ML071410137
| ML071410137 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 05/03/2007 |
| From: | Laughlin G Constellation Energy Group |
| To: | Document Control Desk, NRC/NRR/ADRO |
| References | |
| Download: ML071410137 (8) | |
Text
Constellation Energy Nine Mile Point Nuclear Station P.O. Box 63 Lycoming, NY 13093 May 3, 2007 U. S. Nuclear Regulatory Commission Washington, DC 20555-0001 ATTENTION:
SUBJECT:
Document Control Desk Nine Mile Point Nuclear Station Unit No. 1; Docket No. 50-220 Request for Authorization Under the Provision of 10 CFR 50.55a(a)(3)(i) for Modification of the Core Shroud Stabilizer Assemblies (Tie Rods) -Supplemental Information to Address Deviations Identified During Post-Modification Inspections
REFERENCES:
(a) Letter from G. Harland (NMPNS) to Document Control Desk (NRC), dated February 12, 2007, Generic Letter 94-03, "Intergranular Stress Corrosion Cracking of Core Shrouds in Boiling Water Reactors," Request for Authorization Under the Provision of 10 CFR 50.55a(a)(3)(i) for Modification of the Core Shroud Stabilizer Assemblies (Tie Rods)(b) Letter from G. Harland (NMPNS) to Document Control Desk (NRC), dated March 8, 2007, Request for Authorization Under the Provision of 10 CFR 50.55a(a)(3)(i) for Modification of the Core Shroud Stabilizer Assemblies (Tie Rods) -Supplemental Information in Response to NRC Request for Additional Information By letter dated February 12, 2007 (Reference a), as supplemented by letter dated March 8, 2007 (Reference b), Nine Mile Point Nuclear Station, LLC (NMPNS) requested that the NRC authorize the use of a proposed modification to each of the four core shroud stabilizer assemblies (i.e., tie rods) for Nine Mile Point Unit 1 (NMP1), pursuant to 10 CFR 50.55a(a)(3)(i).
The NRC verbally granted the requested authorization during a telephone conference call on March 26, 2007, and NMPNS completed installation of the tie rod modification during the 2007 NMPI refueling outage. Following installation, post-modification tie rod inspections were performed as outlined in Reference (a), Attachment (1), Section 7.2.1. During these inspections, minor deviations from the requirements of the installation specification were identified.
These deviations were discussed with members of the NRC staff in a telephone conference call on April 11, 2007. At the conclusion of the conference call, NMPNS agreed to submit information describing the identified deviations and summarizing the assessments performed by NMPNS to demonstrate that the tie rods are acceptable for operation with the deviations left uncorrected.
This supplemental information is provided in Attachment (1). This letter contains no new regulatory commitments.
Document Control Desk May 3, 2007 Page 2 Should you have any questions regarding the information in this submittal, please contact M. H. Miller, Licensing Director, at (315) 349-5219.Very truly yours, Ga Jay Laughlin Manager Engineering Services GJL/DEV
Attachment:
(1) Nine Mile Point Unit 1 -Supplemental Information to Address Deviations Identified During Post-Modification Inspections of the Core Shroud Tie Rods cc: S. J. Collins, NRC M. J. David, NRC Resident Inspector, NRC ATTACHMENT (1)NINE MILE POINT UNIT 1 SUPPLEMENTAL INFORMATION TO ADDRESS DEVIATIONS IDENTIFIED DURING POST-MODIFICATION INSPECTIONS OF THE CORE SHROUD TIE RODS Nine Mile Point Nuclear Station, LLC May 3, 2007 ATTACHMENT (1)NINE MILE POINT UNIT 1 SUPPLEMENTAL INFORMATION TO ADDRESS DEVIATIONS IDENTIFIED DURING POST-MODIFICATION INSPECTIONS OF THE CORE SHROUD TIE RODS By letter dated February 12, 2007, as supplemented by letter dated March 8, 2007, Nine Mile Point Nuclear Station, LLC (NMPNS) requested that the NRC authorize the use of a proposed modification to each of the four core shroud stabilizer assemblies (i.e., tie rods) for Nine Mile Point Unit 1 (NMP1), pursuant to 10 CFR 50.55a(a)(3)(i).
Following installation of the tie rod modification during the 2007 NMP1 refueling outage, post-modification inspections were performed as outlined in Attachment (1), Section 7.2.1, of the February 12, 2007, NMPNS letter. During these inspections, minor deviations from the requirements of the installation specification were identified.
This attachment describes the identified deviations and summarizes the assessments performed by NMPNS to demonstrate that the tie rods are acceptable for operation with the deviations left uncorrected.
Diagrams of the tie rod assembly components were provided in General Electric (GE) documents GE-NE-B13-01739-04 and GE-NE-B13-01739-05 that were included in the original 1995 core shroud repair submittal (Niagara Mohawk Power Corporation letter dated January 23, 1995, TAC No. M90102).First Deviation Description of Condition In the February 12, 2007, NMPNS letter, Attachment (1), Section 7.2.1, inspection attribute "e" states: "There is contact between the lower support clevis pin and hook at the 10 o'clock and 2 o'clock points on both sides of the hook." Contrary to the above, the as-left condition of the tie rod clevis pin to lower support hook interface for the tie rods at the 166 degree and 270 degree locations does not meet this criterion.
At the 166 degree location, an approximate 10 mil gap exists on both sides of the tie rod clevis pin to lower support hook interface.
At the 270 degree location, an approximate 20 mil gap exists on one side of the tie rod clevis pin to lower support hook interface.
In addition to this as-left condition, the as-found condition of the tie rods indicated that a slight loss of preload had occurred during the service period from 1999 to 2007. This preload loss was measured by a tie rod nut rotation in the range of 60 degrees, which translates into an equivalent average gap of approximately 30 mils. The assessment of the as-left clevis pin to lower support hook interface condition considered this observed relaxation by conservatively assuming it occurs immediately upon initial heatup from the 2007 refueling outage.Assessment Lack of contact at the clevis pin to lower support hook interface will decrease the tie rod thermal preload.An engineering evaluation has been performed to assess the impact of the as-left condition described above (i.e., lack of clevis pin to hook contact for two tie rods) on the functionality and operability of the tie rods relative to the design basis. The following summary provides the evaluation conclusions relative to key tie rod design criteria.1. The predicted preload reduction (approximately 20 percent due to the conditions described above)decreases the tie rod stresses relative to the original design basis stresses (a positive effect).1 of 5 ATTACHMENT (1)NINE MILE POINT UNIT 1 SUPPLEMENTAL INFORMATION TO ADDRESS DEVIATIONS IDENTIFIED DURING POST-MODIFICATION INSPECTIONS OF THE CORE SHROUD TIE RODS 2. The predicted preload reduction (addressing both the as-found tie rod nut rotation and the as-left gaps) decreases the net clamping force for the design basis "Case 1" H2/H3 core shroud horizontal weld cracking scenario described in Reference 1, for the limiting H6B weld, from 118.7 Kips to 49.4 Kips. Section 5.3.1.1 of BWRVIP-02-A, "BWR Vessel and Internals Project, BWR Core Shroud Repair Design Criteria, Rev. 2," states the following functional requirements for circumferential shroud repairs with regard to allowable displacement of the shroud: " "The shroud repair shall be designed so that there is no separation of 3600 through-wall cracking of the shroud welds during normal operation, as a minimum." The design of shroud repair shall ensure that vertical, horizontal and rotational movement of a shroud with 360' through-wall cracked welds does not impair the ECCS functions during the conditions covered by Section 5.2." The evaluation concluded that a 49.4 Kip net clamping force is adequate to prevent separation of the assumed 3600 through-wall cracked core shroud circumferential (horizontal) welds under normal operating pressure conditions, in accordance with the first BWRVIP-02-A functional requirement stated above. The reduced clamping force does change the previously analyzed weld separation during upset pressure conditions.
The supplemental core shroud repair hardware stress analysis submitted to the NRC in Reference 1 concluded that the NMP1 tie rod design preload was sufficient to prevent weld separation under upset pressure conditions assuming the "Case 1" H2/H3 core shroud horizontal weld cracking scenario.
The reduced preload will allow separation for this condition.
In accordance with the second BWRVIP-02A functional requirement stated above, circumferential weld separation is allowed provided the ECCS functions are not impaired.
The NMPNS evaluation concluded that the weld separation under upset pressure conditions would be momentary with negligible impact on the core spray system piping analysis or the allowable leakage assessment for upset pressure conditions.
- 3. Since shroud weld separation is postulated under upset pressure conditions, the original Upset #2 load case (upset pressure + deadweight
+ seismic) was re-evaluated.
Under postulated weld separation conditions, the shroud weld crack plane is modeled as a "Roller" (as opposed to a "Hinge" when separation is not predicted to occur). The reduced preload creates a potential for weld separation only at the H6B and H7 welds using the design basis Case 1 shroud stiffness.
The impact of this weld separation on the tie rod components is evaluated as follows: a. Upper Spring Since separation is postulated for the Upset #2 load case, seismic lateral load based on a "Roller" condition would apply. From the seismic analysis (Reference 3), of all the "Roller" cases, the maximum load in the upper spring is 21.63 kips. The Upset #2 condition load for which the upper spring has been qualified in Reference 2 is 19.685 kips. The load ratio is 21.63 / 19.685 = 1.10.Scaling the Upset #2 load case stresses from Reference 2 by a factor of 1.10, results in the following:
2 of 5 ATTACHMENT (1)NINE MILE POINT UNIT 1 SUPPLEMENTAL INFORMATION TO ADDRESS DEVIATIONS IDENTIFIED DURING POST-MODIFICATION INSPECTIONS OF THE CORE SHROUD TIE RODS Pm = 470 x 1.10 = 517 psi vs. Allowable
= 47,500 psi, Stress Ratio = 0.01 (OK)Pm + Pb = 25,722 x 1.10 = 28,294 psi vs. Allowable
= 71,250 psi, Stress Ratio = 0.40 (OK)Pm + Pb + Q = 25,722 x 1.10 = 28,294 psi vs. Allowable
= 142,500 psi, Stress Ratio = 0.20 (OK)Substantial margin exists with respect to the allowable.
Therefore, there is no structural integrity concern.b. Lower Spring The analysis performed for the 270 degree tie rod (Reference
- 4) evaluated an eccentric loading because the lower spring vessel contact location was on the nozzle radius. This condition was corrected in 1997. The analysis in Reference 4 clearly documents the application of a maximum lateral seismic load of 63,800 lbs (consistent with an H6B weld "Roller" scenario with separation) and a maximum tie rod axial load of 164,484 lbs. The maximum stress in the lower spring was defined as 0.91 of allowable.
This condition increased the Upset #2 condition load from the original stress analysis for the lower spring from 0.72 of allowable to 0.91 of allowable.
Supplemental analyses that exclude the eccentric loading cannot be completed without extensive reconstitution of the original FEA model. Since this condition is conservative, the overall conclusion is that the analysis in Reference 4 conservatively bounds the Upset #2 load case with separation and an H6B weld "Roller" case assumption, and that this analysis demonstrates that the lower spring stress remains below upset allowable limits.c. Middle Support: The evaluation of the maximum displacement of the shroud considering weld separation conservatively at the H6B weld lower (shell side cracking) and upper (ring side cracking) has concluded that the mid support remains unloaded for the Upset #2 load case and therefore the stress analysis is not impacted.
A significant clamping load (121.9 kips) is maintained with the as-left preload such that the H6A weld location remains a hinged condition for the design basis H2/H3 weld crack stiffness condition (Case 1) and therefore the maximum lateral movement is unchanged.
The overall conclusion is that all tie rod lateral support components remain below upset allowable stress limits assuming the H6B weld "Roller" condition.
- 4. The predicted preload reduction has essentially no impact on the tie rod main steam line break loss of coolant accident and recirculation line break loss of coolant accident load carrying capability.
Thus, original design margins are maintained.
- 5. The predicted preload reduction decreases the preload used in the original flow induced vibration (FIV) analysis.
The evaluation concluded that the original design criteria (i.e., that there be a 3:1 ratio of tie rod natural frequency to vortex shedding frequency) is maintained, thereby minimizing the potential for FIV.In conclusion, the evaluation of the as-left condition of the tie rod clevis. pin to lower support hook interface for the tie rods at the 166 degree and 270 degree locations has determined that the BWRVIP A tie rod design criteria are met with sufficient margin to ensure that the tie rods are capable of 3 of 5 ATTACHMENT (1)NINE MILE POINT UNIT 1 SUPPLEMENTAL INFORMATION TO ADDRESS DEVIATIONS IDENTIFIED DURING POST-MODIFICATION INSPECTIONS OF THE CORE SHROUD TIE RODS performing their design function.
Thus, the two subject tie rods are acceptable for operation.
Consistent with the inspection requirements of BWRVIP-76, a tie rod tightness inspection will be performed after one cycle of operation to confirm that no substantial loss of total preload has occurred in accordance with the BWRVIP-02-A design criteria.Second Deviation Description of Condition In the February 12, 2007, NMPNS letter, Attachment (1), Section 7.2.1, inspection attribute "g" states: "Lower wedge has been pulled up flush with the lower spring." Contrary to the above, the as-left condition of the lower wedge for the tie rods at the 90 degree and 270 degree locations does not meet this criterion.
The actual installation criterion allows a tolerance of +/- 20 mils. However, at the 90 degree location, the tie rod lower wedge was flush with the lower spring on one side but 40 mils below flush on the other side. At the 270 degree location, the tie rod lower wedge was 30 mils below flush on one side and 40 mils below flush on the other side.Assessment The NMP1 tie rod design is such that the lower wedge drops by a maximum of 0.25" during an operating cycle and loads the lower wedge retainer clip. The retainer clip is designed to withstand this loading with significant margin. NMPNS has defined acceptance criterion for as-found conditions after an operating cycle of 0.10" as acceptable for continued service for one operating cycle without corrective actions. This evaluation is based on maintaining the peak stress below the BWRVIP-84 criterion for X-750 IGSCC resistance.
The criterion of "flush +/- 20 mils" is intended only to provide a tolerance for maintenance activities that raise the lower wedge and as such is not a deviation from a design criterion for IGSCC susceptibility.
The as-left condition for the lower wedge maintains the retainer clip within the BWRVIP-84 design margins for X-750 peak principle stress of less than 0.8 Sy. The lower wedge and retainer clip are inspected each outage and the wedge raised as needed to maintain the stress in the retainer clip within BWRVIP-84 requirements.
References
- 1. Letter from C. D. Terry (Niagara Mohawk Power Corporation) to Document Control Desk (NRC), dated February 28, 1995, "Response to Request for Additional Information Regarding the Nine Mile Point Unit I Core Shroud Repair (TAC No. M90102)2. General Electric Report GE-NE-B13-01739-04, "Nine Mile Point Unit 1 -Shroud Repair Hardware Stress Analysis," Rev. 0, January, 1995, DRF B13-01739, Supplement 2, April 1995.4 of 5 ATTACHMENT (1)NINE MILE POINT UNIT 1 SUPPLEMENTAL INFORMATION TO ADDRESS DEVIATIONS IDENTIFIED DURING POST-MODIFICATION INSPECTIONS OF THE CORE SHROUD TIE RODS 3. General Electric Report GE-NE-B13-01739-03, Nine Mile Point 1 Nuclear Power Station -Seismic Analysis:
Core Shroud Repair Modification, Revision 0, submitted in letter from C. D. Terry (Niagara Mohawk Power Corporation) to Document Control Desk (NRC) dated January 6, 1995, "Generic Letter 94-03, Intergranular Stress Corrosion Cracking of Core Shrouds in Boiling Water Reactors" (TAC No. M90102)4. General Electric Report GE-NE-B13-01739-04, Supplement 3.2 to Revision 0, submitted in letter from C. D. Terry (Niagara Mohawk Power Corporation) to Document Control Desk (NRC) dated April 30, 1996, "Generic Letter 94-03, Intergranular Stress Corrosion Cracking of Core Shrouds in Boiling Water Reactors" (TAC No. M90 102)5 of 5