nuclear fuel bundles and the requirement that in 
such nuclear fuel bundles so-called "tie rods" be locked in their threaded 
relationship to upper and lower tie plates. More particularly, a 
conventional lock nut is provided with nut wings which move in 
interference with the fuel bundle channel to effect locking of the tie 
rods with respect to both the upper and lower tie plates. 
BACKGROUND OF THE INVENTION 
Nuclear fuel bundles are held together by tie rods. These tie rods having 
threaded engagement with the upper tie plate and lower tie plate of the 
fuel bundle. Beside contributing to the overall nuclear steam generation 
of the fuel bundles, the tie rods function mechanically in two ways. 
First, they maintain the fuel bundles as a unitary assembly. Secondly, 
they permit disassembly of the fuel bundles for inspection of the 
individual fuel rods interior of the fuel bundle during reactor outages. 
Accordingly, there is disclosed an improved method and apparatus for 
assuring that the tie rods and tie plates remain locking the fuel bundle 
into a unitary assembly. 
In order to understand this invention, the construction of a fuel bundle 
will first be discussed. Secondly, operation of the fuel bundle will be 
briefly set forth. Finally existing practice in the disassembly and 
inspection of such fuel bundles will be set forth. Once this has been 
done, the invention herein and its advantages can be fully understood. 
Fuel bundles for boiling water nuclear reactors include a plurality of 
side-by-side vertically upstanding sealed fuel rods, these fuel rods 
containing the fuel. In so far as the mechanical integrity of the fuel 
bundle is concerned, the fuel rods are divided into two classes. First, 
there is a class of fuel rods that is merely captured between the upper 
and lower tie plate within the fuel bundle These are the "ordinary" rods. 
Secondly, there is a class of fuel "tie rods" that attached through 
threaded engagement to the lower tie plate and through a threaded nut to 
the upper tie plate. These are the so-called "tie rods" which in effect 
tie the fuel bundle into a unitary assembly 
Over simplifying the construction of a nuclear fuel bundle, the lower tie 
plate functions to support the fuel rods and threads in engagement to 
typically eight tie rods. The upper tie plate--together with the lower tie 
plate--traps the ordinary fuel rods in vertical upstanding relation and 
ties to the tie rods at threaded nuts. Spacers are placed at regular 
vertical intervals to maintain designed spacing of the fuel rods for 
maximum nuclear efficiency. So-called water rods are placed at central 
locations in the fuel bundle and filled with water to improve the nuclear 
characteristics of the fuel bundle through improved neutron moderation. 
When all of these components are assembled together by securing the tie 
rods to the upper and lower tie plates, a nuclear fuel bundle which is a 
discrete separately handled unit of fuel for a boiling water nuclear 
reactor is created. 
The fuel bundles must under no circumstance come apart during operation of 
the boiling water nuclear reactor. This being the case, elaborate 
precautions are taken to make sure that the threaded attachment of the tie 
rods does not come apart. The major precaution taken is the so-called 
locking tab at the top of the fuel bundle. These locking tabs fit over 
adjacent paired tie rods on the upper side of the upper tie plate and are 
held between the upper tie plate on the bottom and the fastening nuts on 
the top. The function of these locking tabs is relatively easy to 
understand. 
First, the tie rods each have a slot at their respective upper end plugs. 
These slots mate with corresponding protruding nubs within the holes in 
the fastening tabs that extend around the tie rod end plugs. Remembering 
that each fastening tab fits around at least two tie rods, the tab itself 
cannot rotate. Further, when the nub fits into the slot in the tie rod, 
the tie rod cannot rotate relative to the locking tab and hence cannot 
rotate relative to the lower tie plate. 
The upper tie plate is fastened to the threaded upper end of the tie rods. 
A nut is used which threads over the tie rod at the upper end and 
compresses the upper tie plate onto a spring between the end of the tie 
rod and a hole in the upper tie plate. Once this nut is in place, securing 
the nut against inadvertent rotation is required. 
Securing the nut to the upper end of the tie rod occurs through strips or 
tangs coming up from the fastening tabs. These tangs are bent to fit 
against the sides of the hex nut utilized to fasten the upper tie plate. 
Since the fastening tabs cannot rotate, and the tangs from the fastening 
tabs contacting the sides of the hex nut prevent the nuts from rotation, 
the nuts fastening the upper tie plate are effectively locked in place. 
Thus, inadvertent disassembly of the tie rods, tie plates, and fuel bundle 
components held together by the tie rods and tie plates cannot occur. 
All fuel bundles in boiling water nuclear reactors have a fuel bundle 
channel placed over the assembled fuel bundle. This channel may or may not 
be replaced with the nuclear fuel bundle; channels frequently are utilized 
for at least a second cycle, even though the fuel bundle within the 
channel has been replaced. 
The fuel bundle channel is important when it comes to the operation of the 
fuel bundle. Specifically, the channel surrounds the fuel rods from the 
vicinity of the lower tie plate to the vicinity of the upper tie plate. It 
thus helps each fuel bundle to have an exclusive steam generating flow 
path which is separate from both the immediately surrounding core bypass 
region (which is liquid water moderator for improved nuclear performance) 
and the steam generating flow path through all remaining fuel bundles. In 
operation, water moderator enters the channel by passing through the lower 
tie plate and around the fuel rods. Water and generated steam leave the 
fuel bundle at the top of the channel typically by passage through the 
upper tie plate. 
Boiling water nuclear reactors operate for specific periods of time. 
Thereafter, they are taken off line, depressurized, and serviced at 
intervals herein called "outages." During such outages, 1/3 to 1/5 of 
their respective fuel bundles are replaced. At the same time, inspection 
of other fuel bundles can be required to occur. And when such inspection 
occurs, the fuel bundles need to be disassembled. Given the locking tabs 
now in use, this disassembly is not convenient. 
Once the fuel bundles have been within a nuclear reactor for a full cycle, 
they are the source of radiation. This being the case, disassembly and 
inspection of the fuel bundles must occur under a shielding water layer on 
the order of at least six feet of depth. Thus, all tools utilized function 
remotely under a water depth usually exceeding six feet. 
In order to disassemble the fuel bundles, the upper tie plates must be 
removed. First, the tangs of the locking tabs in contact with the side of 
the hex nuts are bent out of contact with the sides of the nuts. 
Thereafter, the nuts are loosened. Finally, the locking tabs are lifted 
and discarded. Where the fuel bundle is reassembled after inspection, this 
process is reversed with the discarded locking tabs being replaced. 
From the standpoint of reactor servicing, these required and laborious 
steps present two difficulties. This effort is time consuming, taking 
about 20 minutes. Reactors when off line are extremely expensive, costing 
hundreds of thousands of dollars per hour in lost utility revenue. Simply 
stated, time added in disassembly and reassembly is money lost in revenue. 
Secondly, and more importantly, although all disassembly operations are 
done over a holding pool where any radiation is minimal, it is required by 
both regulation and safety that all exposure to such radiation be kept as 
low as reasonably achievable. Accordingly, the scheme for locking of the 
tie rods according to this invention has been developed. 
SUMMARY OF THE INVENTION 
A method and apparatus for locking the tie rods to block rotation relative 
to either the upper or lower tie plates utilizing the required fuel bundle 
channel is disclosed. A conventional lock nut is provided for threading to 
the upper threaded end of the tie rods. This lock nut has attached 
extending wings, these wings when rotated interfering with a placed fuel 
bundle channel. In installation to the fuel bundle, and assuming that the 
tie rod has been fully threaded to the lower tie plate, the winged lock 
nut is rotated overlying the upper tie plate to cause full engagement of 
the upper tie plate to the tie rods. In such fastening, the winged lock 
nuts are rotated to an angle that provides no interference with the 
subsequently placed channel. Thereafter, and when the channel is placed, 
the channel interferes with the rotational path of the wings of the lock 
nut and thus prevents relative rotation between the tie rod and winged 
lock nut. The fastening of the wings to the lock nut leaves portions of 
the hex surface of the lock nut exposed. Thus when bundle disassembly is 
required, only removal of the channel followed by conventional counter or 
unthreading rotation of the winged lock nut with sufficient force to 
overcome the locking characteristic of the nut is required. Consequently, 
disassembly for inspection of an irradiated fuel bundle assembly is 
facilitated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a fuel bundle B is shown having upper tie plate 
T.sub.u, lower tie plate T.sub.l with fuel rods R extending therebetween. 
As is standard, a tie rod R.sub.t threads at lower tie plate T.sub.l. The 
improvement of this invention is illustrated. Tie rod R.sub.t has a Winged 
lock nut W securing tie rod R.sub.t to upper tie plate T.sub.u. Further, 
channel C is shown lowered about the fuel bundle, it being remembered that 
the fuel bundle proper may or may not include the channel when initially 
received at the reactor. 
Referring to FIG. 2, it will be seen that the winged lock nut W has the 
path of its wings 14 swinging in interference with channel C. 
Also, it can be seen that these wings 14 are downward curved at surfaces 
16. Thus if the wings have some interference with channel C, contact 
between wings 14 and surface 16 will cause rotation of winged lock nut W 
to a position of non-interference. 
Two additional embodiments can be seen in FIGS. 3 and 4. 
FIG. 3 illustrates winged lock nut W fastened at the bottom to washer 20. 
Washer 20, at a radial separation from the bottom of winged lock nut W, 
includes bent wire wings 22. The interval of radial separation is 
sufficient to allow socket 30 to screw and unscrew lock nut W. 
FIG. 4 illustrates an alternate embodiment in which winged lock nut W has 
paired wings 36 fastened to opposite hex sides of winged lock nut W. A 
portion of a socket 40 having slots 42 for accommodating wings 36 is shown 
overlying winged lock nut W. 
It will be understood that the winged lock nut W fastened to the top of a 
tie rod R.sub.t has interference with the channel. This interference has 
two effects. 
First, the locking properties of winged lock nut W prevent rotation of that 
nut relative to the upper tie plate T.sub.u. Release of the upper tie 
plate cannot occur. 
Second, the locking properties of winged lock nut W prevent rotation of tie 
rod R.sub.t. This being the case, tie rod R.sub.t rotation relative to the 
lower tie plate is prevented. Thus, where a lock nut is usually utilized 
for simple locking of a nut to a threaded shaft, the interference of the 
winged lock nut W with the channel both locks the nut to the tie rod at 
the upper end as well as locking the tie rod in threaded connection to the 
lower tie plate as well.