ML20058K702
| ML20058K702 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 04/04/1975 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Anthony Giambusso NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III) |
| References | |
| NUDOCS 9102130510 | |
| Download: ML20058K702 (2) | |
Text
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,_f NORTHERN STATES POWER COMPANY t
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Minneapolis, Minnesota 55401 L.
April 4, 1975
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Mr. A. Giambusso, Director g
Division of Reactor Licensing U S Nuclear Regulatoly Commission Washington, D.C.
20555
Dear Mr. Giambusso:
MONTICELIO NUCLEAR GENERATING PIANT 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, hydmgen ignitions occurred on May 20, 1974 and June 10, 1974 which resulted in rupture of both air ejector discharge line rupture discs and a release of off-gas from the reactor building vent. These Abnomal Occurrences were reported by letters from Mr. L. O. Mayer to Mr. J. F. O' leary, on May 29, 1974 and June 20, 1974.
This letter is intended to infom you of the results of subsequent testing and corrective actions taken to prevent recurrence of off gas hydrogen ignitiors.
Prior to resumption of Modified Off-Gas System testing in July,1974, specici monitoring instrumentation was installed between the air ejectors and the recambiner train eductors. The instrumentation included fast response pressure transducers, off-gas flow control valve position, fast response strain gauges and themocouples.
In addition, the air ejector rupture discs were blank flanged to preclude the inadvertent release of activity if another detonation occurred.
Shortly after reaching 25% power on July 8, 1974, a third off-gas hydrogen ignition occurred and the off-gas system automatically isolated due to high The plant was immediately shutdown, the off-gas holdup system was pressure.
returned to the original design configuration and the plant wm returned to operation. Since no physical damage or radioactivity releases resulted from the ignition, the event was determined not to represent an Abnomal 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. 'Ihe 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 1'
I 9102130510 750404 0
+
o instantaneous pressures as high as
-300 psi existing for 1 to 2 seconds and then readjusting to + 3 psi of the initial pressure within 5 to 6 seconds.
One air ejector suction Isolation valves were automatically tripped closed approximately three seconds following the detonation. Analysis of t mperature data disclosed + hat limited recombination had been occurring in the off-gas 24" diameter hoMup pipe and ir the vicinity of the inlet flou control and bypass valves of both recmbiner trains. This recombination action had been most significant in the vicinity of the B train inlet flow control valve.
hhterial sampics from 'he 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 controi valve and in two capped low point drains.
It is postulated that the catalyst pellets were transported from : ocombiner "A" during a system flush and from both reconbinert during system operating transients.
An extensive mechanical and chemical clearing program was developed to renve t
all traces of catalytic material from the off-gas piping and valves. The program involved excavating below the recombiner building, cutting sections of the 6" inlet return piping from the 24" delay pipe, and removing one end of the 24" delay pipe. These lines were cleaned using a 10,000 psi hydro laser dry and wet sand blasting and a final chemical cleaning using a solution of The off-gas piping inside the recombiner building was cut phughric acid.
into sections and cleaned using a 10,000 psi hydro laser and a final chemical cleaning using a phosphoric acid solution. The bonnets and internals of control and isolation valves were removed and hand cleaned. After completion of all cleaning and flushing operations, material removed frar 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 the response received ham material removed from the contre (un-contaminated) piping. 'Ihis was considered as positive evidence that all catalyst had been removed.
To prevent recurrence of catalyst contamination of the off-gas piping, both recombiners were unloaded and the catalyst retention screens were ncdified to prevent pellets from leaving the recombiner. Addition'111y, a second screen was installed at the recombiner inlet norle.
No further ignitions have cccurred and off-ms inlet piping temperatures have remained at norral levels during opei tionai testing conducted following completion of t'e cleanup and recombiner modifications.
Yours very tiuly, fh.f
- J'V L. O. hbyer, hhnager of Nuclear Support Services LC'Jnm 4
cc: J G Keppler G Charnoff Minnesota Pollution Control Agency
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Attn: E A Pryzina
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