United States Nuclear Regulatory Commission - Protecting People and the Environment

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 

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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.    
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(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.  

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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


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