05000333/LER-2013-001

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LER-2013-001, Failed Under Voltage Relay Results In Auto Start of the Emergency Diesel Generators
James A. Fitzpatrick Nuclear Power Plant
Event date:
Report date:
Reporting criterion: 10 CFR 50.73(a)(2)(iv)(A), System Actuation
3332013001R00 - NRC Website
v • d • e

EVENT DESCRIPTION

On January 15, 2013, during the performance of surveillance test (ST), ST-43D, "Remote Shutdown Panel 25ASP-3 Component Operation and Isolation Verification," the James A. FitzPatrick Nuclear Power Plant (JAF) experienced an automatic start of the "B" and "D" emergency diesel generators (EDG). The automatic start of the EDGs was caused by the failure of contact set 3/4 of the 10600 Emergency Bus Under Voltage Relay (71- 271AB-1HOEB04) in the closed position. The relay is designed such that these contacts will close when a loss of voltage is detected on the AB phase of the bus. This closure inserts one channel of the two required to start the EDGs on loss of bus voltage.

The circuitry detected a loss of voltage condition during the test when the BC phase Bus under voltage relay was actuated per the ST and initiated an automatic start of the EDGs. When the EDG output voltage reached 75% of normal output voltage, the voltage monitoring circuit initiated a trip of the 10614 and 10404 breakers which feed the 4kV emergency bus (10600). This resulted in a loss of the 10600 bus voltage, a "B" side half scram, and group II primary containment isolation.

These events are reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), Any event or condition that resulted in the automatic actuation of general containment isolation signals affecting containment isolation valves in more than one system and the automatic start of the emergency diesel generators.

EVENT ANALYSIS

ST-43D section 8.2 demonstrates the ability to take control of the "B" and "D" EDGs from the local remote shutdown control panel, 25ASP-3. This section involves the lineup of isolation switches and the pulling of bus voltage sensing fuses on the respective EDG 4kV breaker panel. With isolation switch (IS-3) in the LOCAL position, removing the fuses verifies that an under voltage condition is not sensed by the remote 10600 bus under voltage circuitry. The fuses that are pulled in this section supply power to two normally energized relays (71-271AB-1HOEB04 and 71-272BC-1HOEB04) which monitor the phase voltage on the 10600 bus. When the fuses were pulled, relay 272BC became de-energized and its 3/4 contact set closed as per design. At this time, IS-3 had been previously placed in the LOCAL such that power was being provided to 271AB. With 271AB energized, its 3/4 contact set should have remained open. However, they had previously failed close.

The closure of the 272BC contacts combined with the failure of the 271AB contacts completed the loss of voltage circuit and resulted in an automatic start of the "B" and "D" EDGs. Once the EDG output voltage reached 75% of normal, the control circuitry initiated a trip of the 10400 to 10600 bus tie breakers (10404 and 10614) which supply power from the 10400 to the 10600 bus. This resulted in a loss of the 10600 bus which in turn resulted in a "B" division half scram and a primary containment isolation of group II isolation valves. Normally, when breakers 10614 and 10404 are open, a permissive to close is provided to the "B" and "D" EDG output breakers (10612 and 10602). However, 10612 and 10602 were unable to close because the breaker closing circuitry had been previously disabled as part of the ST. Since there was no power to the "B" Emergency Service Water (ESW) pump; no cooling was provided to the "B" and "D" EDG jacket water coolers. This resulted in the automatic trip of the "B" and "D" EDGs on high cooling jacket water temperature as designed.

CAUSE OF EVENT

A root cause evaluation (RCE) was completed to determine any human performance, equipment, organizational, or programmatic causes that may have directly caused or contributed to the event. The RCE determined that the root cause was directly attributed to the failure of relay 271AB. Programmatically, a contributing cause was noted in that ST-43D does not direct the operator to perform a voltage or continuity check such that the proper configuration is verified prior to pulling the fuses. There were no human performance errors identified during this evaluation.

The extent of condition review performed as part of the RCE considered the five (5) alternate shutdown panels and the one (1) remote shutdown panel at JAF as being within the scope of the review. There are eleven ST-43 series of surveillance test procedures that verify the capabilities of these panels and corresponding components.

Of the eleven surveillance tests, ST-43D and ST-43H, "Circuit Breaker 10614 Remote Shutdown Operation and Isolation Verification," are the only two STs that affect both the B and D EDGs and the 10400/10600 buses. No other sections of ST-43D contained steps where any other automatic initiations could occur. Also, no sections of ST-43H contained steps where automatic operations would jeopardize the 10400 or 10600 buses. As such, no other conditions were identified where an automatic initiation could occur that would result in a loss of an emergency bus or initiate a half scram.

The extent of condition also included a review of the particular style and model of relay that failed. It was determined that this model relay is used in a total of four (4) locations at JAF. All four locations are related to the EDGs and the two safety related vital buses. The relay that failed was the only one of the four that is placed in the configuration where an under voltage condition is simulated while the breakers that would normally close on loss of bus voltage and subsequent start of the EDGs are defeated. In the other locations, for this same type of event to occur, both under voltage relays in the circuit would have to fail concurrently. In either case, the EDGs would sense an under voltage condition and would automatically start to provide power to the emergency buses.

CORRECTIVE ACTIONS

Completed Actions

  • Operators restored the normal configuration of the "B" and "D" EDGs and returned them to available status.
  • ST-43D was revised to include steps for performing a check for 1/2 logic failures prior to removing fuses.
  • The failed under voltage relay (271AB) was replaced and post work testing was completed satisfactory.
  • 271AB sent to vendor lab for a failure analysis.

Future Actions

  • Selected surveillance test procedures will be reviewed for instances of when control fuses are pulled within safety related circuitry. Any identified procedures will be revised to ensure that relay contact checks are performed prior to removing fuses.

SAFETY SIGNIFICANCE

Nuclear Safety The safety significance of this event is directly linked to the loss of the 10600 4kV Emergency Bus which has a number of safety significant electrical loads. These electrical loads include components for the Residual Heat Removal (RHR) system, Core Spray (CS) system, Emergency Service Water (ESW) system, Reactor Protection system (RPS), and the "B" Station Battery. As required by the Technical Specifications, Limiting Condition for Operation (LCO), 3.8.7, Condition A, the 10600 bus was restored within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. During the approximate four (4) hours while the 10600 bus was de-energized, the plant risk state was orange. After restoration of the 10600 bus, the plant risk state turned yellow for approximately four (4) more hours while the EDGs were being restored.

Once the EDGs were restored, the plant risk state returned to green.

During the event, reactor building differential pressure was greater than zero. Appropriate plant procedures were followed to isolate reactor building ventilation and start the "A" standby gas treatment system. Reactor building differential pressure returned to less than zero and no high radiation alarms were received. Therefore, there was no impact to radiological safety and the health and safety of the public was not challenged.

Industrial Safety This event had no impact on industrial safety. There were no personal injuries or damage to plant equipment.

SIMILAR EVENTS

An internal search of Entergy's corrective action database was performed for a period of time covering the 11 years preceding this event. Four (4) events were found relevant. All four of the events had human performance related causes and were not directly similar to the event described in this LER. LERs at JAF for the preceding five (5) years were also reviewed. LER 2008-003 reports a loss of the 10600 bus voltage and automatic start of the "B" and "D" EDGs. The cause of the event was related to a human performance deficiency and thus no direct correlation can be made between that event and the event described in this LER.

The Institute of Nuclear Power Operations (INPO) Consolidated Events System (ICES) database was also reviewed for similar events across the nuclear industry. Two events were found relevant that pertained to the loss of a safety related electrical bus and the automatic start of EDGs. In particular, insight was gained from an event at Peach Bottom Unit 2 where the loss of the 4kV emergency AC bus occurred due to a failed time delay relay that had exceeded the manufacturer's service life of 10 years.

REFERENCES

  • Licensee Event Report, LER-2008-003, Loss of Emergency Bus and Auto-Start of the "B" and "D" EDGs
  • JAF Technical Specifications and Bases, 3.8.7, "Distribution Systems — Operating"

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