05000498/LER-2006-003

LER-2006-003,

South Texas, Unit 1
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4982006003R00 - NRC Website

A. REPORTABLE EVENT CLASSIFICATION

This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B). The South Texas Project (STP) determined that Control Room Envelope (CRE) Makeup Fan 11B was inoperable and actions had not been taken as required by Technical Specifications. The requirements of Technical Specifications (TS) were not met in that one of the three CRE HVAC trains was not operable, and TS 3.7.7 Action a. was not implemented within the required allowed outage time. Consequently, STP Unit 1 was in a condition prohibited by Technical Specifications.

B. PLANT OPERATING CONDITIONS PRIOR TO EVENT

STP Unit 1 was in Mode 1 at approximately 95% power, in coast-down operations.

THE START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT

No other inoperable structures, systems, or components contributed to the event.

D. NARRATIVE SUMMARY OF THE EVENT

On 9/11/2006, at 1210 hours0.014 days <br />0.336 hours <br />0.002 weeks <br />4.60405e-4 months <br />, with Unit 1 in coast down operations, CRE Make-up Fan 11B was manually actuated from the Control Room in support of maintenance activities and failed to start.

The failure was due to the magnetic circuit breaker (short circuit trip protection) for Fan 11B tripping on instantaneous (starting) over current. As a result, CRE HVAC Train B was declared inoperable.

Following the event, a walkdown of the Fan 11B motor determined the locked rotor code letter specified in the nameplate data to be "J," indicating a higher than normal locked rotor current motor. Energy efficient and high torque motors have inherently higher locked rotor currents than more common older motors with the same full load ampere ratings and, therefore, require a higher instantaneous trip setting for the magnetic breaker. Based on the revision of STP Design Criteria 4E510EQ1005, Class 1 E AC Power Distribution that was in effect when the design change package was prepared, the instantaneous trip setting of the, magnetic breaker was incorrectly selected based on code letter "G" motors. "G" motors have lower locked rotor current requirements than "J" motors. As a result, the magnetic breaker setting was set too low causing a vulnerability to spurious tripping.

During emergency conditions (Safety Injection (SI) or Safety Injection with a Loss of Offsite Power (LOOP)), lower voltages on the bus feeding the fan could have sufficiently decreased the starting current required to allow a successful Fan 11B motor start. However, a high radiation signal could require the motor to start without an SI signal. For a high radiation start, the bus voltages could be high and the motor windings could be at ambient temperature and this condition could also trip the breaker. Therefore, as evident by the Fan 11B breaker trip, the conditions which could potentially trip the breaker have existed since its replacement in 2000.

Investigation determined that the nameplate locked rotor code letters for the Motor Control Center (MCC) fed motors were not documented when the Makeup Fan 11B breaker was replaced. Further, the investigation identified several missed opportunities in previous years to identify the deficiency in the Design Criteria. The causes of these missed opportunities include human performance issues, as well as programmatic issues. These issues are summarized below.

It was also observed that at the time of the 11B Makeup Fan start failure, the bus voltages were high (because of light grid loads due to overcast weather conditions) and the motor windings were at ambient temperature (the motor is typically in standby mode for weeks). Both conditions increase the motor starting current requirements for induction motors. With the 11B Fan breaker set at the incorrect tap setting, review of test data showed that the breaker would trip at the low end of the acceptable breaker setting tolerance. Breakers tripping on the high end of the setting "tolerance range" would have not been affected by the above conditions.

Background Information The Control Room Envelope Make-up Fans are part of the CRE HVAC System. The CRE HVAC System is safety-related and consists of three 50 percent capacity redundant equipment trains.

The subject fan is powered by 480 Volt Engineered Safety Features (ESF) MCC El B1 cubicle M3.

The CRE Makeup Fans supply outside makeup air through the makeup filter units for CRE pressurization and to filter outside air for makeup during emergency mode. The makeup unit fans deliver filtered air to the Control Room Air Cleanup Filter Unit. The fans are not required to operate under normal operating conditions.

The fans are started automatically by the ESF load sequencer after an SI or SI coincident with LOOP signal actuation or by an outside air intake high radiation signal. Surveillance of airborne radioactivity levels of the outside makeup air to the supply system is provided by the Control Room ventilation inlet air radiation monitors. The fans can also be started manually by operator action.

The CRE Makeup Fan motors are provided with overload and short-circuit trip protection by 480 Volt Class 1 E MCCs. The overloads provide long term protection and the short circuit protection is instantaneous for shorts (faults and abnormally high starting current). The 480 Volt ESF MCCs have combination motor starters provided with magnetic, multiple setting instantaneous trip circuit breakers for short-circuit protection. Control Room Make-up Fan 11B MCC breaker El B1/M3 tripped on instantaneous current when manually actuated from the Control Room. The MCC breaker was reset and the motor started and ran successfully. The measured full load current was less than nameplate and the motor physically checked out satisfactorily.

The magnetic breaker in MCC El B1/M3 was replaced as part of the Class 1 E MCC Upgrade Modification in the spring (March 4-May 20) of 2000 (Refueling Outage 1RE09). CRE Makeup Fan 11B had been successfully started and/or tested on multiple occasions up to the discovery date of this event.

PROCEDURAL ERROR

This condition, failure of the 11B CRE Makeup Fan to start when manually actuated from the Control Room pushbutton, was identified while Operations was starting Makeup fans in support of maintenance.

II.� EVENT-DRIVEN INFORMATION

A. SAFETY SYSTEMS THAT RESPONDED

No safety systems were required to respond during this event.

B. DURATION OF SAFETY SYSTEM INOPERABILITY

The instantaneous trip setting for the CRE Makeup Fan 11B breaker was set incorrectly when it was replaced as part of the MCC Upgrade Modification in 2000 (Refueling Outage 1RE09). As evident by the Fan 11B trip, the conditions which could potentially trip the breaker have been possible since its replacement. Therefore, the CRE Makeup Fan 11B was inoperable since the breaker replacement in 2000.

CRE Makeup Fan 11B had been successfully started and/or tested on multiple occasions up to the discovery date of this event.

C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT

A risk assessment performed by the STP PRA group determined that there was no adverse impact to Core Damage Frequency or to the Large Early Release Frequency as a result of this event. The CRE HVAC Makeup fan is used to support the control room operator habitability safety function. This function is not modeled in the plant probabilistic risk assessment (PRA) model; only the CRE HVAC room cooling function is modeled as a plant initiating event. Failure of the CRE HVAC Makeup fan 11B does not impact the room cooling function. Therefore, failure of CRE HVAC Makeup fan 11B does not cause a quantifiable increase in core damage or large early release frequency.

This event resulted in no personnel injuries, no offsite radiological releases, and no damage to other safety-related equipment.

CAUSE OF THE EVENT

The cause of the event was an incorrect instantaneous magnetic breaker setting. This incorrect setting was established as part of the Class 1 E MCC Upgrade Modification, which was implemented during the spring (March 4-May 20) of 2000 during Refueling Outage 1RE09.

It was not recognized at the time the design change was prepared that the magnetic setting criterion was incorrect and should have been revised. The incorrect breaker setting data was also contained in the STP Electrical Setpoint Index. The incorrect design criteria was corrected as part of the corrective actions for a 2002 Condition Report. Additionally, corrective actions associated with a 2000 Condition Report revised several sections of the Electrical Setpoint Index to reflect the correct breaker setting criteria. However, some guidance in the STP Electrical Setpoint Index still reflects the incorrect magnetic breaker setting criteria and will be revised.

A contributing cause is the fact that the Electrical Setpoint Index in some instances provides more than one value for the breaker magnetic setting with direction to set the breaker to the minimum value and increase if required due to spurious trips. This practice masks events where the breaker is adjusted in accordance with the setpoint index because these adjustments are not reviewed by engineering and the work order closure process does not review-for these types of changes. The Setpoint Index will be revised to specify a single magnetic breaker setting for safety related SSCs.

Additional contributing programmatic/procedure/performance causes are as follows:

1. Causal evaluations performed for previous breaker setting events in 2000 and 2002 were too narrowly focused and did not adequately consider the extent of condition.

Since these earlier events occurred, changes have been made to the corrective action program that improve the quality and depth of causal evaluations. Improvements include lowering the threshold for significant event determination; improvements to the apparent cause and root cause processes such as fault tree analysis and error precursor analysis; and a complete rewrite of the apparent cause and root cause investigation guidance manuals.

2. In some cases, for previous events, Engineering did not evaluate potential operability concerns during the design change process. Had these events been recognized as potential operability concerns, further investigation would likely have resulted in identifying the incorrect setting on CRE HVAC makeup Fan 11B. As detailed below, case study training will be presented and the design change procedure will be revised to address this issue.

3. An adverse trend in equipment reliability was not identified. It is an expectation that system engineers trend similar failures for common cause determination. The current System Engineering administrative guidelines are clear on this expectation. At the time of these past events, no programmatic tool existed to assist the system engineer in identification of repetitive issues. Now, all Condition Reports are reviewed to determine if a repeat failure exists and that information is provided to the system engineer.

IV. CORRECTIVE ACTIONS

1. The locked rotor current setting was adjusted for the 11B CRE Makeup Fan breaker, and the 11B Fan was declared operable on 09/12/2006 at 0240 hours0.00278 days <br />0.0667 hours <br />3.968254e-4 weeks <br />9.132e-5 months <br />.

2. Walkdowns were performed on all Unit 1 and Unit 2 safety related 480 Volt motors affected by the MCC Upgrade Modification to verify the correct magnetic trip settings for the breakers. No other motors were found with incorrect magnetic breaker settings.

3. The Electrical Setpoint Index guidance will be revised to apply the adjusted magnetic breaker setting criteria.

4. The Electrical Setpoint Index (Notes) will be revised to specify a single magnetic breaker setting for safety related SSCs.

5. Although no additional motors were found with incorrect magnetic breaker settings, optimization of Unit 1 and 2 motor breaker settings will be performed based on walkdown information.

6. Case study training on this event will be provided to design and maintenance engineers.

7. The design change procedure will be revised to include guidance on addressing potential operability and reportability issues when correcting conditions that support SSC operability (corrective design changes).

V. PREVIOUS SIMILAR EVENTS

No similar reportable events due to incorrect magnetic circuit breaker settings have occurred at STP within the past three years.

VI. ADDITIONAL INFORMATION

None