United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-92, Supplement 1: Potential for Spent Fuel Pool Draindown

                                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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