ML20247C051
| ML20247C051 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 09/01/1989 |
| From: | Shell R TENNESSEE VALLEY AUTHORITY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 8909130226 | |
| Download: ML20247C051 (3) | |
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TENNESSEE VALLEY AUTHORITY.
CH ATTANOOGA. TENNESSEE 37401 SN 157B Lookout Place SEP 011989 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C.
20555 Gen tlemen:
In the Matter of
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Docket Nos. 50-327 Tennessee Valley Authority
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50-328 SEQUOYAH NUCLEAR PLANT (SQN) UNITS 1 AND 2 - FEEDWATER CONTROL STUDY RESULTS
Reference:
TVA letter to NRC dated May 5, 1989, "Sequoyah Nuclear Plant -
Summary of Meeting on Recent Unit 2 Reactor Trips, April 23, 1989" In the referenced letter, TVA committed to provide NRC with the results of the subject study.
The enclosed summary provides those results.
No new commitments are made in this submittal.
Please direct questions concerning this issue to B. S. Schofield at (615) 843-6172.
Very.truly yours, TENNESSEE VALLEY AUTHORITY kanager,NuclearLicensing and Regulatory Affairs Enclosure cc (Enclosure):
Ms. S. C. Black, Assistant Director for Projects TVA Projects Division U.S. Nuclear Regulatory Commission One White Flint, North 11555 Rockville Pike Rockville, Maryland 20852 Mr. B. A. Wilson, Assistant Director for Inspection Programs TVA Projects Division U.S. Nuclear Regulatory Commission Region II 101 Marietta Street, NW, Suite 2900 op30 Atlanta, Georgia 30323 v
i L NRC Resident Inspector Sequoyah Nuclear Plant 2600 Igou Ferry Road l
Soddy Daisy, Tennessee 37379 8909130226 890901 PDR ADOCK 05000327 P
PDC An Equal Opportunity Ernployer
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L RESULTS OF_THE STUDY OF THE~SEQUOYAH NUCLEAR PLANT (SQN)..
' MAIN'FEEDHATER CONTROL SYSTEM A study of the;SQN main feedwater control system was initiated following the
- ttree feedwater-related trips of Unit 2 after the Cycle 3 refueling. outage.
'To perform the study, a team was assembled l consisting of Operations, Systems
. Engineering, Nuclear. Maintenance, and Nuclear Engineering.
This study evaluated the SQ4 feedwater system design and' determined if modifications should be made to improve plant availability _and to reduce the number of feedwater-related reactor trips.
In order to reduce the potential for reactor trips and improve availability, three areas were used as objectives:
widen operating margins; stabilize final control elements (pumps, valves); and reduce required operator actions.
The team made recommendations in each of these categories.
TVA will use a graduated approach in implementing these recommendations based on the benefit i
derived from each modification, starting with widening operating margins.
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That is, if analysis of operation after widening operating margins shows
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significantly reduced probability of reactor trips, then stabilizing final control elements may not produce added benefit and, therefore, may not be j
implemented.
l Below is a discussion of-planned and recommended modifications in each of the i
three areas.
Operating Margin j
The Eagle 21 modification, presently scheduled for implementation during the Cycle 4 refueling' outage for each unit, incorporates the" environmental allowance modifier / trip time delay logic into the reactor protection system.
These changes widen operating margin under nonaccident containment environments and increase the time-to-trip margins when operating below 50 percent power.
j The team.also recommended eliminating the feedwater loop isolation on high steam generator (SG) level (60 percent). Although not a reactor trip, this high SG level isolation causes system perturbations that have resulted in a low-low SG level reactor trip. Although presently scheduled coincidently with the Eagle 21 implementation, this is not a commitment in that the effects of Eagle 21 could be evaluated prior to implementing this modification.
Final Control Element Stability The team made the following six recommendations that would affect final
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control element stability.
1.
Replacing the solenoids on the main feedwater regulator valves.
This recommended replacement will be prioritized by the SQN obsolete equipment program.
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,. ~ '.. 2.
Addition of a voltage reduction network on main feedwater pump turbine components to extend equipment life.
3.
Changing the main feedwater bypass control valves from 4-inch to 6-inch valves.
4.
Changing the main feedwater pump turbine speed controls from mechanical-hydraulic to electro-hydraulic controls.
5.
Addition of a motor actuator for the main feedwater pump recirculation valve.
6.
Addition of an integrated digital feedwater control system.
Reduce 1 Operator Actions 1
The team reviewed the scheduled changes resulting from the control room design review to determine if a reduction.in feedwater operator action would result.
SG wide-range level recorders are being added to main control room Panel M-3 and grouped with the main feedwater controls.
These recorders will be multispeed to allow greater resolution during startups.
This change will enhance operator actions by not requiring the operator to walk to the present recorders located on Panel M-4 or to have a separate operator viewing a high-speed recorder.
The team made the following two recommendations to reduce operator action.
1.
Install a standby main feedwater pump.
This would eliminate the need to use auxiliary feedwater for control of SG levels during startup and the actions required to complete transitica from auxiliary feedwater to main feedwater.
2.
Installation of an integrated feedwater control system, which was discussed above, would reduce operator actions by eliminating the operator actions required to complete the transition from the bypass valves to the l
main control valves.
CONCLUSION The team reviewed the main feedwater system and made recommendations in three areas--widen operating margins, stabilize final control elements, and reduce operator actions. The team recommended staging the modifications to analyze the effects at each stage and thus determine if sufficient benefit would be derived from the next stage.
Currently planned modifications for the Unit 1 Cycle 4 outage include:
Eagle 21 with accelerated implementation of environmental allowance i
monitor / trip time delay.
Hide-range SG level recorders will be enhanced and human factored.
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Elimination of 60 percent SG feedwater loop isolation.
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