ML092190302 

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 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. 
IN 2010-06 

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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 overpiston 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 

licensee’s determination that only 5 control rods (rather than 7 control rods) inserted to an overtravel position.] 
IN 2010-06 

Page 3 of 5 

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 HCU’s 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 
IN 2010-06 

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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 nonlicensed 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 nonlicensed 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 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. 

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 
IN 2010-06 

Page 5 of 5 

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 

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. 
IN 2010-06 

Page 5 of 5 

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 

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