ML15224A340
| ML15224A340 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 11/03/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML15224A339 | List: |
| References | |
| REF-GTECI-A-47, REF-GTECI-SY, TASK-A-47, TASK-OR GL-89-19, NUDOCS 9311100305 | |
| Download: ML15224A340 (4) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION REPORT OF NUCLEAR REACTOR REGULATION STEAM GENERATOR OVERFILL PROTECTION RESPONSE TO GENERIC LETTER 89-19 OCONEE NUCLEAR STATION, UNITS 1. 2. AND 3 DOCKET NOS. 50-269, 50-270, and 50-287
1.0 INTRODUCTION
By Generic Letter (GL) 89-19, the NRC recommended that a system be provided to mitigate main feedwater (MFW) overfill events for all boiling and pressurized water reactors (BWRs and PWRs) that currently do not have such protection.
This action was part of the technical resolution of Unresolved Safety Issue (USI) A-47, "Safety Implications of Control Systems in LWR Nuclear Power Plants."
It is recommended that, (1) all Babcock & Wilcox (B&W) plant designs have automatic steam generator overfill protection to mitigate MFW overfeed events; (2) plant procedures and Technical Specifications (TS) for all B&W plants include provisions to periodically verify the operability of overfill protection, and ensure that automatic mainfeedwater overfill protection is operable during reactor power operation; and (3) plant designs with no automatic protection to prevent steam generator dryout upgrade their design and the appropriate TS, and provide an automatic protection system to prevent steam generator dryout on loss of power to the control system. Automatic initiation of auxiliary feedwater on steam generator low-water level is considered an acceptable design.
By letters dated March 19, 1990, March 19, 1991 and July 28, 1993, the licensee provided a discussion of the automatic steam generator overfill protection system at Oconee, Units 1, 2, and 3, and a description of how the system meets the recommendations of GL 89-19.
2.0 DISCUSSION AND EVALUATION 2.1 Steam Generator Overfill Protection System The overfill protection system is presently provided through the Integrated Control System (ICS) to terminate feedwater when the high water level setpoint is reached. The design for the steam generator overfill protection (SGOP) is a two-out-of-two logic on either steam generator. Four transmitters monitor Once Through Steam Generator (OTSG) water level and trip the main turbine and the main feedwater pumps when high level is sensed. The ICS functions are designed to limit the potential for overfill in two ways. The ICS controls main feedwater control valve position. The ICS has been configured such that upon loss of Hand Power and/or Auto Power the OTSG high level monitoring circuits will de-energize and trip the main turbine and the main feedwater pumps.
9311100305 931103 PDR ADOCK 05000269 P
-2 The design of Oconee calls for a non-safety-related main feedwater pump trip utilizing a two-out-of-two steam generator high water-level trip system from each steam generator. The design is subject to a number of single failures, each of which prevents a feedwater trip on high water-level. The system is designed in an "energized to trip" configuration in such a way that a loss of control power (125 Vdc) to the trip relay would not trip the feedwater pumps.
If 125 Vdc control power to the trip relay (FPTX) is available, a loss of power to the level sensors would cause the main feedwater pumps to trip.
The system modification is proposed to provide an additional trip relay in parallel with the existing trip relay (FPTX) to prevent single failure (loss of control power) from initiating a trip. The main feedwater pump turbine trip circuits will utilize an additional trip device to assure the turbine trips as required upon a loss of the control power to the trip relay. A solenoid valve (SV6) will be connected to the existing steam generator level monitors on separate contacts through an additional auxiliary relay (FPTX1).
The power sources for the existing feedwater pump turbine trip device (SV12) is from the non-safety grade 125 Vdc station power batteries. The additi9nal trip device (SV6) will also be powered from the same source. The power source for the auxiliary relay (FPTX1) will be from the 125 Vdc station instrumentation and control batteries. The proposed modification provides an additional independent MFW trip system actuated from a separate steam generator high water-level channel to isolate the feedwater flow via a trip of the MFW block valves. The modified overfill protection system satisfies the single failure criterion.
The overfill protection circuits and the feedwater control system are located in different ICS Cabinets. The cable routes for the overfill protection and feedwater control systems are routed in the normal plant cable tray system which is designed to minimize common fire related affects.
Fire detection and protection measures are provided in areas where these cables are routed. The signal modules de-energize to actuate the trip circuits of the equipment as the design requires. In addition to the separation provided by the location of the control and monitoring functions in different cabinets, the Smart Automatic Signal Selector (SASS) and the plant computer outputs are connected to the level transmitter signals in a separate cabinet. The SASS will automatically transfer the feedwater control system to a valid transmitter while the computer will alert the operator to the failed signal such that appropriate action may be taken.
2.2 Steam'Generator Dryout Protection Generic Letter 89-19 recommended that a plant design with no automatic protection to prevent steam generator dryout (SGDO) upgrade their design and the appropriate TS and provide an automatic protection system to prevent steam generator dryout on loss of power to the control system.
-3 The steamngenerator level control system (SGLCS) is a two-train safety-related system installed to meet the guidance of NUREG-0737. The power source for each train of the SGLCS is from the safety-related vital instrumentation and control power panelboards. The SGLCS receives four OTSG level signals. Each train receives one signal from each of the two steam generators. Low level setpoints to the OTSG level transmitter instrument loops will provide a diverse means of actuating emergency feedwater (EFW) as the steam generator approaches dryout conditions. These level signals control OTSG level by modulating EFW valves which feed OTSGs. These level signals will be used to start the motor driven emergency feedwater pumps (MDEFWPs) upon two-out-of-two low OTSG level. A time delay at the low level setpoint will delay starting the MDEFWPs for approximately 30 seconds in order to allow the water level in the OTSGs to stabilize after a plant trip to prevent inadvertent starts of EFW.
Isolated outputs from the SGLCS will be arranged such that if low level is sensed in either OTSG both MDEFWPs will start and provide water to both steam generators. At the same time as the pumps are commanded to start, the level control system will be directed to control level to the appropriate setpoint depending upon whether any Reactor Coolant Pumps are running.
Computer alarms will be provided to alert the operator to the initiation of EFW due to low OTSG levels.
2.3 Technical Specifications By letter dated March 19, 1991, the response regarding steam generator overfill protection and dryout protection TS was provided, including the licensee's commitment to provide proposed changes to the TS for overfill protection. In addition, Selected Licensee Commitments (SLC) will be included in FSAR Chapter 16 which describe limiting conditions for operation, actions, and surveillance for EFW actuation on low steam generator level.
By letter dated September 24, 1991, TS issues associated with steam generator overfill protection were deferred pending generic resolution of the issue for B&W plants. The schedule provided by letter dated March 19, 1990, for implementation of the SLC for EFW actuation on low steam generator level is unchanged. Subsequently, it was determined that GL 89-19 should be handled on a plant-specific basis.
By letter dated July 28, 1993, the licensee provided an evaluation of TS requirements for steam generator overfill protection. The steam generator overfill protection system has been evaluated against the Oconee design basis as established in the FSAR, as well as the Oconee specific risk assessments.
The steam generator overfill protection is not credited in any design basis accident evaluated in the Oconee FSAR. Steam generator overfill protection is credited in certain main feedwater overfill events in the Oconee Pressurized Thermal Shock (PTS) evaluations; however, PTS is not considered to be a design basis accident. Therefore, steam generator overfill protection is not part of the primary success path nor does it function to mitigate a DBA or transient that neither assumes the failure of, or presents a challenge to, the integrity of a fission product barrier.
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3.0 CONCLUSION
The modification for overfill protection system provides an additional independent MFW trip system actuated from separate steam generator high water-level channel to isolate the feedwater flow via a trip of the MFW block valves. The modified overfill protection system has been improved to satisfy the single failure criterion. Based on the above, the staff concludes that the steam generator overfill and dryout protection designs at Oconee, Units 1, 2, and 3 satisfy the objectives of GL 89-19, and are therefore, acceptable.
In addition, the Selected Licensee Commitment satifies the objectives of the GL 89-19 recommendation regarding TS and is, therefore, acceptable.
Principal Contributor: S. Rhow Date:
Novenber 3, 1993