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United States Nuclear Regulatory Commission - Protecting People and the Environment


                                                       OMB No.:  3150-0011
                                                       NRCB 89-03

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

                              November 21, 1989


NRC BULLETIN NO. 89-03:  POTENTIAL LOSS OF REQUIRED SHUTDOWN MARGIN DURING 
                         REFUELING OPERATIONS


Addressees:

All holders of operating licenses or construction permits for pressurized 
water reactors.  

Purpose:

This bulletin 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 take the actions in this bulletin to prevent potential violations of 
required shutdown margin and, in extreme cases, inadvertent criticality 
during refueling.  

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 NRC 
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 losing some of the shutdown margin 
required by the plant technical specifications, even if several fresh reload 
assemblies with enrichments as low as 4.1 weight percent were grouped 
together.  Calculations also showed that under extreme conditions an 
inadvertent criticality could occur if a number of highly reactive 
assemblies were grouped together.  In response to the BG&E notification, the 
NRC issued Information Notice No. 89-51, "Potential Loss of Required 
Shutdown Margin During Refueling Operations," dated May 31, 1989.

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 



8911200248
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                                                       November 21, 1989
                                                       Page 2 of 4


allow fuel to be positioned only in intermediate core locations that will 
contain fuel of equal or greater reactivity in the final core configuration.  

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 certain refueling conditions.  Although analyses are 
performed for PWRs 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.  An inadvertent criticality 
could result in fuel failures, system damage, and potentially high 
radiological doses to onsite workers.  

Combustion Engineering, Inc. (CE), the nuclear fuel design consultant for 
Calvert Cliffs, recognized this potential problem and issued an information 
bulletin regarding shutdown margin during refueling to all utilities 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 intermediate 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.  

Requested Actions:

Operating Reactors:

All PWR licensees are requested to assure that adequate shutdown margin is 
maintained during all refueling operations.  This should be accomplished 
through the following actions:

1.   Assure that any intermediate fuel assembly configuration (including 
     control rods) intended to be used during refueling is identified and 
     evaluated to maintain sufficient refueling boron concentration to 
     result in a minimum shutdown margin of approximately 5 percent.

2.   Assure that fuel loading procedures only allow those intermediate fuel 
     assembly configurations that do not violate the allowable shutdown 
     margin and that these procedures are strictly adhered to.
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                                                       NRCB 89-03
                                                       November 21, 1989
                                                       Page 3 of 4


3.   Assure that the staff responsible for refueling operations is trained 
     in the procedures recommended in Item 2 above and understand the 
     potential consequences of violating these procedures.  This training 
     should include the fundamental aspects of criticality control with 
     higher enriched fuel assemblies.

Construction Permit Holders:

All PWR construction permit holders are requested to complete the 
above-listed actions before the date scheduled for fuel loading.  

Reporting Requirements:

Within 60 days of receipt of this bulletin, pursuant to 10 CFR 50.54(f), all 
holders of OLs for PWRs shall notify the NRC by letter whether they have 
taken, or will take, the actions requested above.  Prior to initial fuel 
loading, CP holders shall advise the NRC by letter whether the requested 
actions have been implemented.  

The written reports required above shall be addressed to the U.S. Nuclear 
Regulatory Commission, ATTN:  Document Control Desk, Washington, D.C. 20555, 
and shall be submitted under oath or affirmation pursuant to the provisions 
of Section 182a, Atomic Energy Act of 1954, as amended and 10 CFR 50.54(f).  
In addition, a copy shall be submitted to the appropriate Regional 
Administrator.  

Backfit Discussion

The objective of the actions requested in this bulletin is to ensure that an 
adequate shutdown margin is maintained during the movement and placement of 
fuel during refueling operations.  

The issuance of the bulletin is justified on the basis of the need to 
provide adequate protection to the health and safety of the public and is 
consistent with the provisions of 10 CFR 50.109a(4)(ii).  The actions 
proposed by the bulletin will ensure that an adequate shutdown margin is 
maintained.  Failure to take these actions may result in an inadequate 
shutdown margin during refueling operations.  In extreme cases this could 
result in inadvertent criticality with potential for fuel failures, reactor 
damage, and relatively high radiological doses to onsite workers.  

This request is covered by Office of Management and Budget Clearance Number 
3150-0011 which expires December 31, 1989.  The estimated average burden 
hours are 100 person-hours per licensee response, including assessment of 
the new recommendations, searching data sources, gathering and analyzing the 
data, and preparing the required letters.  These estimated average burden 
hours pertain only to these identified response-related matters and do not 
include the time for actual implementation of the requested actions.  Send 
comments regarding this burden estimate or any other aspect of this 
collection of information, 
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                                                       NRCB 89-03
                                                       November 21, 1989
                                                       Page 4 of 4


including suggestions for reducing this burden, to the Information and 
Records Management Branch, Division of Information Support Services, Office 
of Information Resources Management, U.S. Nuclear Regulatory Commission, 
Washington, D.C. 20555; and to the Paperwork Reduction Project (3150-0011), 
Office of Management and Budget, Washington D.C. 20503.  

If you have any questions about this matter, please contact one of the 
technical contacts listed below or the appropriate NRR project manager. 




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


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

                     N. Prasad Kadambi, NRR
                     (301) 492-1153

Attachment:  List of Recently Issued NRC Bulletins

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