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UNITED STATES l'
NUCLEAR REGULATORY COMMISSION
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WASHINGTON. D C. 20555 0001 a
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION STEAM GENERATOR OVERFILL PROTECTION RESPONSE TO GENERIC LETTER 89-19 ST. LUCIE UNITS 1 AND 2 DOCKET NOS. 50-335 AND 389
1.0 BACKGROUND
Steam generator overfill events have been identified by the NRC as potentially significant transients that could lead to unacceptable consequences. Review i
of how control systems failures contribute to these events was, therefore, a major part of the Unresolved Safety Issue (USI) A-47 program, " Safety Implications of Control Systems in LWR Nuclear Power Plants." This program evaluated control system failures that could be more severe than those previously analyzed in the FSAR. Studies identified potentially safety-significant failure scenarios for Combustion Engineering plants which lead to
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overfilling the steam generator via the main feedwater system. Generic Letter (GL) 89-19 " Request for Action Related to Resolution of Unresolved Safety Issue A-47 ' Safety Implications of Control Systems in LWR Nuclear Power Plants' Pursuant to 10 CFR 50.54(f)" recommends that Combustion Engineering plants provide automatic steam generator overfill protection to mitigate main 3
feedwater (MFW) overfeed events. Additionally, it was recommended that utilities that have plants designed with high pressure injection pump discharge pressures less than or equal to 1275 psi reassess their emergency procedures and operator training programs and modify them, as needed, to ensure that the operators can handle the full spectrum of possible small-break loss of coolant accident scenarios which may occur from control system failures and a resulting steam generator overfill.
GL 89-19 indicated that CE-designed plants do not provide automatic steam generator overfill protection that terminates MFW flow. By "L-90-106, Forwards Response to Generic Ltr 89-19, Request for Action Re Resolution of USI A-47, Safety Implications of Control Sys in LWR Nuclear Power Plants, Per [[CFR" contains a listed "[" character as part of the property label and has therefore been classified as invalid.)|letter dated March 19, 1990]], Florida Power and Light (FPL) stated that with respect to St. Lucie Units 1 and 2, GL 89-19 was incorrect because both units have steam generator overfill protection systems to satisfactorily resolve USI A-47.
The staff's evaluation of the licensee's response to GL 89-19 is dicussed below.
9703260149 970301 PDR FOIA BINDER 96-485 PDR
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, 2.0 DISCUSSTAH St. Lucie Units 1 and 2 use a 2-out-of-4 coincidence logic for steam generator overfill protection from four Reactor Protection System (RPS) level loops per steam generator. The loops provide high and high-high level isolation signals. These signals result in feedwater regulating valve closure on high steam generator level and feedwater pump trip and main turbine trip on high-high steam generator level.
The initiating signals are used in non-safety related overfill protection circuits. The level loops which initiate steam generator isolation (feedwater regulating valve closure, feedwater pump trip and main turbine trip) are different from the level channels used for normal feedwater regulating valve control. The high-high level isolation function is accomplished independently of the feedwater control system via a separate circuit. Upon receipt of a high-high level isolation signal, with a 2-out-of-4 coincidence from either steam generator, a trip relay is energized. This relay trips both feedwater pumps and energizes a second trip relay and an auto stop solenoid. The second trip relay provides annunciation. The auto stop solenoid provides for a main turbine trip by closing the turbine stop/ control valves.
The power for the feedwater control system and the steam generator overfill protection circuit are from different sources. The normal power supplies for a single feedwater control train and the overfill protection system are ultimately fed from a common 480 volt switchgear (Unit 1) or 4160 volt bus (Unit 2); however, the feedwater control system is supplied with 120 volt AC power and.the steam generator overfill protection system is provided 125 volt DC power. The steam generator overfill protection system circuit is provided with back-up emergency power from the 28 station battery in the event normal power is lost.
Each train of the feedwater control system is also provided with an automatic transfer to a back-up power source from the 120 volt vital AC power cabinets to ensure further reliability. Furthermure, the design of the feedwater control system requires the feedwater regulating valve to fail closed on a loss of power such that even in the unlikely event the common bus and the back-up power supplies fail, feedwater flow will still be isolated for the affected train.
St. Lucie plant procedures include provisions to periodically verify steam generator overfill protection system operability during reactor power operation. Testing of the steam generator high and high-high level circuitry is performed on an 18-month refueling outage basis by functionally testing the circuitry using the 2-out-of-4 logic to close the feedwater regulating valve, trip the feedwater pump and trip the turbine. The licensee indicates that the requirements for testing the steam generator overfill protection system are controlled through the plant procedures.
j St. Lucie Plant Emergency Operating Procedure, E0P-3, " Loss of Coolant Accident (LOCA)", provides operator instructions and contingency actions for the full spectrum of LOCA scenarios. This procedure would be employed in the unlikely event of a steam generator tube rupture as a result of a steam generator overifil condition.
Guidance is provided within E0P-3 for cooldown and depressurization of the plant with either a loss of offsite power or the
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. loss of a single emergency electrical train. Upon the loss of normal steam generator cooling safety function, procedural guidance directs the operator to E0P-15 " Functional Recovery". Cooldown and depressurization of the plant would then be affected through use of E0P-15, RCS and Core Heat Removal Success Path 4, once through cooling using the pilot operated relief valves.
The Combustion Engineering plant procedural guidance for cooldown of the plant i
under small break LOCA conditions has been previously approved by the staff.
The licensee also indicated that the St. Lucie plant operators are trained on the above procedures as part of the licensed operator training program.
4.0 CONCLUSION
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Based on the staff's review of the licensee's responses to GL 89-19 as discussed above, the staff concludes that the current steam generator overfill protection system design and associated plant procedures and operator training at St. Lucie Units 1 and 2 are in compliance with the recommendations of GL 89-19 and are, therefore, acceptable.
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l ENCLOSURE 2
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SYSTEMATIC ASSESSMENT OF LICENSEE PERFORMANCE
- FACILITY:
ST. LUCIE UNITS 1 AND 2 GENERIC LETTER 89-19. TAC M75002 AND M75003 i
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SUMMARY
OF REVIEW / INSPECTION ACTIVITIES 1
Generic Letter 89-19 recommends that all Combustion Engineering designed PWR plants provide automatic steam generator overfill protection to mitigate MFW j
overfeed events. The licensee provided a response to GL 89-19 describing their steam generator overfill protection system. The Instrumentation and Controls Branch assessed the licensee's response and found the overfill protection system to be acceptable.
i NARRATIVE DISCUSSION OF LICENSEE PERFORMANCE - FUNCTIONAL AREA i
The licensee performed a detailed evaluation of the Generic Letter 89-19 recommendations against their plant design. Their evaluation demonstrated sour.d technical knowledge and understanding of the issue.
1 Author:
Sano Rhow Date:
2/10/94 4
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