IE Circular 77-10, Vacuum Conditions Resulting in Damage to Liquid Process Tanks


                              July 15, 1977 

J. P. O'Reilly, Director, Region I 
N. C. Moseley, Director, Region II 
J. G. Keppler, Director, Region III 
E. M. Howard, Director, Region IV 
R. H. Engelken, Director, Region V 


The subject document is transmitted for issuance by close of business within
three days after date of this letter. The Circular should be issued to all 
holders of Reactor Licenses. Also enclosed is a draft copy of the 
transmittal letter. 

                                        Harold Thornburg, Director 
                                        Division of Reactor Operations 
                                        Office of Inspection and Enforcement 

1. Circular 77-10 
2. Draft Transmittal Letter 

Transmittal letter for Circular 77-10 to each holder of an NRC Operating 


The enclosed Circular 77-10 is forwarded to you for information. If there 
are any questions related to your understanding of the actions required, 
please contact this office. 

                                        (Regional Director) 

IE Circular 77-10 

                                                       IE Circular 77-10  
                                                       Date: July 15, 1977 
                                                       Page 1 of 2 


Description of Circumstances 

On January 31, 1977, at the Portland General Electric Company's Trojan 
Nuclear Station, a vacuum was pulled on a Chemical and Volume Control System
holdup tank which caused the tank to buckle inwardly and crack at the 
junction of two buckled areas. (LER 50-344/77-02) Radioactive cover gas 
escaped from the tank to the fuel building exhaust ventilation and 
ultimately through the auxiliary building ventilation stack to the 
environment. During the period of January 31 through February 7, 1977, (date 
of discovery) approximately 62.5 curies of noble gases were released to the 
environment. The occurrence was caused by malfunctions of both the primary 
(waste gas decay tank collection header) and the backup (nitrogen 
pressurization system) cover gas supply pressure regulators during pumpout 
of the tank contents. The malfunction of the primary regulator was due to 
moisture accumulation which led to the formation of a water column on the 
discharge side of the pressure regulator. The regulator in the backup 
nitrogen supply system malfunctioned at the same time, but the cause has not 
been determined. There were no alarms or remote indication for the holdup 
tank or the gas control header to indicate abnormal pressure or to indicate 
the operating status of the nitrogen supply system. 

A similar event occurred at the Sacramento Municipal Utility District's 
Rancho Seco nuclear station wherein the Reactor Coolant System Drain Tank 
partially collapsed under vacuum conditions and in doing so cracked an inlet
line attached to the tank (LER 50-312/77-03). The first indication of the 
condition was detected by the radiation monitor that samples the ventilation
exhaust in an area adjacent to the room in which the damaged tank was 
located. Radioactive gases were released through the failed line between 
January 12 and February 16, 1977. During this period, approximately 9 curies
of radioactive, gas were released. The cause of this occurrence was 
attributed to the tank not being properly vented by the operator during 
discharge of its liquid contents. 

                                                       IE Circular 77-10 
                                                       Date: July 15, 1977 
                                                       Page 2 of 2 

It is recommended that you examine the systems of your reactor facility(ies)
that contain low pressure process or holdup tanks and assure that adequate 
measures have been taken to protect against vacuum conditions that could 
result in tank inward buckling and failure with subsequent release of 
radioactive material or cause other detrimental effects with regard to the 
overall safety of plant operations. It is also recommended that you examine 
your capability to detect and locate possible leaks of radioactive material 
from such tanks. 

No written response to this Circular is required. If you desire additional 
information regarding this matter, contact the Director of the appropriate 
NRC Regional Office. 


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