NRC Generic Letter 1986-09
UNITED STATESNUCLEAR REGULATORY COMMISSIONWASHINGTON, D. C. 20555March 31, 1986TO: ALL LICENSEES OF OPERATING BWRs AND PWRs AND LICENSE APPLICANTSGENTLEMEN:SUBJECT: TECHNICAL RESOLUTION OF GENERIC ISSUE NO. B-59-(N-1) LOOPOPERATION IN BWRs AND PWRs (GENERIC LETTER NO. 86-09)The staff has been studying (N-1) loop operation in BWRs and PWRs under GenericIssue No. B-59. We have recently completed our review of this issue and thepurpose of this letter is to inform you of our findings on the resolution ofGeneric Issue No. B-59.The majority of the presently operating BWRs-and PWRs are designed to operatewith less than full reactor coolant flow. If a PWR reactor coolant pump or aBWR recirculation pump becomes inoperative, the flow' provided by the remaining(N-1) loops is sufficient for steady state operation at a power level lessthan full power. Although the FSARs for the licensed BWRs and PWRs present(N-1) loop calculations showing allowable power and protective system tripset-points, the NRC staff has disallowed this mode of operation for most plantsprimarily because of insufficient ECCS analyses as well as thermal-hydraulicstability concerns associated with BWRs. At present, the Technical Specifica-tions for most BWRs and PWRs require shutdown within several hours if one ofthe reactor coolant loops becomes inoperable.The staff recently completed a safety evaluation report (SER) (Ref. 1) for therequest by Beaver Valley Unit 1 for (N-1) loop operation. Based on that SER,it is expected that Beaver Valley Unit 1 will be authorized to operate with(N-1) loops when the Technical Specifications are revised and updatedappropriately in the near future. The review by the staff of the BeaverValley Unit 1 submittal has not identified any unacceptable consequencesassociated with (N-1) loop operation. Therefore, other PWR owners may wish toevaluate the merits of (N-1) loop submittals for their plants based on theBeaver Valley 1 experience or on the approval of (N-1) loop operation forMillstone 3 (Docket No. 50-423), including technical specifications, inconjunction with its operating license review. However, the specific designcharacteristics of each plant must be reviewed in the same detail as BeaverValley 1 and Millstone 3 to assure that all safety considerations relative to(N-1) loop operation are evaluated. Some examples of PWR considerations whichare highly plant specific in nature are:LZID/P-
-2 -1. the impact of the down loop on instrumentation and control systems,including contributions of the down loop instruments to voting logic;2. human factors considerations of indications and alarms from instrumentsin the down loop;3. effects of the down loop on operational systems such as pressurizer spray;4. effects of the down loop on safety systems such as steam to turbinedriven safety pumps;5. considerations relating to piping and status of valves for systemsconnecting to the down loop;6. effects on core flow distribution, potential for cold water reactivityinsertion, etc.Plant specific aspects of the safety analyses, including considerationsrelating to plants which do not have loop isolation valves, may identifysafety questions which could affect decisions regarding the desirability of(N-1) loop operation. Both PWR plants reviewed for (N-1) loop operation wereequipped with loop isolation valves.The review of BWR (N-1) loop operation has been complicated by potentialthermal-hydraulic instability and jet pump vibration problems during singleloop operation (SLO). In low flow operating regions, it has been necessary todevelop special operating procedures to assure that General Design Criteria 10and 12 are satisfied in regard to thermal-hydraulic instabilities. TechnicalSpecifications consistent with these procedures have been accepted by thestaff for reactors which are not demonstrably stable based on analyses usingapproved analytical methods; details of the operating limitations weredeveloped for the General Electric Safety Information Letter (SIL) 380 andcontributed to the technical resolution of Generic Issue B-19, which is thesubject of our Generic Letter No. 86-02 (Ref. 2). In addition, in an effortto resolve certain plant specific concerns about jet pump vibration or thermalhydraulic instability in the single loop operating mode at Browns Ferry Unit1, TVA completed tests for that reactor on February 9, 1985, and those concernshave been resolved. These tests demonstrated that SLO has similar stabilitycharacteristics as two loop operation under the same power/flow operatingconditions. They also confirmed the staff's finding that TechnicalSpecifications based on GE SIL 380 which have been proposed. for some BWRs areappropriate for the detection and suppression of thermal hydraulicinstabilities. Recently Technical Specifications based on GE SIL 380 were
-3 -submitted for Duane Arnold. Permanent SLO has been approved for Duane Arnold(Ref. 3), and the staff expects to approve permanent SLO for other owners whohave submitted SLO ECCS analysis as soon as Technical Specification changessimilar to those for Duane Arnold are submitted.This Generic Letter does not involve any reporting requirements so that no OMBclearance is necessary.Sincerely,evnadSir.ed kj1LL RcntfOWHarold R. Denton, DirectorOffice of Nuclear Reactor RegulationDISTRIBUTIONCentral FilesBWR readingHDentonDEisenhutFMiragliaHThompsonRBerneroRHoustonGLainasLPhillipsBWR:RSBMHodges2/21/36 *SEE PREVIOUS CONCURRENCE PAGEOF*C :R.B:BWR* :BWR:AD* :BWR:DD* :BWR:.*_____- _-_ _ _-______ _______ -----------NAME :LPhillips :GLainas :RHouston :RBernero :H ompson :FMi taDATE : 2/20/86 : 2/25/86 2/26/86 : 2/27/86 /86OFC :NRR:DD :NRRg 4-4fW :___~~- -- ----- --- --------------. __ ---- --- --- :------_-----______:______ _____NAME :DE t :HDe onDATE: [f/86 :Izbrl /86OI FICIAL RECORD COPY
- ---4-REFERENCES1. Letter from S. Varga (NRC) to J. J. Carey (Duquesne Light Company),"Beaver Valley Unit 1 -Operation with Two Out of Three Reactor CoolantLoops -Safety Evaluation", dated July 20, 1984, Docket No. 50-334.2. Generic Letter 86-02, "Technical Resolution of Generic Issue B-19,Thermal Hydraulic Stability," dated January 23, 19863. Letter from Mohan Thadani (NRC) to L. Liu (Iowa Electric Light and PowerCompany), dated May 28, 1985, Docket No. 50-331.