ML20056B738

Ro:On 711114,off Gas Sys Explosion Occurred.No Personnel Injuries Resulted from Occurrance.Procedure Prepared to Purge Off Gas Line Prior to Maint on Sys

ML20056B738
Person / Time
Site:	Monticello
Issue date:	11/24/1971
From:	Mayer L NORTHERN STATES POWER CO.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9102080459
Download: ML20056B738 (2)

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'. . Reguistory File Cy, NORTHERN STATES POWER COMPANY ,

Minneapolis, Minneso T*}h l t /

November 24, 1971 O h tent owc

~ CD g~ i Dr. Peter A. Morris g NOV 201971 > = ,, g l

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- RtsuutoRy e M Division of Reactor Licensing QQEg k { 1 _U United States Atomic Energy Corrnission .

Washington, D. C. 20545 HOV29197W -

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Dear Dr. Morris:

gy nem (f.9 f f.ONTICELLO NUCLEAR GENERATito PLANT Docket No. 50-263 License No. DPR-22 Report on Off-Gas Sysim Explosion A condition has occurred recently at the Monticello Nuclear Generating Plant which we interpret to be reportable in accordance with section 6.6.B of the Technical Specifications. The Region ill Compliance office has been notified of this event.

On Novanber 14, 1971, an explosion occurred in the plant off-gas system.

At the time of the occurrence, the plant had been shutdown for about 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />, and the condenser equalized to the atmosphere for 32 hours3.703704e-4 days <br />0.00889 hours <br />5.291005e-5 weeks <br />1.2176e-5 months <br />. The off-gas system had been isolated from the condenser at the air ejectors and the off-gas line loop seal was being cut with an acetylene torch to install a liquid level switch on the loop seal.

No personnel injuries resulted from this occurrence. Following the event, the stack gas monitor indicated an activity release rate of less than the normal operating activity release rate. (The stack gas monitor showed an increase in the activity release of from ~2500 uci/see to 10 The mechanical vacuum pump was started t thisa,000 uci/sec time to purgeat thethe time of separators, moisture the event) and the change is due to mechanical vacuum pump flog Airborne activity _fn the air ejector room increased slightly (from 8.8 x 10~ uci/cc to 1.7 x 10 uci/cc),

and ihe reactor buiIding venti 1ation radiation monitors did not show any change in ventilation air activity.

The rupture diaphragms on ihe sicam jet air ejector discharge ruptured io relieve the internal pressure created by the explosion and protected the air ejectors from any overpressure damage. The relative location of the air ejectors, rupture diaphragms, and the off-gas pipe protected the air ejectors from the j shock wage. The shock wave would have to follow a 180 bend in the pipe (a tee, '

and a 90 elbow) to enter the air ejector after condenser, but would only have to follow a straight pipe to impact on the rupture diaphragm.

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One pressure switch (PS-1921) used to cause isolation of the off-gas suction valves on high pressure was damaged by the explosion. The remaining iristrumentation connected to the off-gas systm has been tested and found to be in good working order.

, inspection and testing performed on the off-gas system has detected no damage.

l The air ejector condensers have been hydrostatically tested, the off-gas piping l from the air ejectors to the inlet of the stack off gas filters has been pressur-l ized with air and the leakage rate measured, the stack HEPA filters have been tested with DOP, and ultrasonic testing has been performed on welds at pipe t elbows. The piping system has been visually inspected for signs of pipe movement and none can be found.

L General Electric Company and Bechtel Corporation have been involved in the review of the occur rence. Representatives of these two companies support our conclusion that the piping systen has not been damaged.

Two changes will be made as a result of this occurrence. It was found that the off-gas system suction valves ( A01083A & B,1084A & B) isolation logic does not

" seal in" on trip action. The logic circuit will be changed to " seal in" and requirc operator action to reset an isolation. (Note that the isolation of these valves {

was meaningless with respect to this occurrence, since the explosion occurred on the discharge side of the air ejectors, and the manual valves at the air ejectors were closed. The off gas system suction valves are located between the air ejectors and the condenser, or on the suction side of the air ejectors.)

l A procedure will be prepared to purge the off gas line prior to maintenance on the system. This procedure will become a part of the Operations Manual.

Based on all findings, the off-gas system damage was limited to the rupture j diaphragms, and the one pressure switch. These components will be replaced '

before the plant is returned to service. This information has been made available to the Compliance inspector. An Event report has been written and will be reviewed by the Safety Audit Committee. l Yours very truly, h.h L. O. Mayer Director of Nucle.ar Support Services LOM/GHJhrrn

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