ML20236H271

Responds to Generic Ltr 87-12, Loss of RHR While RCS Partially Filled. Procedures Will Be Developed for Lowering Coolant Level in Reactor Vessel Below Top of Loop Piping. W/One Oversize Drawing

ML20236H271
Person / Time
Site:	Maine Yankee
Issue date:	10/26/1987
From:	Randazza J Maine Yankee
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
GL-87-12, MN-87-121, NUDOCS 8711040033
Download: ML20236H271 (15)

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MainsYankee RELIABLE ELECTRICITY FOR MAINE $1NCE 1972 EDISON DRIVE . AUGUSTA, MAdE 04330 . (207) 622-4868 October 26, 1987 MN-87-121 United States Nuclear Regulatory Commission Attention: Document Control Desk Hashington, D. C. 20555

References:

(a) License No. DPR-36 (Docket No. 50-309)

(b) NRC Generic Letter 87-12: Loss of Residual Heat Removal (RHR) while the Reactor Coolant System (RCS) is Partially Filled, dated July 9, 1987 l

Subject:

Response to Generic Letter 87-12 Gentlemen:

Generic Letter 87-12, Reference (b), requested information on plant

, operation when the reactor coolant system (RCS) is below the top of the i

reactor vessel.

Maine Yankee is designed to facilitate repair of most RCS components l' without lowering the level in the reactor vessel. Each of the three RCS loops l contain a motor-operated stop valve in the hot and cold legs. The loop stop l valves permit an RCS loop to be drained to perform inspection, maintenance and repairs on steam generators, reactor coolant pumps and other components, without lowering the coolant level in the reactor vessel.

As depicted in Attachment 1 (RCS/RHR Component Elevation Refueling Print 1), with the exception of the RHR suction valves, not isolated by the loop stop valves, the majority of connections to the RCS come off the top of the RCS piping. Thus, except for very infrequent situations, it is unnecessary to drain down the conlant level in the vessel below the top of the loop piping, avoiding the mid-loop condition for which the NRC expressed concern in l' Reference (b).

Mid-loop operation has never been required at Maine Yankee. Operation in this infrequent condition would, however, be permitted with administrative controls, complete with the appropriate cautions, limitations and requirements, in place. The following are examples of the limitations and requirements to be considered for mid-loop operations: a diverse and redundant level monitoring system would be operable, the pressurizer would be vented with a 2" or larger opening to the containment, a HPSI pump would be operable to provide water "or core cooling in the event of a loss of RHR; the equipment l hatch would be in place which would permit the prompt establishment of g\

L containment integrity to mitigate any radiological consequences.

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PDR ADDCK 05000309 P PDR L_ _ - _ - _ - _ _ _ _ -

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MaineYankee p United States Nuclear Regulatory Commission Page Two l Attention: . Document-Control Desk MN-87-121

... Procedures ~will~be developed to address-'the concerns expressed in the subject letter before lowering the coolant level in the reactor vessel below

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'the top of the loop piping.

Should maintenance of RHR valve RH-H-l' require its disassembly, the reactor would have to be defueled so that loop piping can be completely drained..

Our detailed responses to the information requested in Reference (b) are found in the enclosure to this letter.

Very truly yours, MAINE YANKEE ATOMIC POWER COMPANY nW c John B.' Randazza Executive Vice President TJOR/bjp-  !

Enclosure:

(13 Pag'es)-

cc: Mr. Cecil'0.' Thomas-Mr. William T. Russell Mr.:Cornelius F. Holden Mr.' Pat Sears

' STATE OF MAINE Then personally appeared before me, John B. Randazza, who being duly sworn i Edid state that he is Executive Vice President of Maine Yankee Atomic Power Company. that he is duly authorized to execute and file the foregoing response i

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in'.the name and on behalf of Maine Yankee Atomic Power Company, and that the {

. statements therein are true to the best of his knowledge and belief. )

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/" /'NotaryPublic STEPHEN D. EVANS l h

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MaineYankee i ENCLOSURE 1

'Haine Yankee Atomic Power Company-  !

Response to Generic Letter 87-12~ '

NRC RE00EST (1) 3 A detailed description.of the circumstances and conditions under which

.h :your plant would be entered into and brought through a draindown process and operated with. the RCS partially filled, including any interlocks that could cause a disturbance to the system. Examples of the type of information required are: .

Time'botween full-power operation and reaching a partially filled i condition (used to determine decay heat loads);

Requirements for minimum steam generator (SG) levels; Changes in the status of equipment for maintenance and testing, and coordination of such operations while the RCS is partially filled; Restrictions. regarding testing, operations, and maintenance that could perturb the nuclear ~ steam supply system (NSSS); i

. Ability of the RCS to withstand pressurization if the reactor vessel head and steam generator manway are'in place-Requirements pertaining to isolation of containment; l The time required to replace the equipment hatch should replacement be ,

necessary; and '

Requirements pertinent to reestablishing the integrity of the RCS pressure boundary.

MY RESPONSE At least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> must elapse, since power operation, before the reactor coolant system is drained to a partially filled condition (PFC).

i Prior to draining to a PFC, the reactor coolant system must be cooled to 1 less than 150*F, depressurized to atmospheric pressure and adequately vented. .In addition, redundant residual heat removal trains and redundant level monitoring systems must be operable, and a means of adding borated water to the. system must be available. Normally, prior to draining to a PFC, all reactor coolant loops are isolated from the reactor by closure of the loop stop valves.

Plant design and configuration permit normal refueling and nearly all maintenance activities to be performed without draining the reactor coolant system below the top of the loop piping. Maine Yankee has not had to drain to the mid-pipe level in its 14 years of operation. However, if ,

maintenance activities ever required it, a special procedure would be 9136L-SDE

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developed and approved incorporating the lessons presented in Generic I Letter 87-12. The special procedure may require that a reactor coolant loop remain connected to provide an accessible reliable tap for the f J

redundant and diverse level monitoring system, l The Steam Generators are neither required nor capable of being operable when a PFC-is established. When they are relied upon as a heat sink, the  ;

minimum steam generator level is at the top of the tube bundle. 4 Hork on components such as the reactor coolant pumps and steam generators may be performed with the loops isolated from the reactor coolant system <

by means of the loop stop valves.

During the conduct of refueling or maintenance activities when the reactor coolant system is in a PFC, all operations, maintenance and testing activities must be approved by the Senior Licensed Operations Supervisor ,

on shift. This individual is responsible for the status of the reactor  !

coolant system inventory and is aware of the possible effects of changes f in equipment status. In addition, a Refueling Shift Coordinator provides  !

field oversight to such activities. This individual holds, or has held, a l senior operator license or has otherwise received systems training l providing an additional level of direct oversight.

With the reactor vessel partially filled, repressurization would be controlled through maintaining Low Temperature Overpressurintion Protection (LTOP) limitations and ensuring that the RCS is adequately vented via a 2" or larger pressurizer vent and a 1" or larger reactor head vent.

Core alterations or movement of irradiated fuel is prohibited within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of attaining the subcritical condition. If less than 210 hours0.00243 days <br />0.0583 hours <br />3.472222e-4 weeks <br />7.9905e-5 months <br /> have elapsed since subcriticality, Technical Specification 3.13 requires that the containment equipment hatch be closed and held in place by a minimum of four bolts, at least one containment personnel air lock be closed, and I the containment vent and purge system discharge be through the HEPA filters and charcoal absorbers. It would take approximately four hours to replace the equipment hatch if it were removed.

During a PFC, the reactor coolant system pressure boundary is controlled by the Senior Licensed operations supervisor on each shift. Should the boundry be violated and a loss of inventory ensue, he would be responsible for reestablishing the RCS integrity and injecting sufficient water to ensure that adequate core cooling is maintained.

The largest RCS pressure boundary openings, i.e, SG manways and reactor coolant pump casings, are isolated by the loop stop valves which are depowered and chain-locked closed. All other possible RCS openings would l be above the top of the reactor coolant loop pipe. Water loss through such openings could readily be replaced by HPSI pump operation avoiding a loss of core cooling.

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1 NRC REOUEST (2) 1 A detailed description of the instrumentation and alarms provided to the l operators for controlling thermal and hydraulic aspects of the NSSS during operation with the RCS partially filled. You should describe temporary connections, piping, and instrumentation used for this RCS condition and the quality control process to ensure proper functioning of such connections, piping, and instrumentation, including assurance that they do i not ' contribute to loss of RCS inventory or otherwise lead to perturbation 1 of the NSSS while the RCS is partially filled. You should also provide a description of your ability to monitor RCS pressure, temperature, and i level after the RHR function may be lost. j l

HY RESPONSE Reactor vessel level indication is provided by the Primary Inventory Trend i System (PITS) transmitters (PT-3001 and PT-3002). These differential j pressure transmitters, which receive signals from an ICI tube tap at the '

bottom of the reactor vessel, are recalibrates for refueling conditions.

They provide a positive level indication from the top of the core to the top of the reactor head or to atmosphere if the head is removed. This  ;

instrumentation is designed safety class IE and is environmentally j qualified. Refueling level instrumentation (LT-104), which taps off the top of the hot leg, is also available as back up instrumentation.

In addition, clear tygon tubing is connected and may be valved into '

operation periodically to verify the accuracy of the above mentioned l instrumentation. This tubing may be connected to the same tap as LT-104 i or to a loop drain line if mid-pipe level control is necessary. '

The temperature of the RCS is measured by incore thermocouple and by the ,

RHR system temperature detectors. '

During a PFC, the RCS is vented to atmosphere, therefore, no special l pressure instrumentation is necessary. f Additional instrumentation that is available to the control room operators when performing " partially filled" evolutions is identified in Attachment 2.

Instrumentation utilized to monitor the temperature and level of the RCS during partially filled conditions would not be adversely affected assuming a loss of RHR. The values of these parameters would continue to be available to the control room operator.

All refueling level instrumentation is recalibrates to meet refueling conditions.

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MaineYankee NRC REOUEST (3)

Identification of all pumps that can be used to control NSSS inventory.

Include: (a) pumps you require be operable or capable of operation (include information about such pumps that may be temporarily removed from service for testing or maintenance); (b) other pumps not included in item a (above); and (c) an evaluation of items a and b (above) with respect to applicable TS requirements. .

1 MY RESPONSE  !

Attachment 3 summarizes the pumps that can be used to control NSSS inventory, including applicable operating requirements as required by l Technical Specifications. j i

NRC REOUEST (4)

A description of the containment closure condition you require for the conduct of operations while the RCS is partially filled. Examples of areas of consideration are the equipment hatch, personnel hatches, containment purge valves, SG secondary-side condition upstream of the isolation valves (including the valves), piping penetrations, and electrical penetrations.

MY RESPONSE See response to Request 1.

NHC REOUEST (5) i Reference to and a summary description of procedures in the control room l of your plant which describe operation while the RCS is partially filled.

Your response should include the analytic basis you used for procedures development. He are particularly interested in your treatment of  ;

draindown to the condition where the RCS is partially filled, treatment of minor variations from expected behavior such as caused by air entrainment and de-entrainment, treatment of boiling in the core with and without RCS j pressure boundary integrity, calculations of approximate time from loss of l RHR to core damage, level differences in the RCS and the effect upon instrumentation indications, treatment of air in the RCS/RHR system, including the impact of air upon NSSS and instrumentation response, and treatment of vortexing at the connection of the RHR suction line(s) to the RCS, l 9136L-SDE l

MaineYankee Explain how your analytic basis supports the following'as pertaining to your facility: (a) procedural guidance pertinent to timing of operations, required instrumentation, cautions, and critical parameters; (b) operations control and communications requirements regarding operations that may perturb the NSSS, including restrictions upon testing, maintenance, and coordination of operations that could upset the condition e of the NSSS; and (c) response to loss of RHR, including regaining control i of RCS heat removal, operations involving the NSSS-if RHR cannot be '

restored, control of effluent from the containment if containment was not in an isolated condition at the time of loss of RHR, and operations to provide containment isolation if containment was not isolated at the time of loss of RHR (guidance pertinent to timing of operations, cautions and warnings, critical parameters, and notifications is to be clearly described).

MY RESPONSE The following is a summary of the operating procedures (0P) utilized during a partially filled RCS.

OP-1-17-6 Lowerina of the Reactor Vessel Level (-12") for Head Removal-.

This procedure is utilized to lower RCS level from a pressurizer level of 807. to 12" below the reactor vessel flange in anticipation of reactor head stud detensioning and preparation for head removal.

The NSSS is aligned to permit a partially filled condition to be attained. During the draindown process, a reactor head vent and a special pressurizer vent are opened.

At least two diverse instruments are used to monitor the RCS level. If the deviation between level instruments exceeds the prescribed limit, then the draining of the RCS ceases until the discrepancy is resolved. Level is lowered in segments with hold points to ensure agreement between instrument readings and to ensure that the pressure in the pressurizer and j reactor head is equalized.  ;

I OP-1-17-7 "Lowerina RX Vessel level from 19' to 15' for maintenance l work." This procedure was developed as a result of a review of SOER 85-4 l and NSAC-52. It was used successfully during the 1987 refueling outage to lower level to the top of the hot leg to make repairs on components not ];

isolated by the loop stop valves. As indicated elsewhere in this  !

response, if lowering of reactor vessel level below 15' is ever required,  !

administrative controls taking into account Generic Letter 87-12 will be l developed. l j

In the worst case condition with maximum core burnup and within 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> l of shutdown if the RCS level is lowered below the top of the hot leg and '

if RHR flow is lost, saturation conditions in the core would be reached in about thirty minutes.

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MaineYankee To minimize the effects of air intrusion on level instrumentation, the PITS transmitters are recalibrates during refuelings. Procedures require that the pressure in the RCS and pressurizer are equalized and hold points are established for verifying level. The use of a tap from the bottom of

'the reactor vessel to qualified differential pressure transmitters also ,

minimizes this. problem.

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The procedure requires the RHR pumps to be checked locally after each incremental decrease in level. Also, control room observance of instrumentation for pump operation such as pump flow and amps provides an early indication of vortexing should it occur. Vortexing (and pump cavitation) may be controlled by reducing RHR flow to a predetermined minimum value or increasing RCS level.

OP-1-17-6 and OP l-17-7 contain provisions to help preclude a '"3s of core cooling while 1owering RCS level. Some examples include:

Specific instrumentation is required with re-calibration prior to commencing the evolution.

A nomograph identifies containment elevations and corresponding instrumentation, with their ranges, to be utilized to monitor level from the top of the pressurizer to the top of the core.

Reference to plant drawing 'RCS/RHR Refueling Component Elevation Print #1,' (Attachment 1) is suggested before any work commences on a specific component.

Emergency make up capabilities are identified with transitions to the Abnormal Operating Procedures for long term core cooling.

In the event of a loss of a RHR pump due to possible cavitation the operator is directed not to start the standby pump until the cavitation cause is ccrrected.

Abnormal Ooeration Procedure (AOP) 2-34 " Loss of RCS Coolina or Level l durina RHR Ooerations". This procedure is under revision to provide guidance to the operator in the event that RHR is lost during a partially filled condition. Decay heat curves for the different burnup rates which identify time to saturation in the event RHR cooling is lost, and the requirement to replace the equipment hatch are planned to be addressed in the procedure.

The procedure may also identify the use of SI tanks to assist in reflooding if RCS pressure increases above the RHST head of water, thus preventing gravity make-up from the RHST and other pumping systems are inoperable. Entry into A0P-2-34 requires notification pursuant to the  ;

Maine Yankee Emergency Plan I

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'EBC'REOUEST (11 ,

A brief description of training provided to operators and other affected: )

personnel that is specific to the issue of operation while the RCS is l partially filled. He are particularly interested in such areat as maintenance personnel training regarding avoidance of: perturbing the NSSS and response to loss of decay heat removal while the RCS is partially

. filled. )

-MY RESPONSE During a licensed operator's initial RO training in RHR operations, a '

discussion of RHR degradation and, more specifically, SOER 85-4 is conducted in the classroom. This is enhanced later in the initial certification program during A0P/EOP training through coverage of A0P 2-34, Loss of RHR. This lesson discusses the actions to take should RHR coolihg be lost due to air binding which may occur during a situation with the RCS partially filled. The accompanying simulator lesson requires the students to implement A0P 2-34 upon a loss of RHR suction. Although the initiating event is not due to low level, the indications and actions to be taken are similar. .

Specific training in operations with lowered reactor vessel water level is conducted for all licensed operators,' including appropriate operator actions should'a loss of RHR flow occur.

There is no specific training provided to maintenance personnel regarding potential adverse effects of loss of decay heat removal when the RCS is partially filled. However,' personnel are trained on appropriate administrative and procedural controls which requires that any repair activities affecting plant operation to be approved by plant operations supervisory personnel. Thus, administrative controls ensure that licensed control room personnel are fully cognizant of planned plant repair activities prior to any evolution involving a partially filled RCS.

NRC RE0 VEST (11 Identification of additional resources provided to the operators while the RCS is partially filled, such as assignment of additional personnel with specialized knowledge involving the phenomena and instrumentation.

ELRESE0ESE When entering the partially filled condition, procedures require trained I&C personnel to be on site during these evolutions to resolve any discrepancies in the level monitoring instrumentation.

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, 1 hl8lDOIIWhee NRC REOUEST (81 Comparison of the requirements implemented while the RCS is partially filled and requirements used in other Mode 5 operations. Some l requirements and procedures followed while the RCS is partially filled may I not appear in the other modes. An example of such differences is J operation with a reduced RHR flow rate to minimize the likelihood of j vortexing and air ingestion. '

i MY RESPON_SE ]

LTOP analysis requires a minimum RHR flow of 4,000 gpm if the RCS is not adequately vented. To minimize the potential for vortexing in the partially filled condition, existing procedures permit lowering reactor vessel level only to the* top of the hotleg. Air entrainment into the RHR system is minimized by avoiding mid-loop operation and ensuring that the RCS is adequately vented. The RHR flow may be reduced to a minimum of 2000 gpm when the system is adequately vented as directed by OP 1-17-7 to also minimize the potential for vortexing.

The recalibration of the PITS transmitters (PT-3001 and PT-3002) and the operational calibration and valving in of either the narrow range and wide i range refueling level indicators or a tygon tubing visual indication are additional requirements prior to achieving a partially filled condition.

IL% _Rf0MEST (9)

As a result of your consideration of these issues, you may have made changes to your current program related to these issues. If such changes have strengthened your ability to operate safely during a partially filled situation, describe those changes and tell when they were made or are 1 scheduled to be made.

! MY RESPONSE A review of NSAC-52, SOER 85-4, NUREG 1269, NRC IN-86-101 provided many of i the industry experiences that have been factored into Maine Yankee's l operating and abnormal operating procedures. l+

The following additional enhancements to procedures and administrative controls will be evaluated prior to the next refueling outage to provide a greater degree of control for equipment operability and work activities when in the partially filled condition; Additional requirements to maintain the containment barrier operational when in a partially filled condition.

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iMaineYankee j Removal.ofhidh'pressureautoclosuresignalfromRH-M-1,andRH-M-2 -

while on RHR when the RCS is ' adequately vented. The ability of the- l

. operator to close the valves'should a loss of inventory occur will'be - )

retained. )

' Revision of Procedure 5-300-1, Refueling, to identify.any additional controls and limitations that may be necessary when in_a partially. j c.

' filled condition.

Maine Yankee is. working through the Combustion Engineering Ow'ners Group-(CEOG) to evaluate the potential unanalyzed condition described in Reference,(b). Any additional changes that may result from this effort to i ensure the continued safe operation of Maine Yankee under partia11y' filled J conditions will be described in separate correspondence. 1 l

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ATTACHMENT 1 RCS/RHR COMP _QRENT ELEV. REFUELING PRINT #1 l

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1 OVERSIZE .

DOCUMENT PAGE PULLED 1 SEE APERTURE CARDS I NUMBER OF OVERSIZE PAGES: FILMED ON APERTURE CARDS APERTURE CARD /HARD COPY AVAILABLE FROM DOCUMENT CONTROL BRANCH

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• MaineYankee ATTACHMENT 3 PUMPS ThaT CAN BE USED TO CONTROL NSSS INVENTORY Number OPERATING Required Number Technical Pumo CONDITION Ooerable Available Specification High Pressure Safety Injection 3, 2,1 1 3 3.4.D.3.c (P-14A, -148, -14S) o Low Pressure Safety Injection 3, 2,1 1 2 3.3.A1, 3.130.8 (P-12A, P-128)

Aux. Charging 3,2,1 0 1 (P-7)

Fuel Pool Cooling 3,2,1 0 2 (P-17A, -178)

Boric Acid Transfer 3, 2,1 1 3 3.58 (P-6A, -68, -6C)

Primary Water Makeup 3,2,1 0 2 (P-24A, 248)

Serves as RHR Pumps i

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