05000387/LER-2012-009
| Susquehanna Steam Electric Station Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 3872012009R00 - NRC Website | |
This report is submitted to identify a common cause that resulted from Susquehanna (SSES) Reactor Protection System (RPS) Electrical Protection Assembly (EPA) breakers that failed to trip during surveillance testing.
These RPS EPA breaker failures were determined to be a common cause inoperability of two independent trains of RPS electrical power monitoring associated with several breakers, whose failure could have prevented the fulfillment of a safety function for two or more trains or channels in different systems that are needed to shutdown the reactor and maintain it in a safe shutdown condition.
This condition is being reported in accordance with 10CFR50.73(a)(2)(vii).
There are sixteen RPS EPA breakers currently installed in SSES Units 1 and 2, eight on each unit. The primary function of the EPA breakers is to protect downstream RPS equipment from sustained over / under voltage or underfrequency conditions in the RPS power supplies. Two EPA breakers are installed in series and normally closed during operation. These breakers monitor power quality to each of the RPS buses. Their installation in series requires only one breaker to operate (trip) to disconnect an affected RPS power supply from its associated RPS power distribution bus.
Following is the chronological sequence that lead to this reportable condition:
5/8/2012 RPS EPA breaker 1CBS003A-G failed to trip while performing the procedure for the channel functional test of the RPS EPA breaker trip function. This test is necessary to meet the Unit 1 Technical Specification (TS) Surveillance Requirement (SR) 3.3.8.2.1.
5/9/2012 RPS EPA breaker 1CBS003A-G replaced and tested satisfactory.
5/9/2012 RPS EPA breaker 1CBS003B-B failed to trip while performing the procedure for the channel functional test of the RPS EPA breaker trip function. This test is necessary to meet the Unit 1 TS SR 3.3.8.2.1.
5/10/2012 RPS EPA breaker 1CBS003B-B replaced and tested satisfactory.
5/10/2012 RPS EPA breaker 1CBS003A-C failed to trip while performing the procedure for the channel functional test of the RPS EPA breaker trip function. This test is necessary to meet the Unit 1 TS SR 3.3.8.2.1.
5/12/2012 RPS EPA breaker 1CBS003A-C replaced and tested satisfactory.
9/21/2012 General Electric (GEH) analyses concluding the failures appear to be random calibration issues that fall into the maintenance area and there is no extent of condition.
1/15/2013 RPS EPA breaker 2CBS003B-F failed to trip while performing the procedure for the 24 month channel calibration and functional test. This test is necessary to meet the Unit 2 TS SRs 3.3.8.2.2 and 3.3.8.2.3.
1/16/2013 RPS EPA breaker 2CBS003B-F replaced and tested satisfactory.
3/13/2013 GEH Part 21 analysis concluding the postulated EPA undervoltage device common mode failure was not reportable under Part 21 because it does not represent a substantial safety hazard in the RPS EPA application.
(10-2010) LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
GEH, the RPS breaker manufacturer, performed a forensic analysis of the three failed Unit 1 breakers and determined that the EPA breaker undervoltage release mechanisms (UVR) drifted out of calibration. The apparent cause was that the calibration screws were out of adjustment on the UVR causing an insufficient force balance between the torsional spring and the plunger spring. Because of this, the plunger was not supplying an adequate force to trip the breaker.
Contributing factors were marginal calibration at the GEH factory prior to application of screw holding cement; calibration screw movement over time with repeated cycling of breakers; and curing of the Sauereisen cement on calibration screws was found to be marginal.
Note that the Unit 2 EPA breaker failure to trip on 1/15/2013 exhibited symptoms that indicate the failure mode was the same as the Unit 1 failures evaluated in the September 2012 GEH report.
The GEH report concluded that these failures appeared to be random calibration issues, performed by GEH at the factory, that fall into the maintenance area. GEH further stated there is no extent of condition. However, the calibration issues appear to have caused failures in the under voltage trip devices of the breakers considered to be reportable as events that had a common cause.
Note that a common cause that would impact the operability of individual RPS circuits has not been identified.
ANALYSIS/SAFETY SIGNIFICANCE
Actual Consequences:
There were no actual safety consequences due to the failure of these RPS EPA breakers to trip during surveillance testing. Each of the failed breakers was in different RPS power supply circuits. A failure of one of the two series connected RPS breakers to trip does not result in an entire loss of safety function. The redundant equipment in the same circuit (i.e. the other series connected EPA breaker) was operable and capable of tripping to perform its required safety function.
After replacement of the failed EPA breakers, a retest demonstrated that both of the series connected breakers successfully tripped.
Therefore, since the retest demonstrated that the redundant series breaker was always capable of performing its safety function (i.e. to trip) there were no actual safety consequences.
Potential Consequences:
RPS equipment that remains connected through EPA breakers that fail to trip could be subjected to sustained conditions of under voltage, over voltage, or under frequency. In order for this to occur, two RPS EPA breakers connected in series would need to fail to trip in each of the two RPS trip systems. Therefore, four out of eight EPA breakers, connected as described above would have to fail to trip for this potential condition to occur.
The GEH Part 21 report considered other failures of RPS equipment due to EPA breaker failures and concluded that postulated EPA under voltage device common mode failure does not represent a substantial safety hazard in the RPS EPA application nor is there a substantial increased risk to generation as a result of continued use of EPA breakers.
k10-2010) LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET
Immediate corrective actions were to replace the failed EPA breakers (in the Normal RPS Power alignment) with new EPA breakers from the plant warehouse.
The GEH report identified that the calibration procedures were reevaluated and improved and adjusted. Additionally, further, improvements were made to the breakers to reduce the likelihood of future failures. Thus, a replacement breaker is not expected to be affected by this random calibration issue. Therefore, in the September 2012 report, GEH did not recommend any further replacements or preventive maintenance revisions.at this time.
However, in March 2013, the GEH part 21 report recommended replacing either the entire circuit breaker or the undervoltage release device.
PPL took conservative action to replace all station EPA breakers (Unit 1, Unit 2, and storeroom) with new breakers. Currently, the Unit 1 and Unit 2 Normal Supply EPA breakers have been replaced. The Alternate Supply EPA breaker replacements are scheduled for replacement in accordance with the Susquehanna Work Management process. These replacements address the potential concern regarding the extent of condition related to the remaining EPA breakers.
PREVIOUS SIMILAR EVENTS
Review of industry operating experience (OE) related to failures of EPA breakers to trip identified different distinct causes. Early EPA related failures experienced in the industry (late1990's) led to redesign of the UVR trip arm assembly. Later failures (early 2000's) were related to improper calibration (bending of the trip arm) and grease hardening (beyond design life and improper preventive maintenance). All the breakers at SSES were replaced with new breakers in 2009. The 2009 replacement addressed all the prior issues along with incorporating vendor recommended replacement and preventive maintenance frequencies. The three Unit 1 breakers that failed in 2012 and the Unit 2 breaker that failed in 2013 were installed during the 2009 replacement. These failed breakers were replaced with breakers manufactured by GEH during the same time period.