United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-25: Dynamic Range Uncertainties in the Reactor Vessel Level Instrumentation

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                          WASHINGTON, D.C. 20555-0001

                                  May 9, 1997


NRC INFORMATION NOTICE 97-25:  DYNAMIC RANGE UNCERTAINTIES IN THE REACTOR      
                               VESSEL LEVEL INSTRUMENTATION 

Addressees

All holders of operating licenses or construction permits for Westinghouse
pressurized-water reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a recent incident in which a licensee failed to
provide adequate periodic corrections for analytical uncertainties in the
reactor vessel level indicating system (RVLIS).  This failure could lead plant
operators to take nonconservative actions during an emergency.  It is expected
that recipients will review this information for applicability to their
facilities and consider actions, as appropriate.  However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.

Description of Circumstances

On March 3, 1997, the licensee for Diablo Canyon Units 1 and 2 discovered that
since initial installation of the RVLIS, dynamic range indications had not
been renormalized as described in the Westinghouse operations manuals. 
Because dynamic range uncertainties caused by changes in the reactor coolant
system (RCS) hydraulics were not verified, normalized, or changed periodically
per vendor recommendations, it was initially believed that the RVLIS dynamic
flow range indication readings were 6-8 percent higher than actual values. 
Later, an engineering evaluation performed by the licensee showed that RVLIS
readings were about 10 percent higher than actual values.  These higher-than-
actual readings could have caused plant operators to take non-conservative
actions during a loss of RCS inventory event.

Discussion

The Westinghouse RVLIS is a differential pressure (d/p) measuring system for
determining the water level in the reactor vessel or the relative void content
of the RCS, or both.  In addition to directly indicating of vessel water
level, it provides useful information relating to leaks and small breaks in
the RCS.  This system employs separate sets of three d/p cells, and the
display consists of level indications in three ranges.  The narrow range (also
called full range), displays collapsed water levels from 0 to 100 percent
between the empty and the full vessel with none of the reactor coolant pumps
(RCPs) running.  The upper range displays vessel levels above hot-leg
connections, again with none of the RCPs running.  During 

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normal at-power plant operation with the RCPs running, the displays for these
two ranges are off scale.  The wide range or dynamic range is scaled from 0
percent with the vessel empty and with no RCPs running, to 100 percent with
the vessel full and all RCPs running.  

After initial installation, instruments for the dynamic range are calibrated
using analytical uncertainties associated with the calculated values of the
flow differential pressure head loss.  This calculation is based on head
characteristics of the RCPs and various flow resistances of the RCS.  The
vendor recommended that all calculated values be verified, confirmed, and
adjusted during subsequent plant startups to correct uncertainties involving
system hydraulic changes for each fuel cycle, such as steam generator tube
plugging and change in fuel assembly flow resistance.  This process, known as
"normalizing," returns the outputs to an indication of 100 percent at zero
power with all RCPs running.  In the event of an accident during the cycle
leading to loss of coolant with continued RCPs operation, the coolant would be
circulated as a void mixture, and the RVLIS dynamic range would indicate the
increase in voids as a decrease in dynamic range output.  By renormalizing the
dynamic range to  100 percent at the beginning of each cycle, the dynamic
range output indicates the approximate void content relative to a zero void
condition.  

During an internal review, the Diablo Canyon licensee discovered that, since
initial startup, the calculated uncertainty values associated with the RCS
flow characteristics for the RVLIS dynamic head transmitter had never been
periodically verified as recommended in the Westinghouse (vendor) manual.  The
licensee contacted Westinghouse to discuss the problem, and implemented
corrections for RVLIS indications by recalibrating the instruments through an
online dynamic range normalization.  During this process, the licensee
discovered that the RVLIS readings erred by about 10 percent in the
nonconservative direction.  

This information notice requires no specific action or written response.  If
you have any questions about 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:  S.V. Athavale, NRR          Tai L. Huang, NRR
                     (301) 415-2974              (301) 415-2867
                     E-mail:  sva1@nrc.gov       E-mail:  tlh1@nrc.gov

                     Thomas Koshy, NRR
                     (301) 415-1176
                     E-mail:  txk@nrc.gov
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