Information Notice 1992-39, Unplanned Return to Criticality During Reactor Shutdown: Difference between revisions

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| issue date = 05/13/1992
| issue date = 05/13/1992
| title = Unplanned Return to Criticality During Reactor Shutdown
| title = Unplanned Return to Criticality During Reactor Shutdown
| author name = Rossi C E
| author name = Rossi C
| author affiliation = NRC/NRR
| author affiliation = NRC/NRR
| addressee name =  
| addressee name =  
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| page count = 12
| page count = 12
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{{#Wiki_filter:UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555May 13, 1992NRC INFORMATION NOTICE 92-39: UNPLANNED RETURN TO CRITICALITY DURINGREACTOR SHUTDOWN
{{#Wiki_filter:UNITED STATES
 
NUCLEAR REGULATORY COMMISSION
 
OFFICE OF NUCLEAR REACTOR REGULATION
 
WASHINGTON, D.C. 20555 May 13, 1992 NRC INFORMATION NOTICE 92-39:   UNPLANNED RETURN TO CRITICALITY DURING
 
REACTOR SHUTDOWN


==Addressees==
==Addressees==
All holders of operating licenses or construction permits for nuclear powerreactors.
All holders of operating licenses or construction permits for nuclear power
 
reactors.


==Purpose==
==Purpose==
The U.S. Nuclear Regulatory Commission (NRC) is issuing this informationnotice to alert addressees to recent events involving unplanned returns tocriticality caused by the cooldown of the reactor coolant system during reactorshutdowns. It is expected that recipients will review the information forapplicability to their facilities and consider actions, as appropriate, toavoid similar problems. However, suggestions contained in this informationnotice are not NRC requirements; therefore, no specific action or writtenresponse is required.
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
 
notice to alert addressees to recent events involving unplanned returns to
 
criticality caused by the cooldown of the reactor coolant system during reactor
 
shutdowns. 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==
==Description of Circumstances==
The licensees for the following plants recently experienced unplannedcriticalities during reactor shutdowns: the Monticello Nuclear GeneratingPlant, the Big Rock Point Nuclear Plant, and the Grand Gulf Nuclear Station.MonticelloOn June 6, 1991, an unplanned criticality occurred at the Monticello NuclearGenerating Plant during a reactor shutdown when an unanticipated cooldownoccurred in conjunction with rod insertion. The licensee initiated thisshutdown to repair a leaking safety-relief valve shortly after the plant wasreturning to power following a refueling outage.Since this shutdown occurred shortly after the cycle startup, the reactor didnot generate sufficient decay heat to produce the steam needed to supply thenormal steam system loads and still maintain pressure in the reactor. Ascontrol rods were being inserted using notch insertion, the reactor coolantsystem pressure and temperature began to decrease. As long as the operatorcontinued to insert control rods, the reactor remained subcritical.However, when the operator stopped inserting control rods to review andevaluate plant conditions, the cooldown continued, adding sufficientreactivity to overcome the negative reactivity from the insertion of controlrods and causing the reactor power to increase. The reactor power continued to92050_*,, C j'Ai / 1 ( / /CDQc
The licensees for the following plants recently experienced unplanned
 
criticalities during reactor shutdowns: the Monticello Nuclear Generating
 
Plant, the Big Rock Point Nuclear Plant, and the Grand Gulf Nuclear Station.
 
Monticello
 
On June 6, 1991, an unplanned criticality occurred at the Monticello Nuclear
 
Generating Plant during a reactor shutdown when an unanticipated cooldown
 
occurred in conjunction with rod insertion. The licensee initiated this
 
shutdown to repair a leaking safety-relief valve shortly after the plant was
 
returning to power following a refueling outage.
 
Since this shutdown occurred shortly after the cycle startup, the reactor did
 
not generate sufficient decay heat to produce the steam needed to supply the
 
normal steam system loads and still maintain pressure in the reactor. As
 
control rods were being inserted using notch insertion, the reactor coolant
 
system pressure and temperature began to decrease. As long as the operator
 
continued to insert control rods, the reactor remained subcritical.
 
However, when the operator stopped inserting control rods to review and
 
evaluate plant conditions, the cooldown continued, adding sufficient
 
reactivity to overcome the negative reactivity from the insertion of control
 
rods and causing the reactor power to increase. The reactor power continued to
 
92050_*
,,                                                                 j'A  C
 
i /       1     (                 /     /CDQc
 
K)
                                                                  IN 92-39 May 13, 1992 increase until an intermediate-range monitor (IRM) tripped on a high-high flux
 
setpoint, causing a reactor scram.
 
Further details of the event may be found in Licensee Event Report
 
50-263/91-15 or NRC Inspection Report 50-263/91-13.
 
===Big Rock Point===
On November 30, 1991, an unplanned criticality occurred at the Big Rock Point
 
Nuclear Plant during a reactor shutdown when a cooldown occurred in conjunction
 
with a rod insertion. The event occurred during a scheduled shutdown for a
 
refueling outage.
 
After taking the main turbine off line and bringing the reactor to a
 
subcritical condition, the licensee delayed actions to continue inserting
 
control rods while changing shifts. However, the reactor coolant system
 
continued to cool down because the normal steam system loads were removing
 
heat. The cooldown added sufficient reactivity to eventually overcome the
 
effects of the control rods, and the reactor returned critical. The operators
 
noticed the condition in approximately 2 minutes and took the appropriate
 
actions to return the reactor subcritical.
 
Further details of the event may be found in Licensee Event Report
 
50-155/91-OO9-orJNRC-Inspection-Report 50-155/91-25.        _ _            _
Grand Gulf
 
On December 30, 1991, an unplanned criticality occurred at the Grand Gulf
 
Nuclear Station during a reactor shutdown when a cooldown occurred in
 
conjunction with a rod insertion. The event occurred during a scheduled
 
shutdown for a maintenance outage to replace a recirculation pump shaft. To
 
minimize the release of plated-out radioactive material from the fuel cladding
 
and channels to the reactor coolant system, a slow power reduction and a slow
 
cooldown were prescribed for this shutdown.
 
The slow power reduction and the effects of an earlier outage minimized reactor
 
decay heat and xenon peaking. The operators were driving rods into the core
 
individually because gang drive for the control rods was unavailable. This
 
made the reactivity insertion slower than normal. The operators inserted
 
rods to reduce flux to range 3 on the IRMs and stopped rod insertion in order
 
to perform a source range monitor surveillance. Indicated power reached IRM
 
range 1. However, the reactor coolant system continued to cool down because
 
the normal steam system loads were removing heat. In order not to affect the
 
source range monitor surveillance, the shift supervisor elected to not insert
 
control rods and alerted the operators to the possibility of a return to
 
criticality as the cooldown continued. The reactivity added by the cooldown
 
eventually overcame the effects of the control rods. The reactor returned to a
 
critical condition, and reactor power increased on a reactor period between 300
to 800 seconds. The operators, having been trained on a similar event at
 
another BWR, were monitoring the condition and expected the power to increase
 
to the point of adding heat, where the fuel and moderator temperature coeffi- cients would halt the reactor power increase. As the power increased, the
 
IN 92-39 May 13, 1992 reactor operator ranged the IRMs to prevent a reactor scram. Power level
 
remained on scale. The power increase was terminated with the IRMs reading on
 
range 7 and 8. The operators completed the source range monitor surveillance, then resumed inserting rods and successfully completed the shutdown.
 
Further details of the event may be found in NRC Inspection Report
 
50-416/92-04 and Licensee Event Report 50-416/91-16.
 
Discussion
 
The underlying principle of reactivity management is to maintain the reactor
 
in the desired condition by properly anticipating, controlling, and responding
 
to the plant's changing parameters. The experience at Monticello, Big Rock
 
Point, and Grand Gulf indicates that shutdowns in situations with low decay
 
heat present a unique challenge to reactivity control if inadvertent recritica- lity is to be prevented. These events emphasize the importance of the operator
 
giving continuous attention to plant parameters during a shutdown. Further, accepting recriticality without prior management approval and procedures, i.e.,
an ad-hoc approach, raises concerns due to the lack of opportunity for contin- gency planning.
 
This information notice requires no specific action or written response. If
 
you have any questions about 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.
 
Va&res'E.Rossi, Director
 
Division of Operational Events Assessment
 
Office of Nuclear Reactor Regulation
 
Technical contacts: Melvyn Leach, RIII
 
(708) 790-5559 Geoffrey Wright, RIII
 
(708) 790-5695 Paul O'Connor, NRR
 
(301) 504-1307 Attachment:  List of Recently Issued NRC Information Notices
 
z
 
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m Z      -n      M    Information
 
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0                      92-38            Implementation Date for        05/12/92  All holders of OLs or CPs
 
the Revision to the EPA                    for nuclear power reactors, Manual of Protective                      non-power reactors and
 
Action Guides and Pro-                    materials licensees auth- tective Actions for                        orized to possess large
 
Nuclear Incidents                          quantities of radioactive
 
material.
 
1 92-37            Implementation of the          05/08/92  All Nuclear Regulatory
 
Deliberate Nisconduct Rule                  Commission Licensees.
 
92-16,          Loss of Flow from the Re-        05/07/92  All holders of OLs or CPs
 
Supp. 1          sidual Heat Removal Pump                  for nuclear power reactors.
 
during Refueling Cavity
 
Draindown
 
I 92-36            Intersystem LOCA Outside        05/07/92  All holders of OLs or CPs
 
Containment                                for nuclear power reactors.
 
' 92-35            Higher Than Predicted Ero-      05/06/92  All holders of OLs or CPs
 
sion/Corrosion in Unisol-                  for nuclear power reactors.
 
able Reactor Coolant Pres- sure Boundary Piping Inside
 
Containment at A Boiling
 
Water Reactor
 
92-34            New Exposure Limits for
 
Airborne Uranium and
 
Thorium
 
05/06/92  All licensees whose opera- tions can cause airborne
 
concentrations of uranium
 
(
                                                                                    and thorium.
 
92-33          Increased Instrument            04/30/92  All holders of OLs or CPs
 
-u                      Response Time When                        for nuclear power reactors.
 
0                        Pressure Dampening
 
rn>                          Devices are Installed
 
mD
 
- C > 0
                D2 zzz>                  Operating License
 
u) D-  I CP - 'Construction Permit
 
0
 
IN 92-39 May 13, 1992 reactor operator ranged the IRMs to prevent a reactor scram. Power level
 
remained on scale. The power increase was terminated with the IRMs reading on
 
range 7 and 8. The operators completed the source range monitor surveillance, then resumed inserting rods and successfully completed the shutdown.
 
Further details of the event may be found in NRC Inspection Report
 
50-416/92-04 and Licensee Event Report 50-416/91-16.
 
Discussion
 
The underlying principle of reactivity management is to maintain the reactor
 
in the desired condition by properly anticipating, controlling, and responding
 
to the plant's changing parameters. The experience at Monticello, Big Rock
 
Point, and Grand Gulf indicates that shutdowns in situations with low decay
 
heat present a unique challenge to reactivity control if inadvertent recritica- lity is to be prevented. These events emphasize the importance of the operator
 
giving continuous attention to plant parameters during a shutdown. Further, accepting recriticality without prior management approval and procedures, i.e.,
          an ad-hoc approach, raises concerns due to the lack of opportunity for contin- gency planning.
 
This information notice requires no specific action or written response. If
 
you have any questions about 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. Original Signed by
 
Nhedes E, Rossi
 
Charles E. Rossi, Director
 
Division of Operational Events Assessment
 
Office of Nuclear Reactor Regulation
 
Technical contacts:  Melvyn Leach, RIII
 
(708) 790-5559 Geoffrey Wright, RIII
 
(708) 790-5695 Paul O'Connor, NRR
 
(301) 504-1307 Attachment:  List of Recently Issued NRC Information Notices
 
*SEE PREVIOUS CONCURRENCE
 
OFC  RIII        ADR4-5:PD4-1    SC:DOEA:OEAB    ADM:RPB    C:DST:SRXB      C:DOEA:OEAB
 
NAME  GWright*    PO'Connor*      RDennig*        JMain*    RJones*          AChaffee*
DATE  03/11/92    03/17/92        03/11/92        03/11/92  03/13/92        04/13/92 OFC  C:DOEA:OGCB      D        X,
NAME  CBerlinger*
DATE  04/21/92        05/ 7/92 DOCUMENT NAME:  IN 92-39
 
IN 92-XX
 
April xx, 1992 Further details of the event may be found in NRC Inspection Report
 
50-416/92-04 and Licensee Event Report 50-416/91-16.
 
Discussion
 
The underlying principle of reactivity management is to maintain the reactor
 
in the desired condition by properly anticipating, controlling, and responding
 
to the plant's changing parameters. The experience at Big Rock Point, Grand
 
Gulf, and Monticello indicates that shutdowns in situations with low decay heat
 
present a unique challenge to reactivity control if inadvertent recriticality
 
is to be prevented. These events emphasize the importance of the operator
 
giving continuous attention to plant parameters during a shutdown. Further, accepting recriticality without prior management approval and procedures, i.e.,
          an ad-hoc approach, raises concerns due to the lack of opportunity for contin- gency planning.
 
This information notice requires no specific action or written response. If
 
you have any questions about 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:  Melvyn Leach, RIII
 
(708) 790-5559 Geoffrey Wright, RIII
 
(708) 790-5695 Paul O'Connor, NRR
 
(301) 504-1307 Attachment:  List of Recently Issued NRC Information Notices
 
*SEE PREVIOUS CONCURRENCE
 
OFC  RIII        ADR4-5:PD4-1    SC:DOEA:OEAB    ADM:RPB    C:DST:SRXB    C:DOEA:OEAB
 
NAME  GWright*    PO'Connor*      RDennig*        JMain*    RJones*      AChaffee*
DATE  03/11/92    03/17/92        03/11/92        03/11/92  03/13/92      04/13/92 OFC  C:DOEA:OGCB      D:DOEA
 
NAME  CBerlinger*      CRoss tD
 
DATE  04/21/92              /9 OFFICIAL RECORD COPY                          DOCUMENT NAME:  IN DENKIG
 
IN 92-XX
 
April xx, 1992 Further details of the event may be found in NRC Inspection Report
 
50-416/92-04 and Licensee Event Report 50-416/91-16.
 
Discussion
 
The underlying principle of reactivity management is to maintain the reactor
 
in the desired condition by properly anticipating, controlling, and responding
 
to the plant's changing parameters. Once a reactor has been placed in a
 
subcritical condition during a plant shutdown, it is of primary importance
 
that the reactor be maintained in that subcritical condition. Raising the
 
range of the IRMs in the event of recriticality can be inappropriate if the
 
licensed operators do not have management's oversight, training, and specific
 
procedures for this.
 
The experience at Grand Gulf and Monticello indicate that slow shutdowns in
 
situations with low decay heat present a unique challenge to reactivity
 
control and require the operator and management to give careful attention to
 
avoid recriticality. The events at Monticello and Big Rock Point also highlight
 
the importance of the operator giving continuous attention to plant parameters
 
during a shutdown.
 
This information notice requires no specific action or written response. If
 
you have any questions about 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:  Melvyn Leach, RIII
 
(708) 790-5559 Geoffrey Wright, RIII
 
(708) 790-5695 Paul O'Connor, NRR
 
(301) 504-1307 Attachment:  List of Recently Issued NRC Information Notices
 
*SEE PREVIOUS CONCURRENCE
 
OFC  RIII        ADR4-5:PD4-1  SC:DOEA:OEAB    ADM:RPB    C:DST:SRXB  C:DOEA:OEAB
 
NAME  GWright*    PO'Connor*    RDennig*        JMain*      RJones*    AChaffee*
DATE  03/11/92    03/17/92      03/11/92        03/11/92    03/13/92    04/13/92 OFC  C:DO      J
 
f    D:DOEA
 
NAME  CBerlinger      CRosswa'y
 
DATE  f/;11/ 92        / 92 OFFICIAL RECORD COPY                          DOCUMENT NAME:  IN DENNIG
 
IN 92-XX
 
March xx, 1992 Further details of the event may be found in NRC Inspection Report
 
50-416/92-04 and Licensee Event Report 50-416/91-16.
 
Discussion
 
The underlying principle of reactivity management is to maintain the reactor
 
in the desired condition by properly anticipating, controlling, and responding
 
to the plant's changing parameters. Once a reactor has been placed in a
 
subcritical condition during a plant shutdown, it is of primary importance
 
that the reactor be maintained in that subcritical condition. In the event of
 
recriticality, promptly inserting control rods and terminating the cooldown are
 
more conservative actions than raising the range of the IRMs to prevent scram
 
while waiting on the changing temperature to reverse the direction of the
 
power. Raising the range of the IRMs in the event of recriticality can be
 
inappropriate if the licensed operators do not have management's oversight, training, and specific procedures.
 
The experience at Grand Gulf and Monticello indicate that slow shutdowns in
 
situations with low decay heat present a unique challenge to reactivity
 
control and require the operator and management to give careful attention to
 
avoid recriticality. The events at Monticello and Big Rock Point also highlight
 
the importance of the operator giving continuous attention to plant parameters
 
during a shutdown.
 
This information notice requires no specific action or written response. If
 
you have any questions about 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:  Melvyn Leach, RIII
 
(708) 790-5559 Geoffrey Wright, RIII
 
(708) 790-5695 Paul O'Connor, NRR
 
(301) 504-1307 t'/ '      f'      4.f, IV
 
Attachment:  List of Recently Issued NRC Information Notices
 
*SEE PREVIOUS CONCURRENCEC
 
OFC  RIII        ADR4-5:PD4-1  SC:DOEA:OEAB    ADM:RPB    C:DST:SRXB  C:D0EA:OEAB
 
NAME  GWright*    PO'Connor*    RDennig*        JMain*    RJones*      AChaffee
 
DATE 03/11/92      03/17/92      03/11/92        03/11/92  03/13/92      q1/t3/92 OFC  C:DOEA:OGCB      D:DOEA
 
===NAME  CBerlinger      CRossi===
DATE    / /92            /  /92 OFFICIAL RECORD COPY                          DOCUMENT NAME:  IN DENNIG
 
IN 92-XX
 
March xx, 1992 Further details of the event may be found in NRC Inspection Report
 
50-416/92-04 and Licensee Event Report 50-416/91-16.
 
Discussion
 
The underlying principle of reactivity management is to maintain the reactor
 
in the desired condition by properly anticipating, controlling, and responding
 
to the plant's changing parameters. Once a reactor has been placed in a
 
subcritical condition during a plant shutdown, it is of primary importance
 
that the reactor be maintained in that subcritical condition. In the event of
 
recriticality, promptly inserting control rods and terminating the cooldown are
 
more conservative actions than raising the range of the IRMs to prevent scram
 
while waiting on the changing temperature to reverse the direction of the
 
power. Raising the range of the IRMs in the event of recriticality can be
 
inappropriate if the licensed operators do not have management's oversight, training, and specific procedures.
 
The experience at Grand Gulf and Monticello indicate that slow shutdowns in
 
situations with low decay heat present a unique challenge to reactivity
 
control and require the operator and management to give careful attention to
 
avoid recriticality. The events at Monticello and Big Rock Point also highlight
 
the importance of the operator giving continuous attention to plant parameters
 
during a shutdown.
 
This information notice requires no specific action or written response. If
 
you have any questions about 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:  Melvyn Leach, RIII
 
(708) 790-5559 Geoffrey Wright, 'RIII
 
(708) 790-5695 Paul O'Connor, NRR
 
(301) 504-1307 Attachment:  List of Recently Issued NRC Information Notices
 
*SEE PREVIOUS CONCURRENCE
 
OFC  RIII        ADR4-5:PD4-1    SC:DOEA:OEAB    ADM:RPB    C:DST:SRXB  C:DOEA:OEAB
 
NAME GWright*    PO'Connor (OK RDennig*          JMain*    RJones*      AChaffee
 
DATE 03/11/92    3 /17/92  Xis    192          03/11/92  03/13/92      /  /92 OFC  C:DOEA:OGCB      D:DOEA
 
===NAME CBerlinger      CRossi===
DATE  / /92            / /92
 
IN 92-XX
 
March xx, 1992 Further details of the event may be found in NRC Inspection Report
 
50-416/92-04 and Licensee Event Report 50-416/91-16.
 
Discussion
 
The underlying principle of reactivity management is to maintain the reactor
 
in the desired condition by properly anticipating, controlling, and responding
 
to the plant's changing parameters. Once a reactor has been placed in a
 
subcritical condition during a plant shutdown, it is of primary importance
 
that the reactor be maintained in that subcritical condition. Since recriti- cality has not been avoided, promptly inserting control rods and terminating
 
the cooldown are more conservative actions than raising the range of the IRMs
 
to prevent scram while waiting on the changing temperature to reverse the
 
direction of the power. Raising the range of the IRMs in the event of recriti- cality can be inappropriate if the licensed operators do not have management's
 
oversight, training, and specific procedures.
 
The experience at Grand Gulf and Monticello indicate that slow shutdowns in
 
situations with low decay heat present a unique challenge to reactivity
 
control and require the operator and management to give careful attention to
 
avoid recriticality. The events at Monticello and Big Rock Point also highlight
 
the importance of the operator giving continuous attention to plant parameters
 
during a shutdown.
 
This information notice requires no specific action or written response. If
 
you have any questions about 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.


K)IN 92-39May 13, 1992 increase until an intermediate-range monitor (IRM) tripped on a high-high fluxsetpoint, causing a reactor scram.Further details of the event may be found in Licensee Event Report50-263/91-15 or NRC Inspection Report 50-263/91-13.Big Rock PointOn November 30, 1991, an unplanned criticality occurred at the Big Rock PointNuclear Plant during a reactor shutdown when a cooldown occurred in conjunctionwith a rod insertion. The event occurred during a scheduled shutdown for arefueling outage.After taking the main turbine off line and bringing the reactor to asubcritical condition, the licensee delayed actions to continue insertingcontrol rods while changing shifts. However, the reactor coolant systemcontinued to cool down because the normal steam system loads were removingheat. The cooldown added sufficient reactivity to eventually overcome theeffects of the control rods, and the reactor returned critical. The operatorsnoticed the condition in approximately 2 minutes and took the appropriateactions to return the reactor subcritical.Further details of the event may be found in Licensee Event Report50-155/91-OO9-orJNRC-Inspection-Report 50-155/91-25. _ _ _Grand GulfOn December 30, 1991, an unplanned criticality occurred at the Grand GulfNuclear Station during a reactor shutdown when a cooldown occurred inconjunction with a rod insertion. The event occurred during a scheduledshutdown for a maintenance outage to replace a recirculation pump shaft. Tominimize the release of plated-out radioactive material from the fuel claddingand channels to the reactor coolant system, a slow power reduction and a slowcooldown were prescribed for this shutdown.The slow power reduction and the effects of an earlier outage minimized reactordecay heat and xenon peaking. The operators were driving rods into the coreindividually because gang drive for the control rods was unavailable. Thismade the reactivity insertion slower than normal. The operators insertedrods to reduce flux to range 3 on the IRMs and stopped rod insertion in orderto perform a source range monitor surveillance. Indicated power reached IRMrange 1. However, the reactor coolant system continued to cool down becausethe normal steam system loads were removing heat. In order not to affect thesource range monitor surveillance, the shift supervisor elected to not insertcontrol rods and alerted the operators to the possibility of a return tocriticality as the cooldown continued. The reactivity added by the cooldowneventually overcame the effects of the control rods. The reactor returned to acritical condition, and reactor power increased on a reactor period between 300to 800 seconds. The operators, having been trained on a similar event atanother BWR, were monitoring the condition and expected the power to increaseto the point of adding heat, where the fuel and moderator temperature coeffi-cients would halt the reactor power increase. As the power increased, the
Charles E. Rossi, Director


IN 92-39May 13, 1992 reactor operator ranged the IRMs to prevent a reactor scram. Power levelremained on scale. The power increase was terminated with the IRMs reading onrange 7 and 8. The operators completed the source range monitor surveillance,then resumed inserting rods and successfully completed the shutdown.Further details of the event may be found in NRC Inspection Report50-416/92-04 and Licensee Event Report 50-416/91-16.DiscussionThe underlying principle of reactivity management is to maintain the reactorin the desired condition by properly anticipating, controlling, and respondingto the plant's changing parameters. The experience at Monticello, Big RockPoint, and Grand Gulf indicates that shutdowns in situations with low decayheat present a unique challenge to reactivity control if inadvertent recritica-lity is to be prevented. These events emphasize the importance of the operatorgiving continuous attention to plant parameters during a shutdown. Further,accepting recriticality without prior management approval and procedures, i.e.,an ad-hoc approach, raises concerns due to the lack of opportunity for contin-gency planning.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager.Va&res'E.Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical contacts: Melvyn Leach, RIII(708) 790-5559Geoffrey Wright, RIII(708) 790-5695Paul O'Connor, NRR(301) 504-1307Attachment:List of Recently Issued NRC Information Notices
Division of Operational Events Assessment


zI-0.<OD n; co> 0m Z.4-mrno00zZ G2-cnaEn z* omo) Zen q-n M0n a0 na -h <-Attachment 1IN 92-39May 13, 1992Page I of 1LIST OF RECENTLY ISSUEDNRC INFORMATION NOTICESDate ofInformationinformationNotice No.92-381 92-3792-16,Supp. 1I 92-36' 92-35SubjectImplementation Date forthe Revision to the EPAManual of ProtectiveAction Guides and Pro-tective Actions forNuclear IncidentsImplementation of theDeliberate Nisconduct RuleLoss of Flow from the Re-sidual Heat Removal Pumpduring Refueling CavityDraindownIntersystem LOCA OutsideContainmentHigher Than Predicted Ero-sion/Corrosion in Unisol-able Reactor Coolant Pres-sure Boundary Piping InsideContainment at A BoilingWater ReactorNew Exposure Limits forAirborne Uranium andThoriumIncreased InstrumentResponse Time WhenPressure DampeningDevices are InstalledIssuance05/12/9205/08/9205/07/9205/07/9205/06/92Issued toAll holders of OLs or CPsfor nuclear power reactors,non-power reactors andmaterials licensees auth-orized to possess largequantities of radioactivematerial.All Nuclear RegulatoryCommission Licensees.All holders of OLs or CPsfor nuclear power reactors.All holders of OLs or CPsfor nuclear power reactors.All holders of OLs or CPsfor nuclear power reactors.-u0rn>mD-C > 0u) D-D2zzz>092-3492-33I CP -'05/06/92 All licensees whose opera-tions can cause airborneconcentrations of uraniumand thorium.04/30/92 All holders of OLs or CPsfor nuclear power reactors.(Operating LicenseConstruction Permit
Office of Nuclear Reactor Regulation


IN 92-39May 13, 1992 reactor operator ranged the IRMs to prevent a reactor scram. Power levelremained on scale. The power increase was terminated with the IRMs reading onrange 7 and 8. The operators completed the source range monitor surveillance,then resumed inserting rods and successfully completed the shutdown.Further details of the event may be found in NRC Inspection Report50-416/92-04 and Licensee Event Report 50-416/91-16.DiscussionThe underlying principle of reactivity management is to maintain the reactorin the desired condition by properly anticipating, controlling, and respondingto the plant's changing parameters. The experience at Monticello, Big RockPoint, and Grand Gulf indicates that shutdowns in situations with low decayheat present a unique challenge to reactivity control if inadvertent recritica-lity is to be prevented. These events emphasize the importance of the operatorgiving continuous attention to plant parameters during a shutdown. Further,accepting recriticality without prior management approval and procedures, i.e.,an ad-hoc approach, raises concerns due to the lack of opportunity for contin-gency planning.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager. Original Signed byNhedes E, RossiCharles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical contacts: Melvyn Leach, RIII(708) 790-5559Geoffrey Wright, RIII(708) 790-5695Paul O'Connor, NRR(301) 504-1307Attachment: List of Recently Issued NRC Information Notices*SEE PREVIOUS CONCURRENCEOFC RIII ADR4-5:PD4-1 SC:DOEA:OEAB ADM:RPB C:DST:SRXB C:DOEA:OEABNAME GWright* PO'Connor* RDennig* JMain* RJones* AChaffee*DATE 03/11/92 03/17/92 03/11/92 03/11/92 03/13/92 04/13/92OFC C:DOEA:OGCB D X,NAME CBerlinger*DATE 04/21/92 05/ 7/92DOCUMENT NAME: IN 92-39 IN 92-XXApril xx, 1992 DOCUMENT NAME:IN DENKIG
Technical contacts:           Melvyn Leach, RIII


IN 92-XXApril xx, 1992 Further details of the event may be found in NRC Inspection Report50-416/92-04 and Licensee Event Report 50-416/91-16.DiscussionThe underlying principle of reactivity management is to maintain the reactorin the desired condition by properly anticipating, controlling, and respondingto the plant's changing parameters. Once a reactor has been placed in asubcritical condition during a plant shutdown, it is of primary importancethat the reactor be maintained in that subcritical condition. Raising therange of the IRMs in the event of recriticality can be inappropriate if thelicensed operators do not have management's oversight, training, and specificprocedures for this.The experience at Grand Gulf and Monticello indicate that slow shutdowns insituations with low decay heat present a unique challenge to reactivitycontrol and require the operator and management to give careful attention toavoid recriticality. The events at Monticello and Big Rock Point also highlightthe importance of the operator giving continuous attention to plant parametersduring a shutdown.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical contacts: Melvyn Leach, RIII(708) 790-5559Geoffrey Wright, RIII(708) 790-5695Paul O'Connor, NRR(301) 504-1307Attachment: List of Recently Issued NRC Information Notices*SEE PREVIOUS CONCURRENCEOFC RIII ADR4-5:PD4-1 SC:DOEA:OEAB ADM:RPB C:DST:SRXB C:DOEA:OEABNAME GWright* PO'Connor* RDennig* JMain* RJones* AChaffee*DATE 03/11/92 03/17/92 03/11/92 03/11/92 03/13/92 04/13/92OFC C:DO J f D:DOEANAME CBerlinger CRosswa'yDATE f/;11/92/ 92OFFICIAL RECORD COPYDOCUMENT NAME: IN DENNIG
(708) 790-5559 Geoffrey Wright, RII


IN 92-XXMarch xx, 1992 Further details of the event may be found in NRC Inspection Report50-416/92-04 and Licensee Event Report 50-416/91-16.DiscussionThe underlying principle of reactivity management is to maintain the reactorin the desired condition by properly anticipating, controlling, and respondingto the plant's changing parameters. Once a reactor has been placed in asubcritical condition during a plant shutdown, it is of primary importancethat the reactor be maintained in that subcritical condition. In the event ofrecriticality, promptly inserting control rods and terminating the cooldown aremore conservative actions than raising the range of the IRMs to prevent scramwhile waiting on the changing temperature to reverse the direction of thepower. Raising the range of the IRMs in the event of recriticality can beinappropriate if the licensed operators do not have management's oversight,training, and specific procedures.The experience at Grand Gulf and Monticello indicate that slow shutdowns insituations with low decay heat present a unique challenge to reactivitycontrol and require the operator and management to give careful attention toavoid recriticality. The events at Monticello and Big Rock Point also highlightthe importance of the operator giving continuous attention to plant parametersduring a shutdown.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical contacts: Melvyn Leach, RIII(708) 790-5559Geoffrey Wright, RIII(708) 790-5695Paul O'Connor, NRR IV t'/ ' f'(301) 504-1307 4.f,Attachment: List of Recently Issued NRC Information Notices*SEE PREVIOUS CONCURRENCECOFC RIII ADR4-5:PD4-1 SC:DOEA:OEAB ADM:RPB C:DST:SRXB C:D0EA:OEABNAME GWright* PO'Connor* RDennig* JMain* RJones* AChaffeeDATE 03/11/92 03/17/92 03/11/92 03/11/92 03/13/92 q1/t3/92OFC C:DOEA:OGCB D:DOEANAME CBerlinger CRossiDATE / /92 / /92OFFICIAL RECORD COPYDOCUMENT NAME: IN DENNIG
(708) 790-5695 Paul O'Connor, NRR


IN 92-XXMarch xx, 1992 Further details of the event may be found in NRC Inspection Report50-416/92-04 and Licensee Event Report 50-416/91-16.DiscussionThe underlying principle of reactivity management is to maintain the reactorin the desired condition by properly anticipating, controlling, and respondingto the plant's changing parameters. Once a reactor has been placed in asubcritical condition during a plant shutdown, it is of primary importancethat the reactor be maintained in that subcritical condition. In the event ofrecriticality, promptly inserting control rods and terminating the cooldown aremore conservative actions than raising the range of the IRMs to prevent scramwhile waiting on the changing temperature to reverse the direction of thepower. Raising the range of the IRMs in the event of recriticality can beinappropriate if the licensed operators do not have management's oversight,training, and specific procedures.The experience at Grand Gulf and Monticello indicate that slow shutdowns insituations with low decay heat present a unique challenge to reactivitycontrol and require the operator and management to give careful attention toavoid recriticality. The events at Monticello and Big Rock Point also highlightthe importance of the operator giving continuous attention to plant parametersduring a shutdown.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical contacts: Melvyn Leach, RIII(708) 790-5559Geoffrey Wright, 'RIII(708) 790-5695Paul O'Connor, NRR(301) 504-1307Attachment: List of Recently Issued NRC Information Notices*SEE PREVIOUS CONCURRENCEOFC RIII ADR4-5:PD4-1 SC:DOEA:OEAB ADM:RPB C:DST:SRXB C:DOEA:OEABNAME GWright* PO'Connor (OK RDennig* JMain* RJones* AChaffeeDATE 03/11/92 3 /17/92 Xis 192 03/11/92 03/13/92 / /92OFC C:DOEA:OGCB D:DOEANAME CBerlinger CRossiDATE / /92 / /92 IN 92-XXMarch xx, 1992 Further details of the event may be found in NRC Inspection Report50-416/92-04 and Licensee Event Report 50-416/91-16.DiscussionThe underlying principle of reactivity management is to maintain the reactorin the desired condition by properly anticipating, controlling, and respondingto the plant's changing parameters. Once a reactor has been placed in asubcritical condition during a plant shutdown, it is of primary importancethat the reactor be maintained in that subcritical condition. Since recriti-cality has not been avoided, promptly inserting control rods and terminatingthe cooldown are more conservative actions than raising the range of the IRMsto prevent scram while waiting on the changing temperature to reverse thedirection of the power. Raising the range of the IRMs in the event of recriti-cality can be inappropriate if the licensed operators do not have management'soversight, training, and specific procedures.The experience at Grand Gulf and Monticello indicate that slow shutdowns insituations with low decay heat present a unique challenge to reactivitycontrol and require the operator and management to give careful attention toavoid recriticality. The events at Monticello and Big Rock Point also highlightthe importance of the operator giving continuous attention to plant parametersduring a shutdown.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical contacts: Melvyn Leach, RIII(708) 790-5559Geoffrey Wright, RII(708) 790-5695Paul O'Connor, NRR(301) 504-1307Attachment: List of Recently Issued NRC Information Notices*SEE PREVIOUS CONCURRENCEOFC :RIII :S Oq;:EfAB:ADM:RPB :C:D B :C:DOEA:OEAB :C:DOEA:OGCB :D:DOEA-____ _ _________---- _ --____________ ----_ -:---------------NAME :GWright* :Rbennig :JMain* j Jones :AChaffee :CBerlinger :CRossiDATE :03/11/92 :3 /11/92 :03/11/92 : /13/92 : / /92 : / /92 : / /92OFFICIAL RECORD COPYDocument Name: IN DENNIG
(301) 504-1307 Attachment:         List of Recently Issued NRC Information Notices


Further details of the event may be found in NRC Inspection Report 50-416/92-04and Licensee Event Report 50-416/91-16.DiscussionThe underlying principle of reactivity management is to maintain the reactor inthe desired condition by properly anticipating, controlling, and responding tothe plant's changing parameters. Once a reactor has been placed in asubcritical condition, as part of a plant shutdown, it is of primary importancethat the reactor be maintained in that subcritical condition. Given thatrecriticality has not been avoided, prompt insertion of control rods andtermination of the cooldown are more conservative actions than upranging IRMsto prevent scram while waiting on temperature coefficients to turn power.Upranging IRMs in the event of recriticality can be inappropriate if managementoversight, prior training, and specific procedures are absent.The experience at Grand Gulf and Monticello indicate that slow shutdowns insituations with low decay heat present a particular challenge to reactivitycontrol, and require heightened operator and management awareness to avoidrecriticality. In addition, the events at Monticello and Big Rock Pointhighlight the importance of continuous operator attention to plant parametersin the course of a shutdown.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contact oneof the technical contacts listed below or the appropriate Office of NuclearReactor Regulation (NRR) project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical contacts: Melvyn Leach, RIII(708) 790-5559Geoffrey Wright, RIII(708) 790-5695Paul O'Connor, NRR(301) 504-1307Attachment: List of Recently Issued NRC Information NoticesOFC :RIII :SC:DOEA:OEAB:ADM:RPB :C:DST:SRXB :C:DOEA:OEAB :C:DOEA:OGCB :D:DOEANAME :GWright :RDennig :JMain_ w :RJones :AChaffee :CBerlinger :CRossiDATE ://92 :1/92 :3/11 /92 :1/92 ://92 :1 /92 :/ /92OFFICIAL RECORD COPYDocument Name: IN DENNIG/DUP
*SEE PREVIOUS CONCURRENCE


IN 92-XXMarch xx, 1992 Further details of the event may be found in NRC Inspection Report50-416/92-04 and Licensee Event Report 50-416/91-16.DiscussionThe underlying principle of reactivity management is to maintain the reactorin the desired condition by properly anticipating, controlling, and respondingto the plant's changing parameters. Once a reactor has been placed in asubcritical condition, as part of a plant shutdown, it is of primary importancethat the reactor be maintained in that subcritical condition. Given thatrecriticality has not been avoided, prompt insertion of control rods andtermination of the cooldown are more conservative actions than uprangingIRMs to prevent scram while waiting on temperature coefficients to turn power.Upranging IRMs in the event of recriticality can be inappropriate if managementoversight, prior training, and specific procedures are absent.The experience at Grand Gulf and Monticello indicate that slow shutdowns insituations with low decay heat present a particular challenge to reactivitycontrol, and require heightened operator and management awareness to avoidrecriticality. In addition, the events at Monticello and Big Rock Pointhighlight the importance of continuous operator attention to plant parametersin the course of a shutdown.This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager.Charles E. Rossi, DirectorDivision of Operational Events AssessmentOffice of Nuclear Reactor RegulationTechnical contacts: Melvyn Leach, RIII(708) 790-5559Geoffrey Wright, RIII(708) 790-5695Paul O'Connor, NRR(301) 504-1307Attachment: List of Recently Issued NRC Information NoticesOFC :R111 bC:DOEA:OEAB:ADM:RPB :C:DST:SRXB :C:DOEA:OEAB :C:DOEA:OGCB :D:DOEANAME :GWrightV' :RDennig :JMain :RJones :AChaffee :CBerlinger :CRossiDATE :3 /II/92 ://92 :/ /92 :/ /92 :/ /92 :/ /92 :/ /92OFFICIAL RECORD COPYDocument Name: IN DENNIG
OFC  :RIII         :S              Oq;:EfAB:ADM:RPB         :C:D      B  :C:DOEA:OEAB :C:DOEA:OGCB :D:DOEA


}}
____
            _    _________----_          -- ____________                ----  _  -:---------------
NAME :GWright*    :Rbennig              :JMain*          j  Jones      :AChaffee      :CBerlinger            :CRossi
 
DATE :03/11/92    :3 /11/92              :03/11/92          : /13/92    : / /92        : / /92                : / /92 OFFICIAL RECORD COPY
 
Document Name: IN DENNIG
 
Further details of the event may be found in NRC Inspection Report 50-416/92-04 and Licensee Event Report 50-416/91-16.
 
Discussion
 
The underlying principle of reactivity management is to maintain the reactor in
 
the desired condition by properly anticipating, controlling, and responding to
 
the plant's changing parameters. Once a reactor has been placed in a
 
subcritical condition, as part of a plant shutdown, it is of primary importance
 
that the reactor be maintained in that subcritical condition. Given that
 
recriticality has not been avoided, prompt insertion of control rods and
 
termination of the cooldown are more conservative actions than upranging IRMs
 
to prevent scram while waiting on temperature coefficients to turn power.
 
Upranging IRMs in the event of recriticality can be inappropriate if management
 
oversight, prior training, and specific procedures are absent.
 
The experience at Grand Gulf and Monticello indicate that slow shutdowns in
 
situations with low decay heat present a particular challenge to reactivity
 
control, and require heightened operator and management awareness to avoid
 
recriticality. In addition, the events at Monticello and Big Rock Point
 
highlight the importance of continuous operator attention to plant parameters
 
in the course of a shutdown.
 
This information notice requires  no specific action or written response. If
 
you have any questions about 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:  Melvyn Leach, RIII
 
(708) 790-5559 Geoffrey Wright, RIII
 
(708) 790-5695 Paul O'Connor, NRR
 
(301) 504-1307 Attachment:  List of Recently Issued NRC Information Notices
 
OFC  :RIII          :SC:DOEA:OEAB:ADM:RPB    :C:DST:SRXB  :C:DOEA:OEAB :C:DOEA:OGCB :D:DOEA
 
NAME :GWright      :RDennig    :JMain_ w    :RJones      :AChaffee    :CBerlinger  :CRossi
 
DATE ://92          :1/92        :3/11 /92    :1/92        ://92        :1    /92    :/    /92 OFFICIAL RECORD COPY
 
Document Name: IN DENNIG/DUP
 
IN 92-XX
 
March xx, 1992 Further details of the event may be found in NRC Inspection Report
 
50-416/92-04 and Licensee Event Report 50-416/91-16.
 
Discussion
 
The underlying principle of reactivity management is to maintain the reactor
 
in the desired condition by properly anticipating, controlling, and responding
 
to the plant's changing parameters. Once a reactor has been placed in a
 
subcritical condition, as part of a plant shutdown, it is of primary importance
 
that the reactor be maintained in that subcritical condition. Given that
 
recriticality has not been avoided, prompt insertion of control rods and
 
termination of the cooldown are more conservative actions than upranging
 
IRMs to prevent scram while waiting on temperature coefficients to turn power.
 
Upranging IRMs in the event of recriticality can be inappropriate if management
 
oversight, prior training, and specific procedures are absent.
 
The experience at Grand Gulf and Monticello indicate that slow shutdowns in
 
situations with low decay heat present a particular challenge to reactivity
 
control, and require heightened operator and management awareness to avoid
 
recriticality. In addition, the events at Monticello and Big Rock Point
 
highlight the importance of continuous operator attention to plant parameters
 
in the course of a shutdown.
 
This information notice requires no specific action or written response. If
 
you have any questions about 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:  Melvyn Leach, RIII
 
(708) 790-5559 Geoffrey Wright, RIII
 
(708) 790-5695 Paul O'Connor, NRR
 
(301) 504-1307 Attachment:  List of Recently Issued NRC Information Notices
 
OFC  :R111          bC:DOEA:OEAB:ADM:RPB      :C:DST:SRXB  :C:DOEA:OEAB :C:DOEA:OGCB :D:DOEA
 
NAME :GWrightV' :RDennig        :JMain      :RJones      :AChaffee    :CBerlinger  :CRossi
 
DATE :3 /II/92      ://92        :/    /92  :/    /92    :/    /92    :/    /92    :/    /92 OFFICIAL RECORD COPY
 
Document Name: IN DENNIG}}


{{Information notice-Nav}}
{{Information notice-Nav}}

Latest revision as of 03:22, 24 November 2019

Unplanned Return to Criticality During Reactor Shutdown
ML031200314
Person / Time
Site: Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant  Entergy icon.png
Issue date: 05/13/1992
From: Rossi C
Office of Nuclear Reactor Regulation
To:
References
IN-92-039, NUDOCS 9205060036
Download: ML031200314 (12)


UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 May 13, 1992 NRC INFORMATION NOTICE 92-39: UNPLANNED RETURN TO CRITICALITY DURING

REACTOR SHUTDOWN

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to recent events involving unplanned returns to

criticality caused by the cooldown of the reactor coolant system during reactor

shutdowns. 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

The licensees for the following plants recently experienced unplanned

criticalities during reactor shutdowns: the Monticello Nuclear Generating

Plant, the Big Rock Point Nuclear Plant, and the Grand Gulf Nuclear Station.

Monticello

On June 6, 1991, an unplanned criticality occurred at the Monticello Nuclear

Generating Plant during a reactor shutdown when an unanticipated cooldown

occurred in conjunction with rod insertion. The licensee initiated this

shutdown to repair a leaking safety-relief valve shortly after the plant was

returning to power following a refueling outage.

Since this shutdown occurred shortly after the cycle startup, the reactor did

not generate sufficient decay heat to produce the steam needed to supply the

normal steam system loads and still maintain pressure in the reactor. As

control rods were being inserted using notch insertion, the reactor coolant

system pressure and temperature began to decrease. As long as the operator

continued to insert control rods, the reactor remained subcritical.

However, when the operator stopped inserting control rods to review and

evaluate plant conditions, the cooldown continued, adding sufficient

reactivity to overcome the negative reactivity from the insertion of control

rods and causing the reactor power to increase. The reactor power continued to

92050_*

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IN 92-39 May 13, 1992 increase until an intermediate-range monitor (IRM) tripped on a high-high flux

setpoint, causing a reactor scram.

Further details of the event may be found in Licensee Event Report

50-263/91-15 or NRC Inspection Report 50-263/91-13.

Big Rock Point

On November 30, 1991, an unplanned criticality occurred at the Big Rock Point

Nuclear Plant during a reactor shutdown when a cooldown occurred in conjunction

with a rod insertion. The event occurred during a scheduled shutdown for a

refueling outage.

After taking the main turbine off line and bringing the reactor to a

subcritical condition, the licensee delayed actions to continue inserting

control rods while changing shifts. However, the reactor coolant system

continued to cool down because the normal steam system loads were removing

heat. The cooldown added sufficient reactivity to eventually overcome the

effects of the control rods, and the reactor returned critical. The operators

noticed the condition in approximately 2 minutes and took the appropriate

actions to return the reactor subcritical.

Further details of the event may be found in Licensee Event Report

50-155/91-OO9-orJNRC-Inspection-Report 50-155/91-25. _ _ _

Grand Gulf

On December 30, 1991, an unplanned criticality occurred at the Grand Gulf

Nuclear Station during a reactor shutdown when a cooldown occurred in

conjunction with a rod insertion. The event occurred during a scheduled

shutdown for a maintenance outage to replace a recirculation pump shaft. To

minimize the release of plated-out radioactive material from the fuel cladding

and channels to the reactor coolant system, a slow power reduction and a slow

cooldown were prescribed for this shutdown.

The slow power reduction and the effects of an earlier outage minimized reactor

decay heat and xenon peaking. The operators were driving rods into the core

individually because gang drive for the control rods was unavailable. This

made the reactivity insertion slower than normal. The operators inserted

rods to reduce flux to range 3 on the IRMs and stopped rod insertion in order

to perform a source range monitor surveillance. Indicated power reached IRM

range 1. However, the reactor coolant system continued to cool down because

the normal steam system loads were removing heat. In order not to affect the

source range monitor surveillance, the shift supervisor elected to not insert

control rods and alerted the operators to the possibility of a return to

criticality as the cooldown continued. The reactivity added by the cooldown

eventually overcame the effects of the control rods. The reactor returned to a

critical condition, and reactor power increased on a reactor period between 300

to 800 seconds. The operators, having been trained on a similar event at

another BWR, were monitoring the condition and expected the power to increase

to the point of adding heat, where the fuel and moderator temperature coeffi- cients would halt the reactor power increase. As the power increased, the

IN 92-39 May 13, 1992 reactor operator ranged the IRMs to prevent a reactor scram. Power level

remained on scale. The power increase was terminated with the IRMs reading on

range 7 and 8. The operators completed the source range monitor surveillance, then resumed inserting rods and successfully completed the shutdown.

Further details of the event may be found in NRC Inspection Report

50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor

in the desired condition by properly anticipating, controlling, and responding

to the plant's changing parameters. The experience at Monticello, Big Rock

Point, and Grand Gulf indicates that shutdowns in situations with low decay

heat present a unique challenge to reactivity control if inadvertent recritica- lity is to be prevented. These events emphasize the importance of the operator

giving continuous attention to plant parameters during a shutdown. Further, accepting recriticality without prior management approval and procedures, i.e.,

an ad-hoc approach, raises concerns due to the lack of opportunity for contin- gency planning.

This information notice requires no specific action or written response. If

you have any questions about 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.

Va&res'E.Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contacts: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, RIII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 Attachment: List of Recently Issued NRC Information Notices

z

z

Attachment 1 I-0

IN 92-39

.< Z G2 May 13, 1992 OD n Page I of 1

co
  • a

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LIST OF RECENTLY ISSUED

>0 o) Z NRC INFORMATION NOTICES

Enenz q

m Z -n M Information

m 0n a

.4- information Date of

rn - 0 cn

n Notice No. Subject Issuance Issued to

o a -

0 h <-

0 92-38 Implementation Date for 05/12/92 All holders of OLs or CPs

the Revision to the EPA for nuclear power reactors, Manual of Protective non-power reactors and

Action Guides and Pro- materials licensees auth- tective Actions for orized to possess large

Nuclear Incidents quantities of radioactive

material.

1 92-37 Implementation of the 05/08/92 All Nuclear Regulatory

Deliberate Nisconduct Rule Commission Licensees.

92-16, Loss of Flow from the Re- 05/07/92 All holders of OLs or CPs

Supp. 1 sidual Heat Removal Pump for nuclear power reactors.

during Refueling Cavity

Draindown

I 92-36 Intersystem LOCA Outside 05/07/92 All holders of OLs or CPs

Containment for nuclear power reactors.

' 92-35 Higher Than Predicted Ero- 05/06/92 All holders of OLs or CPs

sion/Corrosion in Unisol- for nuclear power reactors.

able Reactor Coolant Pres- sure Boundary Piping Inside

Containment at A Boiling

Water Reactor

92-34 New Exposure Limits for

Airborne Uranium and

Thorium

05/06/92 All licensees whose opera- tions can cause airborne

concentrations of uranium

(

and thorium.

92-33 Increased Instrument 04/30/92 All holders of OLs or CPs

-u Response Time When for nuclear power reactors.

0 Pressure Dampening

rn> Devices are Installed

mD

- C > 0

D2 zzz> Operating License

u) D- I CP - 'Construction Permit

0

IN 92-39 May 13, 1992 reactor operator ranged the IRMs to prevent a reactor scram. Power level

remained on scale. The power increase was terminated with the IRMs reading on

range 7 and 8. The operators completed the source range monitor surveillance, then resumed inserting rods and successfully completed the shutdown.

Further details of the event may be found in NRC Inspection Report

50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor

in the desired condition by properly anticipating, controlling, and responding

to the plant's changing parameters. The experience at Monticello, Big Rock

Point, and Grand Gulf indicates that shutdowns in situations with low decay

heat present a unique challenge to reactivity control if inadvertent recritica- lity is to be prevented. These events emphasize the importance of the operator

giving continuous attention to plant parameters during a shutdown. Further, accepting recriticality without prior management approval and procedures, i.e.,

an ad-hoc approach, raises concerns due to the lack of opportunity for contin- gency planning.

This information notice requires no specific action or written response. If

you have any questions about 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. Original Signed by

Nhedes E, Rossi

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contacts: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, RIII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 Attachment: List of Recently Issued NRC Information Notices

  • SEE PREVIOUS CONCURRENCE

OFC RIII ADR4-5:PD4-1 SC:DOEA:OEAB ADM:RPB C:DST:SRXB C:DOEA:OEAB

NAME GWright* PO'Connor* RDennig* JMain* RJones* AChaffee*

DATE 03/11/92 03/17/92 03/11/92 03/11/92 03/13/92 04/13/92 OFC C:DOEA:OGCB D X,

NAME CBerlinger*

DATE 04/21/92 05/ 7/92 DOCUMENT NAME: IN 92-39

IN 92-XX

April xx, 1992 Further details of the event may be found in NRC Inspection Report

50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor

in the desired condition by properly anticipating, controlling, and responding

to the plant's changing parameters. The experience at Big Rock Point, Grand

Gulf, and Monticello indicates that shutdowns in situations with low decay heat

present a unique challenge to reactivity control if inadvertent recriticality

is to be prevented. These events emphasize the importance of the operator

giving continuous attention to plant parameters during a shutdown. Further, accepting recriticality without prior management approval and procedures, i.e.,

an ad-hoc approach, raises concerns due to the lack of opportunity for contin- gency planning.

This information notice requires no specific action or written response. If

you have any questions about 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: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, RIII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 Attachment: List of Recently Issued NRC Information Notices

  • SEE PREVIOUS CONCURRENCE

OFC RIII ADR4-5:PD4-1 SC:DOEA:OEAB ADM:RPB C:DST:SRXB C:DOEA:OEAB

NAME GWright* PO'Connor* RDennig* JMain* RJones* AChaffee*

DATE 03/11/92 03/17/92 03/11/92 03/11/92 03/13/92 04/13/92 OFC C:DOEA:OGCB D:DOEA

NAME CBerlinger* CRoss tD

DATE 04/21/92 /9 OFFICIAL RECORD COPY DOCUMENT NAME: IN DENKIG

IN 92-XX

April xx, 1992 Further details of the event may be found in NRC Inspection Report

50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor

in the desired condition by properly anticipating, controlling, and responding

to the plant's changing parameters. Once a reactor has been placed in a

subcritical condition during a plant shutdown, it is of primary importance

that the reactor be maintained in that subcritical condition. Raising the

range of the IRMs in the event of recriticality can be inappropriate if the

licensed operators do not have management's oversight, training, and specific

procedures for this.

The experience at Grand Gulf and Monticello indicate that slow shutdowns in

situations with low decay heat present a unique challenge to reactivity

control and require the operator and management to give careful attention to

avoid recriticality. The events at Monticello and Big Rock Point also highlight

the importance of the operator giving continuous attention to plant parameters

during a shutdown.

This information notice requires no specific action or written response. If

you have any questions about 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: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, RIII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 Attachment: List of Recently Issued NRC Information Notices

  • SEE PREVIOUS CONCURRENCE

OFC RIII ADR4-5:PD4-1 SC:DOEA:OEAB ADM:RPB C:DST:SRXB C:DOEA:OEAB

NAME GWright* PO'Connor* RDennig* JMain* RJones* AChaffee*

DATE 03/11/92 03/17/92 03/11/92 03/11/92 03/13/92 04/13/92 OFC C:DO J

f D:DOEA

NAME CBerlinger CRosswa'y

DATE f/;11/ 92 / 92 OFFICIAL RECORD COPY DOCUMENT NAME: IN DENNIG

IN 92-XX

March xx, 1992 Further details of the event may be found in NRC Inspection Report

50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor

in the desired condition by properly anticipating, controlling, and responding

to the plant's changing parameters. Once a reactor has been placed in a

subcritical condition during a plant shutdown, it is of primary importance

that the reactor be maintained in that subcritical condition. In the event of

recriticality, promptly inserting control rods and terminating the cooldown are

more conservative actions than raising the range of the IRMs to prevent scram

while waiting on the changing temperature to reverse the direction of the

power. Raising the range of the IRMs in the event of recriticality can be

inappropriate if the licensed operators do not have management's oversight, training, and specific procedures.

The experience at Grand Gulf and Monticello indicate that slow shutdowns in

situations with low decay heat present a unique challenge to reactivity

control and require the operator and management to give careful attention to

avoid recriticality. The events at Monticello and Big Rock Point also highlight

the importance of the operator giving continuous attention to plant parameters

during a shutdown.

This information notice requires no specific action or written response. If

you have any questions about 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: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, RIII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 t'/ ' f' 4.f, IV

Attachment: List of Recently Issued NRC Information Notices

  • SEE PREVIOUS CONCURRENCEC

OFC RIII ADR4-5:PD4-1 SC:DOEA:OEAB ADM:RPB C:DST:SRXB C:D0EA:OEAB

NAME GWright* PO'Connor* RDennig* JMain* RJones* AChaffee

DATE 03/11/92 03/17/92 03/11/92 03/11/92 03/13/92 q1/t3/92 OFC C:DOEA:OGCB D:DOEA

NAME CBerlinger CRossi

DATE / /92 / /92 OFFICIAL RECORD COPY DOCUMENT NAME: IN DENNIG

IN 92-XX

March xx, 1992 Further details of the event may be found in NRC Inspection Report

50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor

in the desired condition by properly anticipating, controlling, and responding

to the plant's changing parameters. Once a reactor has been placed in a

subcritical condition during a plant shutdown, it is of primary importance

that the reactor be maintained in that subcritical condition. In the event of

recriticality, promptly inserting control rods and terminating the cooldown are

more conservative actions than raising the range of the IRMs to prevent scram

while waiting on the changing temperature to reverse the direction of the

power. Raising the range of the IRMs in the event of recriticality can be

inappropriate if the licensed operators do not have management's oversight, training, and specific procedures.

The experience at Grand Gulf and Monticello indicate that slow shutdowns in

situations with low decay heat present a unique challenge to reactivity

control and require the operator and management to give careful attention to

avoid recriticality. The events at Monticello and Big Rock Point also highlight

the importance of the operator giving continuous attention to plant parameters

during a shutdown.

This information notice requires no specific action or written response. If

you have any questions about 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: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, 'RIII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 Attachment: List of Recently Issued NRC Information Notices

  • SEE PREVIOUS CONCURRENCE

OFC RIII ADR4-5:PD4-1 SC:DOEA:OEAB ADM:RPB C:DST:SRXB C:DOEA:OEAB

NAME GWright* PO'Connor (OK RDennig* JMain* RJones* AChaffee

DATE 03/11/92 3 /17/92 Xis 192 03/11/92 03/13/92 / /92 OFC C:DOEA:OGCB D:DOEA

NAME CBerlinger CRossi

DATE / /92 / /92

IN 92-XX

March xx, 1992 Further details of the event may be found in NRC Inspection Report

50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor

in the desired condition by properly anticipating, controlling, and responding

to the plant's changing parameters. Once a reactor has been placed in a

subcritical condition during a plant shutdown, it is of primary importance

that the reactor be maintained in that subcritical condition. Since recriti- cality has not been avoided, promptly inserting control rods and terminating

the cooldown are more conservative actions than raising the range of the IRMs

to prevent scram while waiting on the changing temperature to reverse the

direction of the power. Raising the range of the IRMs in the event of recriti- cality can be inappropriate if the licensed operators do not have management's

oversight, training, and specific procedures.

The experience at Grand Gulf and Monticello indicate that slow shutdowns in

situations with low decay heat present a unique challenge to reactivity

control and require the operator and management to give careful attention to

avoid recriticality. The events at Monticello and Big Rock Point also highlight

the importance of the operator giving continuous attention to plant parameters

during a shutdown.

This information notice requires no specific action or written response. If

you have any questions about 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: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, RII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 Attachment: List of Recently Issued NRC Information Notices

  • SEE PREVIOUS CONCURRENCE

OFC :RIII :S Oq;:EfAB:ADM:RPB :C:D B :C:DOEA:OEAB :C:DOEA:OGCB :D:DOEA

____

_ _________----_ -- ____________ ---- _ -:---------------

NAME :GWright* :Rbennig :JMain* j Jones :AChaffee :CBerlinger :CRossi

DATE :03/11/92 :3 /11/92 :03/11/92  : /13/92  : / /92  : / /92  : / /92 OFFICIAL RECORD COPY

Document Name: IN DENNIG

Further details of the event may be found in NRC Inspection Report 50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor in

the desired condition by properly anticipating, controlling, and responding to

the plant's changing parameters. Once a reactor has been placed in a

subcritical condition, as part of a plant shutdown, it is of primary importance

that the reactor be maintained in that subcritical condition. Given that

recriticality has not been avoided, prompt insertion of control rods and

termination of the cooldown are more conservative actions than upranging IRMs

to prevent scram while waiting on temperature coefficients to turn power.

Upranging IRMs in the event of recriticality can be inappropriate if management

oversight, prior training, and specific procedures are absent.

The experience at Grand Gulf and Monticello indicate that slow shutdowns in

situations with low decay heat present a particular challenge to reactivity

control, and require heightened operator and management awareness to avoid

recriticality. In addition, the events at Monticello and Big Rock Point

highlight the importance of continuous operator attention to plant parameters

in the course of a shutdown.

This information notice requires no specific action or written response. If

you have any questions about 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: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, RIII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 Attachment: List of Recently Issued NRC Information Notices

OFC :RIII :SC:DOEA:OEAB:ADM:RPB :C:DST:SRXB :C:DOEA:OEAB :C:DOEA:OGCB :D:DOEA

NAME :GWright :RDennig :JMain_ w :RJones :AChaffee :CBerlinger :CRossi

DATE ://92 :1/92 :3/11 /92 :1/92  ://92 :1 /92  :/ /92 OFFICIAL RECORD COPY

Document Name: IN DENNIG/DUP

IN 92-XX

March xx, 1992 Further details of the event may be found in NRC Inspection Report

50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor

in the desired condition by properly anticipating, controlling, and responding

to the plant's changing parameters. Once a reactor has been placed in a

subcritical condition, as part of a plant shutdown, it is of primary importance

that the reactor be maintained in that subcritical condition. Given that

recriticality has not been avoided, prompt insertion of control rods and

termination of the cooldown are more conservative actions than upranging

IRMs to prevent scram while waiting on temperature coefficients to turn power.

Upranging IRMs in the event of recriticality can be inappropriate if management

oversight, prior training, and specific procedures are absent.

The experience at Grand Gulf and Monticello indicate that slow shutdowns in

situations with low decay heat present a particular challenge to reactivity

control, and require heightened operator and management awareness to avoid

recriticality. In addition, the events at Monticello and Big Rock Point

highlight the importance of continuous operator attention to plant parameters

in the course of a shutdown.

This information notice requires no specific action or written response. If

you have any questions about 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: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, RIII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 Attachment: List of Recently Issued NRC Information Notices

OFC :R111 bC:DOEA:OEAB:ADM:RPB :C:DST:SRXB :C:DOEA:OEAB :C:DOEA:OGCB :D:DOEA

NAME :GWrightV' :RDennig :JMain :RJones :AChaffee :CBerlinger :CRossi

DATE :3 /II/92  ://92  :/ /92  :/ /92  :/ /92  :/ /92  :/ /92 OFFICIAL RECORD COPY

Document Name: IN DENNIG