Information Notice 1993-32, Nonconservative Inputs for Boron Dilution Events Analysis
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
April 21, 1993 NRC INFORMATION NOTICE 93-32: NONCONSERVATIVE INPUTS FOR
BORON DILUTION EVENT ANALYSIS
Addressees
All holders of operating licenses or construction permits for pressurized
water reactors (PWRs).
purpose
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
required.
Background
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),6 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 93-32 April 21, 1993 In NRC Generic Letter 85-05, NInadvertent 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 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> 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 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> 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.
Discussion
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 April 21, 1993 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.
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
Attachment
April 29
1992 Pag te1 Im
LIST OF RECEINLY
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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.
Orginal signed by
Brian K. Grim K. %actor
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
- SEE PREVIOUS CONCURRENCES
- OGCB:DORS:NRR
- SRXB:DSSA:NRR
- SRXB:DSSA:NRR
- C:SRXB:DSS:NRR
PCWen:mkm
LIKopp
LEPhillips
RCJones
03/04/93
03/04/93
03/12/93
03/15/93
- TECH ED
- D:DSSA:NRR
- C:OGCB:DORS:NRR
ACThadani
GHMarcus
03/04/93
03/26/93
04/01/93
0/6/93
IN 93- April
, 1993 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.
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
- SEE PREVIOUS CONCURRENCES
- OGCB:DORS:NRR
PCWen:mkm
03/04/93
- SRXB:DSSA:NRR
LIKopp
03/04/93
- SRXB:DSSA:NRR
LEPhillips
03/12/93
- C:SRXB:DSS:NRR
RCJones
03/15/93
- TECH ED
03/04/93
- D:DSSA:NRR
ACThadani
03/26/93
- C:OGCB:DORS:NRR
GHMarcus
04/01/93 D:DORS:NRR
BKGrimes
i.
03/ /93 , U
Il.
Document Name: BORON.CON
IN 93- March
, 1993 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 BDMS terminates an inadvertent boron dilution
event. This problem may be generic not only for plants that rely on an
automatic system such as the BDMS, but also for 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 BDMS to be activated and the RCS to be borated.
This sequence of
events would have detrimental effects on operations.
On March 23, 1992, the staff met at NRC Headquarters with the licensee and
Westinghouse to discuss issues about the BDMS. The licensee informed the
staff that it had investigated various alternatives and found none readily
available to restore the operability of the BDMS 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 BDMS. It was concluded that the BDMS 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.
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
- SEE PREVIOUS CONCURRENCES
- OGCB:DORS:NRR
- SRXB:DSSA:NRR
- SRXB:DSSA:NRR
- C:SRXB:DSS:NRR
PCWen:mkm
LIKopp
LEPhillips
RCJones
03/04/93
03/04/93
03/12/93
03/15/93
- TECH ED
- D:DSSA:NRR
C:OGCB:DORS:NRR
D:DORS:NRR
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'IN 93- March
, 1993 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 BDMS terminates an inadvertent boron dilution
event. This problem may be generic not only for plants that rely on an
automatic system such as the BDMS, but also for 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 BDMS to be activated and the RCS to be borated. This sequence of
events would have detrimental effects on operations.
On March 23, 1992, the staff met at NRC Headquarters with the licensee and
Westinghouse to discuss issues about the BDMS. The licensee informed the
staff that it had investigated various alternatives and found none readily
available to restore the operability of the BDMS 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 BDMS. It was concluded that the BDMS 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. It
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.
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
- SEE PREVIOUS CONCURRENCES
- OGCB:DORS:NRR
- SRXB:DSSA:NRR
- SRXB:DSSA:NRR
- C:SRXB:DSS:NRR
PCWen:mkm
LIKopp
LEPhillips
RCJones
03/04/93 by
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03/12/93
03/15/93
- TECH ED
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03/04/93 M~y
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03/ /93
03/ /93 Document Name: BRON.CON
..yau'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.
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contact:
Attachment:
List
Larry Kopp, NRR
(301) 504-2879 t of Recently Issued NRC Information Notices
OGCB:DORS:NRR
PCenv:mkm
03/14 /93 SRXB:DSSA:NRR
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BKGrimes
03/ /93