ML20058K815
| ML20058K815 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 06/20/1974 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Oleary J US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| AO-263-74-16, NUDOCS 9102140458 | |
| Download: ML20058K815 (3) | |
Text
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MSIs NORTHERN STATES POWER COMPANY
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M IN N E A POU S. MIN N E SOTA B5401
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Mr. J F O' Leary, Director
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u Directorate of Licensing Office of Regulation U S Atomic Energy Commission
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Washington, DC 20545 e
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ABNORMAL OCCURRENCE REPORT TO THE AEC
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Off Cas Ignition N
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1.
Report Number: A0 263/74-16 f
Y' 2A. Report Date:
June 20, 1974 Y
2B. Occurrence Date:
June 10, 1974 G-3.
Facility: Monticello Nuclear Generating Plant (DPR-22)
Monticello, Minnesota 55362 4.
Identification of occurrence:
This report concerns an off gas ignition resulting in the rupture of both air ejector discharge line rupture discs and a release of off gas from the reactor building vent.
5.
Conditions Prior to Occurrence:
Steady-State Power-the plant was at 257. power and operational testing of the modified off gas system was being conducted.
6.
Description of Occurrence:
On June 10, 1974, an operational test of the modified off gas system was performed wherein off gas flow had been deliberately terminated to establish the rate of vacuum decay in the main condenser subsequent to the loss of a recombiner train.
Shortly after flow was resumed, air ejector off gas high pressure was annunciated in the main control room.
The steam jet air ejector isolation valves isolated due to the high pressure condition.
Following the isolation, there was a sudden loss of off gas flow and a rapid pressure drop in the piping system between the air ejectors and the recombiner inlet flow control valves.
This resulted in a low pressure isolation of the recombiner system.
Immediately thereafter, an increase in the reactor building ventilation plenum radiation monitor was observed.
Following isolation of the air ejectors, reactor power was reduced from 257, to 127. at which time a condenser low vacuum scram occurred.
The plant was subsequently returned to power operation using the original off gas system.
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CKETFILfCOM 9302140458 740620'EGULATORY DO R
NORTHEh.4 CTATES POWER COMPANY 7.
Designation of Apparent Cause of the occurrence:
The increase in reactor building ventilation release rate was caused by a release from the off gas system rupture discs.
The discs ruptured due to an ignition of the off gas.
The exact cause of the ignition is uncertain although the majority of the evidence indicates probable involvement of the off gas inlet flow control valves.
The major impediments to more positive conclusions are the lack of sufficiently accurate time records to establish a definite sequence of events and the lack of information regarding the directions of travel of the ignition wave fronts.
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8.
Analysis of Occurrence:
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The release rate from the vent reached a maximum of 19,500 uCi/sec for one minute, decreasing to background levels af ter two hours. At no time was the Technical Specification release limit (210,000 uCi/sec) exceeded.
The radio-active materials released were primarily isotopes of Xenon and Krypton. An inspection of the components located in the air ejector room and recombiner cubicles revealed no equipment damage.
The portions of the off gas system involved in the ignition are designed to withstand of f gas ignition without loss of integrity (reference FSAR Section 9.3 and Gaseous Radwaste System Modification Report, Revision C, dated October 13, 1971).
It is noted that the pressure surge associated with the of f gas ignition was limited to the piping between the discharge of the air ejectors and the inlet to the o f f gas recombiner eductor.
This is evidenced by the absence of a recombiner high pressure shutdown following the ignition.
There were no equipment malfunctions, operator errors, personnel injuries or significant personnel exposures associated with this occurrence.
9.
Corrective Action:
Following the incident, a task force of experts in the field of off gas system design and in the field of detonable mixtures was established.
The task force performed a cetailea review of this occurrence and a previous similar occurrence t refe rence letter f rom L 0 Mayer to J P 0' Leary, dated May 29, 1974).
Th e exac t cause of the ignitions could not be determined f rom the available information, however, the following actions recommended by the task force, are being taken:
a.
The trip closure time of the recombiner inlet flow control valves will be increaseo ;a reduce the abruptness of flow rate changes and to further reduce the sparking potential (reference letter from L o Mayer to J F O' Leary, dated May 29, 1974, for modifications made ta alves to render them non-sparking).
b.
Adequate grounding of valve internal parts and of the flow orifice plate will be verified.
c.
The electric heaters on the recirculation line to the condenser will be removed from service.
Removal of the heaters does not create an unsafe concition and experience to date indicates that they are not essential to operation.
NORTHERrJ GTATE3 POWER COMPANY d.
The controlled back pressure on the air ejectors will be reduced to less than 16 PSIA so the air ejectors will not be choked under maximum t
differential pressure transients.
This will minimize the potential for flow surges in the system.
e.
The intrinsic safety aspects of the Eastech flowmeters will be reverified.
Prior to further operational testing of the modified off gas system, the rupture discs at the air ejector discharge will be blanked.
This change will prevent the release of radioactive gases from the rupture discs in the event of future ignitions.
The off gas system has been designed to contain, without damage to durable components, the pressures expected as a result of an off gas ignition.
Analysis indicates that reduction of shock wave forces by the rupture discs is insignificant.
The potential radiation exposure to personnel due to disc rupture is felt to outweigh any potential gain in equipment availability.
In addition to these changes, temporary fast response instrumentation will be considered to record exact sequences of key events that might be important to the evaluation of possible future ignitions (this will include instru=ents to record the direction of the initial wave front).
10.
Failure Data:
There were no equipment failures which caused the occurrence or resulted from the occurrence.
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L 0 Mayer, PE Director of Nuclear Support Services La3/kn I
cc:
J G Keppler G Charnoff Minnesota P211utien Control Agencv Attn. E A Pryzina