United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-28: Potential Error in High Steamline Flow Setpoint

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                               April 30, 1990


Information Notice No. 90-28:  POTENTIAL ERROR IN HIGH STEAMLINE 
                                   FLOW SETPOINT


Addressees:

All holders of operating licenses or construction permits for boiling water 
reactors (BWRs).

Purpose:

This information notice is intended to alert addressees to possible errors 
in the method used to determine the proper setpoints for the steamline flow 
differential pressure instruments that initiate isolation of the High 
Pressure Coolant Injection (HPCI) and Reactor Core Isolation Cooling (RCIC) 
systems at a limit corresponding to a specific percentage of the rated steam 
flow.  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 do not constitute NRC requirements; therefore, no specific action or 
written response is required.  

Description of Circumstances:

On November 30, 1989, during a surveillance test of the HPCI system at the 
FitzPatrick Nuclear Power Plant, automatic isolation of the steam admission 
valves on high steamline flow occurred (Docket No. 50-333; LER No. 
89-025-00).  An intensive series of tests was initiated to identify the root 
cause of this event.  To perform these tests, additional instrumentation was 
connected to the steamline flow instruments to determine the differential 
pressure detected by the instrumentation.  This testing led to the 
determination that the signal that resulted from startup of the turbine was 
close to the isolation setpoint and, in some cases, was greater than the 
setpoint.  Because the isolation setpoint was in accordance with the value 
specified in the technical specifications, and because no leakage or reason 
for a high-flow condition could be found, a possible problem either with the 
value specified for the setpoint or with the amount of steam needed to start 
the turbine, or both, was identified.

Discussion:

At FitzPatrick, the HPCI system is a steam turbine driven Emergency Core 
Cooling System designed to supply makeup water to the reactor vessel in 




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the event of a loss-of-coolant accident.  The high steam flow isolation of 
the HPCI turbine is designed to shut the steamline isolation valves to the 
turbine if the steam supply piping ruptures.  At FitzPatrick, this flow 
signal is derived by sensing the differential pressure that results from 
flow through the steam supply piping at an elbow in the piping.  If this 
differential pressure, expressed in inches of water, reaches the value given 
in the technical specifications, the isolation valves close. 

The accepted value of steam flow that indicates that a break in the 
steamline has occurred is typically 300 percent of the rated steam flow.  
The determination of the differential pressure that corresponds to this 
flow, and hence the indication that a steamline break has occurred, is the 
subject of General Electric Nuclear Services Information Letter (SIL) No. 
475.*  The analytic method described in the SIL applies to HPCI and RCIC 
systems and uses measured data from the as-built system operating under 
rated conditions in the surveillance test mode, rather than using purely 
theoretical analysis, to calculate the differential pressure that would 
exist at 300-percent flow.  This differential pressure then becomes the 
setpoint for the switch that initiates isolation of the system.  However, 
other methods based on sound engineering principles may also be appropriate.

At FitzPatrick, as part of the root cause determination, the intensive 
series of tests found that the actual differential pressure setpoint that 
caused the isolations corresponded to approximately 200-percent flow, and 
that the startup of the HPCI system occasionally caused the flow to reach 
this value.  Such a setpoint is conservative from the viewpoint of the 
safety function of the high steamline flow trip, but it adversely affects 
the reliability of the HPCI system.  Performing the calculations in 
accordance with SIL No. 475 resulted in the removal of this conservatism 
from the setpoint and in the restoration of the reliability of the HPCI 
system.

The HPCI system had operated with the setpoint conservatism for 14 years 
until several recent changes were made to the system.  These changes 
included test methodology improvements, hydraulic actuator replacement 
(which resulted in the need for additional steam for turbine startup), and 
turbine stop valve rewiring.  Setting the setpoint according to Revision 2 
of SIL No. 475 provides the proper value for initiation of the isolation 
function.  




                        
*Revision 2 of SIL No. 475 was issued November 28, 1988, by the General 
 Electric Company, San Jose, California.  As noted in Revision 2, errors in 
 the original and in Revision 1 have been corrected in Revision 2.
.

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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 
the technical contact listed below or the appropriate NRR project manager. 




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation


Technical Contact:  D. LaBarge, NRR
                    (301) 492-1421

Attachment:  List of Recently Issued NRC Information Notices
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