United States Nuclear Regulatory Commission - Protecting People and the Environment

Technical Resolution of Generic Issue No. B-59-(N-1) Loop Operation in BWRs and PWRs (Generic Letter No. 86-09)


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

                               March 31, 1986

TO:       ALL LICENSEES OF OPERATING BWRs AND PWRs AND LICENSE APPLICANTS 

GENTLEMEN: 

SUBJECT:  TECHNICAL RESOLUTION OF GENERIC ISSUE NO. B-59-(N-1) LOOP 
          OPERATION IN BWRs AND PWRs (GENERIC LETTER NO. 86-09) 

The staff has been studying (N-1) loop operation in BWRs and PWRs under 
Generic Issue No. B-59. 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 No. B-59. 

The majority of the presently operating BWRs and PWRs are designed to 
operate with less than full reactor coolant flow. If a PWR reactor coolant 
pump or a BWR recirculation pump becomes inoperative, the flow provided by 
the remaining (N-1) loops is sufficient for steady state operation at a 
power level less than full power. Although the FSARs for the licensed BWRs 
and PWRs present (N-1) loop calculations showing allowable power and 
protective system trip sit-points the NRC staff has disallowed this mode of 
operation for most plants primarily because of insufficient ECCS analyses as 
well as thermal-hydraulic stability concerns associated with BWRs. At 
present, the Technical Specifications for most BWRs and PWRs require 
shutdown within several hours if one of the reactor coolant loops becomes 
inoperable. 

The staff recently completed a safety evaluation report (SER) (Ref. 1) for 
the request 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 updated 
appropriately in the near future. The review by the staff of the Beaver 
Valley Unit 1 submittal has not identified any unacceptable consequences 
associated with (N-1) loop operation. Therefore, other PWR owners may wish 
to evaluate the merits of (N-1) loop submittals for their plants based on 
the Beaver Valley 1 experience or on the approval of (N-1) loop operation 
for Millstone 3 (Docket No. 50-423), including technical specifications, in 
conjunction with its operating license review. However, the specific design 
characteristics of each plant must be reviewed in the same detail as Beaver 
Valley 1 and Millstone 3 to assure that all safety considerations relative 
to (N-1) loop operation are evaluated. Some examples of PWR considerations 
which are highly plant specific in nature are: 


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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 instruments 
     in 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 turbine 
     driven safety pumps; 

5.   considerations relating to piping and status of valves for systems 
     connecting to the down loop; 

6.   effects on core flow distribution, potential for cold water reactivity 
     insertion, etc. 

Plant specific aspects of the safety analyses, including considerations 
relating to plants which do not have loop isolation valves, may identify 
safety questions which could affect decisions regarding the desirability of 
(N-1) loop operation. Both PWR plants reviewed for (N-1) loop operation were
equipped with loop isolation valves. 

The review of BWR (N-1) loop operation has been complicated by potential 
thermal-hydraulic instability and jet pump vibration problems during single 
loop operation (SLO). In low flow operating regions, it has been necessary 
to develop special operating procedures to assure that General Design 
Criteria 10 and 12 are satisfied in regard to thermal-hydraulic 
instabilities. Technical Specifications consistent with these procedures 
have been accepted by the staff for reactors which are not demonstrably 
stable based on analyses using approved analytical methods; details of the 
operating limitations were developed for the General Electric Safety 
Information Letter (SIL) 380 and contributed to the technical resolution of 
Generic Issue B-19, which is the subject of our Generic Letter No. 86-02 
(Ref. 2). In addition, in an effort to resolve certain plant specific 
concerns about jet pump vibration or thermal hydraulic instability in the 
single loop operating mode at Browns Ferry Unit 1, TVA completed tests for 
that reactor on February 9, 1985, and those concerns have been resolved. 
These tests demonstrated that SLO has similar stability characteristics as 
two loop operation under the same power/flow operating conditions. They also 
confirmed the staff's finding that Technical Specifications based on GE SIL 
380 which have been proposed for some BWRs are appropriate for the detection 
and suppression of thermal hydraulic instabilities. Recently Technical 
Specifications based on GE SIL 380 were 

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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 
who have submitted SLO ECCS analysis as soon as Technical Specification 
changes similar to those for Duane Arnold are submitted. 

This Generic Letter does not involve any reporting requirements so that no 
OMB clearance is necessary. 

                              Sincerely, 


                              Harold R. Denton, Director 
                              Office of Nuclear Reactor Regulation 

                                    - 4 - 

REFERENCES 

1.   Letter from S. Varga (NRC) to J. J. Carey (Duquesne Light Company), 
     "Beaver Valley Unit 1 - Operation with Two Out of Three Reactor Coolant
     Loops - 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, 1986 

3.   Letter from Mohan Thadani (NRC) to L. Liu (Iowa Electric Light and 
     Power Company), dated May 28, 1985, Docket No. 50-331. 
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