05000366/LER-2007-007
| Edwin I. Hatch Nuclear Plant - Unit 2 | |
| Event date: | 05-18-2007 |
|---|---|
| Report date: | 07-13-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3662007007R00 - 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 May 18, 2007 at 17:45 EDT, Unit 2 was in Mode 1 at 2804 CMWT, 100 percent power. While investigating the source of oil on top of the Unit 2 High Pressure Coolant Injection (HPCI) Turbine Skid, Maintenance personnel found water in the HPCI lube oil system. Consequently, Operations personnel entered the applicable Technical Specifications Limiting Condition of Operation (LCO) for an inoperable HPCI system. The water intrusion was attributed to a clogged bracket drain valve which caused water to accumulate in the mechanical seal cavity. The water came from a small leak in the mechanical seal, and apparently entered the oil system by flowing past the thrust bearing oil deflector. The deflector forms one of the seal cavity walls and is not designed to keep standing water out of the bearing housing. The HPCI oil system was drained and flushed; the duplex filter and the Electronic Governor (EGR) were replaced. In addition, the drain valve was removed from the system to prevent this water accumulation in the future. Testing of the HPCI system was performed on May 19, 2007, and the system was declared operable at 19:49 EDT.
CAUSE OF EVENT
This event was caused when water backed up in the mechanical seal cavity due to a clogged drain valve, and then migrated into the oil system. Following are the root causes of the water intrusion: 1) the component functional design is not optimal in that, according to the vendor, the drain valves are prone to clogging and are not needed; 2) although a modification proposal was approved to remove the valves, they had not been removed as of the time of this event; 3) corrective action to keep the drain line cleared was unsuccessful. The HPCI system inspection and lubrication procedure had been revised to include a section to clear the drain lines and that activity has been performed every 24 months since its inception in 2000. However, the method was inadequate for cleaning the particular type of globe valve in this drain line.
REPORTABILITY ANALYSIS AND SAFETY ASSESSMENT
This event is reportable, per 10 CFR 50.73 (a)(2)(v)(D), because an event occurred in which the HPCI system, a single train safety system, was rendered inoperable.
The HPCI system consists of a steam turbine-driven pump and the necessary piping and valves to transfer water from the suppression pool or the condensate storage tank (EIIS Code KA) to the reactor vessel. The system is designed to inject water to the reactor vessel over a range of reactor pressures from approximately 160 psig through full-rated pressure. The HPCI system starts and injects, automatically, whenever low reactor water level or high drywell pressure indicates the possibility of an abnormal loss of coolant inventory. The HPCI system is designed to replace lost reactor coolant inventory in cases where a small line break occurs which does not result in full depressurization of the reactor vessel.
The backup for the HPCI system is the Automatic Depressurization System (ADS) together with two low pressure injection systems: The Low Pressure Coolant Injection (LPCI, EIIS Code BO) system and the Core Spray (CS, EIIS Code BM) system. The CS system is composed of two independent, redundant, 100 percent capacity subsystems. Each subsystem consists of a motor-driven pump, its own dedicated spray sparger located above the core, and piping and valves to transfer water from the suppression pool to the sparger. Upon receipt of an initiation signal, the CS pumps in both subsystems start. Once ADS has reduced reactor pressure sufficiently, CS system flow begins.
LPCI is an operating mode of the Residual Heat Removal (EIIS Code BO) system. There are two independent, redundant, 100 percent capacity LPCI subsystems, each consisting of two motor driven pumps and piping and valves to transfer water from the suppression pool to the reactor vessel. Upon receipt of an initiation signal, all four LPCI pumps automatically start. Once ADS has reduced reactor pressure sufficiently, the LPCI flow to the reactor vessel begins.
ADS consists of 7 of the 11 Safety Relief Valves (SRV). It is designed to provide depressurization of the Reactor Coolant System during a small break Loss of Coolant Accident (LOCA), if HPCI fails or is unable to maintain required water level in the Reactor Pressure Vessel (RPV). ADS operation reduces the RPV pressure to within the operating pressure range of the low pressure Emergency Core Cooling System (ECCS) subsystems (CS and LPCI), so that these subsystems can provide coolant inventory makeup.
In this event, the HPCI oil system was found to have water in the oil. As a result, the system was conservatively declared inoperable. During the time the HPCI system was inoperable, the Reactor Core Isolation Cooling (RCIC, EIIS Code BN) system was available to inject high pressure water into the reactor vessel. Although not an ECCS, the RCIC system is designed, maintained, and tested to the same standards and requirements as the HPCI system and, therefore, should reliably inject water into the reactor vessel, when required. If a break exceeded the capacity of the RCIC system (400 gallons per minute), the ADS would be available to depressurize the reactor vessel to the point that either the CS or LPCI systems could have been used. The capacity of one loop of the CS system is equal to that of the HPCI system (4250 gpm each); the capacity of one loop of the LPCI system is approximately three times that of the HPCI system. Therefore, either of the two loops of the LPCI system would have provided sufficient injection capacity for a small break LOCA.
Based on this analysis, it is concluded that this event had no adverse impact on nuclear safety. This analysis is applicable to all power levels and operating modes in which a LOCA is postulated to occur.
CORRECTIVE ACTIONS
Maintenance replaced the oil in the HPCI Turbine and reservoir.
The Duplex Filters and the EGR were replaced.
The drain valves were removed from the system, pursuant to an approved modification request. The design of the plant's Unit 1 HPCI system was verified to not have valves on the pump drain lines.
The approved modification proposal for removing the drain valves was closed prior to being implemented. An Engineering Fix-it-Now (EFIN) work request was supposed to have been initiated to remove the drain valves. However, no such work request was generated. A review of closed modification proposals is being performed in accordance with the plant's Corrective Action Program. This review is to ensure that there are not any other cases where modification proposals were closed, cancelled, deferred, etc. without expected alternate resolutions being performed.
ADDITIONAL INFORMATION
Other Systems Affected: None Failed Components Information: None Commitment Information: This report does not create any permanent licensing commitments.
Previous Similar Events:
The root cause of this event centered on a tag out that did not adequately address the system or functional impact associated with the components that were tagged or removed from service. This resulted in a flow path for water to enter the HPCI oil system. The corrective actions for this earlier event would not have prevented the event reported in the current LER.
to a clogged drain line. The corrective action from this LER of periodically cleaning the drain line was unsuccessful in that the line became clogged resulting in the current event.