05000498/LER-2009-001
| South Texas Project | |
| Event date: | 02-24-2009 |
|---|---|
| Report date: | 07-30-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4982009001R00 - NRC Website | |
the Technical Specification 3.7.14 required action time, 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 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 02/24/2009 Unit 1 was in Mode 1. Essential Chiller 12B was secured from operation at 13:51 in preparation for rotating chiller trains. At 22:28, the oil reservoir temperature was found to be 108°F which is below the minimum temperature of 120°F as specified in the system operating procedure. Essential Chiller 12B was declared inoperable at 01:45 on 02/25/2009.
Separately, Essential Chiller 12C was out of service for planned maintenance and was declared inoperable from 02/23/09. Essential Chiller 12C was restored to operable status at 21:26 on 2/24/09.
The Essential Chillers have oil heaters that are used when the chiller is in standby. When the chiller is running, the heaters are off When the chiller is in standby, a thermostat controls the heater to maintain the oil temperature in the oil reservoir hot enough to minimize the refrigerant from absorbing into the oil which can cause oil foaming. Excessive oil foaming can prevent the chiller from starting on demand.
The cause of the low oil temperature condition on Essential Chiller 12B was determined to be inadequate calibration of the oil heater thermostat completed on 02/17/09. The calibration procedure applied was not specific to the Essential Chillers. Several factors not addressed by the procedure can affect the calibration through temperature and pressure changes. These factors include other concurrent work, ambient changes, and other system interactions that can influence the calibration results if not properly controlled. Consequently, the lack of a calibration procedure specific to the essential chillers adversely affected the results of the calibration and subsequent adjustments.
Since the inadequate calibration caused the thermostat to not properly maintain temperature, Essential Chiller 12B was inoperable from the time it was secured at 13:51 on 02/24/09.
Essential Chiller 12C was inoperable for planned maintenance from 0400 on 02/23/09 until its restoration at 21:26 on 02/24/09 at which time Technical Specification 3.7.14 action b was no longer applicable.
E. METHOD OF DISCOVERY OF EACH COMPONENT FAILURE, SYSTEM FAILURE, OR
PROCEDURAL ERROR
The low oil temperature condition on Essential Chiller 12B was discovered during plant operator rounds.
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
Essential Chiller 12B was secured at 13:51 on 02/24/09. Chiller 12C was declared inoperable for planned maintenance from 02/23/09 at 04:00 to 02/24/09 at 21:26. Since Chiller 12B is considered inoperable starting from 13:51 on 02/24/09, both chillers were considered inoperable concurrently for 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> 35 minutes.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
There are three essential chilled water trains. The Essential Chilled Water System is designed to provide chilled water to certain supply air handling units (AHUs) under any normal or emergency conditions.
Essential Chiller 12B was declared inoperable but is considered to have remained functional.
The Essential Chiller oil heaters function to minimize refrigerant absorption in the oil. The amount of time the Essential Chiller 12B low oil temperature condition existed was insufficient to have prevented it from starting. Additionally, because the essential chiller is a required support system for ECCS operability, two trains of ECCS were concurrently inoperable but functional for a time in excess of Technical Specification 3.5.2.
There was no impact on incremental core damage frequency. This event did not have an adverse effect on the health and safety of the public. This event resulted in no personnel injuries, no offsite radiological releases, and no damage to other safety-related equipment.
The root cause of the low oil temperature condition on Essential Chiller 12B was the lack of an oil heater thermostat calibration procedure that was specific to the Essential Chillers.
IV. CORRECTIVE ACTIONS
- Create procedural calibration instructions within OPMP05-CH-0003 that specifically address factors that can affect calibration of the Essential Chiller oil heater thermostat.
- Change the associated PM planning model to reflect no concurrent chiller maintenance activities during calibration activities.
- Add scheduling comments to PMs for oil heater thermostat calibration that the PM is to be done with no other scheduled activities to avoid potential influences of other work.
V. PREVIOUS SIMILAR EVENTS
Review of condition reports did not identify any previous similar events at STP. However, operating history was reviewed for both the oil heater thermostat and the whole heater assembly which includes the oil heater thermostat. The review indicated there have been two replacements due to drift, and one replacement due to erratic behavior following preventive maintenance.
There is no history of thermostat problems until after the oil heater thermostats were changed out in 2006 as a chiller reliability improvement measure.
Because of a recent significant change to the vendor supplied oil heater thermostat, past material history will not be germane to the new part.
VI. ADDITIONAL INFORMATION
None.