Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
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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.
