ML20127L160

RO:follow-up Rept Re Modified off-gas Sys Ignitions Which Occurred on 740520 & 0610

ML20127L160
Person / Time
Site:	Monticello
Issue date:	04/04/1975
From:	Mayer L NORTHERN STATES POWER CO.
To:	Giambusso A US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9211230440
Download: ML20127L160 (2)

Text

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O /b o i NORTHERN STATES POWER COMPANY Minneapolis, Minnesota 55401 g}

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, c April h, 1975 Mr. A. Giambusso, Director Division of Reactor Licensing g '.

U S Nuclear Regulatory Commission Washington, D.C. 20555

Dear Mr. Giambusso:

ENTICELID NUCLEAR GENERATING PIMT Docket No. 50-263 License No. DPR-22 Follow-Up Report - Modified Off-Gas System Ignitions hhile conducting tests of the Modified Off-Gas System, hydrogen ignitions occurred on May 20, 1974 and June 10, 1974 which resulted in rupture cf both air ejector dischtge line rupture discs and a release of off-gas fmm the reactor building vent. These Abnomal Occurrences were reported by letters from Mr. L. O. bhyer to Mr. J. F. O' leary, on hby 29, 1974 and June 20, 1974.

This letter is intended to infonn you of the results of subsequent testing l and corrective actions taken to prevent recurrence of off-gas hydrogen

! ignitiom.

Prior to resumption of Fiodified Off-Gas Systen testing in July,1974, special monitoring instrunentation was installed between the air ejectors and the l recombiner train eductors. The instrumentation included fast response pressure

transducers, off-gas flow control valve position, fast response strain gauges and thermocouples. In addition, the air ejector rupture discs were blank flanged to preclude the inadvertent release of activity if another detonation I occurred.

I

Shortly after reaching 25% power on July 8,1974, a third off-gas hydrogen i

ignition occurred and the off-gas system automatically isolated due to high pressure. The plant was immediately shutdown, the off-gas holdup system was i

returned to the original design configuration and the plant was returned to operation. Since no physical damage or radioactivity releases resulted from the ignition, the event was determined not to represent an Abnormal Occurrence.

An analysis of data received from the special test instrumentation disclosed that the hydrogen ignition originated in the Train B inlet piping near the off-gas flow control valve. The detonation wave traveled to Train A, causing the bypass valve to open slightly, and through the piping to the air ejector discharge. The shock wave traveled at 7,000 to 10,000 feet per second with l

3773 9211230440 750404 i PDR ADOCK 05000263 S PDR

O instantaneous pressures as high as 200-300 psi existing for 1 to 2 seconds and then readjusting to + 3 psi of the initial pressure within 5 to 6 seconds.

He air ejector suction Isolation valves were automatically tripped closed approximately three seconds following the detonation. Analysis of tmperature data disclosed that limited recombination had been occurring in the off-gas 24" diameter loldup pipe and in the vicinity of the inlet flow control and bypass valves of both rec ebiner trains. This recombination action had been most significant in the vicinity of the B train inlet flow control valve.

hbterial sampjes from the off-gas piping and valves were analyzed for the presence of catalyst using neutron activation and differential scanning calorimetry. Both techniques gave positive indication. Further investigation revealed the presence of catalyst pellets in both recombiner vessels below the retention screen, in both preheaters, in the A train pressure control valve and in two capped low point drains. It is postulated that the catal:Tt g liets were transported frm recombiner "A" during a system flush and from both recmbiners during systam operating transients.

An extensive mechanical and chemical cleaning program was developed to remve all trace af catalytic nnterial from the off-gas piping and valves, he program involved excavating below the recmbiner building, cutting sections of the 6" inlet return piping from the 24" delay pipe, and removing one end of the 24" delay pipe. Rese lines were cleaned using a 10,000 psi hydro laser, dry and wet sand blasting and a final chemical cleaning using a solution of phosphoric acid. The off-gas piping inside the recombiner building was cut into sections and cleaned using a 10,000 psi hydro laser and a final c % .a1 cleaning using a phosphoric acid solution. Le bonnets and internals of control and isolation valves were removed and hand cleaned. After completion of all cleaning and flushing operations, material renoved fra off gas system pipes

! and valves, when tested for the presence of catalyst using the differential scanning calorimetry technique, exhibited a response less than or equal to l the response received from material removed frm the control (un-contaminated) piping. His was considered as positive evidence that all catalyst had been removed.

i I

To prevent recurrence of catalyst contamination of the off-gas piping, both recombiners were unloaded and the catalyst retention screens were modified to prevent pellets frm leaving the recombiner. Additionally, a second screen j was installed at the recombiner inlet nozzle.

No further igni+1ons have occurred and off-gas inlet piping temperatures have remained at nonnal levels during operational testing conducted following completion of the cleanup and recombiner 'nodifications.

Yours very truly, l

st's.9 L. O hbyer, hhnager of Nuclear Support Services Im/nnn cc: J G Keppler G Charnoff Minnesota Pollution Control Agency Attn: E A Pryzina

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