ML080280044
| ML080280044 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 01/25/2008 |
| From: | Dave Hills NRC/RGN-III/DRS/EB1 |
| To: | |
| References | |
| Download: ML080280044 (4) | |
Text
Attachment 1 BASIS FOR INVESTIGATORY RESPONSE DECISION FOR A SIGNIFICANT OPERATIONAL POWER REACTOR EVENT PLANT - EVENT DATE - 1/04/08 EVALUATION DATE - 1/04/08 Davis Besse BRIEF DESCRIPTION OF THE SIGNIFICANT OPERATIONAL POWER REACTOR EVENT During a planned pre-emptive weld overlay on the Decay Heat Suction nozzle to elbow weld during the outage, indication of a through wall flaw was noted by a blow out on the first overlay pass and RCS leakage. The characterization of the flaw was unknown at that time. The weld process was stopped and the leak is characterized as weeping. The RV Head was on the Vessel and all the fuel still in the vessel. Subsequently the crack was found to be axially oriented and short by ultrasonic examination. This crack geometry was bounded by the analysis of the structural weld overlay. The licensee proceeded to repair the crack by finishing the planned weld overlay.
Y/N DETERMINISTIC CRITERIA
- a. Involved operations that exceeded, or were not included in, the design bases of N the facility.
- b. Involved a major deficiency in design, construction, or operation having potential N
generic safety implications.
- c. Led to a significant loss of integrity of the fuel, the primary coolant pressure N
boundary, or the primary containment boundary of a nuclear reactor. (Significant loss applies to each boundary)
Based on Ultrasonic testing results the flaw has been characterized as an axial crack in the weld and butter. The leakage was characterized as weepage or very slight throughout the period it was being monitored. These types of flaws have been found to be arrested in the weld material and do not progress into the carbon steel base material, therefore resulting in a short crack. The axial orientation of the flaw prevents catastrophic failure of the pressure boundary and resulting leakage will be readily detected such that it will remain within the make up capability until the plant can be shutdown and the flaw repaired or the plant defueled. Based on this characterization and analysis of this flaw, this is not considered a significant loss of integrity of the primary coolant pressure boundary
- d. Led to the loss of a safety function or multiple failures in systems used to N
mitigate an actual event. (An event has occurred and a safety-function or mitigation system has actually failed) The Decay Heat System was declared inoperable but available for use.
Attachment 1 Y e. Involved possible adverse generic implications.
Remarks: Axial flaws in welds of these materials and geometry have been previously identified and analyzed on numerous occasions in the nuclear industry, therefore the presence of this flaw was not unexpected. The weld overlays were being conducted at Davis Besse due to this industry experience and recommendations. However. this is apparently the first occurrence in a PWR plant where the structural weld overlay process caused the flaw to progress through wall, resulting in a leak. This condition has been noted before in BWR overlay applications.
In this case there are two possible adverse generic implications 1- A deep axial flaw, likely around 90% through wall, existed while the plant was in operations. At this location, a through wall leak during operation would have resulted in an unisolable reactor coolant pressure boundary leak, a small break LOCA. However, due to axial orientation of the flaw, this instance is not considered a significant loss of the primary coolant pressure boundary. See section c.
2- In this case, the leak occurred while the Decay Heat system was required to support safe operation of the plant, irradiated fuel was still in the vessel. The through wall leakage rendered the system inoperable but available. A more significant flaw may have challenged the availability of the system. The possible generic adverse implication is performing weld overlays which create a through wall leak on systems required to be operational, thus increasing risk..
Both conditions have possible adverse generic implications.
The NRC is monitoring the industry inspection and mitigation program for theses welds.
MRP-139, through recently issued TI-172.
Aspects associated with risk assessment while planning to perform weld overlays on systems required to operate are being inspected by the resident inspectors.
This incident has been shared with the industry operating experience programs and input into the NRC operating experience program N f. Involved significant unexpected system interactions.
- g. Involved repetitive failures or events involving safety-related equipment or N
deficiencies in operations.
N h. Involved questions or concerns pertaining to licensee operational performance.
CONDITIONAL RISK ASSESSMENT RISK ANALYSIS BY: David Passehl DATE - January 23, 2008 Not Applicable 1 - Background - Specific facts related to the event or degraded condition:
See Description of Event above.
2 - Safety Impact - The safety impact and any risk insights related to the event or condition, plus the estimated conditional core damage probability:
The estimated leakage was about 2 drops per minute. The through-wall hole and leakage did not adversely affect functionality of the shutdown cooling or makeup systems. During operation, however, a through wall leak would have resulted in an unisolable reactor coolant pressure boundary leak. While the risk significance of this "event" did not rise to the level of a formal special inspection with the plant already shutdown, inspector followup was performed. The risk significance of any performance deficiency resulting from the inspector followup will be evaluated in accordance with the SDP process.
Attachment 1 3 - Risk Analysis/Considerations - Assumptions used in the analysis:
As stated in the deterministic analysis, based on characterization and analysis of this flaw, this was not considered a significant loss of integrity of the primary coolant pressure boundary. The resulting leakage would be readily detected such that it would remain within the make up capability of the plant.
4 - Calculations - Analytical methods used and detailed supporting hand calculations, plus an evaluation of the licensee's analysis and/or their assumptions:
No calculations were performed. Analytical methods used were based on conversation/notes from NRC inspectors and licensee staff.
5 - Conclusions/Recommendations - A recommendation regarding the need for additional inspection:
No additional inspection based on risk is recommended.
6 - References - Any references used in the analysis:
Notes/Communications with NRC Region III engineering inspectors and licensee personnel.
7 - Peer Review - Reviewer Name - Laura Kozak The evaluation will be peer reviewed by a certified SRA or area expert, and this review will be documented and any comments resolved:
8 - DRS Management Concurrence Name - ______________________ Date - ___________
DECISION AND BASIS Routine Followup Using Baseline Inspection Program Y Special Inspection Team N Augmented Inspection Team N
Attachment 1 Basis For Decision:
The risk analysis determined this incident did not meet the threshold for a special inspection.
The flaw was not leaking at operating pressure of over 2000 psig and after progressing through wall during the overlay process, the leakage was never characterized as more than weepage (no measurable active leakage). Ultrasonic examination conducted determined the flaw was an axial crack characteristic of primary water stress corrosion cracking in the weld and buttering. This is a condition that has considerable industry experience and is the focus of an industry inspection and mitigation plan contained in MRP-139. Based on this determination , the flaw was not considered a significant loss of primary coolant pressure boundary integrity.
Generic implications are related to the apparent depth of the flaw while the plant is operating and the performance of the weld overlay process resulting in a through wall crack on a system required to be operational.
Deep axial flaws are rare but not unexpected and are addressed through implementation of the MRP-139 inspection and mitigation program. The NRC is inspecting this program through recently issued TI-172.
The resident inspector is continuing to look at the risk assessment aspects related to performance of the overlay on the operating Decay Heat system.
Therefore, routine baseline followup inspection is recommended through these two programs.
BRANCH CHIEF RECOMMENDATION: /RA/
David E. Hills DATE: 1/24/08 DIVISION DIRECTOR APPROVAL: /RA/
Anne Boland DATE: 1/25/08 LICENSEE NOTIFIED OF RESPONSE DECISION AND BASIS: /RA/
John Jandovich DATE: 1/24/08