05000362/LER-2012-001
| San Onofre Nuclear Generating Station (Songs) | |
| Event date: | 01-31-2012 |
|---|---|
| Report date: | 03-29-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3622012001R00 - NRC Website | |
On January 31, 2012, at 1505 hours0.0174 days <br />0.418 hours <br />0.00249 weeks <br />5.726525e-4 months <br /> Pacific Standard Time (PST), San Onofre Nuclear Generating Station (SONGS) Unit 3 was in Mode 1 operating at 100 percent power, when a high radiation alarm [RA] from the condenser air ejector monitor [MON] indicated a tube leak in one of the two steam generators (SGs) [SG].
As directed by procedure, a rapid power reduction and shutdown was commenced. At 1731 PST when power reached 35 percent, the operators manually tripped the reactor, resulting in actuation of the Reactor Protection System (RPS) [JC], which requires a 60-day written report pursuant to 10 CFR 50.73(a)(2)(iv)(A). Within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of the Unit 3 trip, telephone notification was made to the NRC Emergency Notification System (ENS) as required by 10 CFR 50.72(b)(2)(iv)(B).
B. INITIAL CONDITIONS
At the time of the event on January 31, 2012, SONGS Unit 3 was in Mode 1 with reactor power at approximately 100 percent power. Other than the leaking SG tube, there were no additional inoperable structures, systems, or components at the start of the event that contributed to this event. Unit 2 was in a refueling outage and was unaffected by this event.
SONGS had installed new SGs in both Units 2 and 3 during the previous refueling outages. The replacement SGs were manufactured by Mitsubishi Heavy Industries and the original SGs were supplied by Combustion Engineering. The new Unit 3 SGs were placed into service during plant startup in February 2011. At the time of the event, Unit 2 was in its first refueling outage since SG replacement.
C. DESCRIPTION OF OCCURRENCE
On January 31, 2012, at 1505 PST, SONGS Unit 3 was in Mode 1 operating at 100 percent power, when a high radiation alarm from the condenser air ejector monitor indicated a tube leak in one of the two SGs.
The abnormal operating procedure for reactor coolant system (RCS) leakage was entered due to a SG tube leak indication of greater than 5 gallons per day (gpd). At 1610, based on the air ejector radiation monitors, RCS water inventory balance, and SG chemistry samples, the procedural criteria of greater than 75 gpd leakage with an increasing leak rate exceeding 30 gpd per hour was met requiring a reactor shutdown. No entry was required into Technical Specification (TS) Limiting Condition for Operation (LCO) 3.4.13 (shutdown within 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />) because RCS operational leakage had not exceeded 150 gpd primary to secondary leakage through any one SG. At 1633, a rapid power reduction and shutdown was commenced as directed by procedure. Reactor power was reduced at about 1 percent per minute and, at 1731 at 35 percent power, operators manually tripped the reactor.
Following the reactor trip, all control rods inserted to shutdown the reactor. Safety systems responded as designed. SG level was maintained using the main feedwater system. Decay heat was removed through the main steam bypass valves to the condenser; no atmospheric relief valves lifted during the trip. At 1800, the affected SG was isolated in accordance with the SG tube rupture emergency operating procedure. Plant cooldown continued using the unaffected SG. Mode 4 (hot shutdown) was entered at 0051 and Mode 5 (cold shutdown) at 1647 on February 1, 2012.
Small, monitored radioactive gaseous and liquid releases to the environment occurred, well below allowable limits. The release rates and calculated dose will be included in the 2012 Annual Radioactive Effluent Release Report. The release to the atmosphere via the air ejector vent stack was quantified at 3.75E-2 Curies (noble gases and radioiodine) which is equivalent to 4.25E-5 millirem (mrem) whole body dose. This release was several orders of magnitude below the off-site limit for continuous releases from this vent path of 500 mrem/year.
D. APPARENT CAUSE
A root cause evaluation is being conducted to determine the wear mechanism leading to the tube leak and the extent of condition (see paragraph G below).
E. CORRECTIVE ACTIONS
Immediate/Interim Actions In accordance with plant procedures, reactor power was rapidly reduced and the plant was tripped at 35% power. The affected SG was isolated. The reactor was placed in a stable condition in Mode 5.
Long Term Corrective Actions Inspection, testing, and analysis of SG tube integrity in both Unit 3 SGs is ongoing (see paragraph G below). Defective tubes will be removed from service as required prior to plant startup.
F. SAFETY ASSESSMENT
No significant anomalies and no safety system functional failures occurred during this event. The reactor trip was uncomplicated. All control rods inserted to shut down the reactor. Safety systems responded as designed. The affected steam generator was isolated. The plant was placed in a stable condition in Mode 5. Small, monitored radioactive releases to the environment occurred, several orders of magnitude below the regulatory limits. Based on the above, the safety significance of this event is considered minimal.
G. ADDITIONAL INFORMATION
Previous Similar Events
SONGS had not experienced a steam generator tube leak in the SGs currently installed in Units 2 and 3.
SONGS Unit 3 Steam Generator Inspection Inspection, testing, and analysis of SG tube integrity in both Unit 3 SGs is ongoing. In-situ pressure testing identified eight Unit 3 SG tubes that did not meet the target performance criteria in TS 5.5.2.11 for tube integrity. One of the failed tubes was the leaking tube that required the Unit 3 shutdown reported in this LER. The in-situ testing failures were reported in 8-hour telephone notifications to the NRC in accordance with 10 CFR 50.72(b)(3)(ii)(A) - serious SG tube degradation. A 60-day written report will be submitted under 10 CFR 50.73(a)(2)(ii)(A). The LER reporting the SG tube degradation will address the root cause evaluation being conducted to determine the wear mechanism leading to the tube leak and the extent of condition, as well as additional corrective actions.