ENS 42573
ENS Event | |
|---|---|
04:00 Apr 24, 2006 | |
| Title | Part 21 Notification - Bwr Core Shroud Tie Rod Upper Support Cracking |
| Event Description | {{#Wiki_filter:Summary:
GE Energy, Nuclear (GE) has provided core shroud repairs using tie rods to the US BWR plants identified in Attachment 1 [of the Part 21 notification]. Recently it was discovered during an in-vessel visual inspection (IVVI) that tie rod upper supports at Hatch Unit 1 experienced cracking. The apparent root cause is Intergranular Stress Corrosion Cracking (IGSCC) in the Alloy X-750 tie rod upper support material. Alloy X-750 material is susceptible to IGSCC if subjected to sustained, large peak stress conditions. GE opened an internal evaluation to determine if the potential IGSCC in the X-750 tie rod structural components of other BWR shroud repairs designed by GE could be a reportable condition under 10CFR21. GE used the criterion provided in the BWR Vessels & Internals Project (BWRVIP-84) for the IGSCC susceptibility assessment of the X-750 components in the tie rod vertical load path. GE has concluded that it is not a reportable condition for the plants that were found to be within or not significantly exceed the BWRVIP-84 criterion. These US plants are identified as 'NR' in Attachment 2 [of the Part 21 notification]. GE determined that two US plants exceed the BWRVIP-84 criterion for the upper supports (in addition to the Hatch Unit 1 as-found condition). GE has not completed the evaluation for these plants to assess if a substantial safety hazard (SSH) exists. These plants have been provided a 60-Day Interim Report Notification under �21.21(a)(2) and are identified as '60-Day' in Attachment 2 [of the Part 21 notification]. Safety Basis: Cracking in the tie rod components made of X-750 may render the tie rod ineffective in maintaining core shroud configuration integrity during postulated accident conditions. Loss of core shroud integrity could impact the ability to maintain adequate core cooling for postulated design basis accident conditions. This condition would be reportable under 10 CFR 21 as a substantial safety hazard. Corrective Action: The corrective action which has been, is being, or will be taken; the name of the individual or organization responsible for the action; and the length of time that has been or will be taken to complete the action (note, these are actions specifically associated with the identified deviation or failure to comply): 1. A preliminary cause evaluation has been performed. The apparent cause of the cracking is Intergranular Stress Corrosion Cracking (IGSCC). A material sample is being shipped to the GE Vallecitos Nuclear Center for examination to confirm the apparent cause. GE will report the results of the examination by August 21, 2006. 2. The issue has been communicated to the industry through the BWR Owners' Group and the Electric Power Research Institute (EPRI)/BWR Vessel and Internals Project (BWRVIP). The NRC was informed in a NRC management meeting with EPRI and the BWRVIP Executive Oversight Committee at the NRC offices, Rockville, on March 15, 2006. 3. GE has completed an evaluation of the susceptibility to IGSCC using the BWRVIP-84 criterion. Determination of whether any possible cracking could lead to a substantial safety hazard (i.e., loss of core shroud configuration integrity during a design basis accident condition) depends upon many factors, including the actual extent of cracking in the repair components. Until inspections are completed, the actual extent of cracking is not known. GE is developing a model to predict the postulated extent of tie rod upper support cracking for tie rods with upper supports made of Alloy X-750. For upper supports that exceed the BWRVIP-84 criteria significantly, the model will be used to postulate the extent of cracking. This prediction will be used to determine if a substantial safety hazard could exist. GE will report the results of the evaluation by October 9, 2006. 4. The original design basis stress reports will be reviewed to assess the available margin in the primary membrane + bending stress intensities of the upper supports with respect to ASME code allowable values. Where reasonable margin exists in the original design basis code evaluation (an existing margin of approximately 25 % will be considered as reasonable margin), the existing margin is deemed adequate to offset any engineering assumptions or judgments used in the original analysis. Where the original margin is less than 25%, further review will be performed (including finite element analysis, if necessary) to confirm that the upper support remains qualified. This review will be completed by October 9, 2006. Affected US Plants per Attachments 1 and 2 of the Part 21 notification: Clinton, Nine Mile Point 1, Pilgrim, Dresden 2 & 3, Quad Cities 1 & 2, Hatch 1 & 2.
GE Energy, Nuclear (GE) has completed the failure evaluation of the cracking discovered in the Hatch Unit 1 core shroud repair tie rod upper supports as committed in Reference 2, (GE Part 21 60-Day Interim Report Notification: Core Shroud Repair Tie Rod Upper Support Cracking, MFN 06-133, May 12, 2006). A preliminary cause evaluation had concluded that the apparent cause of the cracking is Intergranular Stress Corrosion Cracking (IGSCC). A material sample was shipped to the GE Vallecitos Nuclear Center for examination to confirm the apparent cause. GE committed to report the results of the examination by August 21, 2006. The fracture was examined by metallographic and scanning electron microscope (SEM) techniques including an analysis of the fracture surface. The examinations revealed the cracking mechanism to be IGSCC. Scanning electron microscopy (SEM) showed the fracture surface had a "rock candy" like appearance, consistent with an IGSCC mechanism. Images of the cross-section of the fracture surface further verified the IGSCC mechanism by showing the path of the crack following the grain boundaries. No hardness or microstrutural anomalies were observed. GE continues to work on the other action items that were committed in Reference 2. If you have any questions, on this information, please call me . . . R2DO (Mark Lesser) notified. E-mailed to NRR Part 21 (Omid Tabastabai & Jack Foster).
Summary of Part 21 Notification: Completion of GE Evaluation on Core Shroud Repair Tie Rod Upper Support Cracking Previous correspondence on this subject identified that GE had concluded that this is not a reportable condition for Hatch Unit 1 and for several other US plants that have core shroud repairs designed by GE. However, GE had not completed the evaluation for two other US plants (Pilgrim and NMP-1). GE committed to complete the evaluation and inform the NRC of the results by October 9, 2006. The purpose of this letter is to inform the NRC of the results of the Pilgrim and NMP-1 evaluations. The GE evaluation concluded the following: a. All other major Alloy X-750 components in the tie rod assembly vertical load path besides the previously identified upper support brackets for the plants with GE designed tie rod repairs are within the BWRVIP-84 criterion for IGSCC susceptibility. Furthermore, the upper supports remain qualified with respect to ASME code allowable values per the original design basis stress analyses. b. It is not a reportable condition for the as found Hatch Unit 1 condition based on the known extent of tie rod upper support cracking. c. It is not a reportable condition for plants that do not use X-750 material for the tie rod upper support brackets. This is applicable to Clinton. d. It is not a reportable condition for plants that have margin to the BWRVIP-84 IGSCC criterion for the tie rod upper support brackets (or which exceed the criteria by a very small amount). These plants are Hatch 2, Quad Cities 1/2, and Dresden 2/3. e. It is not a reportable condition for the plants that exceed the BWRVIP-84 IGSCC criterion for the tie rod upper supports based on a three-pronged composite approach of (1) the shroud repair upper support integrity based on assumed cracking, (2) the shroud horizontal welds integrity, and (3) the previous shroud cracking safety evaluations. These plants are NMP-1 and Pilgrim. Corrective/Preventive Actions: GE recommends that NMP-1 and Pilgrim replace the tie rod upper supports with an improved IGSCC-resistant design at the next regular scheduled outage. R1DO (Decker) notified. E-mailed to NRR Part 21 (Vern Hodge, Ian Jung, John Thorp). }}[[Event description::Description::{{#Regex_clear:Summary: GE Energy, Nuclear (GE) has provided core shroud repairs using tie rods to the US BWR plants identified in Attachment 1 [of the Part 21 notification]. Recently it was discovered during an in-vessel visual inspection (IVVI) that tie rod upper supports at Hatch Unit 1 experienced cracking. The apparent root cause is Intergranular Stress Corrosion Cracking (IGSCC) in the Alloy X-750 tie rod upper support material. Alloy X-750 material is susceptible to IGSCC if subjected to sustained, large peak stress conditions. GE opened an internal evaluation to determine if the potential IGSCC in the X-750 tie rod structural components of other BWR shroud repairs designed by GE could be a reportable condition under 10CFR21. GE used the criterion provided in the BWR Vessels & Internals Project (BWRVIP-84) for the IGSCC susceptibility assessment of the X-750 components in the tie rod vertical load path. GE has concluded that it is not a reportable condition for the plants that were found to be within or not significantly exceed the BWRVIP-84 criterion. These US plants are identified as 'NR' in Attachment 2 [of the Part 21 notification]. GE determined that two US plants exceed the BWRVIP-84 criterion for the upper supports (in addition to the Hatch Unit 1 as-found condition). GE has not completed the evaluation for these plants to assess if a substantial safety hazard (SSH) exists. These plants have been provided a 60-Day Interim Report Notification under �21.21(a)(2) and are identified as '60-Day' in Attachment 2 [of the Part 21 notification]. Safety Basis: Cracking in the tie rod components made of X-750 may render the tie rod ineffective in maintaining core shroud configuration integrity during postulated accident conditions. Loss of core shroud integrity could impact the ability to maintain adequate core cooling for postulated design basis accident conditions. This condition would be reportable under 10 CFR 21 as a substantial safety hazard. Corrective Action: The corrective action which has been, is being, or will be taken; the name of the individual or organization responsible for the action; and the length of time that has been or will be taken to complete the action (note, these are actions specifically associated with the identified deviation or failure to comply): 1. A preliminary cause evaluation has been performed. The apparent cause of the cracking is Intergranular Stress Corrosion Cracking (IGSCC). A material sample is being shipped to the GE Vallecitos Nuclear Center for examination to confirm the apparent cause. GE will report the results of the examination by August 21, 2006. 2. The issue has been communicated to the industry through the BWR Owners' Group and the Electric Power Research Institute (EPRI)/BWR Vessel and Internals Project (BWRVIP). The NRC was informed in a NRC management meeting with EPRI and the BWRVIP Executive Oversight Committee at the NRC offices, Rockville, on March 15, 2006. 3. GE has completed an evaluation of the susceptibility to IGSCC using the BWRVIP-84 criterion. Determination of whether any possible cracking could lead to a substantial safety hazard (i.e., loss of core shroud configuration integrity during a design basis accident condition) depends upon many factors, including the actual extent of cracking in the repair components. Until inspections are completed, the actual extent of cracking is not known. GE is developing a model to predict the postulated extent of tie rod upper support cracking for tie rods with upper supports made of Alloy X-750. For upper supports that exceed the BWRVIP-84 criteria significantly, the model will be used to postulate the extent of cracking. This prediction will be used to determine if a substantial safety hazard could exist. GE will report the results of the evaluation by October 9, 2006. 4. The original design basis stress reports will be reviewed to assess the available margin in the primary membrane + bending stress intensities of the upper supports with respect to ASME code allowable values. Where reasonable margin exists in the original design basis code evaluation (an existing margin of approximately 25 % will be considered as reasonable margin), the existing margin is deemed adequate to offset any engineering assumptions or judgments used in the original analysis. Where the original margin is less than 25%, further review will be performed (including finite element analysis, if necessary) to confirm that the upper support remains qualified. This review will be completed by October 9, 2006. Affected US Plants per Attachments 1 and 2 of the Part 21 notification: Clinton, Nine Mile Point 1, Pilgrim, Dresden 2 & 3, Quad Cities 1 & 2, Hatch 1 & 2.
GE Energy, Nuclear (GE) has completed the failure evaluation of the cracking discovered in the Hatch Unit 1 core shroud repair tie rod upper supports as committed in Reference 2, (GE Part 21 60-Day Interim Report Notification: Core Shroud Repair Tie Rod Upper Support Cracking, MFN 06-133, May 12, 2006). A preliminary cause evaluation had concluded that the apparent cause of the cracking is Intergranular Stress Corrosion Cracking (IGSCC). A material sample was shipped to the GE Vallecitos Nuclear Center for examination to confirm the apparent cause. GE committed to report the results of the examination by August 21, 2006. The fracture was examined by metallographic and scanning electron microscope (SEM) techniques including an analysis of the fracture surface. The examinations revealed the cracking mechanism to be IGSCC. Scanning electron microscopy (SEM) showed the fracture surface had a "rock candy" like appearance, consistent with an IGSCC mechanism. Images of the cross-section of the fracture surface further verified the IGSCC mechanism by showing the path of the crack following the grain boundaries. No hardness or microstrutural anomalies were observed. GE continues to work on the other action items that were committed in Reference 2. If you have any questions, on this information, please call me . . . R2DO (Mark Lesser) notified. E-mailed to NRR Part 21 (Omid Tabastabai & Jack Foster).
Summary of Part 21 Notification: Completion of GE Evaluation on Core Shroud Repair Tie Rod Upper Support Cracking Previous correspondence on this subject identified that GE had concluded that this is not a reportable condition for Hatch Unit 1 and for several other US plants that have core shroud repairs designed by GE. However, GE had not completed the evaluation for two other US plants (Pilgrim and NMP-1). GE committed to complete the evaluation and inform the NRC of the results by October 9, 2006. The purpose of this letter is to inform the NRC of the results of the Pilgrim and NMP-1 evaluations. The GE evaluation concluded the following: a. All other major Alloy X-750 components in the tie rod assembly vertical load path besides the previously identified upper support brackets for the plants with GE designed tie rod repairs are within the BWRVIP-84 criterion for IGSCC susceptibility. Furthermore, the upper supports remain qualified with respect to ASME code allowable values per the original design basis stress analyses. b. It is not a reportable condition for the as found Hatch Unit 1 condition based on the known extent of tie rod upper support cracking. c. It is not a reportable condition for plants that do not use X-750 material for the tie rod upper support brackets. This is applicable to Clinton. d. It is not a reportable condition for plants that have margin to the BWRVIP-84 IGSCC criterion for the tie rod upper support brackets (or which exceed the criteria by a very small amount). These plants are Hatch 2, Quad Cities 1/2, and Dresden 2/3. e. It is not a reportable condition for the plants that exceed the BWRVIP-84 IGSCC criterion for the tie rod upper supports based on a three-pronged composite approach of (1) the shroud repair upper support integrity based on assumed cracking, (2) the shroud horizontal welds integrity, and (3) the previous shroud cracking safety evaluations. These plants are NMP-1 and Pilgrim. Corrective/Preventive Actions: GE recommends that NMP-1 and Pilgrim replace the tie rod upper supports with an improved IGSCC-resistant design at the next regular scheduled outage. R1DO (Decker) notified. E-mailed to NRR Part 21 (Vern Hodge, Ian Jung, John Thorp). }}| ]] |
| Where | |
|---|---|
| General Electric Company Wilmington, North Carolina (NRC Region 1) | |
| Organization: | General Electric Company |
| Reporting | |
| 10 CFR 21.21 | |
| Time - Person (Reporting Time:+450.6 h18.775 days <br />2.682 weeks <br />0.617 months <br />) | |
| Opened: | Jason Post 22:36 May 12, 2006 |
| NRC Officer: | Mike Ripley |
| Last Updated: | Oct 10, 2006 |
| 42573 - NRC Website | |
General Electric Company with 10 CFR 21.21, Notification of failure to comply or existence of a defect and its evaluation | |
WEEKMONTHYEARENS 425732006-04-24T04:00:00024 April 2006 04:00:00
[Table view]10 CFR 21.21, Notification of failure to comply or existence of a defect and its evaluation Part 21 Notification - Bwr Core Shroud Tie Rod Upper Support Cracking 2006-04-24T04:00:00 | |