Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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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May 31, 1989
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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
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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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