Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
November 21, 1991
NRC INFORMATION NOTICE 91-73: LOSS OF SHUTDOWN COOLING DURING DISASSEMBLY
OF HIGH PRESSURE SAFETY INJECTION SYSTEM
CHECK VALVE
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 where loss of reactor shutdown
cooling (SDC) occurred because of inadequate reviews of work instructions
that affected plant conditions and systems configured to establish and
maintain SDC. 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 May 5, 1991, SDC for the Waterford Steam Electric Station, Unit 3, was
lost for 19 minutes when the bonnet of check valve SI 512A (Attachment 1) in
the high pressure safety injection (HPSI) system was opened. This allowed
air to enter the suction piping of the connecting SDC train that was in
service. During this period, the temperature of the reactor coolant
increased from about 100øF to 110øF.
At the time of the event, the plant operators were refilling the reactor
coolant system following refueling. The reactor vessel head had been
installed, but the containment equipment hatch was still open. The "A"
train of the SDC was aligned to fill the reactor coolant system from the
refueling water storage pool, leaving the operating "B" SDC train as the
only immediately available means to provide shutdown cooling. When the
vessel level reached the 19 foot elevation, the operators stopped filling
the system so that maintenance personnel could remove the bonnet from check
valve SI 512A to replace a bonnet seal. Check valve SI 512A is a hot leg
injection check valve in the "A" train of the HPSI system. This valve is
located at an elevation of 23 feet 10 inches and connects to a high point in
the "B" train of the SDC system at the 23 foot elevation. The opening of
the check valve permitted air to enter the SDC suction piping, breaking the
siphon that was holding the water in the high section of the SDC system and
allowing water to run out of the affected SDC train and into the reactor
vessel. This raised the vessel level to 21.3 feet. Since this level was
below the level of
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November 21, 1991
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the high point in the "B" SDC train suction piping, the "B" low pressure
safety injection (LPSI) pump became air bound (the LPSI pumps are also used
for SDC pumping).
After unsuccessfully attempting to vent this pump, the plant operators shut
it down. The licensee evacuated all unnecessary personnel from the
containment and started the process of installing the containment equipment
hatch. The licensee recovered shutdown cooling within 20 minutes by
realigning the "A" SDC train, venting the "A" LPSI pump (a standard
precaution in the procedure for starting up an idle shutdown cooling loop),
and placing the "A" LPSI pump into service. Within a half hour of the start
of the event, the licensee had closed the containment.
Discussion
The root cause of this event was a change in the work procedure that had not
received adequate review.
The licensee determined that check valve SI 512A needed repair about a month
prior to the event. The licensee's initial plans included the use of a
freeze seal to isolate the valve with the reactor coolant level at 44 feet.
A freeze seal was established in a horizontal section of piping
(Attachment 1), using CO2, but it failed when plant personnel attempted to
open the bonnet of the check valve. Another freeze seal was established
later using liquid nitrogen, but it too failed. The licensee found that the
freeze seals failed because this section of pipe had a slight slope,
allowing a small amount of air to be trapped at the elevation of the freeze
seal, which was about an inch above the piping's vent path. The path left
by the trapped air permitted water to leak through and melt the seal.
The licensee then changed the procedure to accomplish the repair without the
use of a freeze seal at a lower reactor coolant level of 19 feet. The
change was discussed and concurred in by the Maintenance, Operations and
Planning, and Scheduling Departments. However, the licensee made the change
without reviewing isometric drawings and system configurations that would
have shown that opening the HPSI check valve would break the suction of
operating LPSI pump "B". The licensee did not perform an adequate review
and did not obtain the required approvals. Some special controls that would
have been applied at a level lower than 18 feet (previously defined by the
licensee as reduced inventory requiring special controls) were not applied
at this level.
Careful review and performance of work procedures, including changes to
previously approved work procedures, are particularly important during
operations which involve opening a partially drained reactor coolant system
when SDC is required. The NRC has discussed loss of SDC in a number of
generic communications including Information Notice 87-23, "Loss of Decay
Heat Removal During Low Reactor Coolant Level Operation," May 27, 1987, and
Generic Letter 88-17, "Loss of Decay Heat Removal," October 14, 1988.
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IN 91-73
November 21, 1991
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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.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: Donald C. Kirkpatrick, NRR
(301) 492-1176
Thomas F. Westerman, RIV
(817) 860-8145
Attachments:
1. Waterford Unit 3 Partial Safety Injection
& Shutdown Cooling Systems
2. List of Recently Issued NRC Information Notices
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