United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 81-27: Flammable Gas Mixtures in the Waste Gas Decay Tanks in PWR Plants

                                                            SSINS No.: 6835 
                                                            Accession No.: 
                                                            8103300415     
                                                            IN 81-27       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555
                                     
                              September 3, 1981

Information Notice No. 81-27:   FLAMMABLE GAS MIXTURES IN THE WASTE GAS 
                                   DECAY TANKS IN PWR PLANTS 

Description of Circumstances: 

In July 1981, hydrogen ignition occurred in one gaseous waste decay tank at 
San Onofre Unit 1 while the plant was in cold shutdown. This resulted in a 
release of about 8.8 curies of noble gases and minor tank damage. The cause 
of the hydrogen ignition was air contamination of the inert nitrogen system 
which is used to control the hydrogen-oxygen concentrations in the tank. 

The source of air was identified as instrument air leaking through check 
valves at the cross connections between instrument air and nitrogen lines. 
Under normal operating conditions, the pressure in the instrument air system
is higher than that of the nitrogen system. These cross connections had been
installed in response to TMI Action Plan requirement item II. E.1.2 of 
NUREG-0737. The nitrogen system provided a backup gas supply to the 
air-operated steam supply valve for the steam-driven auxiliary feedwater 
pump. This backup was installed to provide a "safety grade" auxiliary 
feedwater system that satisfies the single-failure criteria. Other cross 
connections, which apparently did not leak air into the nitrogen system, had
been previously installed in response to TMI Action Plan requirement II.G.1 
of NUREG-0737 to provide a redundant gas supply to the air-operated 
pressurizer relief valves and the associated block valves. 

Following the occurrence, the licensee sampled all potentially affected 
tanks and determined that most of the tanks had oxygen levels above 10 to 15
percent. Generally, the gas in pressurized water reactor (PWR) waste gas 
systems is hydrogen rich and the oxygen concentration is controlled to 
prevent flammable gas mixtures. Flammable concentration of gas mixtures can 
be prevented by limiting either the hydrogen or the oxygen concentration to 
less than 3 percent. 

To eliminate the possibility of recurrence, the licensee has now completely 
separated those portions of the nitrogen system that are a backup supply to 
he air system from the balance of the nitrogen system that supplies cover 
gas. Bottles of compressed nitrogen are now used to provide the backup to 
the air system. 

We are aware of another instance of flammable mixtures in waste gas tanks. 
In August 1980, Arkansas Power and Light Company (AP&L) discovered flammable
concentrations of hydrogen and oxygen in the waste gas decay tanks at 
Arkansas Nuclear One, Unit 1. The flammable gas mixtures were created after 
the primary coolant picked up oxygen from the air during refueling and 
maintenance. No ignition or explosion was reported. 
.

                                                          IN 81-27         
                                                          September 3, 1981 
                                                          Page 2 of 2      

It appears that licensees need to devote more attention to the potential 
effects of nitrogen-air system cross connections when systems are modified 
to use nitrogen as a backup to air systems. If cross connections exist, the 
potential for the formation of flammable gas mixtures should be evaluated. A 
sampling program to assure that flammable gas mixtures do not exist in tanks 
should be considered. 

No written response to this information notice is required. If you need 
additional information with regard to this subject, please contact the 
Director of the appropriate NRC Regional Office. 

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