United States Nuclear Regulatory Commission - Protecting People and the Environment

Technical Resolution of Generic Issue B-19-Thermal Hydraulic Stability (Generic Letter No. 86-02)


                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                           WASHINGTON, D. C. 20555

                              January 23, 1986

TO: ALL LICENSEES OF OPERATING BWRs 

GENTLEMEN:  

SUBJECT:  TECHNICAL RESOLUTION OF GENERIC ISSUE B-19-THERMAL HYDRAULIC 
          STABILITY (GENERIC LETTER NO. 86-02) 

The staff has been studying BWR thermal-hydraulic stability characteristics 
for several years under Generic Issue #B-19 - Thermal-hydraulic Stability. 
We have recently completed our review of this issue and the purpose of this 
letter is to inform you of our findings on the resolution of Generic Issue 
#B-19. 

Specifically, we have recently completed our technical evaluation of topical
reports (Refs. 1 and 2) by General Electric and Exxon which describe their 
analysis methods and have concluded the following: 

     GE/Exxon methods for calculation of core stability decay ratio are 
     uncertain by 20%/25% in predicting the onset of limit cycle 
     oscillations (decay ratio =1.0). Thus a core having a calculated decay 
     ratio of 0.80/0.75 may, in fact, be on the verge of limit cycle 
     oscillations within permissible operating space. The result of this 
     conclusion is that BWR 4, 5, 6s may not be able to show compliance with 
     General Design Criteria 10 and 12 solely using analysis procedures to 
     prove that thermal hydraulic instabilities are prevented by design. BWR 
     1,2,3s with conventional fuel designs and operating restrictions should 
     have sufficient margin; however, licensees should examine each core 
     reload to assure that it is typical of previously evaluated cores which 
     have acceptable stability margin. For cores which do not meet the 
     analytical criteria, we have concluded that operating limitations which 
     provide for the detection and suppression of flux oscillations in 
     operating regions of potential instability consistent with the 
     recommendations of General Electric SIL-380, are acceptable to 
     demonstrate compliance with GDC 10 and GDC 12 for cores loaded with 
     approved fuel designs.


8601290041 

                                    - 2 -

Many owners of BWR 4, 5, and 6s have incorporated into their technical 
specifications provisions which enforce GE SIL 380 recommendations for 
operation of their plants. All BWR owner should review the need for such 
technical specifications in light of the approved stability criteria and the
status of core stability design calculations for specific plants. Licensees 
are advised that the approved stability criteria are applicable to all 
operating reactors, and should be included in future safety evaluations in 
support of 10 CFR 50.59 determinations for all core reloads and other design
or operating modifications which relate to core thermal-hydraulic stability.

This generic letter does not include any reporting requirements so that no 
OMB clearance is necessary. 

                                   Sincerely,  


                                   Robert M. Bernero, Director 
                                   Division of BWR Licensing 
                                   Office of Nuclear Reactor Regulation 
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