Information Notice 1997-33, Unanticipated Effect of Ventilation System on Tank Level Indications and Engineering Safety Features Actuation System Setpoint

Unanticipated Effect of Ventilation System on Tank Level Indications and Engineering Safety Features Actuation System Setpoint
	ML031050580
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:	06/11/1997
From:	Slosson M; Office of Nuclear Reactor Regulation
To:	;
References
	IN-97-033, NUDOCS 9706090325
	Download: ML031050580 (8)
	v • d • e

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001 June 11, 1997 NRC INFORMATION NOTICE 97-33: UNANTICIPATED EFFECT OF VENTILATION

SYSTEM ON TANK LEVEL INDICATIONS AND

ENGINEERING SAFETY FEATURES ACTUATION

SYSTEM SETPOINT

Addresses

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 the potential to affect instrumentation output for certain transmitters by varying

ambient pressure with the operation of plant ventilation equipment. 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

Loss-of-coolant accident (LOCA) mitigation design at most pressurized-water reactors

involves the injection of borated makeup water from a safety-related tank into the reactor

coolant system from emergency core cooling system (ECCS) pumps. When the tank level

falls to a predetermined setpoint, a signal automatically shifts the suction source for the

ECCS pumps to the containment safety injection sump for long-term recirculation. This

swapover setpoint prevents the loss of net positive suction head (NPSH). A delay in the

swapover to the containment sump could result in the common-mode failure of the ECCS

pumps from cavitation or air binding, which would in turn result in the inability to mitigate the

LOCA.

Description of Circumstances

On February 23, 1997, the controlled ventilation area system (CVAS) at Waterford

Generating Station, Unit 3, was undergoing routine testing. The CVAS is a standby system

designed to automatically start following a safety injection signal to provide high-efficiency

particulate filtration and iodine absorption from areas within the reactor auxiliary building

(RAB) subject to potential leakage of post-accident reactor coolant (i.e., high-pressure safety

'injection and low-pressure safety injection pump rooms, shutdown cooling heat exchanger

rooms, etc.). The safety injection signal also simultaneously stops the RAB normal

ventilation system fans. This action stops the flow of all normal ventilation air to the RAB to

permit the CVAS to effectively evacuate its ventilation spaces to a negative pressure of at

least 0.25 inch of water gauge.

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IN 97-33 June 11, 1997 During the CVAS test, control room operators observed an indicated change in refueling

water storage pool (RWSP) level (source of injection water for ECCS) when a CVAS fan was

started. The RWSP level trend recorder indicated a level increase of approximately

2 percent. The shift supervisor contacted the instrumentation and control (I&C) technician on

duty to help investigate why the indicated RWSP level changed when a CVAS fan was

started. The l&C technician determined that the four RWSP level transmitters (Rosemount

Model 11 52DP5A22PB) were located in valve gallery rooms that were part of the CVAS

ventilation spaces. The technician also noted that the reference leg for these transmitters

was vented to the valve gallery room, rather than being routed back to the RWSP, and that

the RWSP was vented to the RAB normal ventilation inlet plenum (which was not subject to

CVAS effects). The operators and the l&C technician determined that the observed level

change had been caused by the slight vacuum produced in the valve gallery rooms by the

CVAS operation; in that the vacuum on the reference leg affected the overall differential

pressure output of the level transmitter. When the CVAS fan was secured, the shift

personnel observed the RWSP indicated level return to is previous position. The control

room operators recognized that the RWSP level instruments provide the input to the

engineering safeguards actuation system to generate a recirculation actuation signal (RAS) at

10-percent RWSP level.

The licensee subsequently determined that the effect of CVAS operation on the RAS setpoint

had not been considered when the setpoint was established. On March 8, the licensee

conducted a series of tests to determine the extent of the interaction between CVAS and

RWSP level instrument output. The tests consisted of running various combinations of RAB

normal and CVAS ventilation trains and measuring the effect on the output of the four RWSP

level transmitters. The tests revealed that, with one train of CVAS running, RAS would occur

within the Technical Specifications allowable value of 9.08 percent. With both trains of CVAS

running, the actual level at the RAS would be below the Technical Specifications

requirements.

The licensee subsequently revised the instrument loop calibration data sheets for the RWSP

instruments to account for/bound the CVAS ventilation effects, and calibrated all four

channels using the new data sheets. The instruments were recalibrated to account for the

worst-case scenario in terms of effect on the RAS; this scenario was determined to be both

trains of CVAS running coincident with a failed makeup damper (would increase vacuum

where instruments were vented). The calibration activity was an interim fix until the level

transmitter reference legs could be rerouted to the RWSP during an outage.

The licensee performed additional tests which revealed that other safety-related system level

indications were affected by ventilation system operation because of the venting arrangement

of the level transmitters.

Discussion

Criterion 13, "Instrumentation and Control," of Appendix A, "General Design Criteria for

Nuclear Power Plants," to 10 CFR Part 50, requires that instrumentation be provided to

monitor variables and systems and that controls be provided to maintain these variables and

S

IN 97-33 June 11, 1997 systems within prescribed operating ranges. Criterion 20, "Protection System Functions,"

requires that the protection system be designed to initiate operation of appropriate systems

to ensure that specified acceptable fuel design limits are not exceeded. Regulatory

Guide 1.105, "Instrument Setpoints," Revision 1, states that the accuracy of all setpoints

should be equal to or better than the accuracy assumed in the safety analysis, which

considers the ambient temperature changes, vibration, and other environmental conditions.

Transmitters that monitor tank level typically route the reference leg tubing to the top of the

tank in order to cancel the effect of tank overpressure on level indication. However, for tanks

that are vented, the level instrument reference legs are often vented to the atmosphere. It is

important that the reference leg accurately reflect the pressure that exists in the space above

the fluid level in the tank so that false levels are not indicated. It does not appear that

Waterford 3 appropriately accounted for the potential effects of different ventilation system

lineups on the RWSP level instruments. Such failure to appropriately account for all potential

environmental effects on protection system setpoints could result in an inability to mitigate

certain accidents.

Because additional testing at Waterford revealed that safety-related systems other than

RWSP level indications were affected by ventilation system operation, other licensees are

reminded that the level instrumentation for many systems at their plant may be subject to

similar problems.

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 contacts

listed below or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

Marylee M. Slosson, Acting Directo

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Phil Harrell, Region IV

(817) 860-8250

E-mail: phhenrc.gov

Lee Keller, Region IV

(504) 783-6253 E-mail: lakenrc.gov

Charles D. Petrone, NRR

301-415-1027 E-mail: cdp@nrc.gov

Attachment: List of Recently Issued NRC Information Notices

X#a 2 A' by( AC

A

Attachment

IN 97-33 June 11, 1997 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

95-36, Potential Problems 06/10/97 All holders of OLs or CPs

Supp. 1 with Post-Fire Emer- for nuclear power reactors

gency Lighting

97-32 Defective Worm Shaft 06/10/97 All holders of OLs or CPs

Clutch Gears in for nuclear power reactors

Limitorque Motor- Operated Valve

Actuators

97-31 Failures of Reactor 06/03/97 All holders of OLs or CPs

Coolant Pump Thermal for pressurized-water

Barriers and Check reactor plants

Valves in Foreign

Plants

97-30 Control of Licensed 06/03/97 All material and fuel

Material During Reorgan- cycle licensees

izations, Employee- Management Disagreements, and Financial Crises

97-29 Containment Inspection 05/30/97 All holders of OLs or CPs

Rule for nuclear power reactors

OL = Operating License

CP = Construction Permit