05000362/LER-2006-006

LER-2006-006, One train of Component Cooling Water considered inoperable for longer than allowed by Technical Specification 3.7.7 due to gas entrainment.

Docket Number
Event date:	12-15-2006
Report date:	4-9-2007
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3622006006R00 - NRC Website

Event Date: December 15, 2006 Reactor Vendor: Combustion Engineering Mode: Mode 1 — Power Operation Power: 0.95 percent

A. BACKGROUND INFORMATION

The Component Cooling Water [CC] (CCW) System consists of two independent critical trains and one non-critical loop (NCL). (Critical trains provide cooling for safety related equipment.) The non­ critical loop can be aligned to either one of the critical loops. Each CCW train is provided with a dedicated pump and a surge tank. A third, swing pump is also provided and can be aligned to either CCW train. The surge tanks are pressurized with a nitrogen blanket.

Technical Specification (TS) 3.7.7, Component Cooling Water System, requires two trains of CCW to be operable when the plant is in Modes 1-4. When one train is inoperable, the TS require SCE to restore the inoperable train within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, or place the plant in Mode 3 within the following 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. TS Bases B3.7.7 states that in Modes 5 and 6, the operability requirements of the CCW System are determined by the systems it supports.

Consistent with 10 CFR 50, Appendix A, General Design Criteria (GDC) 2 and 44, the design basis of the CCW system includes the requirement to be capable of performing its safety function following a Design Basis Earthquake (DBE) concurrent with a single active failure.

B. DESCRIPTION OF EVENT

On March 8, 2006, during routine activities, engineering personnel observed unexpected changes in the Unit 2 CCW Train A surge tank level indication. SCE's troubleshooting efforts concluded surge tank fluctuations were being caused by entrained gas. SCE initiated periodic venting at Unit 2. At was not affected. As a preventative measure, SCE initiated venting at Unit 3 as well. Venting history is provided in the Additional Information section.

On December 15, 2006, 120 gallons was vented from Unit 3 Train B return from the letdown heat exchanger. As a result, SCE contracted with outside consultants (Fauske and Associates) to analyze the potential effects of the gas on Unit 3 CCW system Train B.

SCE received the Fauske and Associates report on February 9, 2007 [Discovery date]. The consultants' report concluded that due to the location of the bubble, it could potentially migrate with the inadvertent opening of the letdown temperature control valve on the output of the letdown heat exchanger which varies CCW flow through the heat exchanger. They determined that should the following conditions occur:

1) Train B is aligned to the non-critical loop and 2) the non-critical loop becomes isolated and 3) the letdown temperature control valve fails fully open, a gas volume of 120 gallons in Train B could result in a void fraction (VF) at the CCW pump suction of 9 percent for 3.5 seconds (the average VF would be about 6 percent for 7.5 seconds, which is the time required for the void to travel through the pump). Generally accepted industry guidance indicates that a void fraction of about 5-6 percent would not permanently degrade pump performance. Because the calculated void fraction was outside the 5-6 percent range, SCE conservatively concluded that Unit 3 CCW Train B was inoperable from the time it was returned to service in Mode 6 following the cycle 14 refueling outage [November 24, 2006] through the initial start up in Mode 4 [December 7, 2006] until the system was vented [December 11 and December 15, 2006], about 21 days.

TS 3.7.7 requires the CCW system to be operable in modes 1 through 4. Unit 3 entered Mode 4 on December 7, 2006. On December 15, 2006, with Unit 3 in Mode 1, SCE vented CCW Train B returning Train B to operable status. Because this period (about 8 days) exceeds the allowable out of service time limit of TS 3.7.7, SCE is reporting this occurrence in accordance with 10CFR50.73(a)(2)(i)(B).

Cause of the Event:

The CCW system is a closed loop system, pressurized by a nitrogen blanket on the surge tank and treated with corrosion inhibitors. SCE believes gas in the system can most likely be attributed to one of three sources: absorption of the nitrogen blanket into the cooling water, air entrained when the system is open during maintenance, or as a product of a chemical reaction. SCE is investigating each of these sources but has not yet been able to determine definitively the source of the entrained gas and our investigation is ongoing.

Corrective Actions:

SCE has taken and is taking the following interim corrective actions:

1. Operating procedures have been changed to perform daily venting on both trains of Unit 3 and Train A of Unit 2. This action ensures that the systems remain operable. As a conservative measure, Unit 2 Train B is vented periodically.

2. Vent extensions are being added to better quantify venting amounts through a more standardized venting procedure.

3. SCE plans to revise the fill and vent procedure before the next planned outage on Unit 3.

SCE may identify and implement additional corrective actions.

Safety Significance

The SONGS Unit 3 Component Cooling Water system has two independent safety-related trains.

During the time Train B CCW was declared inoperable, Train A remained operable. (Amounts vented from Train A were below the amounts determined to be of concern.) Therefore, Train A was available to provide a heat sink for the removal of process and operating heat from safety related components during a Design Basis Accident or transient.

An assessment of the incremental core damage probability (ICDP) and the incremental large early release probability (ILERP) for the inoperability of Train B CCW determined that the Unit 3 ICDP and ILERP were 5E-7 and 2E-8, respectively. Therefore the safety significance of this event was minimal. The assessment was based on the reported component unavailability and operating NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (7-2001) conditions that existed from the time the train was declared operable on November 24, 2006 through the venting of gas on December 15, 2006.

Additional Information:

Although no abnormal CCW surge tank fluctuations had been observed prior to March 3, 2006, it is uncertain whether a similar condition occurred on Unit 3 or at some time in the past. The CCW system configuration has not been changed for many years. Consequently, it is possible that some gas accumulation has occurred in the past. Because it is not possible to quantify any gas volumes that might have accumulated, their potential effect on system operability cannot be assessed.

Venting History Date Unit 2 Train A Unit 2 Train B Unit 3 Train A Unit 3 Train B 8/03/06-9/29/06 No venting No ventingg 9/29/06-10/17/06 Periodic venting Periodic venting Periodic venting Once (9/29/06) 10/16/06-10/24/06 No venting 10/25/06-11/09/06 Daily venting 11/10/06-11/22/06 Daily venting Refueling Outage 11/23/06-12/15/06 Daily venting No venting 12/15/06-2/28/07 Monthly venting Daily venting Previous Events:

In the past three years, SCE has not reported any occurrences where entrained gas caused a plant system to be considered inoperable.