Note: Descriptions are shown in the official language in which they were submitted.
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DIFFERENTIAL PRESSURE BASED LOCK
FIELD OF THE INVENTION
This invention relates to a differential pressure based lock. More
specifically, the
invention is a lock for securing two pressure vessels together. It has
particular application in
securing a fuelling machine, defuelling machine or inspection machine to the
fuel channel of a
nuclear reactor.
BACKGROUND OF THE INVENTION
A pressurized fuel channel type nuclear reactor, such as the CANDU°
nuclear reactor, is
comprised of a pressure vessel containing horizontally oriented fuel channels.
A typical reactor
contains approximately 400 fuel channels. 'The fuel channels are themselves
also pressure
vessels. Each fuel channel contains a plurality of fuel bundles longitudinally
disposed in end-to-
~end relation within a pressure tube. Each fuel bundle comprises a plurality
of elongated fuel rods
containing fissionable material. High pressure heavy or light water coolant
flows through the
fuel channels to cool the fuel rods and remove heat produced by the fission
process. Each end
of each fuel channel has an end fitting to contain the contents of, and
provide the interfaces to,
the fuel channel.
Refuelling of the reactor is carried out with a fuelling machine to remove the
spent fuel bundles and insert fresh replacement fuel bundles. The head of the
fuelling machine is
also a pressure vessel. Fuel bundles are loaded into a fuel transfer mechanism
and then pushed
into the fuelling machine head's fuel loading magazine. The fuel bundles are
loaded into the fuel
channel through the snout of the fuelling machine head. When refuelling is to
take place, the
fuelling machine is remotely moved to the reactor face and the snout of the
fuelling machine
head is clamped to the end fitting of a fuel channel. A snout clamping
mechanism, operated by a
snout drive assembly, clamps the snout to the end fitting. The fuelling
machine closure and the
fuel channel closure are then removed so that the fuel channel is open to the
fuelling machine.
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To avoid the remote possibility of a fuel spill of whole or partial fuel
bundles from the
fuel channel or the leak of reactor coolant, it is important that the fuel
channel not become
disconnected from the fuelling machine while either the fuelling machine
closure or the fuel
channel closure is removed. The snout clamping mechanism and the snout drive
assembly are
designed and tested to ensure that the seal between the end fitting and the
snout is well
maintained unless commanded to unclamp. However, it is possible that under
certain conditions
of temperature or vibration, the snout drive assembly may back-drive. For
example, such
conditions may potentially occur as a result of a seismic event. In addition,
it is possible that the
snout drive assembly is commanded to unclamp at the wrong time as a result of
operator error,
system error, or a system fault such as an electrical short.
To avoid any such inadvertent unclamping, the snout drive assembly is provided
with a
mechanical lock that acts to immobilize the snout drive assembly and thereby
prevent actuation
of the clamping mechanism. In addition, electrical and software interlocks are
commonly used
to prevent the snout drive assembly from being operated.
One of the commonly used mechanical locks is a rod and piston type lock which
engages
when the end of the rod, acting as a lock pin, protrudes into a recess in the
snout drive assembly.
During refuelling, the snout cavity is open to the reactor operating pressure.
The pressure in the
snout cavity activates the piston to drive the lock pin into place in the
snout drive. This
compresses a large spring on the rod side of the piston. Once refuelling is
complete, the snout
cavity is depressurized. When the pressure in the snout cavity falls below a
pre-set level, the
apring re-extends, the lock pin withdraws and the lock disengages. Thus, the
lock is activated by
'the pressure from the reactor and de-activated by re-extension of the spring.
The spring release
must be powerful enough to disengage the lock when lubricants degrade or seals
tear, and
.accordingly, the disengage pressure level is set relatively high.
Although the rod and piston type lock operates effectively and reliably, it
does not
immobilize the snout drive assembly to prevent unclamping of the fuelling
machine from the fuel
channel if the reactor pressure falls below the disengage pressure level.
Thus, if there were a
reduction in pressure in the reactor caused by a loss of coolant accident, a
main steam line break
or other such upset or accident condition, the lock would disengage and there
would be a risk
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that the snout and the end fitting would become disconnected, resulting in a
possible reactor
coolant leak or nuclear fuel spill.
Another scenario in which the rod and piston type lock does not act to secure
the
connection between the fuel channel and the fuelling machine is during
shutdown conditions.
Although refuelling of the fuel channels generally takes place when the
reactor is online, it may
be carried out while the reactor is in shutdown mode. The pressure in the fuel
channels during
shutdown mode is reduced to about 50 psi. This pressure level is insufficient
to cause the rod and
piston type lock to engage and, accordingly, the lock is non-operational
during shutdown
conditions.
Many attempts have been made to develop a lock which overcomes the
deficiencies of
the aforementioned rod and piston type lock. For example, the assignee of the
present
application developed a lock which would operate over a greater pressure
range. The lock
presented operational difficulties as a result of the greater pressure range.
For instance, during
leak checks carried out during the refuelling process, and in other
circumstances, pressurization
in the snout cavity caused the lock to partially engage at the time that the
fuelling machine was to
be removed from the reactor face. In addition, the larger piston which this
lock required made it
difficult to manufacture to a size package that could be retrofitted.
Thus, there is a need for a lock which effectively inactivates the snout drive
assembly
thereby securing the connection between the fuel channel and the fuelling
machine, and which
remains operational in the event of a reduction or loss of pressure in the
fuel channel and during
reactor shutdown conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of the fuel chaimel end fitting connected to
the fuelling machine
snout with the lock of the present invention in situ.
FIG. 2 is a cross-sectional view of the lock of the present invention in its
disengaged position.
1FIG. 3 is a cross-sectional view of the Lock of the present invention in its
engaged position.
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SUMMARY OF THE INVENTION
The lock of the present invention acts to prevent inadvertent unclamping of
the fuelling
machine from the fuel channel in conditions in which the prior art locks do
not. The action of
the lock is based on the differential pressure between the snout cavity and
the fuelling machine
magazine. The lock is engaged when the snout cavity pressure and the fuelling
machine
pressure are substantially equalized. This occurs before the snout and fuel
channel closures are
removed. Thus, the lock remains engaged if there is a drop in reactor pressure
during refuelling
and the lock is also operational during reactor shutdown conditions when the
reactor pressure is
substantially reduced. Once refuelling is complete and the closures are
replaced, the snout cavity
pressure is vented and the fuelling machine magazine pressure acts to
disengage the lock. Thus,
the lock remains engaged if there is a drop in reactor pressure during
refuelling. The lock is also
operational during reactor shutdown conditions when the reactor pressure is
substantially
reduced.
Thus in accordance with the present invention, there is provided a means for
locking a
connecting mechanism, said connecting mechanism adapted to sealingly engage a
first pressure
vessel and a second pressure vessel at an interconnecting passageway,
comprising: closure
means in said passageway effective to maintain a pressure differential between
said first and
second pressure vessels; means for pressurizing said first pressure vessel to
pressure of said
second pressure vessel to eliminate said pressure differential and allow
removal of said closure
into the interior of said first pressure vessel permitting access to the
interior of said second
pressure vessel from said first pressure vessel; a rod and piston
longitudinally movable within a
hydraulic chamber between a first position effective to lock said connecting
mechanism and a
second position effective to unlock said connecting mechanism, the rod end of
said chamber in
fluid communication with the first pressure vessel side of said closure means
and the piston end
of said chamber in fluid communication with the second pressure vessel side of
said closure
means; wherein the elimination of said pressure differential is effective to
move said rod and
;piston to said first position.
In accordance with another aspect of the present invention, there is provided
a means for
llocking a connecting mechanism, said connecting mechanism adapted to
sealingly engage a first
pressure vessel and a second pressure vessel at an interconnecting cavity in
an interconnecting
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passageway, comprising: a first closure means in said passageway effective to
maintain a first
pressure differential between said first pressure vessel and said
interconnecting cavity; a second
closure means in said passageway effective to maintain a second pressure
differential between
said second pressure vessel and said interconnecting cavity; means for
pressurizing said
interconnecting cavity to pressure of said first pressure vessel to eliminate
said first pressure
differential and allow removal of said first closure means into interior of
said first pressure
vessel; means for depressurizing said interconnecting cavity to establish a
pressure differential
between said interconnecting cavity and said first pressure vessel; and a rod
and piston
longitudinally movable within a hydraulic chamber between a first position
effective to lock said
connecting mechanism and a second position effective to unlock said connecting
mechanism, the
rod end of said chamber in fluid communication with said first pressure vessel
and the piston end
of said chamber in fluid communication with said interconnecting cavity;
wherein the
elimination of said pressure differential is effective to move said rod and
piston to said first
position and the establishment of said pressure differential is effective to
move said rod and
piston to said second position.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The fuel channels of a CANDU~ reactor are refuelled by means of a remotely
operated
fuelling machine which removes spent fuel bundles from the fuel channel and
replaces them with
fresh fuel bundles. The spent fuel bundles and the fresh fuel bundles are
stored in fuel ports in
the fuelling machine. In order for the refuelling process to take place, the
head of the fuelling
machine is connected to the fuel channel in the manner illustrated in FIG. 1.
Fuel channel 10 is a
longitudinal cylindrical assembly of approximately 4 inches in inside diameter
and is adapted to
contain a series of fuel bundles in end-to-end relation. High pressure heavy
or light water
coolant enters fuel channel 10 at one end, flows through the fuel bundles to
cool the fuel rods
.and remove heat produced by the fission process, and exits from fuel channel
10 at the opposite
end. An end fitting, one of which is shown by reference numeral 12, is affixed
to each end of
fuel channel 10. Closure plugs, one of which is shown by reference numeral 14,
are removable
closures for fuel channel 10 which fit within the distal portion of each end
fitting 12 and function
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to contain the contents of fuel channel 10. The pressure boundary for fuel
channel 10 is at
closure plug 14.
The fuelling machine head is a pressure vessel which contains a fuel loading
mechanism.
Fuel loading magazine (a portion of which is shown at reference numeral 20)
receives fuel into
vacant positions for insertion into and removal from fuel channel 10. Snout 22
is a barrel-like
extension of fuelling machine magazine 20 through which the fuel bundles pass
to fuel channel
10. Fuelling machine snout plug 24 is a removable closure positioned within
snout 22. The
pressure boundary of fuelling machine is at snout plug 24.
A metallic seal between end fitting 12 and snout 22 is effected by clamp 26.
Clamp 26
consists of clamp jaws 28 on arm 30. Clamp jaws 28 are wedge elements which
are pulled down
into groove 32 on the outside of end fitting 12 to pull end fitting 12 and
snout 22 tightly together.
A snout drive assembly drives clamp 26. In the embodiment illustrated in FIG.
l, the snout drive
assembly is a screw-based linear actuator consisting of clamping barrel 34 on
end fitting 12, ring
gear 36 and gear rack 38. Gear rack 38 drives ring gear 36 which pulls barrel
back towards
fuelling machine and applies pressure to clamp 26 causing activation of jaws
28. Other types of
snout drive assemblies may be used, for example, rack and pinion or sprag disc
type assemblies.
When end fitting 12 and snout 22 are sealed, the volume surrounding the distal
portion of snout
plug 24 is contained and is referred to as snout cavity 16.
The lock of the present invention, illustrated by reference numeral 40 in FIG.
1 and
shown in greater detail in FIGS. 2 and 3, acts on the snout drive assembly to
prevent unclamping
of end fitting 12 and snout 22. With reference to FIG. 2, lock has a rod and
piston-type assembly
housed by casing 42. Piston 44 and spring 46 occupy hydraulic piston chamber
48. Spring 46 is
engaged to floor of piston chamber 48 and is biased to bottom of piston 44
such that when spring
46 is compressed, piston 44 is positioned in the lower part of piston chamber
48 and when spring
46 is extended, piston 44 is positioned in the upper part of piston chamber
48. Rod 52 extends
from top of piston 44 through upper part of casing 42. Upper portion of rod 52
extends through
ambushing 52 to potentially engage gear rack 38.
Rod seal 54 and piston seal 56 provide fluid seals for the rod 50 and piston
44,
respectively. Seals 54, 56 may be O-rings, T-rings or any other suitable seal
type. Rod 50 has
lubricating means (not shown) and grease wiper seal 58. The piston and lock
diameters can be
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any suitable dimension. In a lock used with a typical snout drive assembly,
the piston diameter
is about 2 inches, the rod diameter is about 1 inch and the load of the spring
in its extended
position is about 52 pounds.
Lock 40 is connected to fuelling machine magazine 20 and snout cavity 16 in
the manner
illustrated in FIG. 1. Snout connection tube 60 attaches to piston port 62 and
connects snout
cavity 16 to piston side of piston chamber 48 through piston channel 64.
Magazine connection
tube 66 attaches to rod port 68 and connects fuelling machine magazine 20 to
rod side of piston
chamber 48 through rod channel 70. 'Tubes 60, 66 are connected to snout cavity
16 and fuelling
machine magazine 20 in any suitable manner and preferably, for reasons of
convenience, to pre
existing pressure lines or ports.
A magnet rod 72 is mounted on a threaded rod screwed into piston 44 within
tube 73.
Reed type magnetic switches 74 are mounted on the outside of tube 73 at fixed
positions.
Switches 74 detect the location of the magnet allowing for remote sensing of
the position of
piston 44.
Drain port 76 has horizontal channel 78 to drain fluid if rod seal 54 fails
and vertical
channel 80 to drain any fluid that may have leaked into the snout drive
mechanism.
To refuel a fuel channel, the fuelling machine head magazine 20 is moved to
the reactor
face and snout 22 is connected to end fitting 12 by clamp 26. Assuming that
the reactor is in
operating mode, the pressure in fuel channel 10 is at reactor operating
pressure. The pressure in
fuelling machine magazine 20 is at park pressure and the pressure in snout
cavity 16 is at reactor
lbuilding pressure. Park pressure is the pressure required to cool the spent
fuel contained in the
fuelling machine magazine 20. More specifically, it corresponds to the minimum
pressure at the
:fuelling machine head necessary to ensure coolant flows back through the
cooling pipes which
extend from the fuelling machine head. Once snout 22 and end fitting 12 have
been clamped
together, a leak check of snout cavity 16 is performed. Snout cavity 16 is
pressurized to
magazine park pressure through a line (not shown) which may be independent or
may extend
iErom a T-piece in line 60.
Once the pressures in snout cavity 16 and fuelling machine magazine 20 are
equalized, a
ram assembly withdraws the snout plug into the interior magazine of the
fuelling machine. The
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pressure in the fuelling machine is then raised to reactor operating pressure.
A ram assembly
withdraws closure plug 14 into the interior magazine of the fuelling machine
once the pressures
in fuel channel 10, snout cavity 16 and fuelling machine magazine 20 are
equalized. Once
refuelling of a fuel channel has been completed, the ram assembly replaces
closure plug 14 and
the pressure in the fuelling machine is reduced to fuelling machine park
pressure. The ram
assembly then replaces snout plug 24. Once snout cavity 16 has been vented to
reactor building
pressure, the snout drive assembly operates to disengage clamp 26 and the
fuelling machine is
then moved away from the reactor face.
When end fitting 12 is initially clamped to snout 22, the difference between
the pressure
in fuelling machine magazine 20 and the pressure in snout cavity 16 is
sufficient to generate a
large enough force in the rod side of piston chamber 48 to keep spring 48
compressed and rod 50
within bushing 54. In this state, lock 40 is in its disengaged position as
illustrated in FIG. 2. As
soon as the fuelling machine magazine and snout cavity pressures are equalized
to allow removal
of snout plug 24, as a result of the greater surface area of piston 44, the
pressure in the piston
side of piston chamber 48 together with spring 46 causes piston 44 to rise
within piston chamber
48 and rod 50 to emerge from bushing 54 and overlap with a stop bracket
affixed to the bottom
of gear rack 38. Alternatively, the bottom of gear rack 38 may have a recess
sized for a bolt-like
insertion of rod 50. The engagement of rod 50 with gear rack 38 immobilizes
the snout drive
assembly such that clamp 26 cannot be released. In this state, lock 40 is in
its engaged position.
Lock 40 remains engaged as the fuelling machine pressure is raised to reactor
operating pressure
and channel closure plug 14 is removed, and continues to remain engaged
throughout the
refuelling process. Once refuelling is complete and channel closure plug 14
and snout plug 24
have been replaced, snout cavity 16 is vented to reactor building pressure
whereas fuelling
machine magazine remains at park pressure. hock 40 disengages when a large
enough positive
fuelling machine magazine to snout cavity pressure differential exists. The
fuelling machine
magazine pressure connected to the rod side of piston chamber 48 drives piston
44 down and rod
50 to retract from the stops or recess in gear rack 38 in the snout mechanism
drive.
Magazine connection tube 66 is preferably placed in front of snout connection
tube 60 so
that any mechanical damage to the tubes affects magazine connection tube 66
first. This ensures
that lock 40 remains engaged in such circumstances.
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There are numerous advantages to the present invention.
The lock engages at an earlier stage in the refuelling process and, more
specifically,
engages before the fuelling machine is online with the reactor.
In addition, if there is a reduction or loss of pressure in a fuel channel
while it is open to
the fuelling machine, the magazine to snout cavity pressure differential will
not exist and the
lock will remain engaged. Even if all pressure is lost in the magazine and
snout cavity, the
spring will act to keep the lock engaged. As a result, if a loss of coolant
accident, a main steam
line break or other such upset or accident conditions occur, the lock will
remain engaged and
ensure that the fuelling machine does not become disconnected from the fuel
channel. Not only
is there a greater risk of inadvertent unclamping during such conditions but
it is also during such
an event that it is of particular importance that the lock remain engaged
because if the fuelling
machine were to become disconnected from the fuel channel during upset or
accident conditions,
the severity of any leakage of reactor coolant or spillage of nuclear fuel
would be greater.
Another advantage of the lock of the present invention over prior art locks is
that it
operates during reactor shutdown conditions. When a reactor is in shutdown
mode, the reactor
pressure is generally less than about 50 psi. Refuelling often continues when
the reactor is in this
mode. The procedure is the same as when the reactor is in operating mode and
the relative
pressure sequence is the same. Because the engagement and disengagement of the
lock is
dependent on the snout cavity to fuelling magazine differential, the lock
engages once the
magazine and snout cavity pressure have been equalized. The prior art locks
were not
operational during shutdown conditions.
A further advantage of the lock of the present invention is that it prevents
the fuelling
machine from being removed from a fuel port in the level lowered condition
when the snout plug
is removed. This condition arises when the fluid level in the fuelling
magazine head is lowered
to allow the fuel to be passed through air from the heavy water into light
water. If the snout plug
is not sealing adequately, the leak rate will be too high and the pressure in
the snout cavity will
increase causing the lock to engage. The fuelling machine can then stay in
place and capture the
leaks. The re-engagement of the lock also allows another attempt at inserting
the plug with an
improved seal. Removal of the fuelling machine from a fuel port without the
snout plug in place
creates the opportunity for tritium to be leaked from the snout and for air
and gas to be entrained
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in the magazine coolant which will affect the pH and may cause oxidation and
precipitation, as
well as sloshing of D20.
The lock of the present invention provides a further operational safety
advantage. During
the refuelling procedure, a leak check of the closure plug is generally
performed when the snout
cavity is at park pressure. The lock of the present invention remains engaged
during such a leak
check. Alternatively, the closure plug check may be performed with the snout
cavity at
atmospheric pressure to allow for a combined closure plug/snout plug leak
check. In this case,
the lock retracts prior to the leak check but will re-engage if the pressure
in the snout cavity
increases above a pre-set level.
Although the lock has been described for use with a fuelling machine, it can
also be used
with a defuelling machine or an inspection machine, both of which are pressure
vessels that
connect to the fuel channel in a manner similar to that of the fuelling
machine.
The lock has also been described for use in association with a clamping
mechanism
consisting of clamp jaws on clamp arms. It should be noted that other clamping
or sealing
mechanisms can be used and the lock provides a means of securing any such
alternative
mechanism.
The present invention has been shown and described with reference to preferred
embodiments of the invention. It is to be understood that departures may be
made therefrom
within the spirit and scope of the invention.
