ML20214Q269

Forwards Info Re Loss of RHR While in mid-loop Operation on 870410.Info Provides Guidance for Any Operating Procedure Rewriting Which May Be in Progress & for Incorporation Into Operator Training Program

ML20214Q269
Person / Time
Site:	Diablo Canyon, Rancho Seco, 05000000
Issue date:	05/22/1987
From:	Kalman G Office of Nuclear Reactor Regulation
To:	Andognini G SACRAMENTO MUNICIPAL UTILITY DISTRICT
References
NUDOCS 8706040325
Download: ML20214Q269 (6)

Text

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, i 2 2 MAY 1987 Mr. G. Carl Andognini Chief Executive Office, Nuclear Sacramento Municipal Utilities District 14440 Twin Cities Road Herald, California 95638-9799

SUBJECT:

INFORMATION RELATIVE TO RHR DURING MID-LOOP OPERATIONS Diablo Canyon lost RHR while in mid-loop operation on April 10, 1987, and it was 8 G inuths before RHR was restarted. During more than half of this time, the core was cooled by boiling.

We believe problems associated with this event are generic tt PWR's equipped with "U" tube steam generators, with some a]plicability to P6Rs with once through steam generators. These problems s1ould be addresseo prior to mid-loop operation in any PWR. Enclosure I summarizes the concerns an.f provides recom-mended actions. Enclosure 2 describes the short term Diablo (anyon response to the event, and was implemented prior to our completion of Enclosure 1.

Although we are not aware of any near term plans to conduct mid-loop operations at Rancho Seco, this information is being forwarded to provide guidance for any operating procedure rewriting which may be in progress and for incorporation into your operator training program. Additional NRC correspondence (Information Notice) regarding this matter will be forthcoming.

Sincerely, 9tlelmet slynd by George Kalman, Project Manager Project Directorate V Division of Reactor Projects - Ill/IV/V

& Special Projects

Enclosure:

As stated cc:

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Mr. G. Carl Andognini Rancho Seco Nuclear Generating Chief Executive Officer, Nuclear Station Sacramento Municipal Utility District CC' Mr. David S. Kaplan, Secretary Mr. John Bartus and General Counsel Ms. JoAnne Scott Sacramento Municipal Utility Federal Energy Regulatory Commission District 825 North Capitol Street, N. E.

6201 S Street Washington, D.C. 20426 P. O. Box 15830 Sacramento, California 95813 Thomas A. Baxter, Esq.

Shaw, Pittman, Potts & Trowbridge 2300 N Street, N.W.

Washington, D.C. 20037 Mr. John V. Vinguist Acting Manager, Nuclear Licensing Sacramento Municipal Utility District Rancho Seco Nuclear Generating Station 14440 Twin Cities Road Herald, California 95638-9799 Mr. Robert B. Borsum Babcock & Wilcox Nuclear Power Generation Division Suite 220, 7910 Woodmont Avenue Bethesda, Maryland 20814 Resident inspector / Rancho Seco c/o U. S. N. R. C.

14440 Twin Cities Road Herald, California 95638 Regional Administrator, Region V U.S. Nuclear Regulatory Comission 1450 Maria Lane, Suite 210 Walnut Creek, California 94596 Mr. Joseph 0. Ward, Chief Radiological Health Branch State Department of Health Services 714 P Street, Office Building #8 Sacramento, California 95814 Sacramento County Board of Supervisors 827 7th Street, Room 424 Sacramento, California 95814 Ms. Helen Hubbard P. O. Box 63 Sunol, California 94586

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DISTRIBUTION Uoctet F11e.

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ENCLOSURE I LOSS OF RESIOUAL HEAT REMOVAL SYSTEMS, LOWERED LOOP OPERATIONS, UNRECOGNIZED PHENOMENA, AND LEVEL INSTRUMENTATION ERRORS APPLICABILITY: PWR Operations involving Lowered Water Level CONCERNS Diablo Canyon lost RHR while in mid-loop operation on April 10, 1987, and it was 85 minutes before RHR was restarted. During more than half of this time, the core was cooled by boiling.

We believe problems associated with this event are generic, and the following are of concern should be discussed with the appropriate licensees' staff and the resident inspectors at these facilities.

(1) RCS water level instrumentation "drif t" in all measurements. Higher water level is indicated at the RHR suction nozzle than is actually the case.

(2) Change in RCS inventory when the RCS and RHR systems are interconnected but otherwise isolated. Air appears to rove from the RCS to the RHR system (or the reverse), and displaces RHR water which moves to the RCS.

This has misled operators into taking actions contributing to RHR loss.

(3) Vortexing at the same level as is required to drain steam generator tubes, or at a level higher than that to which operators are instructed to control level. Such vortexing and air entrainment may not affect RHR pump current and flow rate until the pumps become air-bound and continued RHR operation is jeopardized.

(4) Flow dynamics which affects all_ level instrumentation. Of concern are:

(a) Water level difference between RHR injection and suction locations.

The former may be the level indicated by instrumentation.

(b) Level perturbation due to injection water momentum. Water level toward the Reactor Coolant Pump can ba higher than toward the vessel because the former is static, while the latter is a flow region. The former may be the level sensed by level instrumentation.

(c) Pressure increase on the cold leg side of the reactor vessel relative to the hot leg. Air entrained from the hot leg can be pumped to the cold legs, with return flow to the hot leg restricted to core bypass flow from the upper reactor vessel. Instrumentation may reflect the cold leg pressure increase as a level increase.

(5) Level instrumentation design and installation may contribute to erroneous level readings and instrument response delays due to long, small diameter tubing runs with high and low elevation regions. High points trap air, and low points water, both of which can affect level indications.

. 2 (6) If an RHR pump fails due to vortexing, it is likely that starting another will result in its immediate failure for the same reason. Yet operators may persist in attempting to start pumps before increasing level.

(7) RHR system characteristics may differ due to different pipe lengths and plant specific analysis may be necessary to determine acceptable RHR flow / level relationships to preclude vortexing.

(8) Upper vessel thermocouples may have been disconnected, and other tempera-ture indications are usually meaningless when RHR pumps are idle. As a result, operators may be unable to monitor RCS heatup rate following RHR failure. This can lead to inappropriate mitigative actions and failure to comply with technical specifications.

(9) Procedures may not address mid-loop operation and event classification guidance may be inadequate due to failure to consider the unique plant state and the implications of accidental loss of RHR as contrasted to a planned RHR shutdown.

(10) Many operations may be in progress during mid-loop operation. Some operations have the potential to reduce RCS inventory. Others may jeopar-dize containment integrity. Both situations can occur at a time when systems and operators are challenged by unique RCS conditions. Of partic-ular concern are impacts to RCS inventory and the equipment hatch. The fomer can initiate loss of RHR. An open equipment hatch may be incapable of being closed quickly, which could result in an offsite release if an accident occurs.

The licensees should be encouraged to consider the following:

(1) Consider air sucked into RHR System which may affect level indication and give a false reading. Evaluate all level instrumentation with respect to flow phenomena within the combined RCS/RHR systems. Fully consider two component (air /H,0) behavior and RCS/RHR regions which may trap and later release air, thus pemitting water to be displaced from one or more locations to another. Consider additional fully independent instrumentation and display of level in the control room, as appropriate.

(2) Reevaluate vortexing behavior to assure consistency between operations and inventory needed to prevent vortexing associated problems. Consider reduction of RHR flow rate as a means for achieving additional margin.

(3) Postpone removal of key instrumentation, such as several vessel themoccuples, until after mid-loop operation has been completed and RCS water level has been raised.

(4) Review operating and emergency (abnomall procedures and operator training applicable to mid-loop operation. Include appropriate corrective actions following loss of an 1HR pump.

(5) Postpone plant maintenance, test, and related operations which may influence the RCS so they are not conducted during mid-loop operation.

3 (6) Reevaluate containment isolation status during mid-loop operation which closely follows power operation. Consider having containment reasonably isolated or readily isolable under these conditions.

(7) Postpone operations which may negatively influence containment isolation capability so they are not conducted during mid-loop operation.

(8) Minimize time spent in mid-loop operation.

(9) Provide immediate operator access to technical personnel intimately familiar with mid-loop operation and potential problems which may occur while in this condition, or assign such personnal to the control room during mid-loop operation. -7 1

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. . ENCLOSURE 2 FACIFIC GRAS AND E LE C T RI C C O M PANY

. (415)781 4211 . Twx 910-372 6587 FOW3 l 77 BEALE STREET . S AN FRANCISCC. C AUFORNIA 94106

~?, U,"I" May 4, 1987 PGandE Letter No.: DCL-87-099 Mr. John B. Martin, Re'gional Administrator U. S. Nuclear Regulatory Commission, Region V 1450 Maria Lane, Suite 210 Halnut Creek, CA 94596-5368 Re: Docket No. 50-323, OL-DPR-82 Diablo Canyon Unit 2

- April 10, 1987 Interruption of RHR Flow Event

Dear Mr. Martin:

PGandE is' submitting the enclosed information regarding actions to beThese completed prior to resumption of mid-loop operation on DCPP while the plant is in mid-loop operation and to improve definition of corrective action and emergency reporting in the unlikely event of an interruption of RHR flow.

PGandE estimates a resumption of Unit 2 mid-loop operation as early as May 18, 1987, to restore the primary system to its normal operating configuration (removal of steam generator nozzle dams and reinsta11ation of steam generator primary manway covers).

The proposed and completed actions described in the enclosure resulted from PGandE's investigation and discussions with the NRC Augmented Inspection Team PGandE will work with your staff to during the weeks of April 13 and 20.

effect long term resolutions of mid-loop operation issues prior to the next DCPP Unit i refueling outage, presently scheduled for March 1988.

Kindly acknowledge receipt of this material on the enclosed copy of this letter and return it in the enclosed addressed envelope.

Sincerely, J. . hiffer Enclosure .

cc: L. J. Chandler B. Norton J. L. Crews C. M. Trammell G. H. Knighton CPUC M. M. Mendonca Diablo Distribution

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PGandE Letter No.: DCL-87-099 ENCLOSURE

, DIABLO CANYON POWER PLANT UNIT 2 q APRIL 10, 1987, INTERRUPTION OF RHR FLOW EVENT N -

This submittal provides a summary of proposed and completed actions taken by PGandE to address conditions associated with the April 10, 1987, Unit 2 interruption of RHR flow event. These actions are based on the results of PGandE's investigation to date and discussions with the NRC Augmented Inspection Team (AIT). These actions are categorized as

• Actions completed during previous mid-loop operation (April 10-18, 1987) s

• Actions to be completed prior to resumption of mid-loop operation

• Long term actions which involve Westinghouse, the NRC, and PGandE A detailed report of the event including causes and PGandE actions will be submitted at a later date.

' Actions Comoleted Durina Previous Mid-Loon Ooeration (Aoril 10-18. 1987)

1. Evaluation of Reactor Vessel Refueling Level Indication System

--(RVRLIS)

An evaluation of the RVRLIS performance during the interruption of RHR flow event was conducted. This evaluation identified that vortexing occurs at slightly higher RCS water levels than previously

anticipated. Air entrainment resulting from vortexing caused a reduction in level indication accuracy which escalates as a function of the degree of air entrainment.

!' 2. Capability for' Containment Closure i

The major pathways to the envircnment were closed.

. 3. Enhancement of Procedures On April 12, 1987, an on-the-spot change was made to Operating Procedure A-2:II, " Reactor Vessel-Draining the Reactor Coolant System," to include a step for placing the narrow range RVRLIS in

service. This on-the-spot change was revised on April 14, 1987, with another on-the-spot change to give detailed instruction for placing the nar(ow range RVRLIS annunciator in service. The final on-the-spot change to Operating Procedure A-2

II was written April 20, 1987, to retype the procedure for clarity. It

incorporates all the previous on-the-spot changes.

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4 On April 12, 1987, an on-the-spot change was made to AP-16

" Malfunction of RHR System," to include a specific section on system malfunction or interruption of RHR during mid-loop operation.

Previously, the procedure addressed mid-loop operation in a general panner and not in a specific section. On April 13, 1987, a second review of INPO SOER 85-04 and IEIN 86-101 was completed to ensure that all potential immediate corrective actions pe.rtaining to this event were identified. The review concluded that no further imediate procedural revisions were required.

4. Installation of Reactor Coolant System Temperature Indication Two core exit thermocouples were reconnected. Those selected for reconnection were near the top center of the core and on opposite trains. Except for a short period required to facilitate a conoseal removal, they remained connected to the thermocouple monitoring system panels in the control room. The thermocouples were disconnected when the RCS level was raised in preparation for vessel head removal.

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5. Improved Work Planning and Control

' The scope of outage work to be performed on systems connected to'the RCS while in mid-loop operation was restricted to those items which did not (1) communicate between containment and the environment or (2) have the potential to reduce RCS inventory.

6. . Additional Training On-shift operating crews were briefed on the interruption of RHR flow event, including potential causes and procedure changes which were made as an immediate response to this event.

Actions Comoleted or to Be Comoleted Prior to Resumotion of Mid-Loon Ooerittinn Based on the condition of Unit 2 during the forthcoming mid-loop operation, and the actions which have or will be completed to enhance the safety of mid-loop operation, the following information is provided.

PGandE estimates resumption of Unit 2 mid-loop operation on May 18, 1987, at which time the core heat load will be approximately 1.45 MHt (as opposed to 7.5 MHt at the time of the event). At this heat load a loss of decay heat removal would result in the coolant temperature increasing from 90*F to boiling in a minimum of 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. This calculation does not consider any effect of metal heatup or natural circulation.

The following is a summary of those actions which PGandE has completed or will complete prior to the resumption of mid-loop operation to preclude an interruption in RHR flow and to provide for effective emergency action if required. -

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1. RVRLIS Modifications for Use During Mid-Loop Operation The RVRLIS will be modified. This modification will provide for an additional channel of indication.

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a. A narrow range level transmitter will be installed to sense the loop 3 hot leg level, referenced to the reactor vessel head.

This will be indicated in the control room through accumulator 2-3 level indication with high and low alarm capabilities.

b. The existing visual standpipe level system and narrow and wide range level transmitters which provide indication and alarm in the control room will remain as installed except for minor installation enhancements.

Note: PGandE plans to further upgrade the RVRLIS, including the addition of wide range indication sensing of the loop 2 crossover leg level, referenced to the pressurizer vapor space. This will be indicated in the control room through accumulator 2-2 level indication with high and low alarm capabilities. This channel will be installed for mid-loop operation of Unit I during its second refueling outage. ,

2. Enhancement to Procedures Regarding Containment Closure Changes will be made in procedures governing mid-loop operation and in abnormal procedures governing actions to be taken on interruption "of RHR flow to require major pathways which communicate between the containment atmosphere and the outside atmosphere either to be closed (in the case of equipment hatch and steam generator secondary ride isolation) or to have the capability of being closed (e.g.,

airlock door) in a timely manner, less than 30 minutes.

3. Enhancements to Operating Procedure AP-16 The abnormal operating procedure governing the loss of decay heat removal capability, AP-16, will be revised to include:

a. Requirements for not starting the second RHR pump if the first pump cavitates or becomes airbound until proper RHR pump suction is reestablished and verified.

b. Recovery actions to be taken in the event that RCS level decreases belaw acceptable values.

4. Enhancements to Operating Procedure A-2:II Operating Procedure A-2:II will be revised to provide the following:

a. Precautions'to specify minimum reactor vessel water level as a function of RHR flow and RCS level conditions to preclude significant air entrainment due to vortex formation.

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b. Checklists that include proper alignment of the reactor head l and RVRLIS vent systems.

c. RHR flow reduction to a value to be determined with g

Hestinghouse consistent with adequate decay heat removal and other considerations.

5. Enhancement of Emergency Classifications The following clarifies which circumstances would require emergency plan entry and/or notification:

a. A prompt report in accordance with 10 CFR 50.72 if there is any unplanned interruption of RHR flow to the vessel.

b. An Unusual Event if the RHR flow is not restored within 10 minutes,

c. An Alert if RCS temperature exceeds 200*F based on core exit

> thermocouples or the results from a conservatively calculated table indicating the time to reach 200*F. In addition, an Alert will be declared if RHR flow is not restored within 1 -

hour.

6. Emphasis on Procedure Compliance PGandE's policy regarding compliance and adherence to procedures

. will be reemphasized.

7. Instrumentation in Service l A prerequisite for entering mid-loop operation will be that RHR temperature and flow instruments and recorders be available. A minimum of two RCS core exit thermocouples will be in service when the reactor vessel head is in place, except when preparing for removal or replacement of the vessel head. A minimum of one loop's RCS wide range (Th and Tc) temperature monitors and recorders will also be in service.

8. Additional Training Training will be conducted for operating crews on mid-loop operation as described in procedures A-2:II and AP-16. The training will also cover RCS venting and RHR system venting (pumps and piping).

l Note: Training for this mode of operation will be included in the

' formal operator training program.

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9. Control of Work Activities Work activities will be controlled to ensure that:

j. No potential RCS inventory loss path from the reactor coolant system, residual heat removal, or charging system will be l

created. .

b. Effective communication is implemented between the control room and critical work areas.

c. During mid-loop operation, all work activities will be planned so that the time duration in this configuration is minimized (estimated time approximately 5 days). The presently planned activities are (1) to verify proper RHR system operation with installed instrumentation at the specified minimum operating level (2) nozzle dam removal, and (3) steam generator primary manway installation.

d. An engineer or manager knowledgeable with respect to the requirements of mid-loop operation will be present during this period.

Lona Term Actions PGandE, in conjunction with Hestinghouse, intends to resolve mid-loop operation with NRR and Region V prior to the next Unit I refueling outage, presently ~ scheduled for March 1988, or develop all actions to a point where there is no longer a concern or safety issue.

Summary As part of its ongoing investigation efforts, PGandE studied and reviewed the event and its implications. Actions have been agressively taken, and all short term actions will be completed prior to resumption of mid-loop operation. Based upon its investigation to date PGandE has concluded that resumption of mid-loop operation on May 18, 1987, presents no undue risk to public health and safety.

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