Information Notice No. 96-16: BWR Operation with Indicated Flow Less Than Natural Circulation
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
March 14, 1996
NRC INFORMATION NOTICE 96-16: BWR OPERATION WITH INDICATED FLOW LESS THAN
NATURAL CIRCULATION
Addressees
All holders of operating licenses or construction permits for boiling-water
reactors (BWRs).
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to two instances of power operation in which the
core flow appeared to be less than that normally attributed to operation on or
near the natural circulation line. It is expected that recipients will review
the information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems. However, suggestions contained in
this information notice are not NRC requirements; therefore, no specific
action or written response is required.
Description of Circumstances
Nine Mile Point Nuclear Station Unit 1
On February 1, 1995, Nine Mile Point Unit 1 (NMP1) had a recirculation pump
runback event as a result of pump maintenance activities. The reactor was
being operated at 77 percent of rated power and on the 78-percent rod line.
When the runback occurred, one of the five pumps was out of service for
maintenance. Speed of the four operating pumps was reduced to about
10 percent of rated speed. The resulting minimum indicated flow was about
13 percent and the corresponding power level was about 30 percent. (The
facility operating procedures require that flow be greater than 20 percent to
be in the RUN mode.) With four pumps operating at 10-percent speed, the
resulting flow would be expected to be a little higher than at natural
circulation. Natural circulation flow for NMP1 is generally shown as being
about 21 to 26 percent for a power of about 30 percent. Thus, the indicated
flow rate was lower than the rate that would normally be considered for the
natural circulation line on the power/flow map.
There was a turbine trip from a high water level signal as a result of a steam
and feedwater flow mismatch, but no reactor scram occurred since steam flow
was within turbine bypass capacity. Operators reduced power by inserting high
worth control rods and increased the indicated flow to above the natural
circulation line by increasing the speed of the recirculation pumps.
Investigation by the utility, General Electric, and an NRC inspection team
attributed the indicated low flows to inaccurate instrumentation, with lack of
recent low-flow calibration as a suspected cause. The indicated low flow was
9603110159. IN 96-16
March 14, 1996
Page 2 of 4
not compatible with previous analysis and measurements, which indicated the
flow rate would be at least 24 percent following a trip of all 5 pumps from
similar initial conditions.
Peach Bottom Atomic Power Station Unit 2
On June 3, 1995, while the reactor was being operated at about 32 percent of
rated power and in single-loop operation prior to planned maintenance, a full
recirculation pump runback to minimum speed occurred. This was the result of
maintenance activities while attempting to place a reactor feed pump in
standby. Reactor power was reduced to 26 percent, and the indicated core flow
was about 32 percent of rated flow. The operator knew the reason for the
runback and, as directed by the control room supervisor, restored core flow to
conditions that existed before the event. After the event, the reactor
engineer reviewed core performance during the reduced-flow condition and
determined that the power/flow had apparently reached a flow that was 0.5 to
2.0 percent (depending on the instrumentation used) lower than the flow
corresponding to the natural circulation line-flow (as displayed, for example,
in Figure 1.1-1 of the Peach Bottom Technical Specifications).
Discussion
NRC staff investigation of these two events indicates that some plant
operators may not have a complete understanding of at least four aspects of
such events.
(1) Natural circulation may not occur, or may not appear to occur, precisely
where it is depicted on the power/flow maps normally available to
operators, for example, in technical specifications, the Bases or
procedures. Usually, these maps are only intended to be illustrative
and approximate. The correct values change with cycle, time in cycle,
and other core and coolant system characteristics. In addition, the
observed flow also depends on the accuracy of the relevant instruments
and may not be correct because of, for example, calibration problems.
(2) Flow instruments are usually calibrated at high flow rates. Calibration
information obtained at low flows can reduce uncertainties about the
accuracy of the instruments involving low flow situations.
(3) Operation (on a given rod line) at flow lower than that on the natural
circulation line, if real, can be more unstable than operation on the
natural circulation line. For the NMP1 event, the NRC inspection team
calculated that the reactor would have been unstable at the indicated
minimum flow (13 percent) if the event had been initiated from
conditions on a somewhat higher rod line. The stability regions for
NMP1 have now been conservatively extended below the natural circulation
line, and operators are aware of the potential for increased instability
at lower flows.
. IN 96-16
March 14, 1996
Page 3 of 4
(4) It was pointed out in NRC inspection report Reference No. 50-220/95-80
[9504070141] for the NMP1 event that the technical specification for the
NMP1 safety limit with low flow, did not have a logical or sufficient
Basis (B 2.1.1.b) for the safety limit. The specification, which says,
in part, that the power should be less than 25 percent of rated thermal
if the flow is less than 10 percent of rated, does not follow from the
information in the Basis.
The flow related-safety limit addresses flow below 10 percent and flow
above 10 percent. Above 10-percent flow, the limit is stated in terms
of the minimum critical power ratio (MCPR) and the MCPR safety limit.
The Basis states that there are sufficient data to validate the MCPR
correlation down to about 10-percent flow. Thus, with power/flow
information for any given state of the reactor, the relationship to the
MCPR safety limit can be determined. However, no relevant information
for below 10-percent flow is provided as a basis for the stated
25-percent power safety limit. The basis (which, like the specifica-
tion, is essentially the same for most BWRs, and the new standard
specification NUREG-1434) indicates that the critical power ratio
correlations are valid down to about 10-percent flow, but does not
address the below-10-percent-flow critical power correlations or any
other relevant data or analysis for that region. Instead, it states
that minimum flow will be no less than about 20 percent, the lowest flow
that was anticipated under low-flow conditions as a result of the BWR
flow geometry. At this flow, a power of 50 percent was found to be
within minimum critical power limits and thus, it is stated, the
specification of a 25-percent limit is conservative. However, no
information is provided to validate the translation to 10 percent or
lower flow conditions, the flow range of the low-flow specification.
This aspect of the safety limit will be further examined by the NRC, and
action may be taken to correct NUREG-1434.
. IN 96-16
March 14, 1996
Page 4 of 4
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
signed by
Dennis M. Crutchfield, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Howard J. Richings, NRR
(301) 415-2888
Internet:hjr@nrc.gov
T. Jerrell Carter, NRR
(301) 415-1153
Internet:tjc@nrc.gov
Page Last Reviewed/Updated Thursday, March 25, 2021