United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-51: Potential Loss of Required Shutdown Margin During Refueling Operations

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                      OFFICE OF NUCLEAR REACTOR REGULATION
                             WASHINGTON, D.C.  20555

                                  May 31, 1989


Information Notice No. 89-51:  POTENTIAL LOSS OF REQUIRED SHUTDOWN
                                   MARGIN DURING REFUELING OPERATIONS


Addressees:    

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

Purpose:  

This information notice is being provided to alert addressees to the potential 
loss of required shutdown margin during the movement and placement of highly 
reactive fuel during refueling operations.  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 do not constitute NRC requirements; 
therefore, no specific action or written response is required.

Description of Circumstances:

Baltimore Gas and Electric Company (BG&E) submitted a 10 CFR Part 21 report 
to the NRC on March 15, 1989, regarding the potential loss of shutdown margin 
during refueling operations at its Calvert Cliffs Nuclear Power Plant, Units 1
and 2.  BG&E reported that it had increased the fuel enrichment during the 
last several fuel cycles.  In the current cycle, 4.3 weight percent U-235 fuel 
was loaded into the core.  During a review of Information Notice No. 
88-21, "Inadvertent Criticality Events at Oskarshamn and U.S. Nuclear Power 
Plants," BG&E discovered that the Calvert Cliffs refueling procedures allowed 
the placement of fuel assemblies in intermediate positions during core 
alterations.  In addition, BG&E determined that the potential existed for 
placing several fresh 4.3 weight percent reload assemblies together and losing 
some of the required 5 percent shutdown margin.  Calculations also showed that 
under extreme conditions an inadvertent criticality could occur if a number of 
highly reactive assemblies were grouped together and no credit was taken for 
control rods or burnable poisons.

BG&E has revised the Calvert Cliffs refueling procedures to ensure that fuel 
assemblies will not be placed in intermediate positions during core 
alterations without first verifying their potential reactivity.  The revised 
procedures allow fuel to be positioned only in intermediate core locations 
that will contain fuel of equal or greater reactivity in the final core 
configuration.





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Discussion:

As a result of longer fuel operating cycles, utilities have been increasing 
the enrichment of reload fuel.  Some of these fresh reload assemblies may be 
highly reactive under refueling conditions.  Although analyses are performed 
to confirm that the refueling boron concentration is sufficient to maintain 
the required shutdown margin (subcriticality) for the final core 
configuration, these analyses may not be sufficient to assure that the 
shutdown margin will be maintained for all intermediate fuel assembly 
positions.  In addition, explicit procedural controls may not exist to control 
the location and movement of highly reactive fuel assemblies during refueling.  
Because a significant amount of reactivity can be added to subcritical 
configurations by the addition of a single highly reactive assembly, it is 
possible that an inadvertent criticality could occur if a number of such 
assemblies are grouped together.  With this highly reactive fuel, subcritical 
multiplication (inverse count rate) may not provide adequate warning of an 
approach to criticality.

Combustion Engineering, Inc. (CE), the nuclear fuel design consultant for 
Calvert Cliffs, recognized this potential problem and recently issued an 
information bulletin regarding shutdown margin during refueling to all utili-
ties with CE-designed plants.  These utilities have been advised to review 
their fuel shuffle procedures that control the location of highly reactive 
fuel and to be aware that the refueling boron concentration necessary to 
maintain the required shutdown margin, which is based on the final core 
configuration, may not be sufficient to assure that their required shutdown 
margin will be maintained for all intermediate fuel assembly positions.  In 
circumstances in which explicit analyses are not available for each inter-
mediate fuel assembly position, CE recommends positioning fuel only in 
intermediate core locations that will contain fuel of equal or greater 
reactivity in the final core configuration.

No specific action or written response is required by this information notice.  
If you have any questions regarding this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional 
office.




                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation

Technical Contact:  Laurence I. Kopp, NRR
                    (301) 492-0879

Attachment:  List of Recently Issued NRC Information Notices
..                                                            Attachment 
                                                            IN 89-51
                                                            May 31, 1989
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                             NRC INFORMATION NOTICES
_____________________________________________________________________________
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-88,         Degradation of Westinghouse   5/31/89        All holders of OLs
Supp. 1        ARD Relays                                   or CPs for nuclear
                                                            power reactors.

89-50          Inadequate Emergency          5/30/89        All holders of OLs
               Diesel Generator Fuel                        or CPs for nuclear
               Supply                                       power reactors.

89-49          Failure to Close Service      5/22/89        All holders of OLs
               Water Cross-Connect                          or CPs for nuclear
               Isolation Valves                             power reactors.

89-48          Design Deficiency in the      5/22/89        All holders of OLs
               Turbine-Driven Auxiliary                     or CPs for nuclear
               Feedwater Pump Cooling                       power reactors.
               Water System

89-47          Potential Problems With       5/18/89        All holders of OLs
               Worn or Distorted Hose                       or CPs for nuclear
               Clamps on Self-Contained                     power reactors and
               Breathing Apparatus                          fuel facilities.

89-46          Confidentiality of            5/11/89        All holders of 
               Exercise Scenarios                           licenses for fuel 
                                                            cycle facilities 
                                                            and byproduct 
                                                            material licensees 
                                                            having an approved 
                                                            emergency response 
                                                            plan. 

89-45          Metalclad, Low-Voltage        5/8/89         All holders of OLs
               Power Circuit Breakers                       or CPs for nuclear
               Refurbished with Sub-                        power reactors.
               standard Parts

89-44          Hydrogen Storage on the       4/27/89        All holders of OLs
               Roof of the Control Room                     or CPs for nuclear
                                                            power reactors.

88-82,         Torus Shells with Corrosion   5/2/89         All holders of OLs
Supp. 1        and Degraded Coatings in                     or CPs for BWRs.
               BWR Containments
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit 
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