ML20056D747

Summary of 930617 Meeting W/Util to Discuss Steps Being Taken by Licensee to Resolve Reactor Vessel Water Level Instrumentation Degassing Issue.List of Attendees Encl

ML20056D747
Person / Time
Site:	LaSalle
Issue date:	07/27/1993
From:	Kennedy J Office of Nuclear Reactor Regulation
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 9308170425
Download: ML20056D747 (20)

Text

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.;- ( E UNITED STATES 3

5 a! NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001

\ **** j July 27, 1993 Docket Nos. 50-373 and 50-374 f

'f LICENSEE: Commonwealth Edison Company FACILITY: LaSalle County Station. Units 1 and 2

SUMMARY

OF JUNE 17, 1993, MEETING REGARDING BWR WATER LEVEL ,

DEGASSING ISSUE t

On June 17, 1993, representatives of the Commonwealth Edison Company (CECO) met with the Office of Nuclear Reactor Regulation (NRR) staff to discuss steps being taken by CECO at the LaSalle County Station to resolve the reactor-vessel water level instrumentation degassing issue, as described in NRC Generic Letter 92-04 and in NRC Bulletin 93-03. The purpose of the meeting was tc permit CECO to provide plant-specific information regarding laSalle.

Representatives of several other licensees observed the meeting, but did not actively participate.

l A list of attendees at the meeting is provided as Enclosure 1. A copy of the slides used in the licensee's presentation is enclosed as Enclosure 2. Note that pages 7, 8, and 10 of Enclosure 2 were returned, at the licensee's request, because the information presented was potentially misleading without the verbal interpretation provided during the meeting. i The licensee described actions being taken at LaSalle to study the degassing ,

phenomena, and presented some information regarding the degassing phenomena as  ;

observed at LaSalle. The licensee stated that, due to design differences from other BWRs, several reactor vessel water cold-leg level instruments (RVLIS f instruments) provid an indication-only function, without having any automatic .

trip functions associated with them. Maintenance and testing of these  !

indication-only instruments can be performed during power operations without-  ;

the danger of tripping the unit or causing.any ECCS equipment to inadvertently i actuate. These instruments have been used to study the degassing phenomenon, ,

including the accumulation of noncondensible gases in the condensing pots for ,

the reference legs of the water level instruments.  ;

The licensee stated that during the September 1992 shutdown of Unit 1, data.  !

was taken from several of the reactor vessel water level channels. Similar ,

data was taken during the June 1, 1993,. shutdown of Unit 2.. Based upon this-data, the licensee stated that the observed " notching" of indicated water  ;

level for the various instruments correlated well with the behavior predicted

• by tests performed by the BWR Owners' Group. A maximum notch size of approximately 10" was observed during the Unit 1 shutdown, while an 8" notch  ;

was observed on Unit 2. The licensee has used this data to develop procedures i to detect the accumulation of noncondensible gases in individual reference  ;

legs.

The licensee expressed caution in proceeding with modifications to the reactor  :

vessel water level instrumentation following the schedule required by NRC .-

9300170425 930727 PDR ADDCK 05000373 BF4 RE E'dIEWM 1 P PDR ,

?

I i

t Bulletin 93-03. The licensee stated that the backfill modification performed j at Millstone.1 could potentially cause significant problems.with the t instruments and may introduce he potential for a large number -of spurious j reactor trips and ECCS actuations. Unlike Millstone 1, which uses Yarway l instruments for trip functions and only modified indication-only cold-leg- '

instrument channels, trip functions at LaSalle are provided by cold-leg ~!

instruments. Cold-leg instruments are extremely sensitive to pressure perturbations, and the interconnection of the instruments to another system for the purpose of backflushing could expose the instruments to significant

~

pressure pulses. The licensee favored a deliberate, phased approach to any .!

potential modifications. .

The licensee stated that compensatory actions are already being taken at I LaSalle to address the potential for inaccurate reactor vessel water level indications. Licensed operator training currently includes both classroom and i simulator instruction covering rapid depressurization events, and training  ;

modules for slow depressurization events are being developed. The RVLIS  !

instrumentation is periodically backflushed during power operations, and is,  ;

therefore, not susceptible to degassing indication errors. These instruments -[

could be used by operators during plant transients if indication from other i water. level instruments was' determined to be inaccurate. Additionally, the j licensee has developed procedures to identify the accumulation of l noncondensible gases in the water level instruments. i The staff acknowledged the information provided by LaSalle, but did not i comment on the information presented by the licensee. The staff did agree to .l meet again regarding the licensee's written response to Bulletin 93-03 in late t July. Other utility representatives present at the meeting voiced their  !

concerns that the modifications required by the staff in Bulletin 93-03 may j cause more problems than they solved. No other comments or questions were l received.  ;

I aLuY Y Albty Janet Kenedy, Project. Manager  !

Project Directorate 111-2 Division of Reactor Projects Ill/IV/V Office of Nuclear Reactor Regulation -:

Enclosures:

1. List _of Attendees

2. Slide Presentation ,

i cc w/ enclosures:  :

See next page t

+

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Bulletin 93-03. The licensee stated that the backfill modification pe: ormed at Millstone I could potentially cause significant problems with the instruments and may introduce the potential for a large number of spurious reactor trips and ECCS actuations. Unlike Millstone 1, which uses Yarway instruments for trip functions and only modified indication-only cold-leg instrument channels, trip functions at LaSalle are provided by cold-leg instruments. Cold-leg instruments are extremely sensitive to pressure-perturbations, and the interconnection of the instruments to another system for the purpose of backflushing could expose the instruments to significant pressure pulses. The licensee favored a deliberate, phased approach to any potential modifications.

The licensee stated that compensatory actions are already being taken at LaSalle to address the potential for inaccurate reactor vessel water level indications. Licensed operator training currently includes both classroom and simulator instruction covering rapid depressurization events, and training modules for slow depressurization events are being developed. The RVLIS instrumentation is periodically backflushed during power operations, and is, therefore, not susceptible to degassing indication errors. These instruments could be used by operators during plant transients if indication from other water level instruments was determined to be inaccurate. Additionally the licensee has developed procedures to identify the accumulation of noncondensible gases in the water level instruments.

The staff acknowledged the information provided by LaSalle, but did not comment on the information presented by the licensee. The staff did agree to meet again regarding the licensee's written response to Bulletin 93-03 in late July. Other utility representatives present at the meeting voiced their concerns that the modifications required by the staff in Bulletin 93-03 may cause more problems than they solved. No other comments or questions were received. g. gg Janet Kennedy,' Project Manager Project Directorate 111-2.

Division of Reactor Projects III/IV/V Office of Nuclear Reactor Regulation

Enclosures:

1. List of Attendees

2. Slide Presentation cc w/ enclosures:

See next page DISTRIBUTION RLaufer HRood JClifford Docket File- NRC & Local PDRs PDill-2 r/f' . TMurley/FMi raglia JPartlow JRoe JZwolinski JDyer JKennedy CMoore OGC EJordan ACRS(10) GGrant, EDO BClayton, Rill RStransky

, MVirgilio ACubbage i *See previous concurrence

• LA/PDill-2 *PM/PDill-3 *PN/PDill-2 *D/PDill-2 CMoore RStransky.rc JKennedy JDyer i 07/20/93 07/26/93 07/26/93 07/27/93 I

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Bulletin 93-03. The licensee stated that the backfill modification performed at Millstone 1, which is currently favored by the NRC staff, could potentially cause significant problems with the instruments and may introduce the potential for a large number of spurious reactor trips and ECCS actuations.

Unlike Millstone 1, which uses Yarway instruments for trip functions and only modified indication-only cold-leg instrument channels, trip functions at LaSalle are provided by cold-leg instruments. Cold-leg instruments are ,

extremely sensitive to pressure perturbations, and the interconnection of the i instruments to another system fo- the purpose of backflushing could expose the  !

instruments to significant pressure pulses. The licensee favored a i deliberate, phased approach to any potential modifications. {

The licensee stated that compensatory actions are already being taken at l LaSalle to address the potential for inaccurate reactor vessel water level  ;

indications. Licensed operator training currently includes both classroom and j simulator instruction covering rapid depressurization events, and training  ;

modules for slow depressurization events are being developed. The RVLIS l instrumentation is periodically backflushed during power operations, 'and is, i therefore, not susceptible to degassing indication errors. These instruments i could be used by operators during plant transients if indication from other l water level instruments was determined to be inaccurate. Additionally, the l licensee has developed procedures to identify the accumulation of i noncondensible gases in the water level instruments. .j i

The staff acknowledged the information provided by LaSalle, but did not j comment on the information presented by the licensee. The staff did agree to meet again regarding the licensee's written response to Bulletin 93-03 in late  !

July. Other utility representatives present at the meeting voiced their  ;

concerns that the modifications required by the staff in Bulletin 93-03 may v cause more problems than they solved. No other comments or questions were  !

received. ]

Janet Kennedy, Project Manager f Project Directorate III-2 r Division of Reactor Projects Ill/IV/V  !

Office of Nuclear Reactor Regulation  !

Enclosures:

1. ' List of Attendees .

2. Slide Presentation f i

cc w/ enclosures: 1 See next page DISTRIBUTION RLaufer HRood JClifford Docket File- NRC & Local PDRs PDIII-2 r/f TMurley/FMiraglia .j JPartlow JRoe JZwolinski JDyer  ;

JKennedy CMoore OGC EJordan  !

ACRS(10) GGrant, EDO BClayton, Rill .RStransky 1 J(Virgilio ACubbage LA/RD11-2 PM/ 71-3 PM/PDIII-2 D/PDil - [

'doore\ RJS ' sky:rc JKennedyM JDyer f /[f/93 /// 93 7 / 4 /93 ') /g/93 {

1 Commonwealth Edison Company LaSalle County Station Unit Nos_._1 and 2 l I

cc: [

Phillip P. Steptoe, Esquire Robert Cushing  !

Sidley and Austin Chief, Public Utilities Division l One First National Plaza Illinois Attorney General's Office

- Chicago, Illinois 60603 100. West Randolph' Street. j Chicago, Illinois 60601 l Assistant Attorney General 100 West Randolph Street Michael I. Miller,' Esquire Suite 12 Sidley and Austin t Chicago, Illinois 60601 One First National Plaza _i Chicago, Illinois--60690 l Resident inspector /LaSalle, NPS ,

U. S. Nuclear Regulatory Commission Mr. G. Spedl i Rural Route No. 1 LaSalle Station Manager t P. 0. Box 224 LaSalle County Station  !

Marseilles, Illinois 61341 Rural Route I r P. O. Box 220 .

Chairman Marseilles, Illinois _ 61341  :

LaSalle County Board of Supervisors l LaSalle County Courthouse Mr. D. L. Farrar .

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Ottawa, Illinois 61350 Manager, Nuclear Regulatory Services-Commonwealth Edison Company _ _. t Attorney General Executive Towers West Ill, Suite 500 l 500 South 2nd Street 1400 OPUS Place  :

Springfield, Illinois 62701 Downers Grove,-Illinois- 60515 Chairman Illinois Commerce Commission .f teland Building .;

S27 East Capitol Avenue  :

Springfield, Illinois 62706

- Illinois Department of Nuclear Safety Office of Nuclear Facility Safety ~ _;

1035 Outer Park Drive  !

' Springfield, Illinois 62704 f Regional' Administrator,- Region'III  !

U. S. Nuclear Regulatory Commission

~

799 _ Roosevelt Road, Bldg. #4 j Glen Ellyn, Illinois 60137 1 Robert'Neuman' Office of Public Counsel- a State of Illinois Center i 100_W. Rando'ph-Suite 11-300

-- Chicago, Illinois 60601 e

. r I

l{'

L ENCLOSURE 1 l LIST OF ATTENDEES 1

MEETING REGARDING'BWR WATER LEVEL INSTRUMENTATION DEGASSING ISSUE FOR THE LASALLE COUNTY STATION, UNITS 1.AND 2 JUNE 17, 1993 Name Affiliation J. Zwolinski NRR/ADR3 L R. Stransky NRR/PD32 L M. Virgilio NRR/DSSA l A. Cubbage NRR/SRXB J. Clifford NRR/PDS J. Dyer NRR/PD32f H. Rood NRR/PD41 L R. Laufer NRR/PD32 J. Raleigh STS J. Greene NYPA-C. Orogvany CEI B. Ford Entergy Operations' J. Miller CECO J. Gieseker CECO P. Piet Ceco T. Best- Ceco M. Lyster Ceco I:

o n . . , , , , , , , . .. . . , , , , _

,,,'iime-r ' '+ i ' Ia

ENCLOSURE 2 .i i

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Commonwealth Edison i.

t Reactor Water. Level InstrumentDe-gassing l Discussion of Options l

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June 17,1993  :.

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INTRODUCTION Purpose of Presentation:

• Discuss CECO actions in collection, evaluation of data on RWL instrument performance.

• Update NRC on CECO experience / lessons learned which defined effective Compensatory Actions (CA).

• Discuss competing Safety Margin effects related to the available options.

• Discuss approaches CECO considers preferable:

Interim use of highly effective Compensatory Actions Installation of Phased or Interim Modifications L

L Deliberate modification design development, to assure solution to problem without introducing new (or common mode) failures 2

b ____ --

De-gassing Issue

• Reference Leg de-gassing poses concern for 2 classes of events:

1) Rapid Blowdown: Rx accidents and transients Equipment actuations

- Post Accident indications and Emergency Procedure response.

2) Normal Shutdown Conditions:

System Operability, Tech Spec compliance

- Erroneous indications to Operators, esp. during events with multiple challenges (i.e. draindown accompanying depressurization > degassing)

• Evaluations indicated that modifications are the best long term solution. Need to choose the solution path .

that preserves the highest safety margms.  ;

• Industry and CECO evaluations have concluded that Compensatory Actions result in acceptable safety margins.

3

De-gassing Issue (continued)

Each of the above event types has mitigating characteristics:

1) Rapid Blowdown events:

- Abnormal / low frequency -uncontrolled.

Inventory loss in progress Mitigation:

Initial equipment actuations are secure.

Multiple, redundant operator indications are expected to be available.

Emergency Operating Procedures are adequate.

2) Normal Shutdown Conditions: ,

Controlled S/D is frequent event, so:

Each site, and industry in general has extensive experience in this condition.

Inventory loss is a potential event, whose probability is primarily a function of the reliability of hardware interlocks and administrative controls.

Mitigation:

Inherent ' head start' on accident parameters:

Core power, decay heat, ECCS capability (no blowdown phase to reach LP injections).

4 i

1 RWL Monitoring Results

• June 1992

- LaSalle was doing extensive monitoring of RWL ,

sensors, piping, and indications. ,

(Separate effort to improve channel-to-channel offsets)

-Minor additions to this effort allowed CECO to use l LaSalle as lead site for evaluation of de-gassing.

• Data Collection Data spanning greater than 9000 operating hours on both LaSalle Units were acquired, primarily through computer " data logging" at varying intervals (~1/10 min. to 1/ day).

• Parameters Recorded :

1. Condensing pot temperatures: top, bottom, feed leg, reference leg r

2. Configuration (geometry): Initial (design records), and verified with actual measurements during Refuel outage '.

3. Routine operation instrument response data.

4. Reference leg temperature profiles (Unit 2 only)

(24 locations along various legs) s k

5

i RWL Monitoring Results l l

Level instrument Cold legs configuration: RVLIS/ non-RVLIS Dresden 3: Yanvay Columns Dresden 2 LaSalle 1.2 Quad Cities 1.2 l

[ m m) l 2 __

@ Spitto thredesi A .

-N FZ MR NR 9

' PVUS" Jj PostA:xcent gg g) -g]

ir&aton 04 ma es LuaJ LPQ F4 ECCS-e RPS PDS

$0 hRSDQ LaSalle "RVLIS condensing pots feed indication only Post Accident Level Monitors.

This allows periodic manual backfill to keep these indications protected from de-gassing effects.

6

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RWL Monitoring Results

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Unit 1 monitoring and data resulted in Engineering proposal to j periodically backfill RVLIS (Post Accident Monitoring ), to. -l maintain the Post Accident Monitors protected from de-gassing.

Option available to LSCS because "RVLIS" reference legs are -l indication only (no risk of trips). -

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• Unit 2 Maintenance Outage 6/1/93: .

u LaSalle Unit 2 planned outage ended.192 day continuous run, i excellent conditions for de-gassing data collection. ]

Significant effort to enter planned outage with proper. ,

measures in place to:

a

1) Provide Operators with enhanced monitoring procedures,. 3 criteria, and actions for inoperability. j y

2) Approve contingency backfill procedures. l

'l 3)- Collect Unit 2 specific' data both manually and via computer, 1 to validate Operator (manual) monitoring methods and.  ;

1 criteria.

a 9 .

4 w , .. - - , , , , ,

l RWL Monitoring Results ):

1

• Procedures are successful at detecting leg specific ]

behavior.  !

- 1 reference leg at Lasalle exceed criteria (6" notch >5 min duration),  :

experienced 8" notch at ~0 psig. ("B" Channel Unit 2). Unit 1 "B" channel j i

response very similar.

t f

Preventative measure planned: backfill leg prior to < 500 psig.

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- Most legs showed minor notching (2-3"), only weakly affected by cooldown rate. L '

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- Procedure validation very successful. Computer / manual methods j correspond for both minor notches and "B" channel (which exceeded j criteria).

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- Legs which notched (and didn't notch) corresponded very well to expected i behavior based on insights provided through the BWROG test results of ~

j geometry effects. One RVLls leg had intentionally been skipped on  ;

periodic backfill (192 days operating time). That leg did not notch at'all. i l

• Summary of . Notch Monitoring.-LaSalle Data:. j l

Unit 1 A' B C -D RVLIS A RVLIS B .l Notch Size 1 .2" 5-7" 0" 1-10" Not Not i spike monitored' monitored .

f Threshold Pressure 50 # 350 # n/a .150 # -

I Unit 2 A B C D. RVLIS A - RVLIS B ]

Notch Size 0" 8" 1-2" 2-3" computer. 0" data -

unavailable

]

-l Threshold Pressure n/a <10 # <10 # 50 # ~n/a f i

-11 .

A RWL Monitoring Results

• Repeatability / Predictability of Leg response Factors affecting response:

1) Gas inventory (operating time, liquid drawdown).

2) Leg geometry

3) Pressure at onset

4) Depressurization rate P

1. 1) -Condensing pots will gas-saturate within 4-8 weeks after startup or backflushing at power. ,

-Gas saturated legs indicate drawdown by increasing level trend. (2 events ,

at LSCS noted on-line, legs backfilled). Valuable detector of" leaking" instrument valves.

1. 2) -Some legs showed no notching, due to continous sloping (minimal vertical routing).

-At Lasalle " Problem" leg (s) can be backfilled prior to'depressurization

-Possible "B" channel reference leg = convection pump. Evaluations in progress.

1. 3) - Pressure of significant response very low: < 100 psig. This pressure is "by  ;

definition" inside the region where the plant is under manual control. l VERY unlikely to reach this region via uncontrolled evolutions.  ;

'#4) -Minor effect over wide range of cooldown rates (20 - 80 degF / hr).

- Existing procedures for shutdown provide mechanisms to get this cooldown  :

rate under control for nearly all events including significant accidents and transients.

Likely Events are predictable, and readily controlled.  !

12

o CECO Actions June 1992 to PreSent

• Enhanced Operator Training Initial Response to GL 92-04

- Operators trained on concerns related to confusing level indications, determination of correct level, use of Emergency i Procedures.

- Simulator Exercises developed Rapid Blowdown: LaSalle simulator demo (Jan '93) with j level instrument failures and complications.

• Slow Depressurization / Inventory loss events (All Sites)

Simulator exercises developed or in validation cycle.

Issue Updates and Re-training ,

- Instructions issued to implement requirements of completed ,

Operability Evaluations

- Updates on Industry and LaSalle Data

- Detailed training on phenomenon, WNP event,-LaSalle data.

- Update on issuance ofNRCB 93-03, required actions, and ,

review of Admin controls to prevent Inventory loss in Mode 3.

- Reviewed methods for cross-validating instrument indications.

- Plant specific lesson plans Actions in Progress

- Continuing Updates -

- Augmented training as required by NRCB 93-03.

This training includes completion / implementation of slow depressurization conditions with Inventory loss and instrument failure complications.

1 o

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CECO Actions to Present .

• Detailed Testing of Level Instrument Performance >

• Operability Evaluations completed at all 3 CECO BWR sites:  !

-Refined content of Compensatory Actions:

• Enhanced monitoring guidance with acceptance criteria

• Enhanced training for Operators

• Contingency Procedures (Operator guidance, backfill in Mode 3)

• Review, re-emphasis on potential inventory loss (shutdown Cooling) {

during evolutions 1

• (Dresden 3) ~Yarway columns similar to accepted Millstone configuration for trip systems.

• (LaSalle): Regular, routine backfill of Post-Accident level instruments prevents these indications from susceptibility.

• Plant specific guidance implemented.

• Verified ability of Operators to detect notching on installed [

instruments.

• Procedure criteria maintain very large margin to Safety 1 Criteria: (Offset size vs margin to variable leg tap) ,

Compensatory Actions are In Place, Validated, and Effective. ,

14

.,4 a

Compensatory Actions vs Expedited Mods

• Which path best protects plant Safety Margins?

Compensatory Actions

- Safety Assessments have confirmed integrity of design basis .

protective functions ,

-RWL testing and monitoring confirms Compensatory Actions-effectiveness. Safety Margins during interim period are high: i

- Operator awareness and ability to respond is high.  ;

Training on potential events and complications are being i stressed and kept current. ]

- Extensive CECO history with controlled S/D, with a_ wide  !

spectrum of coincident plant conditions, activities. ,

1 RHR system interlocks and administrative controls are effective - .!

1" barrier to unexpected inventory loss.  ;

(> 6000 controlled shutdowns in fleet history) .

- Accurate operator information can be ensured to further -j decrease risk (Dresden 3 Yarway, LaSalle RVLIS)  ;

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- Reference leg leaks / drawdown can be detected.  !

15

Compensatory Actions vs Expedited Mods Expedited Modifications

- Concern over new learning curve resulting in an increased number of.RWL trips due to extremely sensitive application.

-(Narrow margin, fast response trip sensors -GE SIL 463RI).

- The design (pre-installation) cycle needs to address common mode failures, isolation / system interactions, testability, maintainability, and reliability issues. Not suitable asfollow up efforts.

I I - Design inputs are needed to verify that modification will solve the problem. ' (e.g., LaSalle B" channel convection cell)

- Millstone design is not a valid prototype for modifications at CECO plants. Deliberate design input for trip capable channels is essential.

16.

\: -_ _ __ _ _ ____ _____ ___-_______ __ - ___ - _-__________ -_______ -___-______- -_-__ __- ___ _-. -

t CONCLUSIONS

1. CECO response to de-gassing has been to work i towards a technically correct, conservative l solution. It has been based on performing the l proper reviews, with the proper technical inputs. l

2. Compensatory Actions are in place and are l effective.  ;

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3. Expedited modifications do not appear to be l consistent with industry's " lessons leamed" that deliberate design efforts are essential to minimize l challenges to safety. (IN 93-35).  !

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4. Use of Compensatory Actions for interim period is the proper technical response to this concern, l and achieves the lowest total plant risk.  ;

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5. CECO's response to the de-gassing issue must be l based on sound technical footing and  !

conservative operating philosophy. (

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6. Modification schedule will be developed for l Bulletin response. j 17 l

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