Bulletin 89-03: Potential Loss of Required Shutdown Margin During Refueling Operations
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
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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
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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
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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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