1CAN050704, Unit 1, Entergy Actions to Address Once Through Steam Generator Degradation
| ML071420160 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 05/11/2007 |
| From: | Mitchell T Entergy Operations |
| To: | Document Control Desk, NRC/NRR/ADRO |
| References | |
| 1CAN050704 | |
| Download: ML071420160 (5) | |
Text
Entergy Operations. Inc.
E1448 S.R. 333 Russellville, AR 72802 Tel 479-858-3110 Timothy G. Mitchell Vice President Operations ANO 1 CAN050704 May 11, 2007 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001
Subject:
Entergy Actions to Address Once Through Steam Generator Degradation Arkansas Nuclear One - Unit I Docket No. 50-313 License No. DPR-51
Dear Sir or Madam:
During Refueling Outage 1 R20, Eddy Current Testing (ECT) of the "A" replacement Once Through Steam Generator (OTSG), tube-to-tube and tube-to-tie rod proximity indications associated with tube and tie-rod bowing were identified. There are 18 tubes in the "AK OTSG that have been identified, associated with seven tie-rod locations. The proximity indications are located mid-span between the lower tube sheet and the first tube support plate (TSP).
The seven tie-rods are located in an arc pattern on the periphery of the tube bundle centered on the OTSG's "Z" axis.
During the investigation to determine the full extent of the bowed tubes, an apparent related condition was discovered at the 8th TSP. There were multiple mechanical wear indications identified by ECT on the tubes intersecting the 8th TSP indicating an abnormal condition in this area. The wear indications at the 8th TSP are skewed to the side of the OTSG corresponding to the pattern of bowed tie-rods.
The bowed tie-rods and tubes in the 1st span and mechanical wear in the tubes at the 8th TSP are potentially caused by the 8th TSP being restrained to the OTSG shroud. As a result, there are induced loads on the 8th TSP and tie-rods during thermal cycles of the OTSG. The loads are a result of the differential thermal expansion properties between the carbon steel (shroud) and stainless steel (tie-rods). The apparent cause of the 8th TSP being restrained against the shroud on one side of the OTSG is unknown at this time; however, an operability evaluation provided by the OTSG vendor (AREVA) documents a bounding analysis for the affected OTSG components. This analysis addresses the structural aspects of the 8th TSP and the impacted tie-rods and tubes resulting from a postulated bounding case where mechanically induced loads from additional heatup and cooldown cycles are considered. Additionally, full-cycle operation was justified when the mechanical wear was evaluated on the tubes at the 8th TSP and on the tubes in the 1st span. There is no OTSG tube damage identified at the 1st span as a result of this condition. Nor do the involved tubes have any indications throughout their length.
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ICAN050704 Page 2 The Operability Evaluation provided by AREVA which considers an additional heatup and cooldown cycle concludes the following:
- With the conservative assumption that the 7 affected tie-rods provide no structural benefit, the remaining 45 remain structurally intact and there is considerable structural margin available to the internal OTSG assembly as a result.
" Considering additional bending caused by additional thermal cycles and a postulated worst case ratcheting effect due to a thermal growth differential between the tie-rods and the shroud, the strains in the tie-rods will not result in failure and are acceptable regarding fatigue. Additionally, it has been determined that the TSP loads will be redistributed over the TSP and the TSP has sufficient structural capacity.
Stresses on the tubes due to displacement by the bowed tie-rods are acceptable for normal operating and transient conditions. The maximum postulated total strain results in an alternating stress that is acceptable with respect to fatigue.
" Tube wear at the 8th TSP and the locations showing proximity indications will be acceptable for a full cycle of operation.
The initial operability evaluation justifies plant operation for one thermal cycle of the "A OTSG (heatup/cooldown). Evaluation for additional thermal cycles is currently being completed and will be documented in a revised operability assessment. Should a plant cooldown be necessary at the evaluated number of thermal cycles, appropriate corrective actions would be necessary before subsequent plant heatup.
In addition, the following compensatory measures have been established:
" One of two acoustic sensing devices previously located on the upper portion of the "A" OTSG (VBE-1 011 B) was disconnected and its output device was reconnected to a new sensor that is now installed on the lower south side of the "A" OTSG in the immediate vicinity of the bowed tie-rods. Alarm capability in the main control room is established above baseline noise levels. Current procedures specify periodic monitoring and alarm response actions.
ANO-1 Technical Specification 3.4.13.d establishes a primary to secondary leakage limit of 150 gallons per day (gpd) through any one steam generator. For operating cycle 21, ANO has established a conservative administrative limit of 15 gpd primary to secondary leakage through the WA" OTSG to require the initiation of a controlled plant shutdown.
Inspection of the "A" OTSG will be conducted during 1 R21 as dictated by the Root Cause Determination and necessary corrective actions will be completed prior to restart.
Additional analysis of this condition is being performed by AREVA. A Root Cause Evaluation will be performed by AREVA and Entergy. It is expected that this Root Cause Determination will be completed within 3 months. Upon completion of the Root Cause Determination, Entergy will present the results and planned corrective actions to the NRC staff.
ICAN050704 Page 3 New commitments contained in this submittal are summarized in the Attachment.
Should you have any questions regarding this submittal or need additional information, please contact Dale James at 479-858-4619.
Sincerely, TGM/rs Attachment cc:
Dr. Bruce S. Mallett Regional Administrator U. S. Nuclear Regulatory Commission Region IV 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011-8064 NRC Senior Resident Inspector Arkansas Nuclear One P.O. Box 310 London, AR 72847 U. S. Nuclear Regulatory Commission Attn: Ms. Farideh E. Saba Mail Stop 0-8 B1 Washington, DC 20555-0001 U. S. Nuclear Regulatory Commission Attn: Mr. Alan Wang Mail Stop 0-7 D1 Washington, DC 20555-0001 Mr. Bernard Bevill Director, Division of Health Arkansas Department of Health and Human Services PO Box 1437, Slot H-30 Little Rock, AR 72203-1437
Attachment to 1CAN050704 List of Regulatory Commitments
List of Regulatory Commitments The following table identifies those actions committed to by Entergy in this document. Any other statements in this submittal are provided for information purposes and are not considered to be regulatory commitments.
COMMITMENT TYPE SCHEDULED (Check One)
COMPLETION ONE TIME CONTINUING DATE ACTION COMPLIANCE (If Required)
ANO-1 Technical Specification 3.4.13.d establishes a primary to secondary leakage limit of 150 gallons per day (gpd) through X**
For duration of any one steam generator. For cycle Cycle 21 21, ANO has established and will maintain a conservative administrative limit of 15 gpd primary to secondary leakage through the WAU OTSG to require the initiation of a controlled plant shutdown.
Inspection of the "A" OTSG will be conducted during 1 R21 as dictated by the Root Cause Determination and X
1 R21 (Fall 2008) necessary corrective actions will be completed prior to restart Upon completion of the Root Cause Determination, Entergy will present the results and planned corrective X
9/10/2007 actions to the NRC staff. Entergy will provide supporting documents to the NRC staff in advance of this presentation as they become available.