05000361/LER-2006-001
| San Onofre Nuclear Generating Station (Songs) Unit 2 | |
| Event date: | 01-11-2006 |
|---|---|
| Report date: | 03-06-2006 |
| Initial Reporting | |
| ENS 42252 | 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3612006001R00 - NRC Website | |
1. FACILJTV NAME
- . LER NUMBER 2. DOCKET NUMBER PAGE (3) Plant:
Discovery Date:
Reactor Vendor:
Mode:
Power:
Background:
January 11, 2006 Combustion Engineering Mode 5 — Cold Shutdown 0 percent In Modes 1, 2, and 3 with pressurizer pressure greater than or equal to 400 psia, Technical Specification (TS) 3.5.2 requires two trains of Emergency Core Cooling System (ECCS) to be operable. At San Onofre Unit 2, the ECCS includes the Low Pressure Safety Injection (LPSI) [BP] system. The LPSI system also functions as part of the Shutdown Cooling System (SDCS) [BP] when the plant is in Modes 4 through 6. In Mode 4, TS 3.4.6 requires two loops (any combination of SDC loops and/or Reactor Coolant System (RCS) [AB] loops) to be operable and at least one loop in operation. In Mode 5 with the RCS loops filled, TS 3.4.7 requires one RCS loop or one SDC train to be in operation and a second SDC train or RCS loop to be operable. In Mode 5 with the RCS loops not filled, TS 3.4.8 requires two trains of SDCS to be operable and one train operating.
The LPSI/SDC system contains two trains, each with its own pump [P], but sharing a common discharge line. This common line is then split into four individual pathways; each provides a flow path to one of the four RCS cold legs (see attached diagram).
Description of Event
On January 8, 2006, San Onofre Nuclear Generating Station (SONGS) Unit 2 was in Mode 5 for the beginning of a scheduled refueling outage and the SDCS was in service. At about 2300 PST, plant operators discovered a small leak (about 10 drops per minute) from the LPSI pumps common discharge header. Since the piping was covered by insulation, the source of the leak could not be immediately identified.
On January 11, 2006, after removing the pipe insulation, SCE determined the source of the leak to be a small through-wall crack in the 8-inch LPSI system Class 2 pipe [PSP] at the toe of the weld to a 1-inch drain line, (just upstream of valve 2HV9328 [ISV] low pressure cold leg injection isolation valve for RCS branch line 2A). Since the crack was located in the common discharge header and could not be isolated, SCE declared both SDC trains inoperable. This action is consistent with the guidance in Regulatory Issue Summary (RIS) 2005-20, (Appendix C.12 of the NRC Inspection Manual Part 9900), which states in part, "Upon discovery of leakage from a Class 1, 2, or 3 pressure boundary component (pipe wall, valve body, pump casing, etc.), the licensee must declare the component inoperable (...) Following the declaration of inoperability, the licensee may also decide to evaluate the structural integrity of leaking Class 2 or 3 moderate-energy piping using the criteria of Code Case N-513-1.
On January 11, 2006 at 1743 PST, SCE conservatively reported this occurrence to the NRC (NRC Event 42252), in accordance with 10CFR50.72(b)(3)(v)(B) as a condition that could have prevented the removal of residual heat. On January 12, 2006, SCE completed an evaluation of the pipe using the criteria of American Society of Mechanical Engineers (ASME) Code Case N-513-1 and determined the pipe was operable for SDC functions provided the system temperature was maintained below 200 degrees F and below 275 psig as required by the ASME Code Case.
Administrative controls were established by Operations to ensure these conditions were maintained.
1. FACLITY NAME 2. DOCKET NUMBER IL LER NUMBER PAGE (3) SCE also analyzed the pipe integrity at the design basis conditions and determined the pipe would have remained intact and that system leakage through the crack would have remained below the allowable system leak rate. Based on these results, SCE concluded that although the pipe was degraded, it was capable of performing its safety functions under design basis conditions.
Because the system was always capable of performing its safety function, SCE retracted the original phone report on March 3, 2006. SCE is submitting this follow-up Licensee Event Report (LER) to document this condition and inform the NRC of the corrective actions taken and planned.
Cause of Event
Based on laboratory analysis, SCE determined the crack was caused by high cycle fatigue. The crack initiated at a small weld discontinuity on the outside diameter surface of the eight-inch LPSI discharge header pipe at a branch fitting that connects to a one-inch drain line. SCE is continuing to evaluate the cause of the high cycle fatigue (reference AR 060100463-12).
Corrective Actions
SCE performed the following corrective actions:
- The plant was in Mode 5 with the loops not filled and one SDC train in service when the crack location was identified on January 11, 2006. As discussed above, SCE declared both trains of SDC inoperable, complied with the actions required by TS 3.4.8, and continued toward Mode 6.
- SCE performed an evaluation of the structural integrity of the affected pipe and concluded the pipe was operable for SDC functions, provided the system temperature was maintained below 200 degrees F and below 275 psig. SCE later performed a second evaluation that demonstrated the pipe was capable of performing its safety function under all design basis conditions.
- Until SCE was able to remove the SDC system from service (after the core was completely offloaded), administrative controls were established that ensured the conditions assumed in the ASME Code Case N-513-1 evaluation were maintained. The condition of the crack was assessed on a daily basis and no crack growth was observed.
- The affected section of pipe was replaced with new pipe and the cracked pipe was sent offsite for analysis.
The extent of condition investigation is ongoing. The following inspections have been completed or are planned as indicated:
- All accessible sections of Unit 2 SDC system were visually inspected for signs of pipe or weld leakage. No additional leakage was observed.
- A visual inspection of the equivalent branch line was performed in Unit 3 on January 12, 2006.
No leakage was observed.
Eighty-six (86) welds in the SDC, LPSI, Containment Spray (CS), and Containment Penetration systems were inspected using dye penetrant (PT) and/or ultrasonic (UT) inspection techniques and no other cracks were found.
- Further examinations are being planned for Unit 3.
Additional corrective actions will be implemented if they are identified.
Safety Significance
This event had minimal safety significance because:
Although SCE initially declared both trains of the SDC system inoperable, the system was later determined to be operable in Modes 5 and 6, in accordance with ASME Code Case N-513-1.
Therefore, the SDC system was capable of performing its required safety function in its as-found condition.
SCE also determined the pipe was able to perform its safety function under design basis conditions in all Modes (1 — 6). Therefore, the safety significance of this event is minimal.
Additional Information
regenerative heat exchanger. The cause of this crack was elevated vibration due to inadequate charging pump discharge pressure pulsation dampeners and a failed spring can. This event did not require a report to the NRC because the letdown system is isolable and TS leakage limits were not exceeded. Because the SDC/LPSI system pipe crack was not caused by inadequate dampening or a failed spring can, corrective actions for the January, 2004 event would not be expected to prevent this occurrence.
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