Information Notice No. 93-32: Nonconservative Inputs for Boron Dilution Event Analysis

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

                                April 21, 1993

                               BORON DILUTION EVENT ANALYSIS


All holders of operating licenses or construction permits for pressurized
water reactors (PWRs).


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to inform addressees that nonconservative assumptions have been
identified in the analysis of licensing basis boron dilution events.  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


The licensees for several Westinghouse-designed plants have used a boron
dilution mitigation system (mitigation system) to detect and mitigate a boron
dilution event in Modes 3 (hot standby), 4 (hot shutdown), and 5 (cold
shutdown).  The mitigation system is designed to terminate automatically an
inadvertent boron dilution event occurring with the plant in these operating
modes before the reactor coolant system is diluted sufficiently to result in a
total loss of shutdown margin.  A boron dilution event is detected by this
system by monitoring the signals of the source range monitors to determine if
the neutron flux has increased by a specified multiplication factor over a
prescribed time period.  When a dilution event is detected, the mitigation
system isolates known dilution paths to the reactor coolant system and
realigns the reactor makeup water system to the refueling water storage tank
to initiate a re-boration.

NRC Standard Review Plan Section 15.4.6, "Chemical and Volume Control System
Malfunction That Results in a Decrease in Boron Concentration in the Reactor
Coolant (PWR)," specifies acceptance criteria for an unplanned boron
(moderator) dilution.  These acceptance criteria specify that a dilution event
be terminated before total shutdown margin is lost.  If operator action 
is required during Modes 3, 4, and 5, these criteria specify at least 
15 minutes notice be provided to operators from the time at which an alarm
announces an unplanned dilution to the time at which shutdown margin would be
eliminated (criticality).


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In NRC Generic Letter 85-05, "Inadvertent Boron Dilution Events," the staff
concluded that the consequences of boron dilution events would probably not
jeopardize the health and safety of the public and would not warrant
backfitting requirements.  However, the generic letter urged each licensee to
verify that its plants have adequate protection against boron dilution events. 

Description of Circumstances

In late 1991, while performing a quality assurance surveillance of
Westinghouse supporting calculations for the Comanche Peak Steam Electric
Station, the Texas Utilities Electric Company (the licensee) found that it
could not justify using a generic curve for the relationship between the
inverse count rate ratio and the reactor coolant system boron concentration,
as was used in the Westinghouse boron dilution event analysis.  This curve and
a source range flux multiplication setpoint of 2.0 (flux-doubling) were the
bases for detecting and mitigating an inadvertent boron dilution event.  After
further reviewing this concern, in February 1992 the licensee determined that
the generic curve might not be bounding and that the analysis did not include
instrument uncertainties.  These deficiencies may cause Comanche Peak to be
outside the licensing basis in the boron dilution event analysis which shows
that the boron dilution mitigation system response will prevent a return to
criticality.  The licensee declared the mitigation system inoperable.  While
the licensee is working on the long-term solution to correct the problem, it
put in place the following interim actions: 

1)    Within 4 hours of entry into Modes 3, 4, or 5 from Modes 1, 2, or 6 (and
      once per every 14 days thereafter while in Modes 3, 4, or 5), the
      licensee will verify (unless startup is in progress) that those valves
      that could contribute to the boron dilution event are closed and secured
      in position; or

2)    Within 4 hours of entering Mode 5, the licensee will ensure that only
      one reactor makeup water pump (dilution source) is aligned to the supply
      header.  Following entry into Mode 3, 4, or 5 from Modes 1, 2, or 6,
      each crew of the control room staff will receive a briefing to discuss
      the type of reactivity changes that could occur during a boron dilution
      event, the indications of a boron dilution event, and the actions
      required to stop a dilution; commence immediate boration and establish
      the required shutdown margin.  For extended shutdowns, this briefing
      will be repeated for each crew prior to resumption of control room
      duties following an off duty period which exceeds 7 days.  During time
      periods when this option is used, the source range will be monitored for
      indication of unexplained increasing counts and inadvertent boron
      dilution every 15 minutes.         


The inverse count rate ratio used in the Comanche Peak analysis of the boron
dilution event is based on the generic curve supplied by Westinghouse.  The .

                                                            IN 93-32
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                                                            Page 3 of 3

curve is used to predict the time at which a source range flux-doubling signal
would occur as criticality is approached during a boron dilution event.  Using
actual Comanche Peak Unit 1 inverse count rate ratio data, the licensee
determined that the flux-doubling signal and alarm detected by the mitigation
system may not meet the acceptance criteria for the boron dilution event since
an appreciable amount of dilution would already have occurred before the
signal was generated.  

The licensee also found that the flux multiplication setpoint of 2.0 does not
include the instrument uncertainties that should be applied to determine the
flux multiplication setpoint.  These uncertainties include the statistical
variation of neutron flux, the variations in the linearity of the log level
amplifier as a function of counts per second, and uncertainties in numerous
amplifiers and the multiplexer.  Thus, an actual flux signal could exceed the
flux-doubling setpoint used to analyze the boron dilution event.  This
condition could result in a loss of shutdown margin before the mitigation
system terminates an inadvertent boron dilution event.  (This problem may be
applicable not only to plants that rely on an automatic mitigation system, but
also to those that rely on a flux-doubling signal to generate an alarm for
operator intervention.)  Alternatively, setting the setpoint too
conservatively could unnecessarily allow the mitigation system to be activated
and the reactor coolant system to be borated.  This sequence of events would
have detrimental effects on operation.

On March 23, 1992, the staff met at NRC Headquarters with the licensee and
Westinghouse to discuss issues discussed above.  The licensee informed the
staff that it had investigated various alternatives and found none readily
available to restore the operability of the mitigation system but would
continue to evaluate this problem with Westinghouse.  Later, on 
December 15, 1992, the staff met with Westinghouse and licensees of
Westinghouse-designed plants which use the mitigation system.  It was
concluded that the mitigation system was unreliable because of the large
uncertainties associated with the indication of a true flux doubling by the
nuclear instruments.  Therefore, this problem may be common to all PWRs that
rely on a flux-doubling signal to mitigate a boron dilution event before
criticality can occur.  

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.

                                       ORIGINAL SIGNED BY

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

Technical contact:      Larry Kopp, NRR
                        (301) 504-2879

Attachment:  List of Recently Issued NRC Information Notices  

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