ML20127K193
| ML20127K193 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 11/11/1977 |
| From: | Cwalina G Office of Nuclear Reactor Regulation |
| To: | Grimes B Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9211200324 | |
| Download: ML20127K193 (3) | |
Text
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..:(.;V MEMORANDUM FOR:
B. Grimes. Chief. Enytronmentail Evaldaiti
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G. Cwalina. Enytronmental?.Evaluatt 8 'nc 1 '.
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L. Barrett. Section LeaderMEn ronmental ivaluation Branch c "' %
g w7 0,v MNTICELLOTRIP'REPORTh..;
SUBJECT:
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On October 7.1977 I visited the Monticello MucisarePla. t,ith)bservejthe licensee's effort to maintain occupationaldadiationtexposureslto?c"as : low a
as reasonably achievable" (ALARA) during repatr;ofjthelfeedwater nozzles.*'y The Itcensee was engaged in repair of the' nozzles' utilizing aimillin mat chine designed by GE. Because of the generic' nature'ofc,the'problemi see A NUREG-0312. " Interim Technical Report on BWR Feedwater..andjcontrol-Rodl Drive Return Line Nozzle Cracking") and'high occupational" exposures asso n i
clated with past repair efforts. the staff is concerned with Itcenseestf.
efforts to reduce these exposures to' ALARA values.' Therei are2severa1Tvh'k f
l contamination, shielding and development of special tools which reduce expg posure tine. These techniques, as utilized by Monticello, are discussed ' M i
in 'the following paragraphs.
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The majority of work perfonaed was done by contract (GE) personnel.. These i."N*
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__l workers were trained on a full scale mockup using the actual tool which was }
i used to remove the cladding from the nozzle. In addition, any local workers ?
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must view a videotape of the training session before they are allowed to en-).
I ter the work area. Daily meetings are held to plan all activities in order -l 1
to keep occupancy times in high radiation areas as short as possible, i
The work area was decontaminated using a "hydrolasing" (high pressure water I
spray) technique. The decontaminated areas include the reactor vessel walls.
reactor cavity and the operating floor in the vicinity of the reactor cavity.
I Radiation surveys indicated dose rate mduction factors of 2-5 were achieved by hydrolasing.
Extensive shielding was used to reduce dose rates in the work area.
(The work was perforced with the core in place.) GE designed the shield plug to 5
be used over the core region. This plug consisted of 33 inches of concrete d
with a 11/4 inch steel plate on each side. The plug is a single unit and
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weighs about 50.000 pounds. The itcensee stated that an analysis was done to determine the consequences of dropping the shield plug on the core. The result was the consequences would not exceed those of dropping the steam separators and_ dryers. The analysis was reviewed by the plant safety com-mi ttee which concluded that dropping the plug would not constitute an
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unreviewed safety question. Ide understand PSB personnel infonnally re-viewed the plug design, handling and support systems. The shield was supported by a series of small beams which mst on the core shroud. The top of the plug was about 74 inches above the top of the active fuel.
Shielding of the core spray line was accomplished by setting lead shields vertically in front of the 1tne, placing a board resting on the vertical shielefand the spray line and covering the board with lead blankets. This shielding arrangement was about 12 inches below the centerline of the feed-water nozzles. The nozzle area including the feedwater spargers, was shielded by hanging lead shields by pulleys attached to the steam dryer support lugs. These shields are suspended around the entire circumference of the vessel. In order to work oi. a nozzle, the shields in that nozzle area only are raised so that the remaining nozzles and vessel walls are still shielded. The shielding techniques lowered the dose rates in the area by a factor of 8-10. The dose rate at waist level on the shield plug was 500 mr/
hr after shielding and before sparger removal wherea, before shielding and after hydrolasing the level was in excess of 4 R/hr. After sparger removal, the level dropped to 300 mr/hr. A sketch of the shte.1 ding arrangement is included as Figure 1.
The milling tool in use was developed by GE specifically for the BWR feed-water nozzle cracking problem. Past repairs were made by flapper wheel grinding the nozzles to remove all the cladding. This method required long exposure times. Flapper wheel grinding also causes high airborne particu-late activity due to the small sized particles. The new machining technique requires a minimum of flapper wheel grinding to only smooth the nozzle sur-
-l faces and then the use of the milling machine to actually remove the cladding.
The machine operates at 5-10 RPM using either a rough or fine cutting head.
f The resultant chips are about 10 mils thick and are collected in a basket lo-cated under the nozzle. Occupancy in the grinUtng zone is necessary only to set the machine ar.d change the cutting heads. All grinding is done automatic-ally, thus greatly reducing occupancy times in the zone. In addition, a con-tinuous water spray iminges on the nozzle which will wash out any small grinding residue. The area was ventilated by a portable air hose which drew 4000 cfm of air from the work area and exhausted it to the containment after passing it through a HEpA and charcoal filter. Workers in the radiation area were carefully monitored for radiation exposure and occupancy tim. Full pro-tective clothing including respirators equipped with particulate filters, was required at all times in the work zone.
The work ' areas on the refueling floor and inside the vessel were continuously monitored. The refueling floor area was monitored using the instrumentation as designed for the plant during normal operations. The vessel work area was i
monitored by an area radiation monitor located inside the vessel with a read-out on the refueling floor and by a continuous air monitor which drew air from the work area near the nozzle at approximately head level. During milling u
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W s.. y tt;g operations, the area radiation monitor was reading from 150-200 mr/hr.1$'th 6
sel work area increased to about 90 MPC and the refueling floor airborne ~a vessel. During the flapper wheel grinding the airborne activity 6n the;vesG d?M y
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tivity.was about 1 1/2 HPC.
s Radiation protection procedures were formulated in advance of all work andji4 A I
distributed to the plant radiation protection gmup. A copy of the memos "-,
jji sent to personnel are included as Enclosure la and ib.
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.y A copy of the outage timetable. including the nozzle repair work, is'inc1Ededh *.
as Enclosure 2.
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' g., w. ; em The overall plant A1. ARA pmgrams seems to be well planned and executed;,Simi-3;h
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1ar type work was performed at Monticello in 1975 consisting.of, replacement:of.
the feedwater spargers and examining the nozzle areas for cracks;P The'early
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work was done without a core shield and required considerably less; work'than the recent maintenance. Plant personnel estimate exposure of the'present work will be about 1/3 of that previously experienced. It is hoped that' experience 4
gained frca the Monticello repair can be utilized at other facilities to' aid) i,d in reducing their occupational exposures during this anti other repair jobs..'b ff
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G. Cwalina i
Environmental Evaluation Branch i
Division of Operating Reactors
Enclosures:
As stated DISTRIBUTION cc:: w/ encl Central files V. Stello EEB Rdg D. Eisenhut GCwalina R. Snaider JGuibert RCudlin cc: w/o encl.
E. Case T. Murphy K. Goller J. Collins P. Nelson, Region I D. Davis R. Alexander J. Sutherland, Region II R. Muranaka J. Allan, Region III IC,E.Adensam Section B/EEB R. Reid G. Brown, Region IV L. Higgenbotham D. Crutchfield H. Book, Region V
'U OR Br. Chiefs R. Boyd J
H. Denton H. Hulkey
. R. Mattson J. Scinto It2'DPH. DSS.DSEAD's W. Fisher, Region III pp* DPM, DSS DSE Br. Chiefs J. Long W. Kreger R. Fraley, ACRS (16) f yf
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