05000529/LER-2004-001
| Palo Verde Nuclear Generating Station Unit 2 | |
| Event date: | 02-19-2004 |
|---|---|
| Report date: | 06-01-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5292004001R00 - NRC Website | |
1. REPORTING REQUIREMENT(S):
This voluntary LER 529/2004-001-00 is being submitted to report a condition related to equipment performance that does not meet the reporting threshold of 10CFR50.73 (a) for submitting a LER, but may prove useful and be of generic interest to the nuclear industry.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The equipment which experienced a tube leak and is evaluated in this report is the number 1 steam generator (EIIS: AB) in Unit 2. The steam generator is of the vertical recirculating U-tube design with integral economizers for pressurized water reactor systems. Both the primary and secondary sides of the steam generator are designed and constructed in accordance with the requirements for Class 1 vessels as defined in Section III of the ASME Boiler and Pressure Vessel Code, 1989 with no addenda.
The steam generators utilize the heat produced by the reactor core to generate steam. Reactor coolant passing through the reactor vessel is heated by the reactor core (EIIS: AC), and then circulated through the steam generator tubes where heat is transferred to the secondary water. The secondary water is heated to produce steam which leaves the steam generator through the steam outlet nozzles.
The steam generator is designed to transfer approximately 2007 MWt from the RCS to the secondary system, producing approximately 9.0 x 106 lb/hr of 1030 psia saturated steam, when provided with 450 °F feedwater. The steam generators also perform the safety functions of maintaining primary and secondary pressure boundary integrity and heat removal following various postulated design basis events as described in UFSAR Chapters 6.2, 6.3 and 15.
3. INITIAL PLANT CONDITIONS:
On February 19, 2004, at approximately 15:22 Mountain Standard Time (MST), Palo Verde Unit 2 was in Mode 1 (POWER OPERATION), operating at approximately 100 percent power. There were no major structures, systems, or components that were inoperable at the start of the event that contributed to the event. There were no failures that rendered a train of a safety system inoperable and no failures of components with multiple functions were involved.
4. EVENT DESCRIPTION:
During Unit 2 Cycle 12 (U2C12) operation, a primary to secondary leak in SG1 led to a plant shutdown on February 19, 2004. A very small primary-to-secondary leak (PSL) was believed to exist since the startup of U2C12 with Replacement Steam Generators on December 22, 2003. The leak rate remained stable between 0.4 to 0.7 gallons per day (GPD) as trended, since the unit startup (66 day run).
At 15:22 MST on February 19, 2004, the Unit 2 Control Room staff received ALERT radiation level alarms on the Main Steam Line N-16 Radiation Monitors, RU-142 channels 1 and 2, indicating there was a primary to secondary leak in Steam Generator #1. At 15:30 MST, the Control Room staff entered the Excessive RCS Leakrate procedure. At 15:38 MST, the ALERT radiation level alarm was received from the Condenser Air Removal System Radiation Monitor, RU-141 channel 2 indicating primary to secondary leakage of approximately 11 GPD.
At 16:00 MST, Plant Management made a determination to shutdown Unit 2 in response to the apparent increase in Steam Generator primary to secondary leakage.
Preliminary grab sample results (based on a 5 minute sample count time) from the Condenser Air Removal System confirmed that primary to secondary leak rate had increased from the pre-event leak rate of 0.4 to 0.7 GPD to approximately 3 to 5 GPD.
At 16:31 MST, a plant shutdown was commenced. At 16:45, a recount (30 minute sample count time) of the Condenser Air Removal System grab samples was completed, indicating a calculated 2.3 GPD leak rate based on Xe-135 and 4.05 GPD leak rate based on Xe-133.
At 18:23 MST, the Unit 2 reactor was manually tripped from approximately 21°/0 power per station procedures. At 18:28 MST, the Standard Post Trip Actions were completed by the Control Room staff and the Steam Generator Tube Rupture Emergency Operating Procedure was entered. At 18:32 MST, a cooldown of the Reactor Coolant System (RCS) was commenced in accordance with station procedures. At 18:40 MST, SG1 was isolated.
The Steam Generator primary to secondary leak rate did not exceed the Technical Specification limit of 150 GPD (LCO 3.4.14), the EPRI guidelines that would require a plant shutdown or the Station Administrative shutdown limit of 50 GPD.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The nuclear safety significance of this event was minimal. Primary to secondary leakage was detectable (albeit insignificant) and remained below Technical Specification limits at all times. Also, RCS specific activity remained below Technical Specification limits (LCO 3.4.17) at all times. Tests also indicated that there were no other tube leaks in the Unit 2 steam generators. In situ testing was performed on the leaking tube and it met the criteria of retaining a margin of 3.0 against burst under normal steady state full power operation and a margin of 1.4 against burst under the limiting design basis accident concurrent with a safe shutdown earthquake.
The event did not result in any challenges to the fission product barriers or result in any release of radioactive materials. Therefore, there were no adverse safety consequences or implications as a result of this event. This event did not adversely affect the safe operation of the plant or health and safety of the public.
The condition would not have prevented the fulfillment of the safety functions and did not result in a safety system functional failure as defined by 10CFR50.73(a)(2)(v).
6. CAUSE OF THE EVENT:
APS has concluded that the direct cause of the tube leak was a through wall defect of the tube wall for the affected tube, that either immediately created a minute leak at that location or that weakened a tiny area of tube material such that leakage occurred. The evidence that was collected established that deformation occurred during packaging of the tubes, prior to their installation in the steam generator. The package construction utilized spacer and cross brace materials that were assembled as the tubes were loaded using common wood screws.
The design of this packaging placed the wood screws in close proximity to specific locations on some tubes. The tube involved in this event and the location, shape and size of the deformation in that tube are consistent with damage that would occur if the packaging were assembled incorrectly such that a wood screw penetrated completely through the packaging materials and came in contact with the tube.
Root causes for this condition were:
SG1 tube bundles at the tube manufacturer's (Sandvik) facility; and (2) Less than adequate transportability review by the fabricator (Ansaldo) and follow- up by APS for a prior nonconformance report identifying a similar incident.
No unusual characteristics of the work location (e.g., noise, heat, poor lighting) directly contributed to this event.
7. CORRECTIVE ACTIONS:
Corrective actions to address the direct and root causes include:
(1) The leaking tube has been plugged.
(2) Pre-service eddy current results for all tubes in both of the Unit 2 steam generators have been reviewed to ensure that no other tubes exhibiting an examination signature consistent with the identified puncture damage are in-service.
(3) APS will request information from the SG fabricator to identify the Unit 2 steam generator tubes that were shipped in package locations where packaging screw damage was possible.
(4) Tube locations where damage was possible will be evaluated for inclusion of NDE examinations as deemed necessary under the Steam Generator Management program.
(5) A Vendor Corrective Action Report (VCAR) to Ansaldo will be prepared to address improvements to the tube packing processes.
(6) APS will establish a formal review process for nonconformance reports generated at the tubing manufacturer's and steam generator fabricator's facilities.
(7) Damaged tubes from the original Unit 2 shipment that were discovered at the fabricator's facility on 3/18/2004 and 3/22/2004 will be provided to APS for additional testing.
8. PREVIOUS SIMILAR EVENTS:
In the past three years, Palo Verde Nuclear Generating Station has not experienced a Steam Generator tube leak that required a Unit shutdown.
9. ADDITIONAL INFORMATION:
Palo Verde in conjunction with EPRI is developing an acceptance review process for pre-service eddy current examination results that are intended to improve capabilities to steam generator tube.