NLS2024006, Response to Nuclear Regulatory Commissions Request for Additional Information for Relief Request RC3-02
| ML24025A054 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 01/25/2024 |
| From: | Dia K Nebraska Public Power District (NPPD) |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| NLS2024006 | |
| Download: ML24025A054 (1) | |
Text
H Nebraska Public Power District NLS2024006 January 25, 2024 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001
Always there when you need us"
Subject:
Response to Nuclear Regulatory Commission's Request for Additional Information for Relief Request RC3-02 Cooper Nuclear Station, Docket No. 50-298, DPR-46 50.55a
References:
- 1.
Email from Thomas Byrd, U.S. Nuclear Regulatory Commission, to Linda Dewhirst, Nebraska Public Power District, dated December 18, 2023, "Cooper - RAI - Relief Request RC3-02 Drywell Head Inspections (EPID L-2023-LLR-003 5)"
- 2.
Letter from Bill Chapin, Nebraska Public Power District, to the U.S. Nuclear Regulatory Commission, dated June 27, 2023, "10 CFR 50.55a Relief Request RC3-02"
Dear Sir or Madam:
The purpose of this letter is for the Nebraska Public Power District to respond to the Nuclear Regulatory Commission's request for additional information (RAI) (Reference 1) related to the Cooper Nuclear Station relief request for containment drywell head bolting examination requirements (Reference 2).
The responses to the specific RAI questions are provided in the attachment to this letter.
This letter does not contain any new regulatory commitments.
If you have any questions concerning this matter, please contact Linda Dewhirst, Regulatory Affairs and Compliance Manager, at (402) 825-5416.
Sincerely,
~-
Khalil Dia Site Vice President
/dv COOPER NUCLEAR STATION 72676 648A Ave/ P.O. Box 98 / Brownville, NE 68321 http://www.nppd.com
NLS2024006 Page 2 of2
Attachment:
Response to Nuclear Regulatory Commission Request for Additional Information (RAI) cc:
Regional Administrator w/ attachment USNRC - Region IV Cooper Project Manager w/ attachment USNRC - NRR Plant Licensing Branch IV Senior Resident Inspector w/ attachment USNRC-CNS NPG Distribution w/ attachment CNS Records w/ attachment
NLS2024006 Attachment Page 1 of7 Attachment Response to Nuclear Regulatory Commission Request for Additional Information (RAI)
Cooper Nuclear Station, Docket No. 50-298, DPR-46 The Nuclear Regulatory Commission request for additional information regarding Relief Request RC3-02, is shown in italics. The Nebraska Public Power District (NPPD) response to the request is shown in normal font.
RAJ-I Discuss the design analysis or qualitative factors of the containment drywell head bolted connection demonstrating that structural integrity and leak tightness of the drywell is maintained, assuming the bolt connections that could not be examined are degraded (e.g., bolts could not be tightened).
NPPD Response:
During the reassembly process at the end of each refueling outage, the drywell head flange and accessible bolts/nuts/washers are cleaned, visually examined and threads lubricated prior to assembly as a routine maintenance practice. Once the head is in place, the bolting is torqued in three passes to achieve the final 880 ft-lbs of torque. Degraded threads in the nuts would most likely prevent the bolting assembly from achieving the required torque. The combination of torquing the head bolts to required specifications and conducting the As-Left Local Leak Rate Test (LLRT) of the flange joint each refueling outage, provides reasonable assurance joint integrity is established for the subsequent 24 months of operation.
The results of the American Society of Mechanical Engineers (ASME)Section XI direct VT-1 inspections of the disassembled bolting were SAT. The nuts/washers that are tack welded to the bottom side of the drywell flange were examined remotely using the General Electric Hitachi (GEH) VT-1 procedure. The remote visual examination was performed when the refueling cavity was temporarily flooded for refueling operations. High-definition underwater cameras calibrated to VT-1 requirements were lowered from the refueling platform and positioned to examine the accessible portions of each nut/washer including portions of the nut threads that could be observed. In addition, the examination was electronically recorded allowing for improved ability to compare examination results with future results for improved monitoring of potential degradation changes. As stated in the Refueling Outage 32 (RE32) remote VT-1 data sheet VT-IWE32-22-033 comments, the VT-1 examination was considered best effort because of the limited access as well as the observed surface condition related to rust, flaked, blistered, and peeled paint. The tack welded nuts/washers were not cleaned due to dose considerations and risk of introducing debris into the reactor coolant if not completely removed from the refueling cavity. As noted in the Relief Request, Cooper Nuclear Station (CNS) estimates a dose of 3.5 Rem would be accumulated to additionally clean the nuts/washers for little additional gain of examination coverage.
NLS2024006 Attachment Page 2 of7
RAI-2
(a) Discuss the test pressure when performing the leak rate test per 10 CFR 50, Appendix J.
(b) Discuss the hold time to determine potential leakage at the drywell closure head flange.
(c) Discuss how the O-rings are being pressurized to determine the leakage of the drywell closure head flange per 10 CFR 50, Appendix J.
( d) If leakage does occur during the leak rate test, discuss the corrective action.
NPPD Response:
(a) A 10 CFR 50, Appendix J Type B LLRT, which is a pneumatic leakage test of the drywell head to drywell shell flange joint, is conducted after each refueling outage. Per the CNS test procedure, 6.PC.525, steps 27.11 to 27.13, the test volume between the drywell head flange O-rings (approximately 0.475 ft3) is pressurized between 58.0 to 63.8 pounds per square inch gauge (psig) and held until the leakage rate is stabilized. The AS LEFT test for RE32 was completed SAT with an actual test pressure of 60.19 psig. It should be noted that per ASME Section XI, IWE 5223.4(b) a Type A, B, or C test in accordance with 10 CFR 50 Appendix J is considered acceptable pneumatic leakage tests following repair/replacement activities for Class MC components thus providing a high degree of confidence of mechanical joint integrity. Note that the disassembly of the drywell bolting and reassembly is a maintenance activity and not a repair/replacement activity.
(b) A 10 CFR 50, Appendix J Type B LLRT pneumatic leakage test of the drywell head to drywell shell flange joint is conducted after each refueling outage. Per CNS test procedure, 6.PC.525, steps 27.11 to 27.13, the test volume between the drywell head flange O-rings (approximately 0.475 ft3) is pressurized between 58.0 to 63.8 psig and held until the leakage rate is stabilized. This takes approximately 10 - 15 minutes, however, there is no stipulated wait time as it ultimately is based on how long the test volume takes to stabilize.
( c) After each refueling outage, the drywell head bolts are tightened to a torque specification of 880 ft-lbs and drywell head to drywell shell flange joint is tested with a 10 CFR 50, Appendix J Type B LLR T pneumatic leakage test per the CNS Procedure 6.PC.525. Section 27 of the procedure provides the steps to test the volume between the drywell head flange O-rings (approximately 0.475 ft3). The test volume is pressurized to between 58.0 to 63.8 psig and held until the leakage rate is stabilized. The leakage rate is recorded on Attachment 1 of the procedure and compared to the Administrative Limit and Operability Limit. The AS LEFT test for RE32 was completed with an actual test pressure of 60.19 psig and an as-left leakage rate of 0.042 standard cubic feet per hour (scfh), well below the Administrative Limit of 0.5 scfh.
(d) During the 10 CFR 50, Appendix J Type B LLRT pneumatic leakage test of the drywell head to drywell shell flange joint conducted after each refueling outage per CNS Procedure 6.PC.525, the leakage rate is recorded on Attachment 1 per step 27.13. If the leakage rate is greater than the Administrative Limit of::; 0.5 scfh or the Operability Limit of::; 1.0 scfh, a condition report is initiated per step 41.3 of the procedure and will be assessed in accordance with the Corrective Action Program. The corrective action process
NLS2024006 Attachment Page 3 of7 will be entered to determine the cause and appropriate actions to be taken at that time, which could include replacing bolting, as needed. Before plant startup is authorized, a satisfactory test that meets the acceptance criteria of 6.PC.525 would need to be performed.
RAl-3 (a) Discuss how the six lower spherical washers and 33 nuts that were able to be removed and examined by VT-1 visual examination represent a reasonable sample that would detect degradation in these components if degradation occurred.
(b) Are there any asymmetric load conditions with respect to azimuthal bolting location that could cause certain bolting locations to be at higher stress conditions?
(c) Are there any unique environmental conditions around the drywell closure circumference in the subject bolting regions that could cause general corrosion of the nuts and lower spherical washers?
NPPD Response:
(a) The bolting was examined based on a combination of both the ASME Section XI direct and remote examinations that were conducted. All 76 bolts, 33 nuts, and 6 washers were examined by direct inspection with no concerns identified. The 33 nuts that were examined directly achieved 100% coverage and represent 43% of all the nuts in the exam population. ASME Section XI states for bolting that is not disassembled, the bolting can be examined in place under tension. However, in this situation, the 43 nuts and the 70 lower spherical washers that were partially disassembled and partially accessible were examined remotely using a high-definition underwater camera with assistance of the Reactor Pressure Vessel (RPV) invessel visual inspection (IVI) crew to gain as much examination coverage as possible using a VT-1 procedure and IWE NDE Level II examiner. The remote visual examination was performed when the refueling cavity was temporarily flooded for refueling operations. High-definition underwater cameras calibrated to VT-1 requirements were lowered from the refueling platform and positioned to examine the accessible portions of each nut/washer including portions of the nut threads that could be observed. Utilizing high-definition cameras versus performing a direct visual examination utilizing a mirror provides a superior method of conducting the visual examinations because of the enhanced clarity provided by the high-definition cameras. In addition, the examination was electronically recorded allowing for improved ability to compare examination results with future results for improved monitoring for potential changes in degradation. Combined, CNS obtained approximately 63% of the examination coverage based on the number of components examined.
The purpose of conducting ASME Section XI examinations is to detect service induced degradation. ASME Section XI Inservice Inspection Programs rely on a sampling process to provide reasonable confidence of structural integrity of components within the scope of the program. As an example, ASME Section XI requires 15% of all Class 2 supports to be examined in a 10-year ISI interval. CNS achieved 63% coverage of the drywell head bolting and while not 100% as required by Code when disassembled, the 63% sample size does provide a reasonable sample population to ensure overall joint integrity and certainly
NLS2024006 Attachment Page 4 of7 when factoring in the torquing and pressure testing processes that occur after the bolting reassembly. All the bolts were torqued to the same specification so the loading on the bolts is assumed to be uniform. Because of the uniform preload applied to the bolting, it is reasonable to assume that if service induced degradation occurred to one of the 43 tack welded nuts that is only partially accessible for examination, that similar degradation would be evident in at least one or more of the 33 disassembled nuts that are examined directly.
(b) The drywell head is installed per Section 13 of CNS Procedure 7.4REASSEMBL Y. Per step 13.3.18 the hold-down bolts are progressively torqued to 880 ft-lbs in three increments (300 ft-lbs, 600 ft-lbs, 880 ft-lbs) using the progression from Attachment 6, Figure 5. The final torquing pass is performed with a Quality Control (QC) Witness.
Using this consistent process results in symmetrical loading conditions.
( c) Environmentally, the lower nuts and washers reside in a refueling bellows area that is located in the refueling cavity that remains dry during normal operation. However, during refueling operations, the nuts/washers that are tack welded to the bottom of the drywell shell flange, become submerged for several weeks during refueling operations when the refueling cavity is flooded. This cycling of wetting/drying has caused the coating on the nuts/washers to become degraded resulting in indications of surface rust, flaking and peeling paint, and under-deposit corrosion. The areas of under deposit corrosion on the outer surfaces of the tack welded nuts are estimated at less than 5% of the bolting's cross-sectional thickness and therefore do not challenge the ability of the nuts to maintain structural integrity of the drywell head joint. Partial views of the nut threads during the remote examination did not see evidence of degradation that could additionally challenge the ability of nuts to maintain the proper preload when torqued to the required specifications. If significant degradation did occur to the threads and was not observed by the inspections during the maintenance reassembly process, then it would be unlikely that the torquing specification could be met, nor could the joint maintain an acceptable seal when the follow-up Type B pneumatic test is performed. If the bolting could not achieve the required torque specifications, the bolting would be replaced and retorqued to acceptable limits prior to performance of the As-Left LLRT.
Once the joint is reassembled and tested SAT, it is reasonable to assume the bolting will perform its intended design function for the next 24 months until the next refueling outage.
RAI-4
(a) Describe best effort remote VT-1 visual examinations on these remaining tack-welded nuts and washers using underwater cameras in addition to the 63 percent direct VT-1 visual examinations mentioned in the submittal.
(b) Describe the bolting reassembly process and requirements.
NPPD Response:
(a) The drywell head bolting was examined using a combination of both the ASME Section XI direct and remote examinations. All 76 bolts, 33 nuts, and 6 washers were examined
NLS2024006 Attachment Page 5 of7 by direct inspection with no concerns identified. The 33 nuts that were examined directly achieved 100% coverage and represent 43% of all the nuts in the exam population.
ASME Section XI states for bolting that is not disassembled, the bolting can be examined in place under tension. However, in this situation, the 43 nuts and the 70 lower spherical washers that were partially disassembled and partially accessible were examined remotely using a high-definition underwater camera with assistance of the IVI crew to gain as much coverage as possible using a VT-1 procedure and IWE NDE Level II examiner. The remote examination was performed when the refueling cavity was flooded and the nuts/washers submerged. The camera was lowered from the refueling cavity platform and manually manipulated accordingly to maximize examination coverage. The camera was calibration checked to VT-1 character card requirements prior to conducting the examination. The examinations were also recorded allowing those examination results to be compared to future examinations to additionally assess the overall condition of the fasteners. Combined, CNS obtained approximately 63% of the examination coverage based on the number of components examined.
The term "best effort" was noted in the comments on the remote VT-1 data sheet from RE32 (datasheet VT-IWE32-22-033). The note states that the examination was considered a "best effort" VT-1 examination due to the limited access and surface condition that exhibited signs of light to heavy rust and the paint coating to be flaking, blistering, and peeling with some signs of under-deposit corrosion, however, with none reducing the cross-sectional thickness by more than 5%. General Electric Procedure GEH-VT-108, Rev 1 was used for this examination. Conducting a more rigorous cleaning process to improve the surface area for examination was considered however, due to the limited access, it is unlikely the hydro laser would remove enough of the flaky paint and surface corrosion to yield a meaningful increase in coverage.
(b) During reassembly of the drywell head at the end of each refueling outage, the drywell head flange and accessible bolting/washers are cleaned, visually examined and threads lubricated prior to assembly as routine maintenance practice per CNS Procedure 7.4Reassembly. Once the head is in place, the bolting is torqued in three passes to achieve the final 880 ft-lbs of torque with QC Witness applied. Degraded threads in the nuts would most likely prevent the bolting assembly from achieving the required torque. If the bolting could not achieve the required torque specifications, the bolting would be replaced and retorqued to acceptable limits prior to performance of the As-Left LLRT. The combination of torquing the head bolts to required specifications and conducting the As-Left LLRT of the flange joint each refueling outage, provides reasonable assurance that joint integrity is established for 24 months of plant operation until the next refueling outage.
RAI-5
(a) Summarize examination results, conclusions, and disposition of non-conformances for direct and remote VT-1 examinations.
(b) Evaluate whether these lower spherical tack-welded and corroded washers and nuts are acceptable.
(c) Evaluate whether these lower spherical tack-welded and corroded washers and nuts will result in loose nuts or bolts that may affect the intended function of the pressure retaining bolting.
NLS2024006 Attachment Page 6 of7 NPPD Response (a) The drywell head bolting was examined using a combination of both the ASME Section XI direct and remote examinations. All 76 bolts, 33 nuts, and 6 washers were examined by direct inspection with no rejectable indications (Report No. VT-IWE32-22-032). The remaining 43 nuts and the 70 lower spherical washers that were partially disassembled and partially accessible were examined remotely with no rejectable indications (Report No. VT-IWE32-22-032).
The 76 bolts and 6 washers that were examined directly achieved 100% coverage. The 33 nuts that were examined directly achieved 100% coverage and represent 43% of all the nuts in the exam population. Combined, CNS obtained approximately 63% of the examination coverage based on the number of components examined.
ASME Section XI states for bolting that is not disassembled, the bolting can be examined in place under tension. However, in this situation, the 43 nuts and the 70 lower spherical washers that were partially disassembled and partially accessible were examined remotely using a high-definition underwater camera with assistance of the RPV IVI crew to gain as much coverage as possible using a VT-1 procedure and IWE NDE Level II examiner. The term "best effort" was noted in the comments on the remote VT-1 data sheet from RE32 (datasheet VT-IWE32-22-033). The note states that the examination was considered a "best effort" VT-1 examination due to the limited access and surface condition that exhibited signs of light to heavy rust and the coating to be flaking, blistering, and peeling with some signs of under-deposit corrosion, however, with none reducing the cross-sectional thickness by more than 5%. General Electric Procedure GEH-VT-108, Rev. 1 was used for this examination.
Conducting a more rigorous cleaning process to improve the surface condition for examination was considered. However, due to limited access, it is unlikely the hydro laser would remove enough of the flaky paint and surface corrosion to yield a meaningful increase in examination coverage.
(b) During the RE32 remote examination, flaking paint, surface and under-deposit corrosion was observed. The corrosion appears to be on the outer surface of the nuts. The under-deposit corrosion observed on the outer surface of the nuts is estimated at less than 5% of the cross-sectional thickness as noted on the RE32 remote data sheet. During the remote VT-1 examination, some views of the nut threads could be seen with no degradation observed. Direct VT-1 examination of the bolts in the mating location where the nuts would be threaded onto the bolt also did not indicate any significant degradation as well.
The nuts tack welded to the bottom of the drywell shell flange are submerged in water for approximately two to three weeks each refueling outage. The degradation of the coating and surface corrosion is potentially active during that period. However, during the 24 months of normal plant operation, the bellows area where the tack welded nuts/washers reside will be dry so degradation from water is not expected to occur. Based on a combination of examination results from the remote and direct visual examinations, torquing of the bolts during the reassembly process, and the pneumatic leak testing prior to plant operation will
NLS2024006 Attachment Page 7 of7 ensure the drywell head pressure retaining bolting will fully meet its intended structural design function.
( c) During reassembly of the drywell head at the end of each refueling outage, the drywell head flange and accessible bolts/nuts/washers are cleaned, visually examined and threads lubricated prior to assembly as routine maintenance practice per CNS Procedure 7.4REASSEMBL Y. Once the head is in place, the bolting is torqued in three passes to achieve the final 880 ft-lbs of torque with QC Witness applied. At that point, the tack-welded nuts and washers are secure and would perform their intended design function. The observed surface corrosion of the nuts and washers does not impact the load carrying ability of the nuts. Once the bolting assemblies are torqued to specifications, the potential for loose nuts or bolts is eliminated. If the bolting could not achieve the required torque specifications, the bolting would be replaced and retorqued to acceptable limits prior to performance of the As-Left LLRT. The combination of torquing the head bolts to required specifications and conducting the As-Left LLRT of the flange joint each refueling outage, provides reasonable assurance that joint integrity is established for the next 24 months of plant operation until the next refueling outage.
RAl-6 (a)
Summarize examination results, including remote VT-3 inspections on nuts and washers that could not be disassembled, conclusions, and disposition of non-conformances for direct and remote VT-3 examinations.
(b) Describe the bolting reassembly process and requirements.
NPPD Response (a) As noted in the Relief Request, VT-3 examinations of the bolting were conducted in the 2nd 10 Year CISI Interval in refueling outage RE28. Direct VT-3 examinations were conducted of the disassembled bolting. A remote VT-3 examination was also conducted of the nuts/washers that could not be disassembled. The examiner conducted the exam using a handheld video probe that did not record pictures. The remote exam was conducted by the examiner entering the refueling cavity before flood-up and walked around the perimeter of the drywell head flange. The examiner observed light rust and no apparent loss of material thickness. The examination was SAT with no non-conformances identified (See -
RE28 remote data sheet VT-F14-025).
(b) During reassembly of the drywell head at the end of each refueling outage, the drywell head flange and accessible bolts/nuts/washers are cleaned, visually examined and threads lubricated prior to assembly as routine maintenance practice per CNS Procedure 7.4Reassembly. Once the head is in place, the bolting is torqued in three passes to achieve the final 880 ft-lbs of torque. Degraded threads in the nuts would most likely prevent the bolting assembly from achieving the required torque. If the bolting could not achieve the required torque specifications, the bolting would be replaced and retorqued to acceptable limits prior to performance of the As-Left LLRT. The combination of torquing the head bolts to required specifications and conducting the As-Left LLR T of the flange joint each refueling outage, provides reasonable assurance that joint integrity is established for 24 months of plant operation until the next refueling outage.