ML20056C545

Responds to NRC Bulletin 93-003, Resolution of Issues Related to Rv Water Level Instrumentation in Bwrs, Including Summary of Rv Water Level Measurement Capability & Plans Re Errors in Rv Water Level Indication

ML20056C545
Person / Time
Site:	Oyster Creek
Issue date:	06/14/1993
From:	Keaten R GENERAL PUBLIC UTILITIES CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
C321-93-2179, IEB-93-003, IEB-93-3, NUDOCS 9306250076
Download: ML20056C545 (5)

Text

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GPU Nuclear

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TELEX 136 482 Writer's Direct Dial Numt>er:

C321-93-2179 June 14, 1993 U.S. Nuclear Regulatory Commission Att: Document Control Desk Washington, DC 20555

Subject:

Oyster Creek Nuclear Generating Station Docket No. 50-219 Response to Bulletin 93-03 The purpose of this letter is threefold: first, to summarize reactor vessel water level measurement capability at the Oyster Creek Nuclear Generating Station and associated protective functions; second, to address actions requested by NRC Bulletin (NRCB) 93-03, dated May 28, 1993, " Resolution of Issues Related to Reactor Vessel Water Level Instrumentation in BWRs"; and third, to provide a summary of GPUN's plans relative to the issue of errors in reactor vessel water level indication induced by noncondensible gases coming 6 out of solution during depressurization.

OCNGS Reactor Vessel Water Level Measurement Capability OCNGS reactor vessel measurement capability is summarized in.the attached Table 1, which identifies that both hot reference leg (Yarway) instrumentation and cold reference leg (GEMAC) are in use. The Lo and Lo-Lo water level signals for ECCS and RPS (including reactor isolation) are sensed by the hot leg Yarways which are not susceptible to errors induced by non-condensible gasses coming out of solution during rapid depressurization.

The Lo-Lo-Lo setpoint used for Automatic Depressurization System (ADS) actuation and Reactor Building Closed Cooling Water (RBCCW) isolation is sensed by a cold leg instrument. The BWROG generic report reiterates previous conclusions with respect to cold leg instrumentation that safety system actuations occur prior to the introduction of errors associated with non-condensible gases coming out of solution. Therefore, ADS initiation will 1 occur prior to the introduction of errors associated with non-condensible gases coming out of solution. RBCCW isolation occurs if both high drywell pressure and Lo-Lo water level, from Yarways, are sensed. RBCCW isolation initially was based on combined Lo-Lo water level and high drywell pressure -)

only, however, in response to a NUREG 0578 requirement for diverse isolation capability, a feature to isolate on Lo-Lo-Lo water level from cold-leg instruments was added.

Post Accident Monitoring capability, per Regulatory Guide 1.97, is provided by both hot leg Yarways and cold leg Fuel Zone level indication.

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C321-93-2179 Page 2 NRC Bulletin 93-03 NRCB 93-03 requested that BWR licensees take certain actions described in the Bulletin and required that the NRC be notified regarding licensees' corrective actions.

The Bulletin describes an event at the Washington Public Power System, Unit 2 (WNP-2). During a_ normal plant cooldown on January 21, 1993 operators observed a sustained cold reference leg level indication error of 32 inches that gradually recovered over a period of approximately two hours. The  ;

licensee determined that errors of this type could result in failure to ,

automatically isolate a leak in the Residual Heat Removal system during normal l plant depressurization. In NRCB 93-03, the NRC documents their concern that i several paths have the potential to drain the RPV, and automatic isolation signals based on low RPV level are normally credited for terminating such l events. However, automatic isolation of the RHR system, and other systems, would not occur if there were large level errors in multiple instruments.

Although the NRC considers the interim procedures currently in place I appropriate for events initiated from full power, they are not considered adequate for providing protection against events initiated during cooldown-when automatic safety systems may be defeated by level system inaccuracies.

Following the WNP-2 event, the NRC staff requested the BWROG to evaluate the effect of level indication errors on events, such as reactor pressure vessel (RPV) draindown initiated from low pressure conditions. The BWROG submitted a report, " Supplementary Information Regarding RPV Water Level Errors due to Non-condensible Gas in Cold Reference Legs of BWRS", to the NRC on May 20, 1993. The following sentence is taken from the May 20, 1993 submittal: l 1

"BWR/1 and BWR/2 Plants do not rely upon cold reference leg i instrumentation for safety system initiation and are therefore not specifically addressed by this evaluation." {

i GPUN does not consider the concern and associated corrective short term actions described in NRCB 93-03 applicable to the Oyster Creek Nuclear Generating Station (OCNGS), for reasons described in the following paragraphs.

At OCNGS, the draindown scenario during cooldown conditions addressed by NRCB 93-03 is precluded by the fact that drywell isolation valves for the Shutdown Cooling System and the Reactor Water Cleanup System are actuated based on Lo-Lo RPV water level. As noted above, Lo-Lo water level is sensed by Yarways, which are not susceptible to the noncondensible gas phenomenon described in GL 92-04 and NRCB 93-03.

As described in GPUN letter dated September 29, 1992 in response to GL 92-04, the OCNGS emergency operating procedures (EOPs) provide appropriate guidance on how to address level indication discrepancies. Operator guidance directs the operator to flood the' reactor whenever confidence is lost in the level indication. In addition, the operator practice of verifying level' indication l prior to termination of injection for level control has been reinforced. In NRCB 93-03, NRC states that they consider the interim actions currently in place adequate for events initiated from full power. The interim measures implemented at OCNGS in response to GL 92-04 apply equally to full power and cooldown conditions.

C321-93-2179 Page 3 i

GPUN Plans to Resolve Noncondensible Gas Issue Generic Letter (GL) 92-04, dated August 19, 1992, requested licensees to determine the impact of potential level indication errors after a rapid depressurization event on how the plants are operated. GL 92-04 also requested licensees to take short term compensatory measures to mitigate the consequences of potential level indication errors after a rapid depressurization event and provide the staff with plans for long term corrective actions including any proposed hardware modifications. On August 28, 1992, the BWR Owners' Group submitted to the NRC a Generic Report which described offferences between hot and cold reference leg level indication systems, and identified that BWR's use hot reference leg level instrumentation, cold reference leg instrumentation, or a combination of both for safety system actuation.

By letter dated September 29, 1992, GPUN addressed the provisions of GL 92-04 with respect to DCNGS. In this letter GPUN identified that concerns relative to noncondensible gases coming out of solution have no impact on OCNGS automatic safety system response during licensing basis transients or accidents. The Lo and Lo-Lo reactor water level signals associated with the RPS and ECCS are sensed by the hot reference leg Yarway instrumentation. As documented in the BWROG response to GL 92-04, Yarways are not susceptible to the noncondensible gas phenomenon due to their geometric configuration.

As discussed in response to GL 92-04, GPUN has actively participated in BWROG activities relative to errors induced by noncondensible gases. While this issue does not impact OCNGS ability to successfully perform automatic safety

. functions, GPUN recognizes the importance of maintaining all instrumentation as accurate as possible, and of recognizing those situations which impact instrument accuracy.

By letter dated February 18, 1993, the NRC found acceptable GPUN's endorsement of the BWROG commitment regarding plans and schedule to address postulated errors in BWR water level instrumentation. GPUN intends to provide details of ,

plans, by July 30, 1993, to eliminate errors associated with noncondensible gases.

Conclusion In summary, GPUN does not consider the concerns raised in :B 93-03 to be applicable to OCNGS. Reactor isolation,.incluoing shutdown ,

cooling systems, is initiated from hot leg Yarway instruments which are not i susceptible to the noncondensible gas phenomenon. The interim correcthe i actions instituted in response to GL 92-04 apply equally to full power and cooldown conditions.

GPUN relies upon hot reference leg Yarway instruments for all scenarios requiring safety system actuations at significantly reduced pressure.

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C321-93-2179 Page 4 OCNGS Post Accident Monitoring capability is provided by the combination of hot leg Yarways and cold leg Fuel Zone level instrumentation.

GPUN intends to provide details of 3 is, by July 30, 1993, to eliminate ,

errors associated with noncondensible gases in the cold leg fuel-zone level instrumentation.

Should you have any questions, please contact Brenda DeMerchant, Licensing Engineer at 609-971-4642.

Very truly yours,

'k Robert W. Keaten.

Vice President and Director Technical Functions RWK/BDEM:jc cc: Administrator, Region 1 Senior NRC Resident Inspector ,

Oyster Creek NRC Project Manager l

-TABLE I OCNGS REACTOR VESSEL WATER LEVEL MEASUREMENT CAPABILITY AND ASSOCIATED PROTECTIVE FUNCTIONS INSTRUMENT / RANGE FUNCTION SETPOINT PROTECTIVE FUNCTION ACTUATED YARWAY (hot reference 1. Safety System Lo Reactor Trip leg) 86" to 186" TAF Actuation Lo-Lo Recire Pump Trip Reactor Isolation Alt. Rod. Injection Iso Condenser Init.

Containment' Isolation Core Spray

2. Post Accident Monitoring GEMAC (cold reference leg)

1. 56"-186" TAF 1. Safety System Lo-Lo-Lo RBCCW isolation Actuation (Note A)

ADS interlock (Note B)

2. -144*-180" TAF 2. Post Accident - -

Monitoring (Fuel Zone)

3. 90"-186" TAF 1. FW Control - -

(No Safety Function)

4. 90"-490" TAF 1. Cold Shutdown - -

Indication Note A: Alternate RBCCW isolation is initiated on High Drywell Pressure and Lo-Lo level. .In response to a

. NUREG-0578 requirement for diverse isolation capability, a feature to isolate on Lo-Lo-Lo water level was added.

Note B: The BWROG generic report reiterates previous conclusions with respect to cold. leg instrumentation that safety system actuations occur prior to the introduction of errors associated with non-condensible gases coming out of solution.

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