Patent Application
20250100054
This disclosure relates to the field of nuclear facility systems and devices, including their operative safety and maintenance.
In a boiling water nuclear reactor, or BWR, there are eight high-strength alloy socket head cap screws (bolts) that retain Control Rod Drives (CRDs) by means of a seal flange and ring flange located on the underside of the pressure vessel of the reactor. The CRDs are the drive mechanism that inserts and retracts the Boron control rods. The control rods act as a moderator or throttle that slows the fission process between the fuel bundles of a nuclear reactor. These CRDs require maintenance to ensure that this throttling action remains reliable. By way of example, a Boiling Water Nuclear Reactor may have 185 CRDs, such as those made by GE and Hitachi. It is common practice that nuclear facilities undergo routine maintenance (pursuant to their operating licenses, state and federal licenses and guidelines etc.) wherein at least 10% of the CRDs (e.g. 19 CRD units) must be serviced per each refuel outage. In some cases, this routine maintenance occurs every 2 years. Much of the planned maintenance on nuclear reactors occurs during refuel outages because much of the plant is inaccessible during full power operations due to elevated radiation dose rates (i.e. “dose”), heat, and other hazards. Dose rates are tracked on anyone exposed to radiation, and are a key consideration in any repair solution. The goal is to keep this exposure (dose rate) to a minimum. Anything that reduces exposure is highly desired in the nuclear industry. The less time spent in a high dose area doing work, the better. Similar to an X-Ray technician, exposure limits are regulated by Federal guidelines that allow a certain dose per year for a worker in a nuclear facility. Although maintenance and repairs are performed during refuel outages, time is of the essence in scheduling and performing the work, including accounting for unforeseen and emergent issues that may arise.
During removal of the bolting on a given CRD, one or more of the bolts may become unremovable, most likely due to a galled thread formed as a result of the bolt/flange interface being stainless steel on stainless steel. This problem can also be discovered in the context of reactor pressure vessel (RPV) leakage testing (i.e. hydrodynamic testing, or “hydro”) in which the pressure vessel is tested prior to reactor startup. The bolt would need to be drilled out to the tap drill size, the head of the bolt removed, and the CRD extracted and then re-threaded. Contractors report that stuck bolts are an industry wide problem in this context. Related services typically encompass removal of the seal rings, inspection of seal ring surfaces, and replacement of seals and reinstallation. If unrepairable, an engineered thread insert, or other repair method would need to be sought. Since the extent of damage to the threaded area of the bottom mounting surface of the vessel is not yet established, it is critical to maintain the existing hole center and angle in order to repair the damaged thread while minimizing the amount of repair to avoid compromising vessel integrity.
Conventional solutions to this problem involve improvised processes and machines that are risky, heavy, cumbersome, and dangerous in an already less than ideal work environment. Under vessel work comes with a high (radiation) dose, in an uncomfortable confined space, and work is done wearing powered air-purifying respirators (PAPRs). The configuration and contamination risk makes it difficult to get some traditional tools into the area. Thus, extraction times are long, involve a high dose, and are costly. In some instances, a stuck bolt can result in $250,000 of labor and material costs, 3 to 5 days of work, and an excessive dose (e.g. 3.8 man/rem dose expended over 3 days). The industry standard roentgen equivalent man (rem) is a CGS (Columbia Generating Station) unit of equivalent dose, effective dose, and committed dose, which are dose measures used to estimate potential health effects of low levels of ionizing radiation on the human body. Quantities measured in rem are designed to represent the stochastic biological risk of ionizing radiation, which is primarily radiation-induced cancer.
An adaptive drilling device for extracting bolts from a control rod drive mechanism (CRDM) in a boiling water nuclear reactor includes a guide assembly configured to align and secure drilling activity, a drill assembly configured to drive a drill bit into a CRDM bolt, and a thrust assembly configured to linearly and bidirectionally drive the drill assembly. The guide assembly has three guide plates and a pair of guide rods. The drill assembly includes a gear reduction drive motor, drill bearings, a drill bit, and a securing subassembly configured to secure the drill assembly to a first guide plate. The thrust assembly includes a thrust handle connected to the drive motor, a central plate nut, a lead screw running through the central plate nut and connected to the thrust handle, and a feed wheel connected to the lead screw.
In the following discussion, numerous specific details are set forth to provide a thorough understanding of the disclosed subject matter. However, those skilled in the art will appreciate that the present disclosed subject matter may be practiced without such specific details. In other instances, well-known elements, processes or techniques have been briefly mentioned and not elaborated on in order not to obscure the disclosed subject matter in unnecessary detail and description. Moreover, specific details and the like may have been omitted inasmuch as such details are not deemed necessary to obtain a complete understanding of the disclosed subject matter, and are considered to be within the understanding of persons having ordinary skill in the relevant art.
The present invention provides a device and methods designed to address problems with a CRD mechanism (CRDM) located on the underside of a pressure vessel found in a boiling water nuclear reactor (BWR) in a safer, more efficient, and more cost-effective manner. During routine maintenance or hydro procedures, one or more main flange bolts securing a main seal flange of a CRDM can get stuck, the associated flange may leak, and nuclear plant workers may be unable to extract one or more main flange bolts. The present invention provides an adaptive drilling device designed to address such problems with an under-vessel CRDM in a safer, more efficient, and more cost-effective manner.
The device and methods of the present invention were tested in a mock-up work area replicating an actual under-vessel configuration, on the bottom of a nuclear reactor. Typically, CRDMs and associated instrumentation wiring are spaced about 16 inches apart from each other. This working environment presents 185 m/rem per hour dose rates, with 15-minute work stay times per jump. Typically, there is an approximately 90-110° F. ambient temperature in the work area. Technicians wear triple protective jumpsuits having powered air-purifying respirators (PAPRs). Dose rates are tracked on anyone exposed to radiation. Radiation exposure must be kept to a minimum, per industry guidelines. Anything that cuts exposure is highly desired in the Nuclear Industry. The less time spent working in a high dose area, the better. There are federal limits for how much dose-per-year a worker is allowed, similar to the limits for X-Ray technicians. Overall, the present invention allows work (drilling out a stuck bolt) to be done faster and more efficiently, thus reducing time in a high dose area and mitigating the accumulated dose.
An exemplary adaptive drilling device of the present invention is a custom drilling machine that attaches to a bolt pattern, providing a solid, accurate means to drill out, or extract, and rethread an affected fastener. Using the present device for BWR repair work reduces time in a high dose area, therefore mitigating the accumulated dose. Referring to
An exemplary guide assembly comprises three guide plates, including a top spindle plate, or first plate 16, a middle feed (or drive) plate, or second plate 17, and a bottom stabilizer plate, or third plate 18. The guide plates are positioned along a central height axis C running through the adaptive drilling device 1. Each guide plate comprises a top surface and an opposing bottom surface, as well as an edge surface. As seen in
An exemplary guide assembly further comprises a pair of linear guides, or guide rods 35, each rod having a length and a diameter, the guide rods 35 being parallel to each other while running centrally through the guide holes 25 along the rod lengths. A first guide rod 35a runs through the first set of three guide holes, that set axially aligned along guide hole axis G1, while a second guide rod 35b runs through the second set of three guide holes, that set axially aligned along guide hole axis G2. The guide plates are slidably engaged with the guide rods 35. The spindle plate 16 has an attached pair of linear bearings 41 configured to facilitate linear plate motion along the guide rods 35 which run through the bearings 41, the bearings further providing support for axial movement as well. The second/feed plate 17 and third/stabilizer plate 18 each have a lockable position along the guide rods 35. Each of these locking plates includes a pair of sliding locks 28 entering through the edge surface of the plate and running through the body of the plate. Each pair of sliding locks 28 can lock and unlock its respective plate's position along the guide rods 35. Each sliding lock 28 further comprises a rotatable handle 28a and a shaft 28b (shown with dashed lines as it is here hidden within the body of the guide plate), the shaft being the portion of the lock 28 that interfaces with the body of the guide plate. Each guide rod 35 comprises a threaded end 36, these threaded ends projecting away from the first guide plate's top surface 16a. Each guide rod 35 further comprises a cross-drilled pass-through hole, or pin pass-through (see pin pass-through 37 of
An exemplary drill assembly comprises a variable speed, gear reduction drive motor, or drill 55, a drive shaft 56 with shaft opening 56a, a bushing 59 nested within the drive shaft 56, and a drill bit 60 secured within the bushing 59. The drill bit 60 has a length, and is rotatably driven by the drive motor 55. The drill bit 60 is axially aligned with the central height axis C along its bit length. The drill assembly further comprises a securing subassembly (see securing subassembly components of
An exemplary thrust assembly comprises an offset coupler, or thrust handle 80 connected to the drive motor 55, an Acme hex central plate nut 78 (see the labeled example shown in
An exemplary drive motor 55 is positioned between the spindle plate 16 and feed plate 17. The long drill bit 60 runs centrally through the spindle plate's central hole (see central hole 26 and spindle/first plate 16 of
Referring to
The stabilizer plate 18 is depicted with threaded anchoring holes 18e, which run fully through the plate's height. Both the feed plate 17 and stabilizer plate 18 each further comprise the aforementioned pair of receiving holes 17d and holes 18d, respectively, running fully through the plate height, these receiving holes being smaller in size than the guide holes 25, each receiving a roll pin 29. Each exemplary guide hole 25 of the feed plate 17 and stabilizer plate 18 further includes a curved inner contact opening 27a within the guide hole 25 and orthogonally positioned relative to the guide hole's top and bottom surface openings (see bottom surfaces 17b and 18b of
The pair of quick-release detent pins 40 are slidably receivable by the linear guides 35 through their cross-drilled holes 37, with one detent pin received per linear guide. Each detent pin 40 comprises pin portion 40a with ball detent 40d, the pin portion running through the pin pass-throughs 37. The received detent pins 40 provide a stop to prevent the sliding of guide plates and associated assemblies and mechanisms beyond the linear guides 35. The pin pass-throughs 37 can further receive standard steel rods, the inserted steel rods manipulatable to substantially tighten the installed linear guides 35 into a more secure operational configuration.
Referring to
Referring to
The ball screw feed mechanism, or thrust assembly 75 mates with the drill 55 via the aforementioned thrust handle 80. Nested within the thrust handle 80 are a thrust bearing 81 and a standard washer 86, these nested elements secured by a cover plate 82 and securing screws 85. The lead screw 77 runs through the thrust handle 80 and associated/nested components, the lead screw 77 further running through the central plate nut 78 and mating at one end with the hand feed wheel 76 via a standard washer 86 and securing screw 85.
Referring to
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Beginning the method, step 201 includes an assessment of the removability of two main flange bolts on both sides of the affected bolt. Step 202 continues this assessment with a decision on the assessment, if “yes”, the bolts are removable and a user can do so, as indicated by step 203. Following this decision, step 205 includes threading one linear guide 35 into either one of the vacant bolt holes 91a and snugging hand tight. A user can thread the other linear guide 35 into the other vacant hole 91a until it just starts to snug, yet is still somewhat floating.
If in the above assessment of step 202, the bolts 91b are not removable, it may become necessary to use a set of supplied adapter nuts 99, that is if one or more adjacent bolts are nonremovable. The adapter nuts 99 enable the linear guides 35 to still be attached to the head of the existing cap screws of the flange bolts 91b, as indicated by step 204. Step 205 may then follow step 204 normally. It is always preferable to use existing bolt holes 91a or only one adapter nut 99 for safety reasons, except in extreme cases of multiple stuck bolts. Extreme usage scenarios are regarded as a contingency measure, to only be carried out when other options are not available. The offset of the guide rods 35 when using the one hole 91a and one adapter nut 99 configuration will not affect proper functioning of the adaptive drilling device 1. A safety strap or some sort of restraining device should be employed to prevent the device 1 from freefalling should it become unattached. Drilled and tapped holes are placed in the stabilizer plate to provide a secondary anchor point for convenience.
Step 206 includes sliding the top spindle plate 16 and feed plate 17 onto the guide rods 35 as a unit. The guide plates should be placed into a comfortable position mid-way up the guide rods 35. The sliding locks 28 should then be set on the feed plate 17 firmly, as indicated by step 207. The stabilizer plate 18 should be slid on about two inches above the cross-drilled pin pass-through holes 37 in the guide rods 35, as indicated by step 208. The locks 28 should be set on the stabilizer plate 18 firmly, as indicated by step 209.
While keeping one hand on the feed plate 17 for safety, a user should insert the detent pin 40 into the pin pass-through 37 in the first rod 35 that was hand tightened, as indicated by step 210. The detent pins 40 provide a safety stop if all locks 28 were accidently released and the frame/guide assembly was allowed to freefall downward. It is very important to keep a hand or a lock 28 engaged on the guide assembly to prevent gravity from creating a condition that could possibly injure the user/operator, or damage the adaptive drilling device 1. Most of such conditions are the result of operator error. Once both locks 28 on the first guide rod 35 are secure and safety detent pins 40 are in place, the user can release the locks on the second guide rod 35 and snug hand tight. With the use of a steel rod, the cross-drilled pin passthroughs 37 can be used to sufficiently tighten the guide rods 35. The stabilizer plate 18 can now be re-locked. Holes in the stabilizer plate 18 provide an anchor point for a piece of steel strap in case triangulation/bracing to white iron is desired.
At this point, an operator may want to install a supplied foot control pedal 96, as indicated by step 211. Doing so requires the finger switch on the drill 55 to be depressed by a hook and loop binder strap into a full “on” position. Should a drill bit 60 or cutting tool hang up, the foot pedal 96 provides a quick and easy way of shutting the adaptive drilling device 1 off and providing an extra level of control during cutting, allowing the operator to concentrate on feed control for the spindle plate 16 and spindle assembly 50 via the hand feed wheel 76.
Referring to
Step 303 indicates drilling into the head of the bolt 91b. While drilling, an operator or assistant should clear chips and allow heat to subside often, with a goal of cutting to a maximal depth. A chosen approved cutting fluid should be chosen as needed. Quick retraction and setting of the spindle 50 and drill 55 can be achieved by unlocking or locking the drive/feed plate 17 to facilitate power frame/guide assembly movement while the feed/lead screw 77 is traveling linearly. While drilling, the operator should feed at a rate that produces an ample chip, but not so slow that it rubs.
It is crucial to control heat and cutting action, as the bolts 91b can work-harden. Excellent quality cobalt drills are recommended. The operator should not settle for cheap drills. If the drill 55 or bit 60 is not cutting well, it should be promptly replaced. A 0.500 Dia. Jobber Length drill, a 0.625 Dia. Jobber Length drill, and a 0.625 Dia. Extended Length drill can be utilized in the same method as outlined above. A 0.500 Dia. Extended Length drill may also be utilized in the same manner as discussed above, until through the bolt 91b, feeding lightly as the bit 60 breaks through.
At this point, while the head of the bolt 91b is still intact, removal of the bolt should be attempted, as indicated by step 304. It is highly likely that the internal stresses have been relieved from the bolt, such that it may unscrew without further work, this successful result indicated by “yes” at step 305.
If extraction is not achieved, as indicated by “no” at step 305, the head of the bolt 91b can be drilled out to 0.875. About 1″ to 1.250″ in, the head of the bolt 91b will detach from the remaining body of the bolt, as indicated by step 306. Removal of the flange and the attached mechanism will now be possible, as indicated by step 307.
After Flange removal, any excess bolt shank should be removed flush, as indicated by step 308. This will leave only the remaining stuck portion of the screw threads that can be drilled to 0.875 (tap drill Dia. for 1.00″×8 thread), as indicated by step 309.
A 1″-8″ pitch taper tap can then be used with a tap handle as a driver to “chase” the remaining threaded hole, as indicated by step 310. Care should be exercised to gently allow the tap to follow the pre-existing thread, dead center. If there is a positive result in step 310, as indicated by “yes” at step 311, the current project is complete, as indicated by step 312. The tap handle is provided as a guide to ensure the tap is supported and remains straight and on location. Power tapping is not an option and should not be attempted.
The adaptive drilling device 1 is provided with an array of standard size bushings that will accommodate most any drill or annular-type cutter that may be used to prepare the bolt hole 91a for further repair. In the case of a catastrophically damaged thread, an operator should refer to Nuclear Plant Engineering for possible thread repair insert options and preparation, as indicated by step 313. As always, safety should be the number one priority, and the adaptive drilling device 1 should be “doing all of the work”.
In an exemplary embodiment, the drill motor 55 and controls of the adaptive drilling device 1 are made from easily sourced, inexpensive hardware that can be easily replaced if contaminated. The remainder of the components are custom-machined and fabricated in a manner to allow for decontamination.
Testing a full-size mock-up of a BWR's under-vessel, the following design criteria were established for addressing certain needs arising during usage of the adaptive drilling device 1:
Additionally, the above mock-up test yielded the following results:
By way of an exemplary scenario, if a CRDM in question was bolted back up, it could be addressed with the adaptive drilling device 1 at the nuclear facility's next refuel outage. In the short term, this offers a reliable contingency plan in the event that the CRDM in question, or any other CRDM in a set, presents a problem during the next hydrodynamic testing of the pressure vessel prior to startup. If the hydrodynamic testing does not go well going into startup, then the adaptive drilling device 1 can be used to effectively address the problem. As such, the adaptive drilling device 1 can provide a safety net or solution in the event of CRDM leakage during the testing. Otherwise, moving forward with the testing would be risky due to potential CRDM leakage.
Many variations may be made to the embodiments described herein. All variations are intended to be included within the scope of this disclosure. The description of the embodiments herein can be practiced in many ways. Any terminology used herein should not be construed as restricting the features or aspects of the disclosed subject matter. The scope should instead be construed in accordance with the appended claims.
There may be many other ways to implement the disclosed embodiments. Various functions and elements described herein may be partitioned differently from those shown without departing from the scope of the disclosed embodiments. Various modifications to these implementations may be readily apparent to those skilled in the art, and generic principles defined herein may be applied to other implementations. Thus, many changes and modifications may be made to the disclosed embodiments, by one having ordinary skill in the art, without departing from the scope of the disclosed embodiments. For instance, different numbers of a given element or module may be employed, a different type or types of a given element or module may be employed, a given element or module may be added, or a given element or module may be omitted.
It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein.
| Number | Date | Country | |
|---|---|---|---|
| 63584737 | Sep 2023 | US |
