Information Notice 2010-06, Inadvertent Control Rod Withdrawal Event While Shutdown: Difference between revisions

From kanterella
Jump to navigation Jump to search
(Created page by program invented by StriderTol)
(Created page by program invented by StriderTol)
 
(One intermediate revision by the same user not shown)
Line 3: Line 3:
| issue date = 02/17/2010
| issue date = 02/17/2010
| title = Inadvertent Control Rod Withdrawal Event While Shutdown
| title = Inadvertent Control Rod Withdrawal Event While Shutdown
| author name = McGinty T J
| author name = Mcginty T
| author affiliation = NRC/NRR/DPR
| author affiliation = NRC/NRR/DPR
| addressee name =  
| addressee name =  
Line 14: Line 14:
| page count = 6
| page count = 6
}}
}}
{{#Wiki_filter:ML092190302 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, DC  20555-0001
{{#Wiki_filter:UNITED STATES


February 17, 2010
NUCLEAR REGULATORY COMMISSION


NRC INFORMATION NOTICE 2010-06: INADVERTENT CONTROL ROD WITHDRAWAL EVENT WHILE SHUTDOWN
OFFICE OF NUCLEAR REACTOR REGULATION
 
WASHINGTON, DC 20555-0001 February 17, 2010
NRC INFORMATION NOTICE 2010-06:                 INADVERTENT CONTROL ROD WITHDRAWAL
 
EVENT WHILE SHUTDOWN


==ADDRESSEES==
==ADDRESSEES==
All holders of operating licenses or construction permits for nuclear power reactors under the
All holders of operating licenses or construction permits for nuclear power reactors under the


provisions of Title 10 of the
provisions of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing
 
of Production and Utilization Facilities, except those who have permanently ceased operations


Code of Federal Regulations (10 CFR) Part 50, "Domestic Licensing of Production and Utilization Facilities," except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel.
and have certified that fuel has been permanently removed from the reactor vessel.


==PURPOSE==
==PURPOSE==
The U.S. Nuclear Regulatory Commission (NRC) is is
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert
 
addressees of recent operating experience involving an inadvertent control rod withdrawal event
 
caused by a shutdown activity to isolate the water side of the control rod drive (CRD) hydraulic
 
control units (HCUs) at a boiling-water reactor. The procedure utilized to perform this activity
 
inadvertently replicated the normal control rod withdrawal hydraulic sequence for the last three


suing this information notice (IN) to alert addressees of recent operating experience involving an inadvertent control rod withdrawal event
HCUs isolated, resulting in three control rods being driven out of the core. The NRC expects


caused by a shutdown activity to isolate the water side of the control rod drive (CRD) hydraulic control units (HCUs) at a boiling-water reactor.  The procedure utilized to perform this activity inadvertently replicated the normal control rod withdrawal hydraulic sequence for the last three HCUs isolated, resulting in three control rods being driven out of the core.  The NRC expects that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. Suggestions contained in this IN are not NRC requirements; therefore, no specific action or written response is required.
that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. Suggestions contained in this IN are not NRC
 
requirements; therefore, no specific action or written response is required.


==DESCRIPTION OF CIRCUMSTANCES==
==DESCRIPTION OF CIRCUMSTANCES==
On November 3, 2008, with Dresden Unit 3 in cold shutdown, three control rods inadvertently withdrew to positions 06, 18, and 16, respectively (control rod position 00 indicates full-in and
On November 3, 2008, with Dresden Unit 3 in cold shutdown, three control rods inadvertently


control rod position 48 indicates full-out).  The cause of the event was a deficient procedure
withdrew to positions 06, 18, and 16, respectively (control rod position 00 indicates full-in and


performed to isolate all 177 HCUs for CRD system maintenance. The procedure specified sequentially isolating the HCUs with a CRD pump running which established the required
control rod position 48 indicates full-out). The cause of the event was a deficient procedure
 
performed to isolate all 177 HCUs for CRD system maintenance. The procedure specified
 
sequentially isolating the HCUs with a CRD pump running which established the required


hydraulic conditions for outward control rod motion (i.e., insert, unlatch, and withdraw).
hydraulic conditions for outward control rod motion (i.e., insert, unlatch, and withdraw).
Line 45: Line 66:
The event occurred 10 hours after shutdown (approximately peak xenon concentration) at a
The event occurred 10 hours after shutdown (approximately peak xenon concentration) at a


reactor coolant system (RCS) temperature of 168 °F. During the event, the reactor remained shutdown with approximately 4.5 percent shutdown margin. However, an inadvertent criticality event could have occurred under different conditions given that the procedure did not place controls on RCS temperature or xenon concentration. The following describes how sequentially isolating the HCUs with a CRD pump running
reactor coolant system (RCS) temperature of 168 °F. During the event, the reactor remained
 
shutdown with approximately 4.5 percent shutdown margin. However, an inadvertent criticality


established the hydraulic conditions for outward control rod motion (i.e., insertion, unlatching, and withdrawal):
event could have occurred under different conditions given that the procedure did not place
Initial Plant Conditions.  All 177 CRD HCU insert valves (HCU 101) and withdraw valves (HCU 102) were open; all 177 CRD HCU directional control valves 120, 121, 122, and 123 were closed; and one CRD pump was operating applying pressure to each of the 177 unisolated HCU under-piston and over-piston areas (see Figure 1).  Pressure to the under-piston area was


applied through the cooling water header through the open insert valve (HCU 101). Pressure to
controls on RCS temperature or xenon concentration.


the over-piston area was applied through the exhaust header (via the orificed check valve) through a designed reverse flow path in directional control valve 121 (which was shut), and then through the open withdraw valve (HCU 102). While the pressures to the under-piston and over-piston areas were roughly equal, a net inward force was applied because the under-piston has a
The following describes how sequentially isolating the HCUs with a CRD pump running
 
established the hydraulic conditions for outward control rod motion (i.e., insertion, unlatching, and withdrawal):
Initial Plant Conditions. All 177 CRD HCU insert valves (HCU 101) and withdraw valves (HCU
 
102) were open; all 177 CRD HCU directional control valves 120, 121, 122, and 123 were
 
closed; and one CRD pump was operating applying pressure to each of the 177 unisolated HCU
 
under-piston and over-piston areas (see Figure 1). Pressure to the under-piston area was
 
applied through the cooling water header through the open insert valve (HCU 101). Pressure to
 
the over-piston area was applied through the exhaust header (via the orificed check valve)
through a designed reverse flow path in directional control valve 121 (which was shut), and then
 
through the open withdraw valve (HCU 102). While the pressures to the under-piston and over- piston areas were roughly equal, a net inward force was applied because the under-piston has a


larger surface area (see Figure 3a).
larger surface area (see Figure 3a).


Figure 1 Running 3B CRD pump and flow paths available to the unisolated HCUs
Figure 1 Running 3B CRD pump and flow paths available to the unisolated HCUs


Control Rod Insertion. As shown in Figure 2, the CRD header pressure and proportional net inward rod force was initially small because the flow paths to all 177 unisolated HCUs were available.  Using procedural instructions, non-licensed operators began isolating the 177 HCUs one at a time.  As each successive HCU was isolated, available CRD header flow paths
Control Rod Insertion. As shown in Figure 2, the CRD header pressure and proportional net


decreased and since CRD header flow rate remained constant, system back pressure (i.e.,  
inward rod force was initially small because the flow paths to all 177 unisolated HCUs were
 
available. Using procedural instructions, non-licensed operators began isolating the 177 HCUs
 
one at a time. As each successive HCU was isolated, available CRD header flow paths
 
decreased and since CRD header flow rate remained constant, system back pressure (i.e.,
CRD header differential pressure) increased resulting in an exponentially increasing the net
CRD header differential pressure) increased resulting in an exponentially increasing the net


inward force on the remaining unisolated HCUs. After the 170
inward force on the remaining unisolated HCUs. After the 170th (NRC inspector determination)
th (NRC inspector determination) HCU was isolated, the net force to the under-p
HCU was isolated, the net force to the under-piston area of the seven unisolated HCUs was


iston area of the seven unisolated HCUs was
sufficient to lift the CRD and insert the rod into the over-travel position (Figure 3b). [Note: NRC


sufficient to lift the CRD and insert the rod into the over-travel position (Figure 3b).  [Note:  NRC Inspection Report 05000249/2009009 discusses that the NRC inspector disagreed with the licensee's determination that only 5 control rods (rather than 7 control rods) inserted to an over-
Inspection Report 05000249/2009009 discusses that the NRC inspector disagreed with the


travel position.] Figure 2 The number of isolated HCUs versus CRD header pressure and the
licensees determination that only 5 control rods (rather than 7 control rods) inserted to an over- travel position.] Figure 2 The number of isolated HCUs versus CRD header pressure and the


net force applied to the under-piston area of unisolated HCUs
net force applied to the under-piston area of unisolated HCUs


Control Rod Unlatching. When the 174 th HCU was isolated, CRD header differential pressure increased such that the control rods associated with the last three HCUs unlatched because of
Control Rod Unlatching. When the 174th HCU was isolated, CRD header differential pressure
 
increased such that the control rods associated with the last three HCUs unlatched because of


the force applied to the collet piston in the over-piston area was sufficient to compress the collet
the force applied to the collet piston in the over-piston area was sufficient to compress the collet
Line 80: Line 126:
spring (Figure 3c).
spring (Figure 3c).


Control Rod Withdrawal. The withdrawal of the three unlatched control rods occurred due to the order the valves were closed to isolate the HCU. By procedure, each of the 177 HCUs was
Control Rod Withdrawal. The withdrawal of the three unlatched control rods occurred due to the
 
order the valves were closed to isolate the HCU. By procedure, each of the 177 HCUs was
 
isolated by first closing its insert valve (HCU 101) and then closing its withdraw valve (HCU
 
102). Closing the insert valve isolated the HCU under-piston area causing this area to


isolated by
depressurize through designed leakage flow paths. After the 175th HCUs insert valve was shut, the associated control rod was free to move out of the core because the rod was unlatched and


first closing its insert valve (HCU 101) and
a net force was applied to the over-piston area (Figure 3d). Outward control rod motion also


then closing its withdraw valve (HCU 102).  Closing the insert valve isolated the HCU under-piston area causing this area to
occurred for the 176th and 177th HCUs for the same reason. Control rod motion continued until


depressurize through designed leakage flow paths.  After the 175 th HCU's insert valve was shut, the associated control rod was free to move out of the core because the rod was unlatched and
the non-licensed operator shut the associated HCU withdraw valve. The three control rods


a net force was applied to the over-piston area (Figure 3d).  Outward control rod motion also occurred for the 176 th and 177 th HCUs for the same reason. Control rod motion continued until
withdrew to the 06, 18, and 16 positions, respectively.


the non-licensed operator shut the associated HCU withdraw valve. The three control rods withdrew to the 06, 18, and 16 positions, respectively.
Figure 3 Status of CRDs Operator Performance Issues. The licensee found that the main control room was not included


Figure 3 Status of CRDs Operator Performance Issues.  The licensee found that the main control room was not included in the pre-job brief, control room operators were unaware the activity was in progress or that a
in the pre-job brief, control room operators were unaware the activity was in progress or that a


non-licensed operator was performing the HCU valve manipulations. This resulted in non-licensed operators manipulating mechanisms with the potential to affect reactivity without the
non-licensed operator was performing the HCU valve manipulations. This resulted in non- licensed operators manipulating mechanisms with the potential to affect reactivity without the


knowledge and consent of a licensed operator at the controls, as required by 10 CFR 50.54(j).
knowledge and consent of a licensed operator at the controls, as required by 10 CFR 50.54(j).


In addition, when the main control room received multiple unexpected control rod drift alarms and indications of rods being inserted to the over-travel position, control room operators first pursued whether the alarms may have been caused by known instrument maintenance rather
In addition, when the main control room received multiple unexpected control rod drift alarms


than actual control rod movement. As a result, the first rod drift alarm in the control room
and indications of rods being inserted to the over-travel position, control room operators first
 
pursued whether the alarms may have been caused by known instrument maintenance rather
 
than actual control rod movement. As a result, the first rod drift alarm in the control room


preceded the first unplanned control rod motion by almost 18 minutes, and the last three control
preceded the first unplanned control rod motion by almost 18 minutes, and the last three control


rods withdrew over the following 3 minutes. When rod movement stopped after the non-licensed operator shut the last of the HCU withdraw valves, the main control room operators no
rods withdrew over the following 3 minutes. When rod movement stopped after the non- licensed operator shut the last of the HCU withdraw valves, the main control room operators no


longer had control over any of the 177 control rods. The control room operators did not notice
longer had control over any of the 177 control rods. The control room operators did not notice


that the CRD cooling water header pressure was off scale high, which covered in the alarm
that the CRD cooling water header pressure was off scale high, which covered in the alarm


response procedure as potential cause for drifting rods. Approximately 1 hour and 38 minutes
response procedure as potential cause for drifting rods. Approximately 1 hour and 38 minutes


after the first unplanned control rod movement, the operators completed the insertion of each of the three control rods by manually cycling the associated HCU insert valves.
after the first unplanned control rod movement, the operators completed the insertion of each of


Root Cause Evaluation.  The licensee determined that the root cause of this event was latent procedural deficiencies. The procedure did not contain any precautions, prerequisites, selection
the three control rods by manually cycling the associated HCU insert valves.


criteria, or limitations for the quantity of HCUs to be isolated with an operating CRD pump.  The
Root Cause Evaluation. The licensee determined that the root cause of this event was latent


licensee had previously reviewed operating experi
procedural deficiencies. The procedure did not contain any precautions, prerequisites, selection


ence involving control rod withdrawal events at Japanese boiling-water reactors and had implemented procedural changes to prevent this
criteria, or limitations for the quantity of HCUs to be isolated with an operating CRD pump. The


occurrence at the station.  However, the licensee had not revised all of the affected procedures, including the one that was used during this event.
licensee had previously reviewed operating experience involving control rod withdrawal events


The licensee identified the following contributing factors for this event included: 
at Japanese boiling-water reactors and had implemented procedural changes to prevent this
* An over reliance on a computer-based keyword search program system to identify applicable procedures when implementing a procedural change.


* A "shutdown" mindset that did not emphasize that any maintenance on CRD HCUs should be considered for the potential effect on reactivity;  
occurrence at the station. However, the licensee had not revised all of the affected procedures, including the one that was used during this event.
* Operators not trusting their indications and promptly responding to a loss (or potential loss) of control rod control;  
 
* Non-licensed operators performing plant activities (HCU valve manipulations) that could affect reactivity, without the knowledge and consent of a licensed operator present at the
The licensee identified the following contributing factors for this event included:
 
* An over reliance on a computer-based keyword search program system to identify
 
applicable procedures when implementing a procedural change.
 
* A shutdown mindset that did not emphasize that any maintenance on CRD HCUs
 
should be considered for the potential effect on reactivity;
 
* Operators not trusting their indications and promptly responding to a loss (or potential
 
loss) of control rod control;
 
* Non-licensed operators performing plant activities (HCU valve manipulations) that could
 
affect reactivity, without the knowledge and consent of a licensed operator present at the


controls.
controls.


Specific corrective actions taken by the licensee included: identifying and updating all
Specific corrective actions taken by the licensee included: identifying and updating all
 
applicable procedures with key recommendations provided in the operating experience;
specifying a minimum number of HCUs to be kept in service with a CRD pump operating while


applicable procedures with key recommendations provided in the operating experience;
shutdown; monitoring CRD drive pressures, rod positions, and alarms during activities that
specifying a minimum number of HCUs to be kept in service with a CRD pump operating while shutdown; monitoring CRD drive pressures, rod positions, and alarms during activities that


could result in an inadvertent control rod movement; and ensuring that personnel who isolate
could result in an inadvertent control rod movement; and ensuring that personnel who isolate


HCUs are aware that they can affect control rod system drive pressure
HCUs are aware that they can affect control rod system drive pressure and cause inadvertent
 
and cause inadvertent control rod movement.


Additional information is available in
control rod movement.


Dresden Licensee Event Report 50-249/2008-003-00, dated December 31, 2008, and NRC Inspection Report 05000249/2009009, dated August 19, 2009. This information can be found on the NRC's public website in the Agencywide
Additional information is available in Dresden Licensee Event Report 50-249/2008-003-00,
dated December 31, 2008, and NRC Inspection Report 05000249/2009009, dated August 19, 2009. This information can be found on the NRCs public website in the Agencywide


Documents Access and Management System, under Accession Nos. ML090090372 and
Documents Access and Management System, under Accession Nos. ML090090372 and
Line 155: Line 227:


==DISCUSSION==
==DISCUSSION==
This event illustrates the importance of licensees understanding that some field activities at boiling-water and pressurized-water reactors can significantly affect reactivity. The regulation at 10 CFR 50.54(j) requires that the operation of apparatus and mechanisms that may affect the
This event illustrates the importance of licensees understanding that some field activities at
 
boiling-water and pressurized-water reactors can significantly affect reactivity. The regulation at
 
10 CFR 50.54(j) requires that the operation of apparatus and mechanisms that may affect the


reactivity of a reactor shall be manipulated only with the knowledge and consent of a licensed
reactivity of a reactor shall be manipulated only with the knowledge and consent of a licensed


operator present at the controls. In this case, the action of non-licensed operators using a
operator present at the controls. In this case, the action of non-licensed operators using a
 
deficient procedure to manually isolate the HCUs in the field led to multiple control rods moving
 
out of the core without the prior knowledge or consent of the licensed control room operators.
 
Moreover, the licensed operators failure to believe the control room indications unnecessarily


deficient procedure to manually isolate the HCUs in the field led to multiple control rods moving out of the core without the prior knowledge or consent of the licensed control room operators.  Moreover, the licensed operators' failure to believe the control room indications unnecessarily delayed their diagnosis of, and response to, the event. Under a different set of initial conditions, this event could have resulted in an inadvertent criticality.
delayed their diagnosis of, and response to, the event. Under a different set of initial conditions, this event could have resulted in an inadvertent criticality.


==CONTACT==
==CONTACT==
This IN requires no specific action or written response. Please direct any questions about this
This IN requires no specific action or written response. Please direct any questions about this


matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor
matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor
Line 170: Line 252:
Regulation project manager.
Regulation project manager.


/RA/  
/RA/
Timothy J. McGinty, Director
                                              Timothy J. McGinty, Director
 
Division of Policy and Rulemaking


Division of Policy and Rulemaking Office of Nuclear Reactor Regulation
Office of Nuclear Reactor Regulation


Technical Contacts: Mark Ring, RIII Jamie Benjamin, RIII
Technical Contacts:     Mark Ring, RIII                       Jamie Benjamin, RIII


630/829-9703 630/829-9753 E-mail: Mark.Ring@nrc.gov E-mail:
630/829-9703                         630/829-9753 E-mail: Mark.Ring@nrc.gov             E-mail: Jamie.Benjamin@nrc.gov
Jamie.Benjamin@nrc.gov


Carey Brown, RIII
Carey Brown, RIII


630/829-9605 E-mail: Carey.Brown@nrc.gov
630/829-9605 E-mail: Carey.Brown@nrc.gov
 
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections. August 19, 2009. This information can be found on the NRCs public website in the Agencywide


Note:  NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections. August 19, 2009.  This information can be found on the NRC's public website in the Agencywide Documents Access and Management System, under Accession Nos. ML090090372 and
Documents Access and Management System, under Accession Nos. ML090090372 and


ML092320164, respectively.
ML092320164, respectively.


==DISCUSSION==
==DISCUSSION==
This event illustrates the importance of licensees understanding that some field activities at boiling-water and pressurized-water reactors can significantly affect reactivity. The regulation at 10 CFR 50.54(j) requires that the operation of apparatus and mechanisms that may affect the
This event illustrates the importance of licensees understanding that some field activities at
 
boiling-water and pressurized-water reactors can significantly affect reactivity. The regulation at
 
10 CFR 50.54(j) requires that the operation of apparatus and mechanisms that may affect the


reactivity of a reactor shall be manipulated only with the knowledge and consent of a licensed
reactivity of a reactor shall be manipulated only with the knowledge and consent of a licensed


operator present at the controls. In this case, the action of non-licensed operators using a
operator present at the controls. In this case, the action of non-licensed operators using a
 
deficient procedure to manually isolate the HCUs in the field led to multiple control rods moving
 
out of the core without the prior knowledge or consent of the licensed control room operators.
 
Moreover, the licensed operators failure to believe the control room indications unnecessarily


deficient procedure to manually isolate the HCUs in the field led to multiple control rods moving out of the core without the prior knowledge or consent of the licensed control room operators.  Moreover, the licensed operators' failure to believe the control room indications unnecessarily delayed their diagnosis of, and response to, the event. Under a different set of initial conditions, this event could have resulted in an inadvertent criticality.
delayed their diagnosis of, and response to, the event. Under a different set of initial conditions, this event could have resulted in an inadvertent criticality.


==CONTACT==
==CONTACT==
This IN requires no specific action or written response. Please direct any questions about this
This IN requires no specific action or written response. Please direct any questions about this


matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor
matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor
Line 204: Line 299:
Regulation project manager.
Regulation project manager.


/RA/  
/RA/
Timothy J. McGinty, Director
                                              Timothy J. McGinty, Director
 
Division of Policy and Rulemaking


Division of Policy and Rulemaking Office of Nuclear Reactor Regulation
Office of Nuclear Reactor Regulation


Technical Contacts: Mark Ring, RIII Jamie Benjamin, RIII
Technical Contacts:     Mark Ring, RIII                       Jamie Benjamin, RIII


630/829-9703 630/829-9753 E-mail: Mark.Ring@nrc.gov E-mail:
630/829-9703                         630/829-9753 E-mail: Mark.Ring@nrc.gov             E-mail: Jamie.Benjamin@nrc.gov
Jamie.Benjamin@nrc.gov


Carey Brown, RIII
Carey Brown, RIII


630/829-9605 E-mail: Carey.Brown@nrc.gov
630/829-9605 E-mail: Carey.Brown@nrc.gov
 
Note:  NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
 
ADAMS Accession Number:  ML092190302      TAC ME0707 OFFICE RIII TECH EDITOR
 
BC:RIII:DRP
 
BC:IOLB:DIRS
 
D:DIRS NAME JBenjamin KAzariah-Kribbs


MRing JMunro FBrown DATE 9/1/09 e-mail 8/21/09 e-mail 9/1/09 e-ma
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.


il 9/2/09 9/2/09 OFFICE LA:PGCB:NRR
ADAMS Accession Number: ML092190302                                          TAC ME0707 OFFICE   RIII              TECH EDITOR      BC:RIII:DRP        BC:IOLB:DIRS    D:DIRS


PM:PGCB:NRR
NAME      JBenjamin          KAzariah-Kribbs  MRing              JMunro          FBrown


BC:PGCB:NRR
DATE      9/1/09 e-mail      8/21/09 e-mail  9/1/09 e-mail      9/2/09          9/2/09 OFFICE    LA:PGCB:NRR        PM:PGCB:NRR      BC:PGCB:NRR        D:DPR:NRR


D:DPR:NRR
NAME      CHawes            DBeaulieu        MMurphy            TMcGinty


NAME CHawes DBeaulieu MMurphy TMcGinty  OFFICE 2/16/10 9/2/09 2/16/10 2/17/10 OFFICIAL RECORD COPY}}
OFFICE   2/16/10           9/2/09           2/16/10           2/17/10
                                      OFFICIAL RECORD COPY}}


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

Latest revision as of 04:25, 14 November 2019

Inadvertent Control Rod Withdrawal Event While Shutdown
ML092190302
Person / Time
Issue date: 02/17/2010
From: Mcginty T
Division of Policy and Rulemaking
To:
David Beaulieu
References
IN-10-006
Download: ML092190302 (6)
v • d • e


UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001 February 17, 2010

NRC INFORMATION NOTICE 2010-06: INADVERTENT CONTROL ROD WITHDRAWAL

EVENT WHILE SHUTDOWN

ADDRESSEES

All holders of operating licenses or construction permits for nuclear power reactors under the

provisions of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing

of Production and Utilization Facilities, except those who have permanently ceased operations

and have certified that fuel has been permanently removed from the reactor vessel.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert

addressees of recent operating experience involving an inadvertent control rod withdrawal event

caused by a shutdown activity to isolate the water side of the control rod drive (CRD) hydraulic

control units (HCUs) at a boiling-water reactor. The procedure utilized to perform this activity

inadvertently replicated the normal control rod withdrawal hydraulic sequence for the last three

HCUs isolated, resulting in three control rods being driven out of the core. The NRC expects

that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. Suggestions contained in this IN are not NRC

requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

On November 3, 2008, with Dresden Unit 3 in cold shutdown, three control rods inadvertently

withdrew to positions 06, 18, and 16, respectively (control rod position 00 indicates full-in and

control rod position 48 indicates full-out). The cause of the event was a deficient procedure

performed to isolate all 177 HCUs for CRD system maintenance. The procedure specified

sequentially isolating the HCUs with a CRD pump running which established the required

hydraulic conditions for outward control rod motion (i.e., insert, unlatch, and withdraw).

The event occurred 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> after shutdown (approximately peak xenon concentration) at a

reactor coolant system (RCS) temperature of 168 °F. During the event, the reactor remained

shutdown with approximately 4.5 percent shutdown margin. However, an inadvertent criticality

event could have occurred under different conditions given that the procedure did not place

controls on RCS temperature or xenon concentration.

The following describes how sequentially isolating the HCUs with a CRD pump running

established the hydraulic conditions for outward control rod motion (i.e., insertion, unlatching, and withdrawal):

Initial Plant Conditions. All 177 CRD HCU insert valves (HCU 101) and withdraw valves (HCU

102) were open; all 177 CRD HCU directional control valves 120, 121, 122, and 123 were

closed; and one CRD pump was operating applying pressure to each of the 177 unisolated HCU

under-piston and over-piston areas (see Figure 1). Pressure to the under-piston area was

applied through the cooling water header through the open insert valve (HCU 101). Pressure to

the over-piston area was applied through the exhaust header (via the orificed check valve)

through a designed reverse flow path in directional control valve 121 (which was shut), and then

through the open withdraw valve (HCU 102). While the pressures to the under-piston and over- piston areas were roughly equal, a net inward force was applied because the under-piston has a

larger surface area (see Figure 3a).

Figure 1 Running 3B CRD pump and flow paths available to the unisolated HCUs

Control Rod Insertion. As shown in Figure 2, the CRD header pressure and proportional net

inward rod force was initially small because the flow paths to all 177 unisolated HCUs were

available. Using procedural instructions, non-licensed operators began isolating the 177 HCUs

one at a time. As each successive HCU was isolated, available CRD header flow paths

decreased and since CRD header flow rate remained constant, system back pressure (i.e.,

CRD header differential pressure) increased resulting in an exponentially increasing the net

inward force on the remaining unisolated HCUs. After the 170th (NRC inspector determination)

HCU was isolated, the net force to the under-piston area of the seven unisolated HCUs was

sufficient to lift the CRD and insert the rod into the over-travel position (Figure 3b). [Note: NRC

Inspection Report 05000249/2009009 discusses that the NRC inspector disagreed with the

licensees determination that only 5 control rods (rather than 7 control rods) inserted to an over- travel position.] Figure 2 The number of isolated HCUs versus CRD header pressure and the

net force applied to the under-piston area of unisolated HCUs

Control Rod Unlatching. When the 174th HCU was isolated, CRD header differential pressure

increased such that the control rods associated with the last three HCUs unlatched because of

the force applied to the collet piston in the over-piston area was sufficient to compress the collet

spring (Figure 3c).

Control Rod Withdrawal. The withdrawal of the three unlatched control rods occurred due to the

order the valves were closed to isolate the HCU. By procedure, each of the 177 HCUs was

isolated by first closing its insert valve (HCU 101) and then closing its withdraw valve (HCU

102). Closing the insert valve isolated the HCU under-piston area causing this area to

depressurize through designed leakage flow paths. After the 175th HCUs insert valve was shut, the associated control rod was free to move out of the core because the rod was unlatched and

a net force was applied to the over-piston area (Figure 3d). Outward control rod motion also

occurred for the 176th and 177th HCUs for the same reason. Control rod motion continued until

the non-licensed operator shut the associated HCU withdraw valve. The three control rods

withdrew to the 06, 18, and 16 positions, respectively.

Figure 3 Status of CRDs Operator Performance Issues. The licensee found that the main control room was not included

in the pre-job brief, control room operators were unaware the activity was in progress or that a

non-licensed operator was performing the HCU valve manipulations. This resulted in non- licensed operators manipulating mechanisms with the potential to affect reactivity without the

knowledge and consent of a licensed operator at the controls, as required by 10 CFR 50.54(j).

In addition, when the main control room received multiple unexpected control rod drift alarms

and indications of rods being inserted to the over-travel position, control room operators first

pursued whether the alarms may have been caused by known instrument maintenance rather

than actual control rod movement. As a result, the first rod drift alarm in the control room

preceded the first unplanned control rod motion by almost 18 minutes, and the last three control

rods withdrew over the following 3 minutes. When rod movement stopped after the non- licensed operator shut the last of the HCU withdraw valves, the main control room operators no

longer had control over any of the 177 control rods. The control room operators did not notice

that the CRD cooling water header pressure was off scale high, which covered in the alarm

response procedure as potential cause for drifting rods. Approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and 38 minutes

after the first unplanned control rod movement, the operators completed the insertion of each of

the three control rods by manually cycling the associated HCU insert valves.

Root Cause Evaluation. The licensee determined that the root cause of this event was latent

procedural deficiencies. The procedure did not contain any precautions, prerequisites, selection

criteria, or limitations for the quantity of HCUs to be isolated with an operating CRD pump. The

licensee had previously reviewed operating experience involving control rod withdrawal events

at Japanese boiling-water reactors and had implemented procedural changes to prevent this

occurrence at the station. However, the licensee had not revised all of the affected procedures, including the one that was used during this event.

The licensee identified the following contributing factors for this event included:

  • An over reliance on a computer-based keyword search program system to identify

applicable procedures when implementing a procedural change.

  • A shutdown mindset that did not emphasize that any maintenance on CRD HCUs

should be considered for the potential effect on reactivity;

  • Operators not trusting their indications and promptly responding to a loss (or potential

loss) of control rod control;

  • Non-licensed operators performing plant activities (HCU valve manipulations) that could

affect reactivity, without the knowledge and consent of a licensed operator present at the

controls.

Specific corrective actions taken by the licensee included: identifying and updating all

applicable procedures with key recommendations provided in the operating experience;

specifying a minimum number of HCUs to be kept in service with a CRD pump operating while

shutdown; monitoring CRD drive pressures, rod positions, and alarms during activities that

could result in an inadvertent control rod movement; and ensuring that personnel who isolate

HCUs are aware that they can affect control rod system drive pressure and cause inadvertent

control rod movement.

Additional information is available in Dresden Licensee Event Report 50-249/2008-003-00,

dated December 31, 2008, and NRC Inspection Report 05000249/2009009, dated August 19, 2009. This information can be found on the NRCs public website in the Agencywide

Documents Access and Management System, under Accession Nos. ML090090372 and

ML092320164, respectively.

DISCUSSION

This event illustrates the importance of licensees understanding that some field activities at

boiling-water and pressurized-water reactors can significantly affect reactivity. The regulation at

10 CFR 50.54(j) requires that the operation of apparatus and mechanisms that may affect the

reactivity of a reactor shall be manipulated only with the knowledge and consent of a licensed

operator present at the controls. In this case, the action of non-licensed operators using a

deficient procedure to manually isolate the HCUs in the field led to multiple control rods moving

out of the core without the prior knowledge or consent of the licensed control room operators.

Moreover, the licensed operators failure to believe the control room indications unnecessarily

delayed their diagnosis of, and response to, the event. Under a different set of initial conditions, this event could have resulted in an inadvertent criticality.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor

Regulation project manager.

/RA/

Timothy J. McGinty, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contacts: Mark Ring, RIII Jamie Benjamin, RIII

630/829-9703 630/829-9753 E-mail: Mark.Ring@nrc.gov E-mail: Jamie.Benjamin@nrc.gov

Carey Brown, RIII

630/829-9605 E-mail: Carey.Brown@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections. August 19, 2009. This information can be found on the NRCs public website in the Agencywide

Documents Access and Management System, under Accession Nos. ML090090372 and

ML092320164, respectively.

DISCUSSION

This event illustrates the importance of licensees understanding that some field activities at

boiling-water and pressurized-water reactors can significantly affect reactivity. The regulation at

10 CFR 50.54(j) requires that the operation of apparatus and mechanisms that may affect the

reactivity of a reactor shall be manipulated only with the knowledge and consent of a licensed

operator present at the controls. In this case, the action of non-licensed operators using a

deficient procedure to manually isolate the HCUs in the field led to multiple control rods moving

out of the core without the prior knowledge or consent of the licensed control room operators.

Moreover, the licensed operators failure to believe the control room indications unnecessarily

delayed their diagnosis of, and response to, the event. Under a different set of initial conditions, this event could have resulted in an inadvertent criticality.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor

Regulation project manager.

/RA/

Timothy J. McGinty, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contacts: Mark Ring, RIII Jamie Benjamin, RIII

630/829-9703 630/829-9753 E-mail: Mark.Ring@nrc.gov E-mail: Jamie.Benjamin@nrc.gov

Carey Brown, RIII

630/829-9605 E-mail: Carey.Brown@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

ADAMS Accession Number: ML092190302 TAC ME0707 OFFICE RIII TECH EDITOR BC:RIII:DRP BC:IOLB:DIRS D:DIRS

NAME JBenjamin KAzariah-Kribbs MRing JMunro FBrown

DATE 9/1/09 e-mail 8/21/09 e-mail 9/1/09 e-mail 9/2/09 9/2/09 OFFICE LA:PGCB:NRR PM:PGCB:NRR BC:PGCB:NRR D:DPR:NRR

NAME CHawes DBeaulieu MMurphy TMcGinty

OFFICE 2/16/10 9/2/09 2/16/10 2/17/10

OFFICIAL RECORD COPY


Retrieved from "https://kanterella.com/w/index.php?title=Information_Notice_2010-06,_Inadvertent_Control_Rod_Withdrawal_Event_While_Shutdown&oldid=2142779"