05000499/LER-2009-001
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. N/A | |
| Event date: | 07-09-2009 |
|---|---|
| Report date: | 09-14-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4992009001R00 - NRC Website | |
I. DESCRIPTION OF EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B). South Texas Project (STP) Technical Specification 3.7.14 allows one train of Essential Chilled Water to be inoperable in Modes 1 through 4 for seven days before taking action to begin shutdown without extending the allowed outage time using the Configuration Risk Management Program. However, STP Unit 2 Essential Chilled Water Train A was determined to have been inoperable longer than the allowed outage time. Consequently, STP Unit 2 was in a condition prohibited by Technical Specifications.
B. PLANT OPERATING CONDITIONS PRIOR TO EVENT
STP Unit 2 was in Mode 1 at 100% power.
C. STATUS OF STRUCTURES, SYSTEMS, AND COMPONENTS THAT WERE INOPERABLE
AT 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 July 9, 2009, Unit 2 was in Mode 1. At 11:21, Essential Chiller 22A tripped due to low oil pressure approximately 105 seconds into its startup sequence. Prior to the event, Essential Chiller 22A had been idle for over 14 days. Station personnel were present during the start attempt. Chiller parameters were observed and a stopwatch was used to correlate chiller parameters such as lube oil pressure with known startup sequence actions such as the opening of the equalizing valve.
During the prelube portion of the startup cycle, the oil pressure was noted to be approximately 36 psig. When the compressor started approximately 30 seconds later, the oil supply pressure dropped to 16 psig and then started to recover to about 24 psig. At the point where the lube oil equalizing solenoid valve is timed to open (i.e., at approximately 105 seconds after the Control Room started the chiller), the oil supply pressure was observed to rapidly drop past 8 psig and the chiller tripped. The trip setpoint for low oil pressure occurs when the pressure differential between the lube oil supply and the compressor suction vacuum is less than 15 psid so the trip occurred approximately when expected.
Following the Essential Chiller 22A trip, a second start was attempted at 11:53. The observed oil pressures remained higher this time and the chiller started successfully. Despite the successful start on the second attempt, Chiller 22A was declared inoperable pending resolution of any material conditions associated with the chiller.
Subsequent investigation determined that there were no material conditions or degraded components associated with this event. However, it was determined that the Essential Chillers are sensitive to long idle conditions, especially during warmer weather conditions and a maximum idle time limit of 6 days was imposed on all Essential Chillers. Because Essential Chiller 22A had been idle for greater than 14 days prior to the event, and Engineering concluded that low pressure trips of the type experienced could occur after 6 days, the 22A Essential Chiller was considered to be inoperable for longer than the action times permitted in 4 Technical Specifications.
E. METHOD OF DISCOVERY OF EACH COMPONENT FAILURE, SYSTEM FAILURE, OR
PROCEDURAL ERROR
The trip of Essential Chiller 22A occurred during its planned start as part of routine equipment rotation of the Essential Chilled Water trains.
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
Prior to the trip of Essential Chiller 22A, the chiller had been idle for over 14 days (secured on June 24, 2009 at 20:41). Assuming that the Chiller became inoperable after 6 days of idle time, and the Chiller was declared operable on July 11, 2009 at 16:48, this would result in a maximum of approximately 11 days of inoperability time.
The B and C trains of Essential Chilled Water remained operable during this event.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
System Information:
There are three Essential Chilled Water trains for each Unit. The Essential Chilled Water system is designed to provide chilled water to certain supply air handling units during normal and emergency conditions.
Technical Specification 3.7.14 requires at least three independent Essential Chilled Water trains to be operable in Modes 1, 2, 3, and 4. With only two Essential Chilled Water loops operable, the inoperable loop is to be restored to operable status within seven days or the Configuration Risk Management Program applied to justify an extension. Otherwise, the unit is to be in at least hot standby within the next six hours and in cold shutdown within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.
Because Unit 2 Essential Chilled Water Train A was inoperable longer than allowed under the Technical Specifications without entering the appropriate action statements, this event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B).
Risk Assessment:
This event resulted in no personnel injuries, no offsite radiological releases, and no damage to other safety-related equipment. The other two trains of Essential Chilled Water remained operable for the duration of the event. The non-functional time of Essential Chiller 22A did not result in an increase in incremental core damage probability greater than 1 E-6.
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III. CAUSE OF THE EVENT
The root cause of the Essential Chiller 22A trip on low oil pressure was refrigerant saturation in the lube oil due to a long idle period prior to the attempted start.
IV. CORRECTIVE ACTIONS
A 6-day idle time maximum has been implemented for all Essential Chillers.
Purge and equalizing solenoid valve timing changes have been implemented on all Essential Chillers, which are expected to mitigate the effects of refrigerant saturation. Startup lube oil pressure profile data will be collected and evaluated, and if supported by the data, the idle time restrictions will be relaxed or removed. If not supported by the data, appropriate idle time restrictions will be implemented. (Expected completion 11/16/2009)
V. PREVIOUS SIMILAR EVENTS
Essential Chiller 12A (Unit 1) tripped on low oil pressure twice in 2007 and twice in 2008. Prior to this event, Chiller 22A had not experienced a trip due to low oil pressure, and other than Chiller 12A, none of the other essential chillers had tripped on low oil pressure since 2003.
VI. ADDITIONAL INFORMATION
In addition to the Essential Chiller 22A trip on low oil pressure, there have been several other issues with Essential Chillers in 2009 that challenged chiller reliability. An Essential Chiller Task Force has been established to evaluate current station practices and industry experience to make recommendations to improve the reliability of the Essential Chillers.