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
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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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