ML20058L313
| ML20058L313 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 05/29/1974 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Oleary J US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| AO-263-74-11, NUDOCS 9105140451 | |
| Download: ML20058L313 (2) | |
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I Regu!N Docket File NSD NORTHERN STATES POWER COMPANY M I N N E A f*O t.l s. M S N N E S OTA SS401 m
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May 29, 1974
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Mr. J F O' Leary, Director
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Office of Regulation A 1 U S Atomic Energy Commission 9 Washington, DC 20545 v, -
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Dear Mr. O' Leary:
MONTICELLO NUCLEAR GENERATING PIANT Docket No. 50-263, License No. DPR-22, A0 263/74-11 Off Gas Ignition A condition occurred at the Monticelle Nuclear Generating Plant which we are reporting to your office in accordance with Section 6.7.B.1, Abnomal Occurrence Reports, of Appendix A. Technical Specificatior.s, of Provisional Operating License DPR-22.
During plant startup on May 20, 1974, while operating at 575 psig reacto-pressure and conducting on-line testing of the modified off gas holdup t istem, an off gas ignition occurred resulting in the rupture of both air ejector discharge line rupture discs and a release of off gas from the reactor buildirg vent.
The release rate reached a maximum of 6200 uci/sec for five minutes, then decreased to less than 100 uci/sec after 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. At no time was the Technical Specification release limit of 210,000 uci/see approached.
The ignition occurred i= mediately af ter a rapid closure of 'B' train recombiner flow control valve (FCV 7489B) from an intermediate position as evidenced by the rupture of both air ejector discs and isolation of the air ejector suction valves on high pressure.
It is noted that a high pressure recombiner train shutdown, which is initiated by either of two high pressure switches set at 20 psia, did not occur during the incident indicating that the pressure surge did not propagate downstream of the recombiner inlet. flow control valve.
The ignition was, therefore, limited to the air ejector discharge-recombiner train inlet piping.
j Investigation into the occurrence revealed that the apparent cause of the ignition was the impact of the 'B' train inlet flow control valve on the valve seat.
Other ignition sources (including heat tracing, inlet hydrogen analyzers, and flowmeters) were considered and eliminated as the cause of the ignition based on inspection of the components and based on the fact that the ignition occurred simultaneously with the fast closure of the flow control valve.
The flow control valve is a rapid-closing heard-seated valve and the presence of scale or other interfering material j
on the seating surfaces may have caused a sparking condition.
9105140451 740529 CF ADOCK 05000263 1,.
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NORTHERN CTATEG PCWER COMPANY Mr. J F O' Leary May 29, 1974 Page 2 To prevent a recurrence the power operated and check valves which are normally exposed to undiluted off gas hydrogen-oxygen mixture were modified as follows:
Ihe plug material of recombiner train
'A' and 'B' flow control valves FCV 7489A and B, was changed from ste111ted stainless steel to aluminum bronze. The plug eages and seats of the off-gas pressure control valves, PCV 7496A and B were changed from stellited stainless steel to bronze. Check valves (OG 51-1 and
- 2) downstream of the pressure control valves were modified by welding a silicon bronze alloy material on the strib point of the disc hanger. These changes render the valves non-sparking under operating conditions.
An inspection of the components located in the air ejector room and recombiner cubicles revealed no equipment damage.
It is noted that the off-gas system is designed to withstand an ignition of the off-gas mixture.
The only previous off-gas combustion, which occurred at Monticello in November
! 1971, involved welding on an unpurged off-gas line and is therefore unrelated
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to this occurrence.
Testing of the modified off-gas holdup system will continue with the above described changes.
Yours very truly, jm. O-g L 0 Mayer, PE Director of Nuclear Support Services LQ4/kn cc:
J G Keppler G Charnoff Minnesota Pollution Control Agency Attn. E A Pryzina
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