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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IN 90-28
April 30, 1990
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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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April 30, 1990
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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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