ML20046B857
| ML20046B857 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 07/30/1993 |
| From: | Zeringue O TENNESSEE VALLEY AUTHORITY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-93-003, IEB-93-3, NUDOCS 9308060321 | |
| Download: ML20046B857 (10) | |
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vee nevaent moes t er, Neew Pant JUL 3 01993 U.S.
Nuclear Regulatory Commission ATTN:
Document Control Desk
.l Washington, D.C.
20555 Gentlemen:
In the matter of
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Docket Nos. 50-259 Tennessee Valley Authority
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50-260 50-296 BROWNS FERRY NUCLEAR PLANT (BFN) - RESPONSE TO NRC BULLETIN (NRCB) 93-03, RESOLUTION OF ISSUES RELATED TO REACTOR VESSEL WATER LEVEL INSTRUMENTATION IN BOILING WATER REACTORS (BWR)
The purpose of this letter is to provide TVA's response to the reporting requirements 2(a) and 2(b) of NRCB 93-03.
Reporting requirements 2(a) and 2(b) are the reports required by July 30, 1993.
Since the issue of noncondensibles in solution in reference legs was identified, TVA has vigorously pursued evaluation and solution of this potential problem via TVA's own design and. review processes and by extensive participation in industry wide efforts.
As discussed in a meeting between TVA and NRC in Rockville on July 28, 1993, because of conditions outside TVA's control, TVA will not be able to implement a plant modification to address this issue until the next cold-shutdown after design issues and material procurement issues have been resolved.
Complete resolution of these-issues.is expected in early November 1993.
Reporting requirements _'and implementation of. hardware modifications 1are discussed further in Enclosure-1.
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_i Page 2 JUL 3 01993 A summary of commitments in this letter is included in. contains the conceptual hardware modification, Enclosure 4 contains design and material constraints to the proposed modification, and Enclosure 5 contains operational and licensing conditions required to implement the proposed modifications.
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Sincerely, Mfd l
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Zering e i
Subscribed and sworn to before me on this 3pV4 day of $< /f/
1993 ra E )milnd Notary Public '
My Commission Expires 7 ' 7 ~74 Enclosures cc (Enclosures):
Mr.
R.
V.
Crlenjak, Project Chief U.S.
Nuclear Regulatory Commission l
Region II 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323 NRC Resident Inspector Browns Ferry Nuclear Plant 3
Route 12, Box 637 Athens, Alabama 35611 Mr. Thierry M. Ross, Project Manager U.S.
Nuclear Regulatory Commission One White Flint, North 11555 Rockville Pike j
Rockville, Maryland 20852 i
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ENOWSURE 1 Tennessee Valley Authority Browns Ferry Nuclear Plant (BFN)
Reportina Reauirements 2(a) and 2 (b) i This enclosure addresses reporting requirement- '(a) and 2 (b) of NRCB 93-03 for BFN Units 1, 2,
and 3.
Report og equirements 2(a) and 2 (b) are the reports required by July 1993.
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.Repottina Reauirement 2(a):
Describe the short term compensatory actions taken.
Short term compensatory actions specified by NRCB 93-03 are listed below, along with a description of how TVA implemented them.
The short term compensatory actions will remain in force l
until a hardware modification is completed.
Recuested action 1(a)(1):
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Establish enhanced monitoring of all RPV level instruments to f
provide early detection of level anomalies associated with degassing from the reference legs.
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Response
TVA BFN Operations trends water level indications when decreasing pressure below 450 psig.
This is done via special dedicated displays on the integrated computer system (ICS). The displays trend the response of reactor vessel level instruments from each reference leg (except the flood up range).
Variances in response due to degassing can thus be easily !dentified.
Operating procedures contain instructions to mon:ttor a specified display for level anomalies when decreasing reactor pressure below 450 psig.
Feauested action 1(a)(2):
Develop enhanced procedures or additional restrictions and controls for valve alignments and maintenance that have a potential to drain the RPV during Mode 3.
Response
TVA previously installed electrical interlocks on Unit 2 which will prevent (1) simultaneous alignment to shutdown cooling suction and suppression pool suction or, (2) shutdown i
cooling suction and injection into the suppression pool via the test return line or, (3) alignment of the shutdown cooling suction to the suppression pool via minimum flow lines for idle residual heat removal (RHR) pumps.
TVA also added. precautions to the RHR system operating procedure to address a potential drain path from the vessel to the suppression pool via the RHR minimum flow line.
Work Control system evaluators have been instructed that when work with the potential for draining the reactor vessel l
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is reviewed, the work shall not be scheduled during hot shutdown at less than 450 psig without Plant Manager approval.
Reauested action 1(a)(3):
i Alert operators to potentially confusing or misleading level indication that may occur during accidents or transients i
initiating from Mode 3.
For example, a drain-down event could lead to automatic initiation of high-pressure emergency core cooling systems (ECCS) without automatic system isolation or low pressure ECCS actuation.
Response
TVA has conducted training regarding level anomalies associated with degassing of reference legs.
TVA has expanded this training to include events occurring while the plant is in the shutdown cooling (SDC) mode.
This includes special operator briefings which were completed prior to June 12, 1993.
A precaution has been added to the operating procedure for SDC to address level indication anomalies which could require operator response.
Beauested action lib):
Complete augmented training on loss of RPV inventory scenarios during mode 3 including RPV drain-down events and cracks or breaks in piping.
Response
TVA has completed training regarding noncondensibles in reference legs from both high pressure events and normal cooldown events (including events which could initiate from SDC mode).
Reportina reauirement 2(b):
Describe'the hardware modifications to be implemented at the next cold shutdown after July 30, 1993.
If an addressee chooses not to take the requested actions specified in the Hardware Modifications section, the report shall contain a description of the proposed alternative course of action, the schedule for completing it and a justification for any deviations from the requested actions.
Response
TVA intends to complete a modification to BFN Units 1, 2,
and 3 which will prevent the buildup of noncondensibles in water level reference legs by injecting Control Rod Drive' system charging water into the reference legs.
A conceptual design is shown in Enclosure 3.
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TVA intends to complete the modification on BFN Units 1 and 3 before restart of these units.
TVA intends to complete the modification on BFN Unit 2 at the earliest possible time subject I
to design constraints, material constraints, license requirements, and operational conditions.
A discussion of the material and the design constraints is included in Enclosure 4.
A discussion of the license and operational requirements is included in Enclosure 5.
TVA does not intend to attain the necessary operational conditions solely for the performance of l
the proposed modification.
Since this phenomena was originally identified, TVA has vigorously pursued its resolution. TVA has maintained membership in several BWROG resolution committees including the Water Level Issues Steering Committee and subcommittees, and the Emergency Procedures Committee.
TVA originally planned to install a periodic backfill system.
In November 1992, a design was issued to reassign instrument channels among reference legs to support periodic backfill.
After events at other utilities resulted in
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the issuance of NRC Bulletin 93-03, it became obvious that TVA's design would not fulfill the BWROG recommendations and TVA developed the alternate plan now proposed.
At this time, the design is essentially complete, except for the issues listed in Extension of the NRC Bulletin 93-03 requested implementation
- t schedule is further justified by BFN plant specific design features as follows:
1.
BFN has electrical interlocks to prevent inadvertent draindown events while in shutdown cooling mode.
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BFN has one-inch reference legs over at least 70 i
percent of the instrument leg runs.
Investigations by Norris Labs has shown that smaller diameter tubing can trap a greater amount of noncondensibles than larger tubing.
This investigation was conducted prior to this bulletin in response to TVA concerns related with level instrumentation problems experienced in 1985.
As a result of this investigation, TVA upgraded the size of the lines to one inch tubing. The data from the BWROG testing has shown that the one-inch reference legs do not preferentially distribute noncondensibles which e
come out of solution in the vertical piping runs, i
so the observed error is less from the degassing j
phenomena.
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3.
Prior to restarting unit 2 from cycle 5, TVA t
completed a reevaluation of instrument leg slope in accordance with the guidance of the nuclear steam supply system manufacturer, and made appropriate changes.
TVA has reviewed data since the instrument leg work and there has been no observed notching at BFN Unit 2 during subsequent controlled shutdowns.
4.
TVA backfills the reference legs prior to restart l
from refueling outages to ensure that non-3 condensibles are flushed from the reference legs.
5.
BFN Operations monitors water level discrepancies and backfills the narrow range level instrumentation while the unit is online if differences between channels exceed predetermined limits.
l The short term compensatory actions will remain in force until a hardware modification is completed.
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ENCLOSURE 2 Tennessee Valley Authority Browns Terry Nuclear Plant (BFN)
Summary List of Commitments Related to NRCB 93-03 1
1.
The short term compensatory actions will remain in force until a hardware modification is completed.
2.
TVA intends to complete a modification to BFN Units 1, 2,
and 3 which will prevent the buildup of noncondensibles in water level reference legs by injecting CRD system charging water into the reference legs.
TVA intends to complete the modification on BFN Units 1 and 3 before restart after i
refueling of these units.
TVA intends to complete the above modification on BFN Unit 2 during the next cold shutdown which occurs after resolution of the design issues and material restraints identified in this letter.
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ENCLOSURE NO. 3 BFN-UNIT 2
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ENCLOSURE 4 Tennessee Valley Authority Browns Ferry Nuclear Plant (BFN)
Design and Material Constraints to Proposed Water Level Monitoring System Modification The design and material constraints to modification implementation are as discussed in a meeting between TVA and the NRC in Rockville on July 28, 1993.
Desian constraints include:
1.
Testing and analysis by the BWROG to validate the design requirements for backfill rate and allowable out of service time.
Final receipt and analysis of testing is due in late August 1993.
2.
Evaluation of CRD system transients and noise, due September 1993.
3.
Resolution of appendix R considerations (appendix R consideration is whether heatup of backfill lines due to a fire could cause indication errors due to injection of hotter than analyzed water temperature),
i due September 1993.
3.
Generic thermal stress evaluations of vessel nozzle, steam leg piping and condensate pot, due August 15, 1993.
Materi:0 constrajnts 1
1.
Flow measuring instruments expected delivery November 1993 2.
Code class valves expected delivery August 20, 1993 3.
Cabling expected delivery November 1993.
e ENCLOSURE 5 Tennessee Valley Authority Browns Ferry Nuclear Plant (BFN)
Operational and Licensing conditions Beauired to Implement Modifications Implementation of the proposed modification will require that part of the automatic initiation logic of ECCS and the protective function of parts of the Primary Containment Isolation System (PCIS) be removed from service.
The license and operational requirements for removal of these instruments from service are not met each time the plant attains cold shutdown condition.
For this modification, in addition to being in cold shutdown, Technical Specifications require that the reactor head be removed, the vessel cavity flooded, and the fuel pool gates removed.
In addition, equipment and personnel considerations dictate that the reactor coolant temperature must be less than 150 degrees Fahrenheit.
It is possible to safely complete the modification without i
removing the reactor head, flooding the vessel cavity, and removing the gates to the spent fuel pool provided that certain compensatory actions are taken, and provided that NRC grants license relief from TS requirements.
Depending on the other conditions associated with a cold shutdown, li'ense relief would c
be justified by undue hardship in that the requirement to perform the above license conditions would add 13 days to any forced shutdown schedule if reactor head removal was not otherwise required.
Compensatory actions would effectively' limit risk.
Based on the above, TVA will request, at the appropriate time, license relief to allow more than one instroment. channel on a reference leg to be out of service without requiring that the reactor vessel head be removed.
Specific _ required compensatory actions may be dependent on plant conditions at the time of shutdown and will be provided with the request for license relief.
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