Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
November 29, 1991
NRC INFORMATION NOTICE 88-92, SUPPLEMENT 1: POTENTIAL FOR SPENT FUEL POOL
DRAINDOWN
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement to
Information Notice (IN) 88-92 to inform addressees of additional information
regarding the potential for spent fuel pool draindown. 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 supplement are not NRC
requirements; therefore, no specific action or written response is required.
Background
The NRC issued IN 88-92 to alert addressees to problems that could result
from the failure of pneumatic-type boot seals used to separate the spent
fuel pool (SFP) from other cavities such as the fuel transfer canal and the
refueling cavity. IN 88-92 described events involving the loss or potential
loss of this type of seal that occurred at Surry Power Station, Unit 1, and
Arkansas Nuclear One (ANO), Unit 2. A description of a recent event and two
potential scenarios with safety significance follows.
Description of Circumstances
On September 23, 1991, Wolf Creek Generating Station (WCGS) was in cold
shutdown in preparation for refueling when it experienced a SFP draindown.
The gate between the SFP and the fuel transfer canal was in place with the
dual boot seals inflated (Figure 1). The fuel transfer canal was partially
filled (about half full) with borated water and the fuel transfer tube which
connects to the refueling cavity was closed. The air supply for the SFP
gate seals comes from the nonsafety-related service air system. The event
was initiated by the loss of a nonsafety-related electrical bus which caused
the service air to isolate from its source. The loss of this bus also
caused a loss of the instrumentation that provided SFP level indication.
The SFP gate seals subsequently depressurized through leaks in the service
air system (Figure 2). The leaks occurred at isolation valve packings,
check valves, and at "Chicago" quick-connect fittings that were not in
accordance with design drawings.
9111250106
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IN 88-92, Supplement 1
November 29, 1991
Page 2 of 3
The SFP level decreased as water passed through the seals and into the fuel
transfer canal. Finally, cooling to the SFP was lost when the SFP
circulating pump tripped on low level. The operators were not alerted to
the loss of SFP water in the early part of the event, because the SFP
low-level alarm had previously annunciated three days earlier indicating
that the SFP level was lower than normal. However, it was still above
technical specification (TS) requirements. At that time, the operators did
not refill the SFP because of the need for adding makeup water to the
reactor coolant system as cooldown progressed. By the time the operators
recognized that the SFP gate seals had failed and they had completed actions
to repressurize the seals, the SFP level had dropped about 44 inches. This
was about 16 inches below the TS required level of 23 feet above the top of
the spent fuel. An NRC Augmented Inspection Team (AIT) was dispatched to
WCGS to evaluate the event. The results of the inspection and further
details of the event may be found in AIT Inspection Report 50-482/91-28.
Discussion
During the AIT site visit, the NRC identified two additional scenarios of
safety significance that applied to WCGS.
The first scenario involved possible failure of the SFP gate seals while
performing preventive maintenance on the fuel transfer system with the fuel
transfer tube open. The licensee did not have established administrative
controls over the refueling cavity drains or over the reactor
vessel-to-cavity seal for this activity. Under the above conditions, the
licensee estimated that, without operator intervention, the failure of the
SFP gate seals could have allowed the level in the SFP to drop to about 1
foot above the top of the spent fuel assemblies. The level in the SFP would
have dropped to about 9 feet above the spent fuel assemblies with the
reactor vessel-to-cavity seal installed and the refueling cavity drains
closed.
The second scenario involved possible failure of the SFP gate seals during
the periodic inspection or reconstitution of fuel assemblies performed in
the SFP or the cask loading pool with the fuel transfer canal drained. At
WCGS, the SFP is connected to both the fuel transfer canal and the cask
loading pool through removable gates with pneumatic-type seals. The
licensee indicated to the AIT that the gate between the SFP pool and the
cask loading pool was seldom used. The licensee estimated that without
operator intervention, the failure of the SFP gate seals could have allowed
the level in the SFP and the cask loading pool to drop enough to uncover a
fuel assembly held by the fuel handling bridge crane or in the fuel
inspection stand.
To mitigate the risk of the first scenario, the licensee committed to
perform the following whenever the fuel transfer tube is open and the
refueling cavity and the fuel transfer canal are drained: (1) have a backup
gas supply in place to repressurize the boot seals if service air is lost,
(2) have a dedicated operator in place to install the backup gas supply and
close the fuel transfer tube gate valve, and (3) have the reactor
vessel-to-cavity seal in place and the refueling cavity drains blanked or
sealed.
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IN 88-92, Supplement 1
November 29, 1991
Page 3 of 3
To mitigate the risk of the second scenario, the licensee committed to
either have all three fuel building cavities i.e. the SFP, the cask loading
pool, and the fuel transfer canal, full or to notify management and
establish other compensatory measures when performing fuel inspections or
reconstitutions.
This information notice supplement requires no specific action or written
response. If you have any questions about the information in this
supplement, please contact the technical contact 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 contact: Dr. Dale A. Powers, RIV
(817) 860-8195
Attachments:
1. Figure 1. Fuel Transfer System
2 Figure 2. Spent Fuel Pool Gate Seal Air Supply Lines
3. List of Recently Issued NRC Information Notices
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