05000400/LER-2007-003
| Docket Number Sequential Revmonth Day Year Year Month Day Year N/A 05000Number No. | |
| Event date: | 09-28-2007 |
|---|---|
| Report date: | 11-19-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 43676 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 4002007003R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text within brackets [ ].
I. DESCRIPTION OF EVENT
At 22:32 on September 28, 2007, while reducing power for Refueling Outage (RFO)-14, a fault pressure trip on "1A" Startup Transformer (SUT) [EA] caused a reactor trip/turbine trip with a loss of all Reactor Coolant [AB] Pumps (RCP) and a subsequent Auxiliary Feedwater (AFW) [BA] actuation. There was no inoperable equipment at the start of this event that contributed to this event.
The plant was at approximately 27 percent power and was reducing power to begin RFO-14. All plant auxiliary loads had been transferred to the SUTs in accordance with plant procedures. "1A" SUT was supplying the "1A" and "1C" electrical buses. The "1A" SUT tripped due to the actuation of its Qualitrol 930 series sudden pressure fault relay. This deenergized the "1A" and "1C" buses causing a loss of the "A" and . The condition also tripped "B" RCP as designed, resulting in a loss of all forced flow in the reactor coolant system.
The AFW system actuated immediately following the reactor trip at 22:32 and operated as expected to stabilize steam generator levels. All systems functioned as required and no other safety systems were actuated. All control rods fully inserted on the reactor trip. The operations staff responded to the event in accordance with applicable plant procedures. The plant stabilized at normal operating no-load reactor coolant system temperature and pressure following the reactor trip.
This condition is being reported as an unplanned reactor protection system actuation and specified system actuation in accordance with 10 CFR 50.73(a)(2)(iv)(A).
II. CAUSE OF EVENT
The investigation found no evidence of an actual fault condition in the "1A" SUT. Post trip discovery revealed that 2 out of 3 channels on the 930 series pressure relay actuated causing a lockout of the "1A" SUT and the subsequent reactor trip. The investigation for this event determined that available margin for the low rate of rise pressure changes was not adequate. The HNP transformer internal pressure profile was likely near the limit for relay sensitivity when set at 4 (0.35 psig/sec for about 10 seconds) which resulted in two of the three channels actuating.
The cause of the available margin for low rate of rise pressure changes being inadequate was that the data to determine the available margin was not available at the time of the design. The basis for the range of the 930 relay was that it was designed to bound the old 900 series relay. There is no documentation available from the relay vendor or the original equipment manufacturer of the transformer on the original basis for the relay range.
Data is not available on what pressure rates would be seen in the transformer during normal operation. During this investigation contact was made with original equipment manufacturers and with consultants in an attempt to determine what normal operating pressure rates would be seen by the relay sensors. The original equipment manufacturers and consultants stated that the information is not available and that they do not have the analytical tools to determine what rates of pressure change are expected. Therefore, it is concluded that data to determine the available margin was not available at the time of the design.
It was also determined that the 930 series fault pressure relay installed in the "1A" SUT was out of calibration in the more sensitive direction at the low pressure rate.
The potential narrow available margin, combined with the fact that the relay was out of calibration on the low pressure rate (.35 psig/sec) when the sensitivity was set at 4, were the causal factors that were in place that led to the event.
III. SAFETY SIGNIFICANCE
This event is not significant because the Reactor Protection System [JC] functioned as designed. This event is noteworthy because a single component malfunction caused a reactor and turbine trip.
Actual Safety Consequences:
The transient was an automatic reactor trip from 27 percent power generated by logic for a RCP Underfrequency due to a loss of Auxiliary buses 1A-NNS, 1C-NNS and 1D-NNS. Although only one train (fed from the 'A' Startup Transformer) of non-emergency power was lost, this event is bounded by a Loss of Non-emergency AC Power to the Station Auxiliaries which- is analyzed in the HNP FSAR Section 15.2 . A Loss of Non-emergency AC Power to the Station Auxiliaries is classified as an American Nuclear Society (ANS) Condition II event — a fault of moderate frequency. The plant is designed for this type of event and responded as expected. The initial plant conditions prior to the trip were well within the bounding conditions for the plant design. The plant attained normal operating no-load temperature and pressure, and no unusual conditions were observed for plant equipment following the event. All safety equipment functioned as required. The 'A' EDG started and loaded the 'A' safety buses (as designed). No additional or compensatory measures were required for this event. The operating staff performed the required actions for the trip. Other than the transient induced by the trip, there are no adverse safety consequences.
Potential Safety Consequences:
The potential safety consequences under alternate conditions are bounded by the FSAR Chapter 15 events.
IV. PREVIOUS SIMILAR EVENTS
The Qualitrol 930 two-out-of-three sudden pressure relay misoperation experienced at Harris is a first of a kind event in the industry for this model relay.
V, CORRECTIVE ACTIONS Completed corrective actions include replacing the Qualitrol 930 series relay with the Qualitrol 900 series relay and conducting testing to validate that the "1A" SUT was not damaged. Additional corrective actions are to use a temporary modification to gather data on the ."1A" SUT to determine operating margin and implement appropriate design changes to the sudden pressure relays on the SUTs.
VI. COMMITMENTS
This document contains no new regulatory commitments.