05000250/LER-2013-005
| Docket Number | |
| Event date: | 3-12-2013 |
|---|---|
| Report date: | 5-10-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 48817 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 2502013005R00 - NRC Website | |
On March 12, 2013 at approximately 1431 with Unit 3 in Mode 2 at approximately 3% rated thermal power (RTP), an automatic reactor [AC, RCT] trip occurred due to a spike sensed at Pressure Transmitter [JC, PT] PT-3-447, Turbine Inlet Pressure, during turbine control valve (TCV) [TA, TRB, FCV] testing. PT-3-447 provides input to the P-7 (greater than 10% power) permissive, which enables the Reactor Protection System (RPS) [JC] at power reactor trips (At Power Trips). One of the At Power Trips is a turbine [TA, TRB] trip initiated by 2 out of 2 turbine stop valves (TSV) [TA, TRB, ISV] closed, or 2 out of 3 emergency trip header pressure transmitters greater than or equal to 901 psig. At the time, the TSVs were already closed, which resulted in the RPS actuation.
All control rods inserted. The Auxiliary Feedwater (AFW) System [BA] did not actuate. Plant parameter response was as expected.
Because an RPS actuation occurred, this event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A).
The event was previously reported in Event Notification 48817 in accordance with 10 CFR 50.72(b)(2)(iv)(B).
CAUSE OF THE EVENT
The direct cause of the reactor trip was the unexpected pressure spike resulting from a quick opening of the No. 3 TCV of sufficient magnitude to satisfy the P-7 interlock [JC, IEL] with the TSVs closed.
The root cause is a failure to recognize the risk associated with TCV testing that can cause a pressure transient sufficient to enable the At Power Trips.
ANALYSIS OF THE EVENT
During the event, the reactor was at approximately 3% RTP (below P-7 permissive) with the main steam isolation valves (MSIV) [SB, ISV] open and steam header pressurized. The turbine was latched with TSVs closed and turbine stop drain valves [TF, V] open. The open stop drain valves effectively bypass the TSVs and pressurize the line between the TSVs and TCVs to main steam header pressure.
With the TSVs closed, post maintenance testing and calibration was being performed on the newly installed valve position indicator for the No. 3 TCV, which was incrementally exercised from fully closed to fully open. During the final test checks the No. 3 TCV was being opened from full close to full open, which resulted in the steam that had built up behind the TCV to rapidly reach the turbine inlet pressure transmitter causing indicated turbine power to exceed the P-7 set point which enabled the At Power Trips. With the input to the RPS for a turbine trip already met (two TSVs closed), the P-7 interlock satisfied RPS logic to actuate and cause a reactor trip.
ANALYSIS OF SAFETY SIGNIFICANCE
Upon reactor trip, response procedures were entered. Reactor and turbine trips, and unit stability were verified. Operators then transitioned to the normal shutdown procedure. All control rods inserted. AFW did not actuate because all SG levels remained above 16%.
Reactor operation at reduced power levels results in a smaller transient immediately after the automatic reactor trip. Plant parameter response was as expected for this transient. Therefore, the safety significance and contribution to plant risk from this event are very low.
CORRECTIVE ACTIONS
Corrective actions are documented in AR 1856035 and include the following:
1. Procedure changes to require MSIVs and turbine stop drain valves to be closed when testing TCVs in Modes 2 or 3 with reactor trip breakers closed.
2. Procedural precautions identifying that TCV testing can lead to turbine inlet pressure spikes, and reactor trip if the P-7 interlock is enabled with the turbine latched and reactor trip breakers closed.
3. Revision of the post maintenance testing procedure to ensure that the main steam header is depressurized prior to cycling a TCV.
FAILED COMPONENTS IDENTIFIED: None PREVIOUS SIMILAR EVENTS: None