Information Notice No. 81-27: Flammable Gas Mixtures in the Waste Gas Decay Tanks in PWR Plants
SSINS No.: 6835
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.
September 3, 1981
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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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