Information Notice No. 92-54: Level Instrumentation Inaccuracies Caused by Rapid Depressurization

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                                 July 24, 1992

                                   CAUSED BY RAPID DEPRESSURIZATION


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice
to alert addressees to potential inaccuracies in water level indication during
and after rapid depressurization events.  This problem may affect the
indication of pressurizer level for pressurized water reactors (PWR) and
reactor vessel level for boiling water reactors (BWR).  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.

Description of Circumstances

On April 17, 1991, Northeast Utilities (NU) filed a licensee event report (LER)
for Millstone Unit 3, documenting pressurizer level instrument inaccuracies. 
According to the LER the inaccuracies result from non-condensible gases
collecting in the condensing pots of the instrument reference legs.  The LER
stated that pressurizer level would be used to make decisions concerning
operator actions directed by the Emergency Operating Procedures (EOP).  

During the previous operating cycle, NU monitored the accuracy of the
pressurizer level instrumentation and observed a worst case error of 3.6% of
full scale and also confirmed that non-condensible gases had accumulated inside
the condensing pots.  The root causes for the accumulation of non-condensible
gases in the condensing pots were 1) the instrument lines sloped upward from
the pressurizer to the condensing pots and, 2) a restricting orifice in each
instrument line prevented the free flow of steam and non-condensible gases
between the pressurizer and the condensing pots.  NU corrected the problem by
removing the condensing pot and changing the instrument line slope.

Westinghouse and Combustion Engineering performed further engineering
evaluations and concluded that during a rapid depressurization of the Reactor 


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Coolant System (RCS), during certain design basis accidents, the release of
non-condensible gases could result in a level indication error of about 
+40 percent of full scale.  The staff has evaluated the effects of this error
and determined that the only unacceptable actions that could be taken by
operators as a result of this error are to prematurely terminate safety
injection (SI) or to fail to re-initiate SI if required.  The staff further
determined that the pressurizer level instrumentation is not used as the
primary parameter evaluated by operators for safety injection termination and
that PWR emergency operating procedures direct operators to consult  other
instrumentation and parameters (reactor vessel level monitoring system, RCS
subcooling and a stable or increasing RCS pressure) prior to terminating SI.  

For BWRs, reactor vessel level indication system (RVLIS) errors had also been
identified in the past and the staff issued  Generic Letter (GL) No. 84-23,
"Reactor Vessel Water Level Instrumentation in BWRs" to address the concern. 
This GL was based on the BWR Owners Group (BWROG) report (SLI-8218 issued in
November 1982), "Inadequate Core Cooling Detection in Boiling Water Reactors." 
However, these documents do not specifically address the non-condensible gas
evolution concern associated with rapid depressurization.  The staff has
requested the BWROG to address this issue and GE is preparing a report on
behalf of the BWROG.

On July 15, 1992, Northeast Utilities (NU) made a notification to the NRC under
Section 50.72 of Title 10 of the Code of Federal Regulations (10 CFR 50.72)
regarding inaccuracies in reactor vessel level indication at Millstone Unit 1. 
This notification indicated that the level instrumentation may not provide
accurate indication following a rapid depressurization event as a consequence
of the expulsion of water from the reference leg due to the release of non-
condensible gases in the instrument reference leg.  In a conference call with
NU on July 21, 1992, the licensee stated that the Yarway level measurement
instrumentation which provides the automatic actuation of safety systems at
Millstone Unit 1 would not be affected by this phenomenon. However, the GE/MAC
level instrumentation, which is used for indication, feedwater control, and
containment spray pump interlocks, would be affected.   Following a rapid
depressurization event, the operator might receive inaccurate information from
the GE/MAC instrumentation leading the operator to perform inappropriate manual
actions.  The licensee has estimated, based on a conservative analysis, that
the upper bound of the error in the GE/MAC instrumentation could be as much as
15 to 20 feet.  Millstone Unit 1 is currently in cold shutdown for service
water repairs, and NU is reviewing the error analysis and a possible
modification to the condensing pot arrangement in order to reduce inaccuracies
in the level indication to an acceptable level before restart.  

In a conference call on July 22, 1992, the staff informed the BWROG of the
results of the Northeast Utilities' analyses and the licensee's planned
actions.  The BWROG indicated its position that the error would not exceed 
4 inches if the reference leg configuration is installed in accordance with
vendor recommendations. .

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The NRC has activated the BWR Regulatory Response Group and scheduled a meeting
to discuss this issue at NRC headquarters in Rockville, Md., on July 29, 1992.


Inaccuracies in level instrumentation in PWRs and BWRs could affect the
performance of safety functions.  GL 84-23, BWROG report SLI-8218, and vendor
recommendations are intended to provide guidance to preclude the operators from
taking improper actions during normal plant operation.  The inaccuracies caused
by rapid depressurization events in PWRs have limited safety significance
because instrumentation other than that for pressurizer level is used by the
operators to determine appropriate manual actions.  For BWRs, however, large
errors in level indication may have greater safety significance.  An evaluation
by the staff is continuing and when the evaluation is completed the staff will
determine if additional regulatory actions will be necessary.

This information notice requires no specific action or written response.  If
you have any questions regarding 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.

                               Charles E. Rossi, Director
                               Division of Operational Events Assessment
                               Office of Nuclear Reactor Regulation

Technical contacts:  Hukam C. Garg, NRR
                     (301) 504-2929

                     Tim Collins, NRR
                     (301) 504-2897

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