United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-39: Unplanned Return to Criticality During Reactor Shutdown

                                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 


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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 
50-155/91-009 or NRC 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 coefficients would 
halt the reactor power increase.  As the power increased, the 
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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 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 contingency 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
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