05000361/LER-2011-001
| San Onofre Nuclear Generating Station (Songs) Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| 3612011001R00 - NRC Website | |
On January 14, 2011, SCE identified the electrical alignment of the non-Class 1 E 6.9 kV electrical system at SONGS was shared between Units 2 and 3 during two recent refueling outages without an adequate evaluation that considered the impact on the UFSAR Chapter 15 safety analyses. This alignment was considered during the original design of the plant and is allowed in accordance with UFSAR Section 8.3.1 "AC Power Systems" and station operating procedures, but did not consider General Design Criterion 5.
The 6.9 kV busses supply the Reactor Coolant Pumps (RCPs) [P] and are normally aligned to the output of the main generator. The 6.9 kV busses automatically transfer to off-site power on a loss of power from the main generator, as described in the SONGS 2/3 Updated Final Safety Analysis Report (UFSAR). With both units at power, the Unit 2 off-site 6.9 kV source was redirected to Unit 3 to support maintenance on the Unit 3 transformer. If Unit 2 had tripped, the RCPs would have lost power, resulting in a natural circulation cooldown; Unit 3 would not have been affected. A similar configuration was established in 2009 during the Unit 2 Cycle 16 refueling outage, when the Unit 2 6.9 kV busses were aligned to Unit 3.
During the brief period the non-Class 1 E 6.9 kV busses were aligned through a single unit, there was a potential for more than a minimal increase in the probability or consequences of previously evaluated transient analysis events. The event is being reported under 10 CFR 50.73 (a)(2)(ii) (B) "Any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
B. INITIAL CONDITIONS
At the time of discovery on January 14, 2011, SONGS Unit 2 was in Mode 1 with the reactor at approximately 100 percent power, and SONGS Unit 3 was in the Cycle 16 Steam Generator Replacement refueling outage with the plant in Mode 5 (cold shutdown). The issue was identified through engineering reviews of a request to change the protective interlocks on the 6.9 kV system in support of RCP operation for startup from the Unit 3 Cycle 16 refueling outage.
There were two instances found in the past three year reporting period when the non-Class 1 E 6.9 kV system was shared between the units to support maintenance. At the time of each occurrence, both units were in Mode 1.
C. DESCRIPTION OF OCCURRENCE
Each SONGS unit has four RCPs circulating water through the Reactor Coolant System (RCS), transferring the heat from the reactor to the steam generators. The RCPs continue to operate as the reactor is shutting down for decay heat removal. Should the RCPs lose power, 'natural circulation' flow through the RCS, driven by the temperature difference across the steam generators, is also sufficient to remove the decay heat. Forced circulation of the RCS is not credited, nor required for accident mitigation, allowing the RCPs and associated power supplies to be designed as non-Class 1 E components.
The four RCPs on each unit are powered from two non-Class 1 E 6.9 kV AC electrical busses, with each bus supplying two RCPs. The two 6.9 kV busses on each unit are normally supplied by the Unit Auxiliary Transformer (UAT) from the output of the main generator. Alternately, the 6.9 kV busses may be supplied from the switchyard off-site power source through the Reserve Auxiliary Transformer (RAT) on each unit. Each RAT has two windings, and each winding can supply one 6.9 kV bus.
In the event of a main generator trip or failure, a fast transfer scheme automatically switches the 6.9 kV busses from the main generator output to the available off-site source. The design allows off-site power to be automatically provided from either unit to maintain power to the RCPs. Protective interlocks prevent placing more than a single 6.9kV bus on the reserve transformer winding. Consequently, when a 6.9 kV bus supplied from the main generator loses power, the RCPs will trip if the off-site power at that unit has been connected to supply the 6.9 kV bus on the other unit.
Preceding the Unit 2 and Unit 3 Cycle 16 refueling outages, the 6.9 kV busses were realigned from the normal main generator output to off-site power supply on the opposite unit to start scheduled maintenance on the reserve transformer before shutting the units down for the refueling outage. There were two occurrences:
On September 26, 2009, with Unit 2 at 99 percent power, and Unit 3 at 100 percent power, at approximately 1340 PDT, the Unit 2 6.9 kV busses were transferred to the Unit 3 RAT. At 2358 PDT, the Unit 2 reactor was shutdown for the refueling outage. Unit 2 entered Mode 4 on September 27, 2009, at 0606 PDT, and Mode 5 on September 28, 2009, at 0240 PDT. The Unit 2 RCPs were secured and the 6.9 kV busses were de-energized on September 29, 2009, at 0154 PDT.
On October 9, 2010, with Unit 3 at 82 percent power and Unit 2 at 94 percent power, at approximately 2158 PDT the Unit 3 6.9 kV busses were transferred to the Unit 2 RAT. On October 10, 2010, at approximately 0126 PDT, the Unit 3 reactor was shutdown for the refueling outage. Unit 3 entered Mode 4 on October 10, 2010, at 1505 PDT, and Mode 5 on October 11, 2010, at 0447 PDT. The Unit 3 RCPs were secured and the 6.9 kV busses de-energized on October 12, 2010, at 1110 PDT.
The configuration did not violate the Technical Specifications, which, in summary, require operation of the RCPs in Mode 1 until the Shutdown Cooling System is in operation.
D. APPARENT CAUSE
The ability to cross connect the non-Class 1 E 6.9 kV busses between the units is part of the original plant design as described in UFSAR Section 8.3.1.1.1, "Non-Class 1 E AC System," and was implemented through the plant procedures. In the event of an electrical failure, a loss of generation, or a failure of an off- site power source, the design provides the ability to retain power to the RCPs on both units. However, with the electrical distribution system of a single unit carrying the 6.9 kV busses for both units, a generator trip would result in the loss of forced coolant flow on that unit. The alignment of the 6.9 kV cross connection for normal operation, such as maintenance, would initiate natural circulation conditions for a routine plant trip.
The SONGS UFSAR Section 15.3 classifies the "Total Loss of Reactor Coolant Forced Flow," as an Infrequent Incident. The loss of forced reactor coolant circulation upon plant trip would result in more than a minimal increase in the probability or consequences of previously evaluated UFSAR Chapter 15 "Accident Analysis" events.
The Loss of Forced Reactor Coolant Flow event is one of several infrequent category events evaluated in the UFSAR as a concurrent single failure of an active component. The analysis for some of the infrequent events, notably the Increase in Main Steam Flow with a Concurrent Single Failure of an Active Component, indicates fuel damage may result. With a loss of forced reactor coolant flow occurring as a direct result of a plant trip (i.e., no additional failure), these events shift to the moderate frequency category, for which the Standard Review Plan does not allow fuel damage.
10 CFR 50 Appendix A, General Design Criteria (GDC) 5 "Sharing of Structures, Systems and Components," requires that the sharing of systems does not prevent an orderly cooldown and shutdown of one unit given an accident in the other unit.
The SONGS UFSAR identified the shared systems between Units 2 and 3 in Appendix 3B Table 3B-1, concluding "No shared system or failure of a shared system will impair the capability to safely shut down and cool down either or both units." Table 3B-1 does not include the non-Class 1 E 6.9 kV AC system that may be shared between Units 2 and 3. As such, neither SCE, nor the NRC, evaluated the non-Class 1 E 6.9 kV system with respect to GDC 5. Because GDC 5 was not considered in the design of the non-Class 1 E 6.9 kV system, the electrical power for the RCPs with respect to GDC 5 was not evaluated, and the impact to UFSAR Chapter 15 safety analyses was not recognized.
E. CORRECTIVE ACTIONS
Immediate Corrective Actions - Operating procedure changes were implemented to restrict alignment of the 6.9 kV electrical busses between the units to periods when the opposite unit is shutdown (Modes 5, 6, or defueled).
Long Term Corrective Actions - None are required, the existing condition was corrected.
F. SAFETY ASSESSMENT
Sharing the non-Class 1 E 6.9 kV AC busses between the units did not result in an actual loss of forced circulation flow, and was of minimal safety significance. The duration of the cross connection of the 6.9 kV busses during the past three years was limited to a brief period when the busses were aligned to permit maintenance of the Reserve Auxiliary Transformers. The short period of time the 6.9 kV busses were shared between units (total of 2.6 days) resulted in a very small increased risk. The Incremental Core Damage Probability (ICDP) and Incremental Large Early Release Probability (ILERP) due to loss of 6.9 kV power to RCPs following a reactor trip were calculated to be 7.1E-10 and 1.3E-9, respectively.
G. ADDITIONAL INFORMATION
Previous Similar Events
There have been no similar reportable events in the past three years at SONGS with the same underlying cause, nor have similar events caused by sharing a non-Class 1 E electrical system been reported as industry operating experience.