United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-93: Potential for Loss of Water from the Refueling Cavity

                                                           SSINS No. 6835  
                                                           IN 84-93        

                               UNITED STATES  
                       NUCLEAR REGULATORY COMMISSION 
                   OFFICE OF INSPECTION AND ENFORCEMENT  
                           WASHINGTON, DC 20555  

                              December 17, 1984

Information Notice No. 84-93:   POTENTIAL FOR LOSS OF WATER FROM THE 
                                   REFUELING CAVITY 

Addressees: 

All holders of a nuclear power reactor operating license (OL) or 
construction permit (CP) except Fort St. Vrain. 

Purpose: 

This notice is provided to alert licensees and applicants to features in 
some PWRs and BWRs that may have a significant potential to cause loss of 
water in the refueling cavity. It is expected that recipients will review 
the information for applicability to their facilities and consider actions, 
if appropriate, to preclude similar problems from occurring at their 
facilities. However, suggestions contained in this information notice do not 
constitute NRC requirements and, therefore, no specific action or response 
is required. 

Description of Circumstances: 

On August 21, 1984, the Haddam Neck plant experienced a failure of the 
refueling cavity water seal with the refueling cavity flooded in preparation
for refueling.  The refueling cavity water level (23 feet) decreased to the 
level of the reactor vessel flange in about 20 minutes, which flooded the 
containment with approximately 200,000 gallons of water. The leak developed 
when the pneumatic seal assembly was forced out of the normal position as a 
result of static water pressure. The pneumatic seal assembly remained intact
but was extruded through the gap for about 25 percent of its circumference. 

No fuel was being transferred at the time of this seal failure. If fuel had 
been in transfer at the time, it could have been partially or completely 
uncovered with possible high radiation levels, fuel cladding failure, and 
release of radioactivity. In addition, if the fuel transfer tube had been 
open, the spent fuel pool could have drained to a level that would have 
uncovered the top of the fuel assemblies stored in the pool. IE Bulletin No.
84-03, "Refueling Cavity Water Seal," was issued on August 24, 1984, as a 
result of the above incident. 

While evaluating the potential for loss of water from refueling cavities at 
other plants, the NRC staff learned from the Electric Power Research 
Institute (EPRI) that reactor cavity seal development testing had been 
previously 



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                                                       December 17, 1984  
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performed. This seal testing was sponsored by EPRI as part of a "Refueling 
Outage Availability Improvement Program."  These tests (completed in 1981) 
initially resulted in a failure mode very similar to that experienced by the
Haddam Neck plant. However, this failure mode was not observed in further 
testing with a modified seal design. This EPRI testing indicates that the 
performance of pneumatic seals is very sensitive to seal design details and 
to plant-specific refueling cavity design details, including variations in 
cavity gap dimensions.  

Other potential failure modes of the refueling cavity seal have been 
identified, since the incident at the Haddam Neck plant, which could cause a 
rapid loss of water in the refueling cavity at some plants. San Onofre Unit 
2 recently experienced several problems while installing the reactor cavity 
seal in preparation for the unit's first refueling. This unit has redundant 
(inner and outer) pneumatic seals. The inner pneumatic seal was punctured 
during installation. The seal was replaced with a spare. The spare seal also
failed during testing as a result of a manufacturing defect in the seal 
wall. Both the above failures were discovered and corrected before flooding 
the reactor cavity. Failures, like those reported at San Onofre Unit 2, 
could cause a rapid loss of cavity water (if the cavity were flooded) at 
plants with nonredundant pneumatic seals. Some pneumatic/flexible seals also 
may be susceptible to damage from the impact of dropped objects after the 
cavity is flooded. If the dropped object were radioactive, significant 
radiation damage to the pneumatic seal also could occur after a period of 
time. 

In addition to the refueling cavity seal, pneumatic seals also are used as 
hot and cold leg nozzle dams in PWRs and, for some plants, in gates between 
the spent fuel pool and the fuel transfer canal. The failure modes and 
concerns expressed above for the pneumatic refueling cavity seal also apply 
in many cases to these other pneumatic seals. Nozzle dams are of particular 
concern, when the steam generator primary is open during refueling. 

The refueling cavity also can be partially drained (PWR or BWR) by certain 
misalignments of the residual heat removal system (RHR) valves while in the 
shutdown cooling mode (assuming that shutdown cooling is in use when the 
cavity is filled). GE SIL No. 388, "RHR Valve Alignment During Shutdown 
Cooling Operation For BWR 3/4/5 and 6," dated February 1983, and IE 
Information Notice 84-81, "Loss of Reactor Pressure Vessel Coolant Inventory
in Boiling Water Reactors," dated November 16, 1984, discuss these 
possibilities in a BWR. Nuclear Safety Analysis Center report, NSAC-52, 
"Residual Heat Removal Experience Review and Safety Analysis, Pressurized 
Water Reactors," dated January 1983, discusses these possibilities in a PWR.

Finally, there are numerous ways in which the refueling cavity of a PWR or 
BWR could be drained at a slower rate through one of the attached drain 
lines. Adequate emergency procedures and properly calibrated refueling 
cavity water level instrumentation are considered to be important in the 
mitigation of any loss-of-cavity-water accident. 

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                                                       IN 84-93           
                                                       December 17, 1984  
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No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate NRC regional office or this 
office. 



                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  H. A. Bailey, IE 
                    (301) 492-9006 

Attachment:    List of Recently Issued IE Information Notices
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