United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-33: Unanticipated Effect of Ventilation System on Tank Level Indications and Engineering Safety Features Actuation System Setpoint

                                 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.

9706090325.                                                            IN 97-33
                                                            June 11, 1997
                                                            Page 2 of 3


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 I&C
technician determined that the four RWSP level transmitters (Rosemount Model
1152DP5A22PB) 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 I&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 its
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 co-
incident 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 .                                                            IN 97-33
                                                            June 11, 1997
                                                            Page 3 of 3


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.


                                            signed by S.H. Weiss for

                                       Marylee M. Slosson, Acting Director
                                       Division of Reactor Program Management
                                       Office of Nuclear Reactor Regulation

Technical contacts:  Phil Harrell, Region IV 
                     (817) 860-8250
                     E-mail:  phh@nrc.gov

                     Lee Keller, Region IV 
                     (504) 783-6253
                     E-mail:  lak@nrc.gov

                     Charles D. Petrone, NRR
                     301-415-1027
                     E-mail:  cdp@nrc.gov
Page Last Reviewed/Updated Tuesday, December 03, 2013