Flammable Gas Mixtures in the Waste Gas Decay Tanks in PWR PlantsML070220261 |
Person / Time |
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Issue date: |
09/03/1981 |
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From: |
NRC/IE |
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To: |
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References |
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IN-81-027 NUDOCS 8103300415 |
Download: ML070220261 (2) |
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Category:NRC Information Notice
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I-
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 IE 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 ga' ;ystems 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
the 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 Liqht Company (AP&L) di,.covered flammable
concentrations of hydro(len and oxyqen in the w,1st, (las de(yv tanks at Arkar,;as
Nuclear One, Unit I. The flammatble qas mitx ture wert, crealted after the primary
coolant picked up oxv(jen from the air durinq rf,? u, Iinq anl maint,-n1vn 1(e. No
iqnition or ePplosion.wa'. r-pporle(l
IN 81-27 September 3, 1981 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.
Attachment:
Recently issued IE Information Notices