NRC Generic Letter 86-02, Technical Resolution of Generic Issue B-19 - Thermal Hydraulic Stability
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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.
8601290041Many 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