United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-16, Supplement 2: Loss of Flow from the Residual Heat Removal Pump During Refueling Cavity Draindown

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

                                August 23, 1993

                                             HEAT REMOVAL PUMP DURING          
                                             REFUELING CAVITY DRAINDOWN


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement to
Information Notice 92-16 to alert addressees to several events that could have
resulted in loss of shutdown cooling.  This supplement describes three events
where the licensees approached or entered reduced inventory operation without
reliable reactor coolant system (RCS) level monitoring instrumentation.
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

Description of Circumstances

On September 11, 1992, Carolina Power and Light Company, the licensee for H.B.
Robinson, was lowering the RCS level to 63.5 cm (25 inches) below the vessel
flange so that a reactor coolant pump seal could be replaced.  The licensee
procedure erroneously directed the use of 34.5 kPa (5.0 psig) nitrogen
overpressure instead of 3.5 kPa (0.5 psig) overpressure while the RCS is being
drained.  During the draindown, repeated inquiries from the NRC resident
inspector about the validity of the RCS level indication led to securing the
draindown and venting the RCS overpressure.  The tygon tube level indication
dropped 304 cm (10 feet) when the RCS pressure was fully vented.  On September
13, 1992, the licensee lowered the RCS level to 91.4 cm (3 feet) below the
vessel flange, the entry point for mid-loop operation, while the level
indicators continued to show an error of 5 cm (2 inches).

On March 19, 1993, the New York Power Authority, licensee for Indian Point
Unit 3, began RCS draindown so that the ultrasonic level monitoring system
could be functionally tested.   In response to Generic Letter (GL) 88-17,
"Loss of Decay Heat Removal," the licensee committed to have two 
independent means of level indication when RCS level was in the reduced


                                                      IN 92-16, Supplement 2
                                                      August 23, 1993
                                                      Page 2 of 3

inventory range.  One system was a standpipe system and the other was the
ultrasonic level monitoring system.  The ultrasonic transducer, which had been
recently replaced, was bench-tested prior to installation; however, a
functional test with RCS level in the mid-loop range was needed to consider
the monitoring system operable.  

In the early part of the draindown, the indicated level differed by as much as
17.5 cm (7 inches) between the permanent standpipe system and a temporary
standpipe that was in service but settled to within the acceptance value, 15
cm (6 inches).  With RCS level reduced to the mid-loop area to functionally
test the ultrasonic level monitoring system, the only operable level
indication was the permanent standpipe system.  While lowering the RCS level
further to a second data point for the level monitoring system functional
test, the indicated level differed by 7.5 cm (3 inches) between the standpipe
and the monitoring system.  The acceptable difference in level indication in
the mid-loop range was 2.5 cm (1 inch).  Therefore, the licensee began
troubleshooting the standpipe system and, in the process, isolated the only
operable means of level indication with RCS level at mid-loop.  The resident
inspectors, who were in the control room during portions of the draindown,
questioned the reliability of the level indicators and notified licensee
management.  As a result, RCS level was raised above the mid-loop range and
further troubleshooting revealed that a loop seal, which had been overlooked,
existed in the original standpipe vent.  Shutdown cooling was not lost during
this event.   

On March 9, 1993, at Wolf Creek Nuclear Generating Station, divergence between
the two types of level sensing instrumentation occurred during RCS draindown. 
The licensee determined that the divergences were due to problems involving
both the tygon tube and permanently installed level transmitters.  Nitrogen
added to the reactor coolant system during the draindown in order to assist
the removal of fission gases, was determined to have formed a loop seal in the
tygon tubing.  The problem with the level transmitters was water droplets
which adhered to the instrument tube walls during draining of the reference
legs and, subsequently, collected in the reference legs when the RCS was


Generic Letter 88-17, "Loss of Decay Heat Removal," recommended specific
actions to ensure reliable RCS level indication because it is very important
in a PWR when approaching a mid-loop condition.  The high potential for the
loss of shutdown cooling in mid-loop operation due to inadequate net positive
suction head in the shutdown cooling pumps necessitates precise RCS level
control.  Level measurement of a closed RCS can be sensitive to pressure
differences between the RCS and the instrumentation vent tubes.  Diversity in
level measurement methods can reveal some of the potential errors.

In the events addressed above, the licensee procedures lacked specific
guidance to ensure sufficient accuracy for the level monitoring system before
entering risk-significant RCS level manipulations.  As discrepancies were
noted between level indications they were not successfully resolved before.

                                                      IN 92-16, Supplement 2
                                                      August 23, 1993
                                                      Page 3 of 3

continuing with additional RCS draindown.  When level readings were
subsequently found to be outside the acceptable range, the suitability of
remaining in a reduced inventory condition during the instrumentation repair
was not considered.  Entering or remaining in a reduced inventory condition
without reliable instrumentation increases the potential for a loss of
shutdown cooling.

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.

                                    /s/'d by CIGrimes/for

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contact:  Thomas Koshy, NRR
                    (301) 504-1176

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