Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 48,631
DECONTAMINATION METHOD
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to ~ Appli-
cation Serial No. ~co,~.3 , entitled "Decontamination
Apparatus" filed~ n the name of E. H. Smith et
al. and assigned to the same assignee.
BACKGROUND OF THE INVENTION
This invention relates to decontamination
methods and more particularly to a method for decontam-
inating heat exchange tubes in a nuclear steam generator.
A typical nuclear steam generator comprises a
vertically oriented shell, a plurality of U-shaped tubes
disposed in the shell so as to form a tube bundle, a
tubesheet for supporting the tubes at the ends opposite
the U-like curvature, and a dividing plate that cooperates
with the tubesheet orming a primary fluid inlet plenum at
one end of the tube bundle and a primary fluid outlet
plenum at the other end of the tube bundle. The primary
fluid having been heated by circulation through the nu-
clear reactor core enters the steam generator through the
primary fluid inlet plenum. From the primary fluid inlet
plenum, the primary fluid flows upwardly through first
openings in the IJ-tubes near the tubesheet which supports
the tubes, through the U-tube curvature, downwardly
through second openings in the U-tubes near the tubesheet,
and into the primary fluid outlet plenum. At the same
time, a secondary fluid known as feedwater, is circulated
around the U-tubes in heat transfer relationship there-
with, thereby transferring heat from the primary fluid in
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t:he tubes to the secondary fluid surrounding the tubes
causlng a portion of the secondary fluid to be converted
to steam. Since the primary fluid contains radioactive
particles and is isolated from the secondary fluid by the
U-tube walls and the tubesheet, it is important that the
U-tubes and tubesheet be maintained defect-free so that no
breaks will occur in the U-tubes or in the welds between
the U-tubes and the tubesheet, thus preventing contamina-
tion of the secondary fluid by the primary fluid.
Occasionally, it is necessary to inspect or
repair the U-tubes or the tubesheet welds by way of access
through the primary fluid inlet and outlet plena. For
this purpose, manways are provided in the vertical shell
so that working personnel may enter the inlet and outlet
1~ plena to perform operations on the U-tubes and tubesheet.
Ilowever, since the primary fluid which is generally water
contains radioactive particles, the inlet and outlet plena
become radioactive, which thereby limits the time that
working personnel may be present therein. In addition,
since the primary fluid is conducted through the U-tubes,
the U-tubes also become contaminated. Accordingly, it
would be advantageous to be able to perform operations on
the U-tubes and tubesheet without being exposed to high
levels of radiation. Consequently, it is important to
decontaminate lhe inlet and outlet plena together with a
portion of the U-tubes so that working personnel may enter
the inlet and outlet plena o~ the nuclear steam generator
to perform operat:ions thereon.
Therefore, what is needed is a method that i.s
capable of decontaminating the U-tubes so as to reduce the
radiation level in the inlet and outlet plena of the
nllclear steam generator so that work may be performed
thereon.
SUMMARY OF THE INVENTION
The decontamination method comprises inserting a
rotatable hone into a tube while rotating and moving the
hone through the tube at a rate of approximately 6-12
revolutions of the hone per inch of tube.
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RIEF DESCRIPTION OF THE ~RAWINGS
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 drawings
wherein:
Figure 1 is a partial cross-sectional view in
elevation of a typical steam generator;
Figure 2 is a diagram of the decontamination
apparatus shown in relation to a typical steam generator,
Figure 3 is a partial cross-sectional view of
the rotatable hone disposed within a heat exchange tube of
a nuclear steam generator;
Figure 4 is an enlargement of the rotatable
hone;
Figure 5 is a detailed cross-sectional view of
the rotatable hone apparatus;
Figure 6 is a partial cross-sectional view in
elevation of the cleaning apparatus and drive mechanism;
Figure 7 is a view along line VII-VII of Figure
6;
Figure 8 is a view along line VIII-VIII of
Figure 6; and
Figure 9 is a view along line IX-IX of Figure 6.
DESCRIPTION OF THE PREFERRED FMBODIMENT
In a tube-type steam generator, a tubesheet
supports a bundle of heat transfer tubes. The invention
described herein provides a method that is capable of
remotely decontaminating the heat transfer tubes in a
nuclear steam generator.
Referring to Figure 1, a nuclear steam generator
referred to generally as 20, comprises an outer shell 22
with a primary fluid inlet nozzle 24 and a primary fluid
outlet nozzle 26 attached thereto near its lower end. A
generally cylindrical tubesheet 28 having tube holes 30
therein is also attached to outer shell 22 near its lower
end. A dividing plate 32 attached to both tubesheet 28
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and outer shell 22 defines a primary fluid inlet plenum 34
and a primary fluid outlet plenum 36 in the lower end of
the stea~ generator as is well understood in the art.
Tubes 38 which are heat transfer tubes shaped in a U-like
curvature are disposed within outer shell 22 and attached
to tubesheet 28 by means of tube holes 30. Tubes 38,
which may number about 3,500 form a tube bundle 40. In
addition, a secondary inlet nozzle 42 is disposed on outer
shell 22 for providing secondary fluid such as water while
steam outlet nozzle 44 is attached to the top of outer
shell 22. In operation, the primary fluid which may be
water having been heated by circulation through the nu-
clear reactor core enters steam generator 20 through
primary fluid inlet nozzle 24 and flows into primary fluid
15 inlet plenum 34. From primary fluid inlet plenum 34, the
primary fluid flows upwardly through tubes 38, through
tubesheet 28, up through the U-shaped curvature of tubes
38, down through tubes 38 and into the primary fluid
outlet plenum 36, where the primary fluid exits the steam
generator through primary fluid outlet nozzle 26. While
flowing through tubes 38, heat is transferred from the
primary fluid to the secondary fluid which surrounds the
tubes 3~, causing the secondary fluid to vaporize. The
resulting steam then exits the steam generator through
steam outlet nozæle 44. On occasion, it is necessary to
inspect or repair tubes 38 or the welds between the tubes
38 and the tubesheet 28 to assure that the primary fluid,
which may contain radioactive particles, remains isolated
from the secondary flu-id. Therefore, manways 46 are
provided in outer shell 22 to provide access to both
primary fluid inlet plenum 34 and primary fluid outlet
plenum 36 so that access may be had to the entire tube-
sheet 28.
Referring now to Figure 2, the decontamination
apparatus is referred to generally as 50 and comprises a
rotatable hone 52 capable of being disposed within tubes
38. Rotatable hone 52 has an attachment mechanism 54
disposed around it for guiding the rotatable hone in rela-
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~ionship to tubesheet 28. Attachment mechanism 54 has aplurality of camlocks 56 attached thereto for being dis-
posed in tubes 38 so as to support attachment mechanism 54
from tubesheet 28. Camlocks 56 may be chosen from those
well known in the art. As an alternative to the use of
attachment mechanism 54, rotatable hone 52 may be sup-
ported and by guided remote handling devices (not shown).
Decontamination apparatus 50 also comprises a first flexi-
ble conduit 58 attached to attachment mechanism 54 and
n capable of being extended through manway 46 to cleaning
mechanism 60. Cleaning mechanism 60 is provided for
removing contamination from rotatable hone 52 as rotatable
hone 52 is removed from steam generator ~0. Decontamina-
tion apparatus 50 also comprises advancement mechanism 62
for advancing and withdrawing rotatable hone 52. Advance-
ment mechanism 62 may comprise a tandem set of motorized
rubber belts in contact with rotatable hone 52 for moving
rotatable hone 52 in a desired direction. In addition
decontamination apparatus 50 comprises drive mechanism 64
and cable holding apparatus 66. Drive mechanism 64 serves
as the mechanism to rotate rotatable hone 52 while cable
holding apparatus 66 provides a mechanism for winding and
unwinding the cable of decontamination apparatus 50.
Referring now to Figures 3, 4, and 5, rotatable
hone 52 comprises helical wound tubing 68 capable of
conducting a f:Luid therethrough. ~ nozxle 70 is attached
to one end of helical wound tubing 68 while a screw fit-
ting 72 is connected to the other end thereof. A spirally
wound brush 74 is disposed on helical wound tubing 68
between screw fittings 72 and nozzle 70 for contacting the
inside of tubes 38 and removing contaminants therefrom
when rotatable hone 52 is rotated. Brush 74 may be chosen
from those well known in the art of honing. Brush 74 is
chosen such that it is capable of removing a thin layer of
contaminated me~al from the insides of tubes 38 to thereby
reduce the contamination of the tubes 38. Nozzle 70 has a
plurality of channels 76 disposed therein for conducting
the fluid from nozzle plenum 78 and directing the fluid
_ ~ ~ 3
toward the inside of tubes 38 to thus lubricate brush 74
and to flush the contaminated particles that have been
removed by brush 74. Channel 76 may have a diameter of
approximately 1/32 inch and may be arranged at approxi-
mately between 20 to 40 angle with respect to the longi-
tudinal axis of nozzle 70 and preferably at approximately
30. As rotatable hone 52 is rotated and as brush 74
contacts the inside of tubes 38 a fluid such as water is
conducted through helical wound tubing 68, into nozzle
plenum 78, and out through channel 76 into contact with
the inside of tube 38 and brush 74. In this manner a thin
layer of metal is removed from the inside of tubes 38 and
flushed through first flexible conduit 58 to cleaning
mechanism 60. The fl.uid conducted through helical wound
tubing 68 serves to not only flush away the contaminated
particles, but it also serves to lubricate and cool brush
7~
Stil.l referring to Figures 3, 4, and 5, rotat-
able hone 52 also comprises an outer member 80 that is
attached to second flexible conduit 82 in a fluid-tight
manner and is capable of being disposed in first flexible
conduit 58. A bushing 84 is disposed in outer member 80
and has inner member 86 rotatably disposed therein. Inner
member 86 has a bore 88 therethrough for conducting a
fluid. Screw fitting 72 is capable of being attached to
inner member 86 by means of threads 90 in inner member 86.
Inner member 86 also has a plurality of passages 92 in its
lower end for a allowing the fluid to enter inner member
86 and to flow through bore 88 into helical wound tubing
68. At its lower end, inner member 86 is firmly attached
to cable 94 which is attached to drive mechanism 64 lo-
cated outside the steam generator for rotating inner
member 86 and helical wound tubing 68 thus rotating brush
74 of rotatable hone 52. Cable 94 is disposed in second
flexible conduit 82 so as to allow a fluid such as water
to be conducted through second flexible conduit 82 and
into the annulus between inner member 86 and outer member
80 so as to force the fluid through passages 92 and
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through bore 8~. lhe fluid is condu~ d ~ hrou~h ~<:)r~
through helical wound tubing 68, and into nozzle plenum
78. From nozzle plenum 78, the fluid is forced through
channel.s 76 of nozzle 70 and into contact with brush 74
and the inner surface of tubes 38. The fluid entrains the
contaminants that are removed by brush 74 and carries the
contaminants to the bottom of tube 38 where they are
carried away through first flexible conduit 58 to cleaning
mechanism 60.
Referring now to Figures 6-9, cleaning mechanism
60 comprises a housing 96 with flexible conduit 58 at-
tached thereto at one end. Second flexible conduit 82 and
cable 94 are arranged to slide through first flexible
conduit 58, through a central passage in housing 96, and
into attachment with drive mechanism 64. A first fluid
return chamber 98 is defi.ned in housing 96 and in fluid
communication with first flexible conduit 58 so that the
fl.uid that is being returned by first flexible conduit 58
flows int.o first fluid return chamber 98 and into drain
~n piping 100. From drain piping 100, the fluid is conducted
to ei~.her a waste disposal treatment facility or to a
recircu:l.ation facil:ity chosen from those well known in the
art. A spray chamber 102 is also defined within housing
96 for spraying a se~ond fluid which may also be water
onto second flexible conduit 82 for removing contamination
from second flexible conduit 82. Spray chamber 102 com-
prises an outer sleeve 104 which may be a substantially
cylindr:ical member and an inner sleeve 106 disposed within
outer sleeve 104. Inne~ sleeve 106 has holes disposed
therein for conducting the second fluid therethrough. An
inlet. port 108 i.s attached to the outside of outer s:leeve
104 for conducting the second ~luid illtO the annulus
defined between outer sleeve 104 and inner sleeve 106.
From the annulus defined between inner sleeve 106 and
outer sleeve 104, the second fluid is conducted there-
through and out of the holes in inner sleeve 106. In this
manner, the second fluid is sprayed against second flexi-
ble cond~lit 82 for removing the contamination therefrom.
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A spray water return chamber 110 is also disposed within
housing 96 and connected to drain piping 100. Spray water
returll <~h~lmber 110 serves to collect the spray of the
secon{l fLuid ~Ind conduc~ the second fluid into drain
piping 100 thereby removing contaminants from second
flexible conduit 82. A wiper section 112 is attached to
the end of housing 96 such that second flexible conduit 82
passes therethrough. Wiper section 112 comprises a plur-
ality of buffing cloths 114 which are disposed within
wiper section 112 and in contact with second flexible
conduit 82 for removing fluid and contaminants from second
flexible conduit 82. The excess fluid that is collected
by buffing cloths 114 is allowed to drip into drip pan 116
and from there allowed to flow into drain piping 100.
Therefore, it can be seen that cleaning mechanism 60
serves to conduct the first fluid from first flexible
conduit 5~ to drain piping 100 and serves to clean second
f'lexible conduit 82.
Referring now to Figure 6, drive mechanism 64
comprises a chuck 118 having an opening 120 therein.
Cable 94 is rotatably disposed within chuck 118 and at-
tached to motor 122 by means of a rotatable connector 124.
Motor 122 may be a 0.5 horsepower motor chosen from those
well kr.own in the art and is provided for ro~ating cable
94 and rotatable hone 52. I`he second flexible conduit 82
is also attached to chuck 118. Opening 120 is provided
for conducting a ~'luid into chuck 118 and into second
~lexible conduit 82 while cable 94 is bei.ng rotated by
n~otor 122. Thus, chuck 118 provides a mechan.ism whereby
3 cable 94 can be rotated without second f'lexible conduit 82
being rot.ated thereby allowing a fluid to be conducted
through second flexible conduit 82 and around cable 94.
Motor 122 serves to rotate cable 94 which in turn rotates
rotatable hone 52 thus removing the contaminatlon from the
i.nside of tubes 38. Drive mechanism 64 is supported on
cable holding apparatus 66. Cable holding apparatus 66
may be a rotatable platform or a take-up wheel capable of
rotating so as to accommodate the excess cable 94 and
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excess second f,lexible conduiL 82 under the influence ot
advancement mechanism 62. Thus, drive mechanism 64, the
excess o~f cable 94, and the excess of second flexible
(onduit 82 are stored on cable holding apparatus 66 such
rj that when advancement mechanism 62 advances cable 94 and
second f:l,exible conduit 82, cable holding apparatus 66
rotates to allow such advancement. On the other hand,
when advancement mechanism 62 retracts second flexible
conduit 82 and cable 94, cable holding apparatus 66 ro-
tates in the opposite direction, thus storing the excesscable and conduit.
OPERAI'ION
When it becomes necessary to inspect or repair
st.eam generator 20, steam generator 20 is deactivated and
:l.5 drainecl of its primary fluicl. Next, personnel enter steam
generator 20 through manways 46 ~.o position decontamina-
~ion app~rclt-us 50 in relationship thereto so as to decon-
tamin.l~e t.he steam generatoL. A~tachment mechani.sm 54 i.s
at-r;lnged near tubesheet. 28 so that camlocks 56 are in-
,?~) serted i.nl:o t.ubes ~8. Camlocks 56 are then activatedthereby support:i.ng attachment mechanism 54 from tubesheet
28. In ~his manner first ~lexible conduit 58 is attached
to a particular tube 38 i.n a fluid-tight manner.
Once first flexible condui~ 58 has been con-
nected to the sel.ected tube 38, rotatable hone 52 is theninserted i.nto f;.rst ~lexib'l.e conduit 58 and advanced
therethrouKh by means o~' advancement mechanism 62. When
rotatable honc 52 has reached the desired location within
t:ube 38~ drive mechan;sm 64 is activated which results in
3(~ motor 122 rotaling cable 9l~ and rotatable hone 52 a~ the
desired angular speed. ~t the same time, a first fluid
which may be wa~er is conducted under a pressure o~ be-
tween apl~roximately 1800 psi to 2,000 psi through opening
120 and illtO chuc~k 118. From chuck 118, the first fluid
~'j is collducLed through second flexible conduit 82 and into
:i.nner n~ember 86~ From inner member 86 the first fluid is
then collducted through helical wound tubing 68 and out
through channels 76 of nozzle 70. The first fluid co~n-
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~-~ct~; brush 74 and the inside of tubes 38 as brush 74
rotates in contact with the inside ~f tul>e 3~. I`hc .
of brush 74 remo~,7es d thin lclyt~L ~ t~ ~n~ lin~
from the inside of tube 38 which thereby lowers the radia-
tion level of tube 38. The first fluid entrains the
contaminated metal that has been removed and carries it to
the bottom of tube 38 where the first fluid enters first
flexible conduit 58. First flexible conduit 58 carries
the first fluid with contaminants entrained therein to
]() cleaning mechanism 60 and into drain piping 100.
Advancement mechanism 62 moves rotatable hone 52
through tube 38 at a linear rate slow enough to ensure
proper decontamination but at a fast enough rate to pre-
vent degradation of tube 38. The rate at which rotatable
hone 52 is moved through tube 38 is chosen so as to
achieve a sufficient decontamination factor (DF) where:
DF - Initial Activity
Fina Activity
It has been found that it is important for rotatable hone
52 to rotate at between approximately 6-12 revolutions per
inch of tube 38. At approximately 6-12 revolutions per
inch of tube, a decontamination factor of between approxi-
mately 20-25 may be achieved. Thus, a decontamination
factor o between 20-25 can be attained for various com-
binations of rotational speed and linear speed of rotat-
able hone 52. The following table illustrates some of thecombinations that will yield approximately 6-12 revolu-
tions per inch and thus yield decontaminations factors of
between 20-25.
Rotational Speed Linear SpeedRevolutions
3(~of hone (rpm) of ho~ 5~ inch of tube
800 5 12
3500 25 12
800 10 6
3500 50 6
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I'h~re~or-~, d~contaminations factors of approximately 20-25
can be achi~ved ~or rotational speeds of between 800-3500
rpm and for linear speeds of between 5 ft/min to 50
~t/min. The actual choice of rotational speeds and linear
speeds may depend upon the time available to complete the
operation.
Once a sufficient portion of tube 38 has thus
been decontaminated, advancement mechanism 62 is reversed
which causes rotatable hone 52 to be withdrawn from tube
38. As second flexible conduit 82 is withdrawn under the
action of advancement mechanism 82, a second fluid which
may also be water is introduced into spray chamber 102
through inlet port 108. The second fluid is sprayed
against second flexible conduit 82 thereby washing contam-
inants therefrom and into spray water return chamber 110.
From spray water return chamber 110, the second fluid is
conducted through drain piping 100. As second flexible
conduit is withdrawn, it also contacts buffing cloths 114
which wipe residual fluids and contaminants therefrom.
2n When rotatable hone 52 has thus been withdrawn
from the selected tube 38, the same process may then be
performed on another tube. Of course, several rotatable
hones 52 may be arranged on a sing:le attachment mechanism
5~ so that several tubes 38 may be decontaminated at once.
In this manner, tubes 38 are decontaminated thereby reduc-
ing the radiation level in primary fluid inlet plenum 34
and primary flu:id outlet plenum 36 so that working person-
nel may enter and perform opera~ions in steam generator
20. There~ore, it can be seen that the invention provides
3 a decontamination method capable of decontaminating tubes
in a nuclear steam generator.
