05000498/LER-2002-002
| I | |
| Event date: | 11-14-2002 |
|---|---|
| Report date: | 01-10-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 4982002002R00 - NRC Website | |
DESCRIPTION OF EVENT
On November 8, 2002, Unit 1 was operating at 100% reactor power and Unit 2 was defueled.
Troubleshooting was in progress for a condition where the Unit 2 Electrical Auxiliary Building (EAB) supply fan 21C had previously tripped on overload on more than one occasion within the past few days. Running current data indicated that the fan motor breaker's Amptector long time overload trip setting may have been set too low and could result in tripping of the fan motor during an offsite degraded voltage condition.
Each South Texas Project unit has three trains of EAB HVAC. In order to correct this condition, the Amptector long time overcurrent settings for each fan motor in both units were set to a higher value to protect the motor and allow acceptable performance during the range of design grid voltage conditions. The last fan motor breaker was reset on November 9, 2002.
The EAB Heating, Ventilation and Air Conditioning (HVAC) system's design safety function is to maintain ambient temperature conditions to provide operator comfort and to satisfy environmental requirements of safety equipment under normal operating, transient and postulated accident conditions.
A subsequent investigation that was completed on November 14, 2002 determined that the EAB HVAC supply fans would not have fulfilled their design safety function if the following conditions existed:
- A reduced voltage grid condition existed,
- A Mode 1 (loss of offsite power does not occur) safety injection condition occurred, and
- The EAB HVAC supply fan overcurrent overloads were set sufficiently low so that the fans would trip on overload during the Mode 1 safety injection demand event.
Notification of this event was made to the Nuclear Regulatory Commission on November 14, 2002 at 1624 hours0.0188 days <br />0.451 hours <br />0.00269 weeks <br />6.17932e-4 months <br />.
The EAB HVAC supply fans were upgraded with 190 horsepower motors during construction of the South Texas Project units. The Amptector long time overcurrent setting was adjusted to 100% of the maximum motor amperage rating of 204 amps with a —10% to +0.5% tolerance setting. Vendor documentation stated that the fan motor should not exceed design amperage during operation. Plant design documentation for this application specified that the Amptector long-time overcurrent setting comply with vendor instructions to not run the motors at an amperage above their operational rating.
Setting the Amptector at 100% of the motor full load current for this application was inconsistent with the general instructions in the facility's design calculation document for the overcurrent setting of motor circuit breakers. The general instructions in the design calculation document requires that the Amptector long-time overcurrent setting be set at 115% to 130% of the motor rating to account for Amptector tolerances and potential undervoltage conditions. The original trip setpoints were set over fifteen years ago and little information was available in the documentation. As a result, it could not be determined why the specific design documentation instructions for the EAB HVAC supply fan motors deviated from the general instructions in the facility's design calculation document for setting motor long-time overcurrent settings.
' NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (1-2001) 2002 � 02 � 00
EVENT SIGNIFICANCE
This event resulted in no personnel injuries, radiation exposure, offsite radiological releases or damage to important safety related equipment. The event is reportable pursuant to 10CFR50.73(a)(2)(v)(D) because it resulted in a condition that could have prevented the fulfillment of a safety function.
The mitigative function that was affected by this condition was postulated to be the overheating of equipment in the EAB leading of a failure to the Class 1 E Switchgear and Class 1 E 120 volt AC power by exceeding their equipment qualification temperature limits over a 24-hour period. The Train-A EAB supply fan should not be lost under the conditions for the occurrence of this event because its power is regulated by the unit auxiliary transformer which should compensate for any degraded grid voltage conditions. Operator recovery from this event is very likely because the event is slow moving and plant procedures and operator training were in place to respond to a loss of EAB HVAC. Data from the offsite grid transmission provider conservatively concluded that the degraded grid voltage condition for this event existed no more than 10 days per year. These considerations resulted in a change in core damage frequency of 2.17E-7 for Mode I safety injection demand events.
This change in core damage frequency is less than the 1E-6 threshold established in Regulatory Guide 1.174. Based on this small change, the risk significance of this event is considered low.
CAUSE OF EVENT
The root cause of this event was setting the Amptector long time overcurrent setting at an amperage that did not allow sufficient margin for preventing inadvertent trips under all design voltage conditions.
CORRECTIVE ACTIONS
1. The Amptector long time overcurrent settings for each EAB supply fan motor in both units were set to a higher value (i.e., 115% to 130% of motor full load amperage) that meets design criteria to protect the motor and allow acceptable performance during the range of design grid voltage conditions. The last fan motor breaker was reset on November 9, 2002.
2. Other load center breakers were verified to have sufficient trip setpoint margins to protect their associated loads and allow operation under all design voltage conditions.
ADDITIONAL INFORMATION
Safety and non-safety 480-volt load centers and 4160-volt switchgear were evaluated to ensure that the protection device trip settings provided adequate operating margin and component protection. A review was conducted to ensure that no other motors had been upgraded that could have resulted in a condition similar to that described in this event report.