Bulletin 90-02: Loss of Thermal Margin Caused by Channel Box Bow

                                                       OMB No.:  3150-0011
                                                       NRCB No. 90-02

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

                               March 20, 1990


NRC BULLETIN NO. 90-02:  LOSS OF THERMAL MARGIN CAUSED BY CHANNEL BOX BOW


Addressees:

All holders of operating licenses (OLs) or construction permits (CPs) for 
boiling water reactors (BWRs).  

Purpose:

The purpose of this bulletin is to request that addressees determine whether 
any channel boxes are being reused after their first bundle lifetime and, if 
so, ensure that the effects of channel box bow on the critical power ratio 
(CPR) calculation are properly taken into account.  

Background:

As a result of information obtained at a meeting on fuel failures caused by 
dryout at a foreign BWR facility and at meetings with BWR fuel vendors, the 
NRC issued the attached Information Notice 89-69, "Loss of Thermal Margin 
Caused by Channel Box Bow," on September 29, 1989, to alert addressees to 
the potential problems associated with excessive channel box bow that could 
result in a loss of thermal margin.  The main concern applicable to the U.S. 
BWRs at this time is that the channel box bow effect has not been taken into 
account in the CPR calculation for channel boxes in their second bundle 
lifetime operation.  

Discussion:

Because channel box bow has a non-negligible impact on BWR thermal limits 
calculations, it is important that actions are taken to prevent the 
possibility of any fuel failures because of this effect.  The staff 
acknowledges that the possibility of fuel failure as a result of channel box 
bow for channel boxes in the first fuel bundle lifetime is remote for U.S. 
BWRs.  However, channel boxes in use for a second bundle lifetime may be 
severely bowed with a loss of CPR margin of up to 0.20 delta-CPR.  
Therefore, the staff has concluded that all addressees that currently use 
channel boxes for a second bundle lifetime should inform the NRC of the 
number and disposition of such channel boxes in the core, and describe the 
actions taken to assure compliance with the technical specification CPR 
limits.  Because of the new information about channel box bow, the staff 
expects that reuse of channel boxes in future core reloads will be addressed 
as an unreviewed safety question in reload core safety evaluations.  



9003130012 
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                                                            NRCB 90-02
                                                            March 20, 1990
                                                            Page 2 of 3


Addressees who use channel boxes for only a single bundle lifetime are 
expected to take into account the effects of channel box bow in analysis 
supporting the next fuel reload application.  This can be done by either the 
use of an approved methodology that takes the effects of channel box bow 
into account in the CPR calculations, or by the use of a bounding value of 
delta-CPR.  Preliminary results have been obtained by both General Electric 
Company (GE) and Advanced Nuclear Fuels Corporation (ANF) in their proposed 
methodologies.  It is expected that the delta-CPR caused by channel box bow 
would not exceed 0.04 for D-lattice plants and would not exceed 0.02 for 
C-lattice and S-lattice plants.  These results are based on the maximum 
expected channel box bow for channel boxes that are used for only a single 
bundle lifetime.  Because of the increased rate of channel growth observed 
at the foreign BWR facility after about 50 to 60 GWd/MTU burnup, the 
delta-CPR effect is expected to be much larger for channel boxes being used 
in their second bundle lifetime.  However, GE and ANF have not provided 
recommendations for the use of second bundle lifetime channel boxes.  

Both GE and ANF have provided the NRC with reports describing their 
methodology for incorporating the effects of channel box bow in their CPR 
analysis.  The GE report was submitted to the NRC by letter dated November 
15, 1989, and is entitled "Effect of Channel Bow on Margins to Core Thermal 
Limits in BWRs."  The ANF report was submitted to the NRC by letter dated 
November 30, 1989, and is entitled "Critical Power Methodology for Boiling 
Water Reactors - Methodology for Analysis of Assembly Channel Bowing 
Effects," ANF-524(P), Revision 2, Sup-plement 1.  These two methodologies are 
presently under staff review, which is expected to be completed by May 1990.  
In the interim, the staff will consider approval of proposed plant- and 
cycle-specific applications of these new methodologies, if it can be 
demonstrated that the new methods are conservative in comparison to 
previously approved methods and are being correctly implemented.  

In summary, all BWR licensees are expected, in evaluation of the design and 
operation of reload cores, either to implement an approved methodology or to 
implement an acceptable bounding value of delta-CPR, as needed, to assure 
that the effects of channel box bow on CPR limits are taken into account to 
assure compliance with technical specification CPR limits.  

Requested Action:

All BWR licensees that currently use channel boxes for a second bundle 
lifetime should verify that current CPR technical specification operating 
and safety limits are met.

Reporting Requirements:

Within 30 days of receipt of this bulletin, all holders of OLs for BWRs that 
currently use channel boxes for a second bundle lifetime shall advise the 
NRC of the number of such channel boxes and their disposition in the core.  
In addition, the response shall describe the methods, and the associated 
data base, used to account for the effects of channel box bow during the 
second bundle lifetime use of channel boxes to ensure conformance with the 
CPR technical specification operating and safety limits. 

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                                                            NRCB 90-02
                                                            March 20, 1990
                                                            Page 3 of 3


The written reports required above shall be submitted under oath or 
affirmation pursuant to Section 182 of the Atomic Energy Act of 1954, as 
amended (42 U.S.C. 2232), and shall be addressed to the U.S. Nuclear 
Regulatory Commission, ATTN:  Document Control Desk, Washington, D.C. 20555.  
In addition, a copy shall be submitted to the appropriate Regional 
Administrator.  

Backfit Discussion:

The action requested in this bulletin ensures licensees' compliance with 
their technical specifications as required under 10 CFR 50.36(c)(1)(i)(A).  
The safety limit CPR comes under this regulation.  A known deficiency in 
current calculation of the CPR could result in fuel failures because the 
operators are misled to believe they are in compliance with the technical 
specification operating CPR limit.  The requested action was evaluated 
consistent with the provisions of 10 CFR 50.109 and found to be covered by 
the provisions of paragraph (a)(4)(i). 

This request is covered by Office of Management and Budget Clearance Number 
3150-0011 which expires January 31, 1991.  The estimated average burden 
hours are 100 person-hours per licensee response, including assessment of 
the new recommendation, 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.  Send 
comments regarding this burden estimate or any other aspect of this 
collection of information, 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:  Peter C. Wen, NRR 
                     (301) 492-1172 

                     Daniel B. Fieno, NRR 
                     (301) 492-3236

Attachments:
1.  Information Notice 89-69, "Loss of Thermal Margin Caused by Channel
      Box Bow" 
2.  List of Recently Issued NRC Bulletins
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