05000458/LER-2010-001
| River Bend Station -- Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4582010001R00 - NRC Website | |
On April 22, 2010, an engineering evaluation determined that safety-related chillers (**CHU**) serving the main control room envelope had been operated in a configuration outside design parameters, such that the chillers could not be assumed to be operable under all conditions. Specifically, that evaluation and review of operating logs concluded that there was a period beginning in November 2009 when one of the operating chillers was actually inoperable, and was in that condition for a time greater than the allowable outage time cited in the applicable Technical Specifications. Also during this time, a second chiller was inoperable for approximately ten days. This condition is being reported in accordance with 10CFR50.73(a)(2)(1)(B) as operations prohibited by Technical Specification 3.7.3 Conditions A and B.
BACKGROUND
The control building chilled water system (VI) is designed to remove heat to maintain the required environmental conditions. The control building chilled water system is a closed loop cooling water system and provides continuous flow of chilled water to the building air conditioning units. The chilled water is supplied by two independent trains. Each chilled water train consists of two 100-percent capacity electric driven centrifugal liquid chillers (HVK-CHL1A through D), two 100-percent capacity chilled water pumps, two 100-percent capacity service water pumps, one chilled water compression tank, associated piping, valves, instrumentation, and controls. The service water pumps provide cooling water to the chiller condensers and the safety-related standby service water system acts as the ultimate heat sink.
Each HVK chiller is served by a service water recirculation pressure control valve (PCV) that is automatically modulated by chiller condenser pressure, which maintains condenser pressure in the acceptable range. With the PCV is gagged open (allowing full service water flow), and the service water temperature is lower than design values, the overall pressure differential from saturated evaporator to saturated condenser decreases. Each of the two chiller internal refrigerant flow control valves will experience a lower differential pressure and will not be able to accommodate refrigerant full flow at extremely low differential pressures. Subsequently, the chiller is susceptible to an automatic trip due to low evaporator pressure.
CAUSAL ANALYSIS
In 1988, an equipment modification (M87-0719) uprated the HVK chillers from 50% capacity to 100%.
Prior to this modification, the minimum required condensing water temperature supplied by,normal service water (SWP) and the safety-related standby service water (SSW) to the HVK chillers was 55 degrees F. M87-0719 changed the following:
- Service water flow rate from 530 gpm to 630 gpm
- , Leaving chilled water temperature from 45 deg. F to 52.5 deg. F
- Entering chilled water temperature from 58.9 deg. F deg. F to 67.5 deg. F
- Chiller evaporator temperature from 41.3 deg. F to 48.6 deg. F
- Return chilled water temperature switch set-point to operate the hot gas bypass valve from 46 deg. F to 53.5 deg. F.
M87-0719 did not address the required minimum condenser cooling water temperature. No request was made to the chiller's manufacturer to determine whether the minimum required condensing water temperature of 55 deg. F was affected by the modification. Hence, the minimum condensing water temperature listed in the vendor manual was not changed by M87-0719.
In 1993, an engineering evaluation was performed to determine the effects of operating the HVK chillers with low service water temperature with the PCV gagged open. Based on input from the vendor, this evaluation erroneously determined that full service water flow through the chiller was acceptable with standby service water temperature above 55 deg. F.
In 1995, another evaluation was performed concerning operation of the HVK chillers with the PCV gagged open. As a result of not using the updated information developed in M87-0719, it was again concluded that operation of the chillers in this configuration was acceptable provided that standby service water temperature was above 55 deg. F.
On February 10, 2010, the PCV on HVK chiller "C" failed in the full open position. On March 2, 2010, a Condition Report was initiated to provide a documented evaluation of chiller availability with a standby service water temperature of 53.5 deg. F. The chiller vendor was contacted for assistance.
The vendor subsequently provided an analysis which concludes that the revised chiller design parameters implemented by M87-0719 limited the control building chiller's ability to operate at lower entering condensing water temperatures. The analysis determined that the minimum required condensing water temperature for a chiller operating at full load and the PCV gagged in the full open position was 70 deg. F.
A review of operating logs was performed to determine whether there were periods in the past during which a chiller was operated with its PCV gagged open while it was being considered operable. Also, daily recorded values for standby cooling tower basin temperature were retrieved and compared to the chiller data. This review determined that for a period beginning on November 17, 2009, chiller "D" was operated with its PCV gagged open while standby service water temperature was less than 70 deg. F. There were other periods in 2009 during which the "D" chiller was operated in this condition, but none of those other periods exceeded the allowable outage time.
The investigation of this event determined that it was caused by the failure of the equipment modification performed in 1985 to identify the need for increased service water temperature requirements resulting from the chiller design change. The subsequent evaluations performed in 1993 and 1995 represent missed opportunities to correct this error.
IMMEDIATE CORRECTIVE ACTIONS
Operations procedures have been revised to require that chillers be declared inoperable and unavailable when their PCVs are gagged open.
A review of reportable events at River Bend Station since 2005 found no previous occurrences of a similar nature.
SAFETY SIGNIFICANCE
During the times when the PCV for chiller "D" was gagged, the chiller continued to operate and provide cooling to the main control room envelope using the normal service water system. While the allowable outage time for chiller "D" was exceeded, the redundant "B" chiller in that division was available to be returned to service to provide the safety function should that have been necessary.
The availability of the redundant "B" chiller ensured that there was no loss of safety function. This event was of minimal significance with regard to the health and safety of the public.
(NOTE: Energy Industry Component Identification codes are annotated as ("XX"))