Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 20, 1992
NRC INFORMATION NOTICE 92-61: LOSS OF HIGH HEAD SAFETY INJECTION
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to an event involving the loss of High Head Safety
Injection (HHSI) function due to water hammer 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). The root cause of the degradations appears to be water hammer
due to air left in the AMF system piping following system maintenance and
testing activities.
The AMF system (Attachment 1) is designed to prevent a deadhead condition on
the CSIPs by providing a flow path for these pumps to the refueling water
storage tank if the reactor repressurizes following a main steam line break or
feedwater line break accident. The AMF system is designed only to pass flow
when the CSIP discharge pressure is above the lift setpoint of 2300 +/- 69
psig for relief valves 1CS-744 and 1CS-755. The AMF system was a modification
to the original design in response to a Westinghouse 10 CFR Part 21 report.
This report identified the potential for deadheading the CSIPs during
repressurization of the RCS following secondary side breaks.
The licensee identified the following damage to the alternate minimum flow
system:
1) Relief valve 1CS-744 had a broken bellows and a cracked spring and
was found to have a reduced relief setpoint of 1100 psig.
9208180039.
IN 92-61
August 20, 1992
Page 2 of 3
2) Relief valve 1CS-755 had a broken bellows. Its relief setpoint
could not be determined due to valve seat leakage.
3) The piping connection upstream of 1CS-754 failed due to a water
hammer during engineered safety feature (ESF) testing. A small
leak had previously existed in this weld and was scheduled for
repair during the 1991 refueling outage.
Discussion
The physical layout of the AMF system piping at the Shearon Harris Nuclear
Power Plant results in air being trapped upstream and downstream of the relief
valves when they are removed and reinstalled in the system. The upstream
isolation valves 1CS-746 and 1CS-752 remain closed until an SI signal is
received. This prevents water from refilling this piping. Also, piping
upstream of the relief valves does not have high point vents to remove the
trapped air.
The licensee has taken corrective actions by revising plant procedures to
require the piping upstream of the relief valves to be refilled prior to
installation of the relief valves and vented through the relief valves by
hydraulic pressure to eliminate the air. The licensee has also repaired the
damage identified above. The licensee reported this event to the NRC in
LER 050-400/91-008-01 in May of 1991. The licensee is currently evaluating
the potential for water hammer downstream of the relief valves.
The NRC Office for Analysis and Evaluation of Operational Data (AEOD) included
this event in their Accident Sequence Precursor Program. A preliminary
evaluation estimated the conditional core damage probability to be
approximately 6 x 10E-3. The estimate is based on the unavailability of the
HHSI system for a year prior to discovery. The NRC staff considers this to be
a significant operational event.
The NRC sent a special inspection team to the Shearon Harris site to review
the event circumstances. The team determined that several water hammer events
could have occurred in the AMF system over the past 6 years. They concluded
that the water hammer events likely occurred when the CSIP pressurized the AMF
system piping which contained air during ESF testing and during ESF system
actuations.
The AMF system design weaknesses identified at Shearon Harris were:
1) The potential for water hammer events upstream and downstream of
the relief valves had not been analyzed.
2) The AMF system piping had not been analyzed for transient or water
hammer loads.
3) The potential for relief valve chatter and setpoint drift had not
been analyzed..
IN 92-61
August 20, 1992
Page 3 of 3
Also, it is questionable whether operators would have been able to detect and
isolate the flow diversion from the HHSI system in a timely manner due to the
lack of flow indication in the AMF system piping and the lack of adequate
directions in the Emergency Operating Procedures. Flow diversion could have
caused the CSIPs to operate at runout, and they could be potentially damaged
prior to diagnosis and corrective action.
Similar damage to AMF system components has previously been identified at
other plants.
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.
ORIGINAL SIGNED BY
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: J. Jacobson, NRR
(301) 504-2996
D. Gamberoni, NRR
(301) 504-1171
Attachments:
1. Shearon Harris High Head Safety Injection System
2. List of Recently Issued NRC Information Notices
.
