ML20128B022
| ML20128B022 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 06/17/1974 |
| From: | Goller K US ATOMIC ENERGY COMMISSION (AEC) |
| To: | Loper R CLEAN AIR, CLEAN WATER - UNLIMITED |
| Shared Package | |
| ML20128B025 | List: |
| References | |
| NUDOCS 9212030541 | |
| Download: ML20128B022 (3) | |
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Distribution M<rcket File AEC PDR Local PDR JW 171974 JR0' Leary, L DRnuth, R0 Docket No. 50-263 Solicitor FDAnderson, L:0RB #2 GErtter DRA (#7107)
MGroff, DRA Mr. Rodney G. loper, President TJCarter, L:0R clear Air - Clear Water Unlimited DLZiemann, L:0RB #2 Post Office box 311 JJShea, L:0RB #2 South St. Paul, Minnesota 55102 RMDiggs, L:0RB #2 EHughes, L:RP
Dear Mr. Loper:
SECY Mail Fac. (3)(#74-5514; We are pleased to reply to your request of May 20, 1974, to Chairman Ray regarding the hydrogen explosion experienced at the Monticello Nuclear Generating Plant at Monticello, Minnesota. Enclosed for your information is a copy of the report of the incident to which you have referred.
The Monticello plant recently experienced hydrogen detonations in the off-gas system. The detonations occurred during the procass of activating the augmented cff-gas system which is designed to reduce the level of radioactive affluents (such gases include primarily hydrogen and oxygen with some radioactive gases) released to the environs. Although hydrogen detonations in the off-gas system were not considered to be routine occurrences, the of f-gas system was designed to withstand such occurrences.
The system performed as designed to protect the remainder of the off-gas equipment from exoassivo overprescure. The releases of radioactivity from the plant due to these detonations were within permissible limits as specified in Monticello's operating license Technical Specifications and do not represent a risk to the health and safety of the public or to the water supply for Twin Cities.
The cause of these detonations is presently under investigation so that the system may be placed in continuous service as soon as possible. The Directorate of Regulatory Operations will continue to follow up on these occurrences as it does on all such occurrences at nuclear plants.
If we can be of further assistance, please let us know.
N Sincerely.
Originni signed by; Earl R. Goller i
[g Karl R. Go11er, Assistant Director Cs for Operating Reactors Oes Directorate of Licensing copy sent PDo
[fd SEE ATTACHED YELLOW FOR PREVIOUS CONCURRENCES:
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Docket No. 50-263 l
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Mr. Rodney C. Lop President
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Clear Air - Clear W ar Unlimited Post Office Box 311
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South St. Paul, Minnes ta 55102 Dear Mr. Loper We are pleased to reply to your request May 20, 1974, to Chairman Ray i
regarding the hydrogen exp1 sion experi mced at the Monticello Nuclear f
Generating riant at Montical, Minnesota.
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l The Monticello plant recently nced a hydrogen detonation in the j
off-gas system during reactor _
tt '. We detonation occurred during the process of activating the a ated off-gas system which is designed i
to reduce the level of radioacti
~ effluents released to the envireas.-
l The off-gas system is used to r a gases from the reactor system j
during operation.
Such gases el e primarily hydrogen and oxygen j
with some radioactive gases.
1tho gh hydrogen detonations in the off-gas system were not cons red t be routine occurrences, such i
conditions were considered td be cre ble and were included as a design condition for the off-gas e Th efore, the off gas system was
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designedwitharupturedis,fstem.
i k in the ai ejector subsystem which would j.
i' preferentially fail under/such high-pr sure (explosive) conditions.
l Tne cause of the detonation has not been determined but operation of the steel valves in the' system at that-t may have caused a spark (s)
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which could have resulted in the hydrogen tona tion. The rupture i
disk did perform as designed to protect the emainder of the off-gas system from excessive overpressure. ' The rol ses of radioactivity from the plant due to this detonation were well vi in the limits specified in Monticello's operating license Technical Sp fications and do not represent a risk. to the health and safety of the ublic or to the water supply for Twin Cities.
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f The licensee, Northern States Power Company, has estimated a==wh radioactive affluent release rate of 6200 uci/sec for less than five minutes. Within less than three hours, tha' release rate was less 4 9 l
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Mr. Rodney C. Imper I 1
than 100 oC1/sec. During the highest release rate, the release rate would have been less than 30 percent of the most restrictive affluent release limi for the plant.
The Monticello lent has been returned to power with the a6geanted off-gas system eolated until the cause of the incident inn be determined i
and corrective action taken to prevent a recurrence off he event. The l
Directorate of Reg,ulatory Operations will continue to follow up on this occurrance as 4t does on all such occurrences
't nuclear plants.
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l If we can be of further assiatance, please let. know.
5 Sincerely, Karl Coller, Assistant Director fo Operating Reactors DirpcLorateofLicensing Distribution Docket File AEC PDR Local PDR JF0' Leary, L
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DKnuth, RO Solicitor FDAnderson, L:0RB #2/
GErtter, DRA (#7107)'
MGroff, DRA TJCarter, L:0R DLZiemann, L:0RB,#2 JJShea, L:0RB #1 RMDiggs, L 0RB [l2 EHughes, L:RP k
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NOT;"rl lERN GTATES POWER C )MPANY
- g lir. J F O' Leary May 29, 1974 Page 2 To prevent a recurrence the power operated and check valves which are norbally exposed to undiluted off gas hydrogen-oxygen nixture were modified as follows:
1 The plug material of recombiner train ' A' and 'B' flow ~ control valves FCV 7489A and B, was changed from stellited stainicss stect to aluminum bronze.
The plua cages and seats of the off-gas pressure control valves, PCV 7496A and B vere changed from sec111ted stainicss steel to bronze.
Chech valves (OC 51-1 and
- 2) dounstream of the pressure control valves vero codified by velding a silicon bronze alloy ranterial on the strike 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 roem and recombiner cubicles revealed no equipment damsge.
It is noted that the off-gas system is designed to withstand an ignition of the of f-gas mixture.
The only previous of f-gas combu'stion, which occurred at Monticello in November of 1971, involved uciding on an unpurged off-gas line and is therefore unrelated to this occurrence.
Testing of the modified off-gas holdup system vill continue -
with the above described changes.
Yours very truly,
' L.i / f [
W ef L 0 Mayer, PE Dircetor of Nucicar Support Services LOM/kn f
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J G 1:eppler G Cha rno f f Minnesota Pollution Control Agency Attn. E A Pry:ina l
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Dear Hr. O' Leary:
i MONTICELLO KUCLEAR GENEPATEG PIANT Dochet No. 50-263, Licence No. DPR-22, A0 263/74-11 Off Gas Ignition A condition occurred at the Monticello Suclear Generating Plcat which we are reporting t., yiur office in accordance with Section 6.7.11.1, Abnorr.a1 occurrenco neportf, or oppcnu m A. Technical Specitications, of Provisional i
Operating Licenec DTR-22.
l During plant st artup on May 20, 1974, while operating at 575 peig reactor prescure and conducting en-line testing of the nodified off gas holdup systen, i
an of f gas ignition occurred resulting in the rupture of both air ejector discharge line rupture discs and a release cf off gas frota the recctor buildin; vent. The relcaec rate reached a raxitau.n of 6200 uci/sec for five minutes, t.hcn decreased to 1cas than 100 uci/sec af ter 2h hours. At no tire was the Technical l
Specification release li=it of 210,000 uci/sec approached.
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The ignition occurred iccediately a f ter a rapid-closure of 'B' train recoubiner flow control valve (FCV 745Sh) from cn inter.nadiate-position as evidenced by thc:
rupture of both air ejecter discs and ' isolation of the air ejector suction w.ives on high pressure.
It is noted that a high pressure recenbiner train shutdovu, which is initiated by either of two high-prcosure switches cet e.t 20_ psia, did a
not occur durin.';, the incident indicating that tVe pressure surge did not pror.npate downstream of the recoabiner inlet flow control _ valvo.
The ignition vas, thereforc, limited to the air ejector ditcharge-recombiner train inict piping.
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Invectigation into the occurrence revealed that the apparent cause of the ignillon vac the impact of the
'L' train inlet ficv centrol valve en the valve sest.
0"her ignition courcen (including heat tracing, inlet hydrogen analyr.crs, and fici de:crs) vere considered and cli:-innt ed as the cause of the ignition haced on inspectien ci the components sud based on the fact that the ignition occurred cinultanecur1
- ith the fact cic> cure of the tivr control = valve.
The flou control valve is a rcpi' -
clor,ing heard-scared valve and the preconce of seale or other interfering r.aterial on the seating suricccc ma; have caused a spat kirg condition.
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