Information Notice 1981-27, Flammable Gas Mixtures in the Waste Gas Decay Tanks in PWR Plants

Flammable Gas Mixtures in the Waste Gas Decay Tanks in PWR Plants

ML070220261
Person / Time
Issue date:	09/03/1981
From:	NRC/IE
To:
References
IN-81-027 NUDOCS 8103300415
Download: ML070220261 (2)

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.

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