United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-61, Supplement 1: Loss of High Head Safety Injection

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                               November 6, 1992

                                             SAFETY INJECTION


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to additional developments relating to the loss of
high head safety injection (HHSI) function because of anomalies in the
associated alternate minimum flow (AMF) system.  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

On April 3, 1991, Carolina Power & Light Company (the licensee) determined
that the HHSI system for the Shearon Harris Nuclear Power Plant had been in a
degraded condition during the previous operating cycle.  The degraded
condition resulted from relief valve and drain line failures in the AMF system
for the charging/safety injection pumps (CSIPs) which would have diverted a
significant amount of safety injection (SI) flow away from the reactor coolant
system (RCS).  Additional description of the existing AMF system and
additional discussion regarding the specifics of this event are contained in
Information Notice 92-61, "Loss of High Head Safety Injection," dated 
August 20, 1992, and in NRC Special Inspection Report 50-400/92-201, dated
September 1, 1992.  In addition, Figure 1 provides a schematic of the existing
HHSI system at Shearon Harris.  

To address NRC staff concerns regarding long-term operability of the existing
HHSI system, the licensee committed to implement procedures to minimize the
potential for gas or air intrusion into the AMF system and to perform
additional testing of the AMF system.

On September 17, 1992, the licensee tested the HHSI AMF system.  When the 
1CS-746 motor-operated isolation valve was opened to test train "A" of the
HHSI AMF system (1CS-746 will stroke fully open in approximately 7 seconds),
relief valve 1CS-744 chattered slightly for several seconds and then 


                                                       IN 92-61, Supplement 1
                                                       November 6, 1992
                                                       Page 2 of 3

successfully opened.  When the 1CS-752 motor-operated isolation valve was
opened (also requiring approximately 7 seconds) to test train "B" of the HHSI
AMF system, relief valve 1CS-755 chattered significantly, the valve bellows
failed, and the test was halted.  Additional discussion regarding these tests
is contained in NRC Inspection Report 50-400/92-17, dated October 1, 1992.


The licensee has indicated that the chattering of the 1CS-755 relief valve is
believed to have been caused by hydraulic effects associated with opening the
1CS-752 motor-operated isolation valve.  The NRC staff believes that the fluid
frictional and dynamic pressure loss characteristics of the long inlet piping
upstream of ICS-752 may have also contributed to the chattering.  The licensee
indicated that the testing, disassembly, and examination of the 1CS-755 relief
valve identified that the valve lift setpoint (2300 psi) had not changed, and
that the bellows failure was caused by cyclic fatigue of an inside weld. 
Static testing to verify the valve lift setpoint had been routinely performed,
during the plant lifetime, whereas testing to simulate flow through the valve
to assure proper operation and reseating apparently never was performed.  

The Crosby relief valve technical manual states that chattering can affect the
life, operation, and performance of a relief valve.  The manual recommends
that relief valves not have restricted inlets because restricted inlets can
cause valve chatter.  The opening and closing of the motor-operated isolation
valve in the existing HHSI AMF design at Shearon Harris restricts the inlet to
the relief valve.  In addition, relief valves are generally designed to
provide system overpressure protection and are not designed for continuous
fluid throttling purposes.

The licensee also indicated that modifications will be made to the HHSI AMF
system before restarting the plant from the current refueling outage.  Figure
2 provides a schematic of the proposed modifications to the HHSI AMF system at
Shearon Harris.  The licensee intends to install flow restricting orifices
upstream of the motor-operated isolation valves and to remove the relief
valves.  The motor-operated isolation valve actuation logic will also be
modified so that the isolation valves will open when a safety injection signal
is present coincident with RCS pressure above approximately 2300 psi. 

The NRC staff is aware that other facilities may employ an HHSI AMF design
similar to the existing system at Shearon Harris.  These include, but may not
be limited to, Millstone Unit 3 Comanche Peak Units 1 and 2, Beaver Valley
Unit 2, and Vogtle Units 1 and 2.  In addition, systems other than the HHSI
AMF system may contain relief valves with restricted inlets, thus also making
these valves potentially susceptible to chatter.

                                                       IN 92-61, Supplement 1
                                                       November 6, 1992
                                                       Page 3 of 3

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 Office of Nuclear
Reactor Regulation (NRR) project manager.

                                           ORIGINAL SIGNED BY

                                        Brian K. Grimes, Director
                                        Division of Operating Reactor Support
                                        Office of Nuclear Reactor Regulation

Technical contact:  J. Ramsey, NRR
                    (301) 504-1167

1.  Figure 1, Existing Shearon Harris High Head Safety Injection System
2.  Figure 2, Modified Shearon Harris High Head Safety Injection System
3.  List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Tuesday, November 12, 2013