05000321/LER-2007-002
| A. Docket Number 3. Page1. | |
| Event date: | 03-07-2007 |
|---|---|
| Report date: | 2-0-5000 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3212007002R00 - 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 March 7, 2007 at 8:50 pm EST Hatch Unit 1 was in the run mode at approximately 100 percent of rated thermal power. At that time during the performance of the Analog Transmitter Trip System (ATTS) panel Functional Test and Calibration procedure the High Pressure Coolant Injection (HPCI) inboard isolation valve 1E41-F002 closed. The HPCI system was declared inoperable and the appropriate Technical Specifications Required Action Statement was entered. Troubleshooting of the HPCI isolation logic circuit was performed. The most probable cause of the valve closure was determined to be a faulty ATTS card, 1E41-N670A. The failed ATTS card and an associated ATTS card were replaced. In addition, related ATTS Agastat relays were replaced to avoid age-related problems or failures.
The circuitry was functionally tested and HPCI was declared operable on March 10, 2007, at 4:25 p.m. EST.
CAUSE OF EVENT
The most probable cause of the HPCI isolation was an erroneous trip signal being sent by ATTS card 1E41- N670A. 1E41-N670A serves as the ATTS master trip unit (MTU) for the 'A' channel of the HPCI area high temperature isolation, which causes closure of the inboard isolation valve, 1E41-F002.
As a part of the investigation, all inputs feeding into the HPCI 1E41-F002 closure logic were investigated.
There are four different inputs that energize the applicable relays to initiate the HPCI isolation valve closure signal. Ten percent of the Unit 1 ATTS cards and ten percent of the Unit 2 ATTS cards have been inspected, with no other card damage noted. The inspections were initiated in 2006 to look for "tin whiskers". These inspections would identify the type of damage seen on the two E41 ATTS cards described in this LER. In addition the recent failure rate of ATTS cards is approximately 0.3 percent per year. It is concluded that the ATTS cards, on a whole, are highly reliable.
Visual inspections of ATTS cards 1E41-N670A and penetration room high temperature MTU 1E41-N671A revealed damage to components on the cards. For example, components found on card 1E41-N670A had fractured solder connections, and chipped insulation. Furthermore, relays 1E21-K361C and 1E21-K362C, which are Agastat relays located in the logic circuitry downstream of 1E41-N670A and 1E41-N671A, respectively, were bench-tested and noise was seen across the voltage signal of the relays. This disturbance is an early indication of aging.
U.S. NUCLEAR REGULATORY COMMISSIONNRC FORM 364A (1
- 2001) �
REPORTAIMITY ANALYSIS AND SAFETY ASSESSMENT
This event is reportable per 10 CFR 50.73 (a)(2)(v) 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 160 psig through full rated pressure. The HPCI system starts and injects automatically whenever low reactor water level or high dry well pressure indicates the possibility of an abnormal loss of coolant inventory. The HPCI system, in particular, 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, MIS Code BO) system and the Core Spray (CS, EDS 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 subsystems (CS and LPCI), so that these subsystems can proyide coolant inventory makeup.
In this event, the HPCI system was declared inoperable when the 1E41-F002 inboard isolation valve closed.
During the time the HPCI system was inoperable, the Reactor Core Isolation Cooling (RCIC, MIS Code BN) system was available to inject high pressure water into the reactor vessel. Although not an emergency core cooling system, 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 had exceeded the capacity of the RCIC system (400 gpm), the ADS was available to depressurize the reactor vessel to the point that either the Core Spray or LPCI systems could have been used to provide water to the reactor core. The capacity of one loop of the Core Spray 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, any one of the four loops of the low pressure injection systems would have provided sufficient injection capacity for replacement of the HPCI flow.
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
ATTS cards 1E41-N671A and 1E41-N670A were replaced.
ATTS Agastat relays 1E21A-K361C and 1E21AK362C were replaced to avoid age-related problems or failures.
Inspection of all HPCI system ATTS cards will be performed during the next HPCI system outage. This will be tracked under the plant's Corrective Action Program.
Replacement of all ATTS Agastat relays is in progress and will be tracked under the plant's Corrective Action Program.
All ATTS cards are now inspected prior to installation into the plant.
ADDITIONAL INFORMATION
Other Systems Affected: None Failed Components Information:
Master Parts List Number: 1E41-N670A EIIS System Code: BJ Manufacturer: General Electric Reportable to EPIX: Yes Model Number: 184C5988G101 Root Cause Code: X Type: Board, Printed Circuit EIS Component Code: PB Manufacturer Code: G082 Commitment Information:
This report does not create any permanent licensing commitments.
Previous Similar Events:
failure of an ATTS card.