05000321/LER-2010-002
| Edwin I. Hatch Nuclear Plant Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3212010002R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor Energy Industry Identification System codes appear in the text as (EIIS Code XX).
DESCRIPTION OF EVENT
On April 13, 2010 Unit I was at 2800 CMWTh, 99.9 percent power and Unit 2 was in Mode 5, Refuel mode. On this date it was determined that the plant had previously operated in condition prohibited by the Plant Technical Specification (TS). The plant service water (PSW, EIIS Code BI) piping which provides cooling for the Main Control Room Air Conditioning (MCR AC, EIIS Code VI) was degraded due to internal fouling of the cooling water lines. This fouling developed over a period of time resulting in less than adequate cooling flow to the Main Control Room Air Conditioners. The river temperature reached a point at which the Main Control Room Air Conditioning operability would be adversely impacted during a postulated design basis accident. In this case the DBA is a loss of coolant accident (LOCA) concurrent with a river temperature of 95 degrees Fahrenheit and a river level of 60.7 feet mean sea level.
CAUSE OF EVENT
This event was caused by fouling of the cooling water pipe over a period of time.
REPORTABILITY ANALYSIS AND SAFETY ASSESSMENT
This event is reportable under 10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by the planes Technical Specifications. The cooling water flow for the Main Control Room Air Conditioning was degraded due to internal fouling. Due to the degraded cooling flow, the operability of the Main Control Room Air Conditioning would be adversely impacted during a postulated design basis accident.
The Control Room AC portion of the Main Control Room Environmental Control System (hereafter referred to as the Main Control Room AC System) provides temperature control for the control room.
The Main Control Room AC System consists of three 50 percent capacity subsystems that provide cooling and heating of control room supply air. Each subsystem consists of an air handling unit (AHU) (i.e., refrigerant, cooling coils and fan), water cooled condensing units, refrigerant compressors, ductwork, dampers, and instrumentation and controls to provide for control room temperature control.
The condensing units receive cooling water from the Plant Service Water System. Each PSW system, Unit 1 or Unit 2, is designed to provide adequate flow through one of the following paths: one condensing unit receives cooling from Division I PSW, one unit is capable of receiving cooling from both Divisions I and II PSW, the other condensing unit receives cooling from Division II PSW. The Control Room AC System is designed to provide a controlled environment under both normal and accident conditions. Two subsystems provide the required temperature control to maintain a suitable control room environment for a sustained occupancy of 14 persons. The design conditions for the control room environment are 72-79 degrees Fahrenheit and less than 75 percent relative humidity.
A Main Control Room Air Conditioning PSW Flow Evaluation was performed to evaluate the "as found" PSW flow issues associated with the 'loss of ability' to isolate the Shift Manager's office cooling unit due to the failure of isolation valves 1P41-F123 A/B. The "as found" piping condition for the 'B' and 'C' coolers with the 'A' cooler turned off allowed flows of 63.9 gpm and 78.9 gpm for 'B' and 'C' coolers, respectively, with a simulated break in the piping to the shift manager's cooling unit.
The cooling capacity of the main control room air conditioning unit is 40 tons or 480,000 BTUH. The calculated heat load of the main control room is 328,125 BTUH. Design of the main control room air conditioning unit established the minimum flow for the rated capacity at 95 degrees Fahrenheit inlet water condition as 100 gpm. The design water supply flow, from plant service water was specified to be 120 gpm to maintain a flow margin. On reducing the cooler capacity to the calculated heat load value, the minimum PSW flow to the main control room air conditioning unit was established as 81 gpm at 95 degrees Fahrenheit. Through a more detailed evaluation, given the degraded PSW flow of 63.9 gpm to the main control room air conditioning unit, a maximum river temperature of 91.8 degrees Fahrenheit was established in order for the cooler to satisfy the calculated control room heat load, with 10 percent heat load added to the calculated heat load for margin.
A qualitative assessment was performed by Engineering to determine if the Main Control Room condensing units, and thus the Main Control Room AC System would operate to provide Main Control Room cooling with the degraded flow of 63.9 gpm if the PSW water temperature increased from 91.8 degrees Fahrenheit to 95 degrees Fahrenheit. It was determined that the condensing units would likely continue to operate. Increasing the PSW water temperature would cause the refrigerant temperature and pressure to increase in the condensing unit. The assessment was performed by reducing the MCR heat load to the calculated value of 328,157 BTUH (removing the margin from the operability assessment), and assuming the maximum temperature and pressure that would not trip the condensing unit. Then, the heat transfer capability was checked and confirmed to be within design values for the heat load, thus confirming that the maximum assumed temperature and pressure would not be exceeded.
Based on this analysis, it is concluded that this event had no adverse impact on nuclear safety because even with the as-found Service Water Flow, the Main Control Room Air Conditioning Units would perform their intended function during a DBA.
CORRECTIVE ACTIONS
Cleaning of associated piping. Some portions have already been cleaned and additional piping sections are planned to be cleaned. This will be tracked in the corrective action program.
Radiography points will be evaluated based on flow modeling to provide early detection of internal pipe fouling. This will be tracked in the corrective action program.
Sampling of the PSW supply to Main Control Room AC System chillers through Unit 2 to confirm adequate chlorine is present will be performed. This will be tracked in the corrective action program.
ADDITIONAL INFORMATION
Other Systems Affected: None Failed Components Information:
None Commitment Information: This report does not create any new permanent licensing commitments.
Previous Similar Events:
identified a condition where during a seismic event the Main Control Room AC System was not single failure proof. While the mode of failure is different, this is an instance where the Main Control Room AC System could be rendered inoperable. At the time of this event the low flow condition had not been previously identified therefore the corrective actions for this event did not prevent the condition being reported under the current LER.