Information Notice 1993-27, Level Instrumentation Inaccuracies Observed During Normal Plant Depressurization

Level Instrumentation Inaccuracies Observed During Normal Plant Depressurization

ML031080007
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	04/08/1993
From:	Grimes B Office of Nuclear Reactor Regulation
To:
References
IN-93-027, NUDOCS 9304020319
Download: ML031080007 (8)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 April 8, 1993 NRC INFORMATION NOTICE 93-27: LEVEL INSTRUMENTATION INACCURACIES OBSERVED

DURING NORMAL PLANT DEPRESSURIZATION

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to inaccuracies in reactor vessel level indication

that occurred during a normal depressurization of the reactor coolant system

at the Washington Nuclear Plant Unit 2 (WNP-2) and to the fact that errors in

level indication may result in a failure to automatically isolate the residual

heat removal (RHR) system under certain conditions. It is expected that

recipients will review the information for applicability to their facilities

and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements;

therefore, no specific action or written response is required.

Background

As discussed in NRC Information Notice 92-54, "Level Instrumentation

Inaccuracies Caused by Rapid Depressurization," and Generic Letter 92-04,

"Resolution of the Issues Related to Reactor Vessel Water Level

Instrumentation in BWRs Pursuant to 10 CFR 50.54(f)," noncondensible gas may

become dissolved in the reference leg of water level instrumentation and lead

to false indications of high level after a rapid depressurization event.

Reactor vessel level indication signals are important because these signals

are used for actuating automatic safety systems and for guidance to operators

during and after an event. While Information Notice 92-54 dealt with

potential consequences of rapid system depressurization, this information

notice discusses level indication errors that may occur during normal plant

cooldown and depressurization.

Description of Circumstances

On January 21, i993, during a plant cooldown following a reactor scram at

WNP-2, "notching" of the level indication was observed on at least two of four

channels of the reactor vessel narrow range level instrumentation. "Notching

is a momentary increase in indicated water level. This increase occurs when a

gas bubble moves through a vertical portion of the reference leg and causes a

temporary decrease in the static head in the reference leg. The notching at

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IN 93-27 April 8, 1993 WNP-2 was first observed on channel NC" at a pressure of approximately

827 kPa (120 psig]. Channel IBS experienced notching starting at

approximately 350 kPa [50 psig]. At these pressures, the level error was on

the order of 10 to 18 centimeters (4 to 7 inches] and persisted for

approximately one minute.

Beginning at a pressure of approximately 240 kPa [35 psig], the level

indication from channel IC' became erratic and, as the plant continued to

depressurize, an 81-centimeter (32-inch] level indication error occurred.

This depressurization was coincident with the initiation of the shutdown

cooling system. The 81-centimeter [32-inch] level error was sustained

gradually recovered over a period of two hours. The licensee postulatedand was

this large error in level indication was caused by gas released in the that

reference leg displacing approximately 40 percent of the water volume. The

licensee also postulated that the slow recovery of correct level indication

was a result of the time needed for steam to condense in the condensate

chamber and refill the reference leg. The licensee inspected the IC"

reference leg and discovered leakage through reference leg fittings. This

leakage may have been a contributing factor for an increased accumulation

dissolved noncondensible gas in that reference leg. of

The licensee determined that the type of errors observed in level indication

during this event could result in a failure to automatically isolate a leak

the RHR system during shutdown cooling. The design basis for WNP-2 includes in

postulated leak in the RHR system piping outside containment while the plant a

Is in the shutdown cooling mode. For this event, the shutdown cooling suctlon

valves are assumed to automatically isolate on a low reactor vessel water

level signal to mitigate the consequences of the event. For the January

1993 plant cooldown, the licensee concluded that, with the observed errors21, level indication, the shutdown cooling suction valves may not have in

automatically isolated the RHR system on low reactor vessel water level

designed. The licensee has implemented compensatory measures for future as;

plant

cooldowns to ensure that a leak that occurs in the RHR system during shutdown

cooling operation would be isolated promptly. These measures include

touring

the associated RHR pump room hourly during shutdown cooling and backfilling

the water level instrument reference legs after entry into mode 3 (hot

shutdown). The licensee is also evaluating measures to minimize leakage

the IC' reference leg. from

Discussion

The event described above is different than events previously reported because

of the large magnitude and sustained duration (as opposed to momentary

notching) of the level error that occurred during normal plant cooldown.

large sustained level error is of concern because of the potential for A

complicating long-term operator actions. In addition, the scenario of

postulated leak in the RHR system evaluated by WNP-2 suggests that some a

safety

systems may not automatically actuate should an event occur while the reactor

is in a reduced pressure condition. Generic Letter 92-04 requested, in

that licensees determine the impact of potential level indication errors part, on

IN 93-27 April 8, 1993 automatic safety system response during licensing basis transients and

accidents. The information in this notice indicates that sustained level

instrument inaccuracies can occur during a normal reactor depressurization.

Therefore, events occurring during low pressure conditions may also be

complicated by level indication errors.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment:

List of Recently Issued NRC Information Notices

Attachnent

IN 93-27 April 8, 1993 Pge I of I

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LIST OF RECENTLY ISSUED ME0

HRC INFORMATION NOTICES

inroruauon a- Inforuti0n u te OT

Notice No. Subject Issuance Issued to

93-26 Grease Solidification 04/07/93 All holders of OLs or CPs

Causes Molded Case for nuclear power reactors.

Circuit Breaker

Failure to Close

) 93-25 Electrical fenetration

Assembly Degradation

04/01/93 All holders of OLs or Cps

for nuclear power reactors.

93-24 Distribution of 03/31/93 All holders of operator and

Revision 7 of NUREG-1021, senior operator licenses at

• Operator Licensing nuclear power reactors.

Examiner Standards'

93-23 Veschler Instruments 03/31/93 All holders of OLs or CPs

Model 252 Switchboard for nuclear power reactors.

Meters

93-22 Tripping of Ilockner- 03/26/93 All holders of Ots or CPs

toeller Molded-Case for nuclear power reactors.

Circuit Breakers due to

Support Level Failure

93-21 Sumary of NRCStaff 03/25/93 All holders of Ots or CPs

Observations Compiled for light water nuclear

'during Engineering Audits power reactors.

or Inspections of Licen- see Erosion/Corrosion

Programs

2

93-20 Thermal Fatigue Cracking 03/24/93 All holdefs of Os or CPs ° _

of Feedwater Piping to for PFRs supplied by

) Stem Generators Westinghouse or Combustion

Engineering.

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93-19 Slab Hopper Bulging 03/17/92 All nuclear fuel cycle

licensees. II

93-18 Portable Moisture-Density 03/10/93 All U.S. Nuclear Regulatory

Gauge User Responsibilities Couuission licensees that

during Field Operations possess moisture-density a.

gauges.

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OL - Operating License z aL.

CP

• Construction Permit

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IN 93-27 April 8, 1993 automatic safety system response during licensing basis transients and

accidents. The information in this notice indicates that sustained level

instrument inaccuracies can occur during a normal reactor depressurization.

Therefore, events occurring during low pressure conditions may also be

complicated by level indication errors.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Original signed by

Brian K.Grime:

Brian K. Grimes, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment:

List of Recently Issued NRC Information Notices

• See previous concurrence

• OGCB:DORS:NRR *C/OGCB:DORS:NRR *TECH:ED

JLBirmingham GHMarcus RSanders

04/01/93 04/01/93 03/18/93

• SRXB:DSSA:NRR *C/SRXB:DSSA:NRR *D/DSSA:NRR

ACubbage RJones AThadani

03/19/93 03/26/93 03/26/93 Document name: 93-27.IN

IN 93-XX

March XX, 1993 errors on automatic safety system response during licensing basis transients

and accidents. The information in this notice indicates that sustained level

instrument inaccuracies can occur during a normal reactor depressurization.

Therefore, events occurring during low pressure conditions may also be

complicated by level indication errors.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

(one of) the technical contact(s) listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment: List of Recently Issued NRC Information Notices

• See previous cconcurrence

OGCB:DORS:NRR C/OGCB:DORS:NRR D/DORS:NRR *TECH:ED

JLBirmingham GHMarcusas BKGrimesrMk RSanders

03"' gY.Z-

3 °f 1 /93C tW 03/ /931' 03/18/93

• SRXB:DSSA:NRR *C/SRXB:DSSA:NRR *D/DSSA:NRR

ACubbage RJones AThadani

03/19/93 03/26/93 03/26/93 Document name: RVLEVEL.IN

\ I

This information notice requires no specific action or written response. If

you have any questions regarding the information in this notice, please

contact the technical contact listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment: List of Recently Issued NRC Information Notices

Document name: RVLEVEL.IN

• SEE PREVIOUS CONCURRENCE

OGCB:DORS:NRR C/OGCB:DORS:NRR D/DORS:NRR *TECHED:ADM

JLBirmingham GHMarcus BKGrimes RSanders

03//903/ 93 03 /93 03/ /93

• SRXB:DSSA:NRR B:DSSA:NR& D/DSSA:NR

ACubbage RJ nes Thadani

03/ /93 0 t;//93 03 X/931

IN 93-XX

March XX, 1993 This information notice requires no specific action or written response. If

you have any questions regarding the information in this notice, please

contact the technical contact listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: Amy Cubbage, NRR

(301) 504-2875 Attachment: List of Recently Issued NRC Information Notices

Document name: INFONOT2.RVL

OGCB:DORS:NRR C/OGCB: DORS:NRR 0/DORS:NRR TECHED LADM

JLBirmingham GHMarcus BKGrimes JMain

03/ /93 03/ /93 03/ /93 03/8 /93 SRXB:DSSA~:NlRR C/SRXB:DSSA:NRR D/DSSA:NRR

ACubbagqAtf-~ RJones AThadani

03/lcj/93 03/ /93 03/ /93