Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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l 48,392
~ECONT~INATION METHOD
mis invention relates to decontamination meth~
ods and more particularly to methods ~or decontaminating
components of nuclear power plants.
During the operation of nuclear power plants and
similar apparatus, certain components become e~posed to
radiation and may develop a thin redioactive film on the
surface o~ the component. From time ~o time, it is neces-
sary to either inspect or repair these components o~ the
1~ nuclear reactor power plant~ During the inspection or
repair of the components, 1t is necessary for working
personnel to enter the compon~nt or to be sta*ioned in
cloæe proximity to the component where~y working personnel
may be exposed to radiation emitted ~rom the contaminated ;`
component~ In some circumstances~ the radiation ~ield
emitted from these somponents is such that a worker would
receive the maximum permiss1ble radiation dose in les~
than ~ive minutes of working time. Such a situation means
that a given worker may only spend a relatiYely short
amount of time working on the inspection or the repair
operation ~f the nuolear component. Having each worker
spend a relatively ~hort ~mount of time in the repair or
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inspection procedure, necessitates the use of many workers
with each worker working a short time period in order to
accomplish the desired procedure. While this may be an
acceptable practice ~or minor inspections or repair proce-
dures, this is not an acceptable prac-tice where there is
an extensive inspection or an extensive repair job to be
performed. Where the procedure to be performed is a time
consuming procedure, it is l:ikely that an unusually large
number of highly trained personnel would be necessary to
carry out -the task. Such a situation may not only be
unacceptable from a financial aspect, but may also be
unacceptable from a man-power level aspect. Therefore,
what is needed is a decontamination method that reduces
the radiation field in components of nuclear reactor power
plants so that working personnel may perform operations
thereon.
SUMMARY OF THE INVENTION
A method for decontamina-ting radioactive compon-
ents comprises the use of a high pressure water jet con-
taining abrasive grit to remove radioactive oxide filmfrom the surface of the component. The flow rate and
pressure of the water jet are controlled so that a thin
layer of metal oxide deposits may be removed from the
component without damaging the metal componen-t itself
thereby reducing the radiation field of the componen-t.
The grit types, size, and concentration in -the water jet
are also controlled along with -the distance of the nozzle
from the surface of the component and the angle of im-
pingement so that an optimum level of decontamination is
achieved without deterioration of the metal surface.
BRIEF DESC~IPTION OF THE D~AWING
While the specification concludes with claims
particularly pointing out and distinctly claiming the
subject matter of the invention, it is believed the inven-
tion will be better understood from the following descrip-
tion, taken in conjunction with the accompanying drawing,
wherein the single fig~re of the drawing is a diagram of
apparatus used to perform the decontamination method.
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DESCRIPTION OF THE PR~F~RRED EMBODIMENT
. . ~
During operation of typical nuclear power
plants, certain components of the nuclear power plant such
as the nuclear s-team generators, become radioactively
contaminated. Since certain repair or inspec-tion opera-
tions that must be performecl periodically on the nuclear
power plan-t components require working personnel to be
present in or near the components, it is important that
the radiation field associated with the component be at a
level that is compatible w:ith the presence of working
personnel for an extended period of time. The invention,
described herein, is a method for decontaminating compon-
ents of nuclear power plants so tha-t working personnel may
perform operations thereon.
15Referring to the drawing, apparatus for perform-
ing the decontamination method comprises a platform 10 for
supporting container 12 which may be a steel drum having a
closure plate 14. Container 12 comprises s-upports 16 for
supporting the specimen 18 to be decontaminated. Contain-
2~er 12 also has a vent 20 and a drain 22. Container 12 is
arranged such that the front portion thereof is supported
by a bar 24 so tha-t container 12 is tilted such that
liquid within container 12 will flow through drain 22.
Drain 22 is connec-ted to filter 26 which may be a 5-25
micron filter bag chosen from those well known in the art.
Filter 26 is connected to a pipe 28 which is connected to
a drain for disposing of or recirculating -the water used
in the decontamination process. A half-inch diameter tube
30 extends through universal joint 32 and closure plate 14
so that the fron-~ end of tube 30 is disposed within con-
tainer 12. A nozæle 34 is mounted on the fron-t end of
tube 30 and is also connected to flexible hose 36. Nozzle
34 may be chosen from those well known in the art such as
a "Dynajector" manufactured by the Aqua-Dyne Engineering,
Inc. of Houston, Texas. Flexible hose 36 extends through
closure plate 14 and is connected to gri-t supply 38 for
supplying g:rit to nozzle 34. Tube 30 is connected to pump
40 which may be a 40 horsepower pump which is also con-
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nected to water supply 42 :Eor supplying water to nozzle34. Nozzle 34 provides a mechanism for mixing the grit
and wa-ter and for emikting the water-grit mixture from
nozzle 34 toward specimen 18. Tube 30 is also attached to
drive mechanism 44 which may be a mechanism for controll-
ing the horizontal movemen-t of tube 30 and nozzle 34.
Drive mechanism 44 may be chosen from those well known in
the art.
The decontamination me-thod comprises introducing
water through tube 30 at a pressure between approximately
2,000 psi and approximately 2,700 psi. The water flow
rate at -this pressure should be approximately 8 to 9
gallons per minute through nozzle 34. Several types of
grit may be used for mixing with the water such as alumina
or magnetite. However, the grit size should be approxi-
mately 120 to 325 mesh size in accordance with United
States Sieve Series Mesh Sizes. It is importan-t to note
that the grit concentration in the water spray should be
approximately 3% to approximately 7% by weight. In order
to provide effective decontamination without excessive
metal deterioration, it is important that the nozzle be
placed approximately 6 inches to 10 inches from the sur-
face of specimen 18. It has also been found that nozzle
34 should be arranged at approximately between 30 to 70
with respect to the longitudinal axis of tube 30 so that
the water-grit mixture impinges the surface of specimen 18
at approximately be-tween a 30-70 angle and preferably at
about 45.
OPERATION
In operation, a test specimen 18 having a top
surface that has been contaminated with radioactivity is
placed within container 12 and supported by supports 16 as
shown in the drawing. Closure plate 14 is then attached
to container 12 such that tube 30, nozzle 34, and ~lexible
hose 36 are arranged as shown in the drawing. Universal
joint 32 which also comprises a clamping mechanism is
arranged such that the flow of -the water-grit mixture from
nozzle 34 is direc-ted to the proper elevation of specimen
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18. At this point, nozzle 34 is approximately 6 to 10
inches from the surface of the specimen 18. Next, pump 40
is activated which causes water to be pumped from water
supply 42 -through tube 30 and into nozzle 34. The flow of
water through nozzle 34 creates a vacuum in nozzle 34
which draws the grit from gr:it supply 38 through flexible
hose 36 where i-t mixes with the water in nozzle 34. The
water-gri-t mixture is then directed toward the specimen
18. Simultaneously, arive mechanism 44 is activated which
causes tube 30 and nozzle 34 to move in a horizontal ].ine
across specimen 18 at a speed of approxima-tely 1 foot per
minute to approximately 3 feet per minute. The speed of
travel of nozzle 34 is correlated with the water-grit flow
rate so as to provide effective decontamina-tion witho-ut
excessive deterioration of the metal of specimen 18. The
water and grit mixture impinges on the surface of specimen
18 and removes a -thin oxide layer from the specimen which
is carried away by the wa-ter-grit mixture and drains
through drain 22 into filter 26. Once nozzle 34 has made
a complete horizontal pass of specimen 18, universal joint
32 is readj~tsted so that nozzle 34 is directed toward a
different vertical elevation of specimen 18 and at the
same 6 to 10 inch distance. Then, drive mechanism 44 is
reversed so that nozzle 34 makes a similar horizontal pass
f specimen 18 but at a different elevation. In this
manner, an entire sweeping of specimen 18 may be made. Ln
addition to the apparatus as shown in the drawing, an .
additional nozzle 34 may be attached to tube
and also arranged at approximately a 45 angle and
opposing the first nozzle 34 so as to provide a dual
nozzle arrangement.
A test of the decontamination me-thod was made
with the following parameters: ::
1. Nozzle Pressure - 2400 psig
2. Water Flow ~ate - 8.5 gal/min/nozzle
3. Grit Types - Alumina and Magnetite
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4. Grit Size - 220 mesh and 325 mesh
5. Grit Concentration
in Spray - 3% to 7% by weigh-t
6. Spray Traverse Velocity
across Surface - 16 in/min to 38 in/min
7. NozY.le to Surface
Distance - 6 inches
8. Impingement Angle - 45
Two passes were made across the surface of the specimen,
one pass each at opposing 45 angles. This ensured clean-
ing of bo-th sides of the irregularities in the surface of
the specimen. Results of -the tests show that 98.3% to
99.9% of the radioactive contamination was removed from
the sample simulating nuclear reactor componen-t surfaces.
None of the samples suffered perceptible surface damage.
Tests and analysis have determined that with the
use of the decontamination process, it is likely that less
than .001 inch of metal surface will be removed from -the
specimen. It appears that the use of an alumina grit on
Inconel metal will remove a metal layer of less than
.0002-.0003 inches and that the use of a magnetite grit on
stainless steel will remove a metal layer of less than
.0005-.001 inch.
While the decontamination method has been des-
cribed as being used in a test apparatus, the decontamina-
tion me-thod may be used on nuclear reactor components such
as nuclear steam generators with the equipment adapted to
be placed within the nuclear component. Therefore, the
invention provides a decontamina-tion method for lowering
the radiation field of nuclear reactor power components so
-that working personnel may enter the componen-t and perform
operations thereon.
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