Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a clarifying
apparatus for clarifying insoluble Emission products or
corrosion products in nuclear fuel reprocessing or clad
processing of reactor cooling water.
An insoluble fission product is present in a spent
fuel solution (nitric acid solution). The spent fuel
solution is clarified to remove the insoluble fission
product. The resultant clarified solution is subjected
to the next extraction process.
I pulse filter apparatus is frequently used in the
conventional clarifying process. In the pulse filter
apparatus, the spent fuel solution is filtered through a
filter obtained by sistering a stainless steel powder.
The insoluble fission product is thus separated from the
solution containing nuclear fuel. The sludge of the
fission product deposited on the filter is removed from
the filter by supplying pulsated air in a direction
opposite to the direction of the fuel flow.
The pulse filter apparatus of this type is capable
of removing relatively small particles of the insoluble
fission product from the spent fuel solution and is
capable of decreasing the size of particles left in the
clarified solution below a predetermined value, thereby
keeping the uniform quality of the solution. In
addition, the pulse filter apparatus does not have a
movable component, so that the apparatus is free from
mechanical problems and has high reliability.
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However, the pulse filter apparatus has a
disadvantage in that the filter thereof tends to clog.
When the sludge is deposited on the filter surface, -the
filtrat10n rate is decreased. If this occurs, the
filter must be replaced with a new one. The filter can
be used for two or three months for clarification.
However, at the time when the filter is replaced, the
clarification process must be stopped for about a week,
thus increasing clarification cost. In addition to this
disadvantage, maintenance personnel tend to be exposed
to radiation. The atmosphere is also subject to
radioactive contamination. The filter, as a high level
radioactive waste, limits waste disposal. For the above
reasons, there arises a strong demand for eliminating
replacement of the filter in the pulse filter apparatus.
Meanwhile, the centrifugal clarifier has recently
received a great deal of attention. The spent fuel
solution is supplied to a cylindrical rotating bowl and
is separated by centrifugal separation in the
centrifugal clarifier. More particularly, an insoluble
fission product is separated from the solution by
utilizing the difference between the specific gravities
of the insoluble fission product and the nitric acid
solution. In a centrifugal clarifier of this type, the
flow rate per unit time can be increased. In addition
to this advantage, the sludge deposited in the bowl can
be easily removed by the remote control
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However, in the centrifugal clarifier, it is
difficult -to decrease below a certain value a minimum
size (to be referred to as a critical particle size do)
of insoluble fission product particles which can be
5 processed. The critical particle size do is given by
equation (1) below:
do = (lS-~-Q/~p.g.~l/2 ... (1)
where
: the viscosity of spent fuel solution
Q : the flow rate of spent fuel solution
UP : the difference between the specific gravities of
the particles and the nitric acid solution
g : the acceleration of gravity
: the centrifugal separation/deposition area which
is given as follows:
= I ~2-(r22 _ rl2)/g [ln(r2/r1)]
... (2)
where
Q : the length of the bowl along its rotating
axis
: the rotational velocity of the bowl
Al : the rotational radius between the
rotational axis of the bowl and a position
at which the solution is extracted
r2 : the rotational radius between the
rotational axis of the bowl and the inner
surface thereof.
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As is apparent from equations (1) and (2), in order to
decrease the critical particle size do, the radius r2 or
the rotational velocity must be increased. When the
radius r2 or the rotational velocity is increased, the
peripheral speed of the howl is increased, so that the
bowl is subjected to a large centrifugal force. For this
reason, the mechanical strength of the bowl must be surf-
ficiently increased, and the wall thickness of the bowl
must be increased. However, when the wall thickness is
increased, the weight of the bowl is increased As a
result, a high-power bowl drive device is required, and a
bearing which withstands a high stress must be used. In
addition, the critical particle size do can be decreased
when the length of the bowl is increased. However, the
weight of the bowl is increased. For these reasons, it
is impractical to design the centrifugal clarifier to de-
- crease the critical particle size do. In addition to
this disadvantage, the sludge is deposited in the bowl
and is gradually increased. As a result, the effective
length of the bowl is gradually decreased, and the an-
tidal particle size do is increased. The sludge must
therefore be removed from the centrifugal clarifier from
time to time. If sludge removal is left too long, the
insoluble fission product is not fully removed in the
centrifugal clarification process and will flow into
the next process. Consequently, neither the conventional
pulse filter apparatus nor the conventional centrifugal
clarifier provides a full solution.
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The present invention seeks to provide a clarifying
apparatus which minimizes the frequency of filter replace-
mint, improves safety and decreases the cost of the elan-
lying process. It further seeks to provide a compact and
light-weight clarifying apparatus capable of removing
small fission product particles and providing a high
quality clarified solution.
According to the present invention, there is pro-
voided a clarifying apparatus for removing insoluble radio-
active particles from a solution containing the particles said clarifying apparatus comprising (a) a centrifugal
clarifier for centrifugally separating the radioactive
particles from the solution; (b) filter means having a
filter with a secondary clarified liquid outlet side for
removing particles each having a size of not more than a
critical particle size of the radioactive particles which
can be separated by said centrifugal clarifier; (c) sup-
plying means for supplying the solution to said centric
frugal clarifier; (d) a buffer tank which receives the
primary clarified liquid from said centrifugal clarifier;
(e) a feed tank for receiving the primary clarified liquid
from said buffer tank and feeding the primary clarified
liquid to said filter means at a predetermined flow rate;
(f) means for feeding an overflow portion of the primary
clarified liquid from said feed tank back to said buffer
tank; (g) a compressed air supplying means for supplying
compressed air to the secondary clarifying liquid outlet
side of said filter in a pulsated manner; and (h) control
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means for supplying the compressed air to said filter in
the pulsated manner when the primary clarified liquid is
not being supplied to said filter means, thereby removing
sludge from said filter.
With such an arrangement, most of the insoluble
radioactive particles (i.e., fission products or corrosion
products are removed by the centrifugal
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clarifier. The filter means need only filter smaller
insoluble particles. Therefore, the rate of the sludge
deposited in the filter of the filter means is low,
thereby decreasing the frequency of replacement of the
filter which is required upon filter clogging. A
decrease in replacement frequency of the filter
guarantees safe operation during the clarifying process,
thus providing practical advantages. The filter
replacement frequency is decreased, so that the
utilization efficiency of the clarifying apparatus is
improved, thereby decreasing the clarifying cost. The
particle size range of the filter can be narrowed, so
that the pore size of the filter can be comparatively
small. As a result, even smaller particles can be
effectively eliminated, and a high-quality clarified
liquid is obtained. The smaller particles are removed
by the filter means. In this sense, even if the
centrifugal clarifier having a larger critical particle
size is used, the small particles can also be removed
from the liquid. In other words, the filter means
serves as a backup means of the centrifugal clarifier,
so that the safety and high reliability of the
clarifying process are assured. For example, even if
particles having a size larger than the critical
particle size flow through the centrifugal clarifier due
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to the fact that ho waste removal~er~ y~b7
I' these particles can be removed by the filter means and
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will not flow in the next process. Therefore, the
reliability and waste removal timing requirements are
not so strict as in the case where only a centrifugal
clarifier is used, thereby improving the reliability of
the apparatus and allowing easy operation thereof.
Furthermore, should the centrifugal clarifier break
down, the pulse filter serves as a backup member. As
a result, the particles will not flow into the next
process.
This invention can be more fully understood from
the following detailed description when taken in
conjunction with the accompanying drawings, in which:
The figure is a block diagram of a clarifying
apparatus according to an embodiment of the present
invention. The clarifying apparatus has a centrifugal
clarifier 12 and a pulse filter device 18 connected in
tandem therewith at its downstream side. The spent fuel
is dissolved in nitric acid in the dissolving process.
The resultant nitric acid solution containing the spent
fuel is subjected to various apes of treatments and is
supplied to a first buffer tank 10 through a line 42.
The insoluble fission product is contained as particles
in the spent fuel solution. An aqueous solution of
nitric acid is supplied from a reservoir (not shown) to
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the buffer -tank 10 through a line 44. The nitric acid
solution containing -the spent fuel is mixed with the
aqueous solution of nitric acid in the buffer tank 10.
mixture is supplied to -the centrifugal clarifier 12
through a line 46. The centrifugal clarifier 12 has the
same construction as the conventional centrifugal
clarifier.
In the centrifugal clarifier 12, the following
conditions are given: the bowl owe the centrifugal
clarifier 12 has the radius Al of 7 cm, the radius r2 of
10 cm and the length Q of 20 cm; a rotational frequency
N of the bowl is 2,500 rum, and the flow rate Q is
500 Q/hr; the viscosity of the liquid to be clarified
is 0.015 poise, the specific gravity pup of the sludge is
7 g/cm3, and the specific gravity p of the aqueous
solution of nitric acid is 1.4 g/cm3. The rotational
velocity of the bowl then becomes 261.7 rad/sec, and
the difference up between the specific gravities becomes
5.6 g/cm3. Therefore, according to equations (1) and
(2), the critical particle size do becomes 1.04 em. The
insoluble fission product particles each having a size
of not less than 1.04 em are separated by the
centrifugal clarifier 12 from the nitric acid solution.
Cleaning water is supplied to the centrifugal clarifier
12 to wash the bowl by means of a high-pressure gas
through a line (not shown). The used cleaning water is
discharged through a drain 48 disposed in -the
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centrifugal clarifier 12.
A primary clarified liquid is obtained such that
the large particles are removed by the centrifugal
clarifier 12 and is supplied to a second buffer tank 14
5 through a line 50. The primary clarified liquid is then
supplied to a feed tank 16 through a line 52. The
primary clarified liquid is supplied at a predetermined
flow rate from the feed tank 16 to the pulse filter
device 18 through a line 56. An overflow portion of the
primary clarified liquid returns to the second buffer
tank 14 through a line 54. The pulse filter device 18
has a filter 20 having a pore size of, for example,
0.8 em so as to remove small particles each having a
particle size smaller than the critical particle size
do. The primary clarified liquid is filtered through
the filter 20, so that the insoluble fission product
particles having a size falling within -the range of 1 -to
0.8 em are removed. A secondary clarified liquid
obtained from the pulse filter device 18 returns to a
third buffer tank 24 through a line 58. A valve 28 is
inserted in the line 58. A compressed air source 22 is
connected to a clarified liquid outlet side of the
filter 20 of the pulse filter device I through a line
60 so as to supply compressed air to the clarified
liquid outlet side in a pulsated manner. A valve 26 is
inserted in the line 60~ A drain 64 is connected
through a valve 30 to a clarified liquid inlet side of
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the filter 20 of the pulse filter device 18. The second-
cry clarified liquid is supplied from the third buffer
tank 24 to the next step through a line 62. Suitable
control means control the opening and closing of the
valves 26, 28 and 30 and the actuation of the compressed
air source 22.
The operation of the clarifying apparatus having
the construction described above will now be described.
The nitric acid solution containing spent fuel is mixed
with the aqueous solution of nitric acid in the first buff
for tank 10, and the mixture is supplied to the centric
frugal clarifier 12. The centrifugal clarifier 12 is
constructed to have a critical particle size do of, for
example, 1.04 em. The centrifugal clarifier 12 removes
comparatively large insoluble fission product particles.
! The primary clarified liquid which has been subjected
by the centrifugal clarifier 12 to primary clarification
is supplied to the feed tank 16 through the second buffer
tank 14. The primary clarified liquid is supplied at a
predetermined flow rate from the feed tank 16 to the pulse
filter device 18. The primary clarified liquid is sub-
jetted to further clarification by means of the filter 20.
The small particles each having a size between OWE em and
1.04 em are removed by the filter 20. The secondary
clarified liquid is supplied from the pulse filter device
18 to the next process through the buffer tank 24.
When the clarification operation is performed by
the pulse filter device 18, the valves 26 and 30 are
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closed and the valve 28 is opened by the control means.
On the other hand, when the sludge is deposited in the
filter 20 of the pulse filter device 18, the valve 28 is
closed, the valves 26 and I are opened, and the come
pressed air source 22 is actuated by the control means The pressurized air is supplies from-the compressed air
source 22 to the clarified liquid outlet side of the lit-
ton 20 in a pulsated manner The sludge deposited on the
clarified liquid inlet side of the filter 20 is blown out
and is discharged through the drain 64.
The insoluble fission product contained in the
nitric acid solution of spent fuel is mostly removed by
the centrifugal clarifier 12. Only small particles are
removed by the pulse filter device 18. Since the small
particles need not be removed by the centrifugal elan-
lien 12, the bowl, the bowl drive device and the bearing
need not be designed to withstand a high load. A normal
performance centrifugal clarifier can be used. Only a
small amount of fission product need be removed by the
pulse filter device 18, so that the deposition rate of
sludge in the filter 20 it low. For this reason, even if
compressed air is blown in a pulsated manner t the sludge
can be easily removed, thereby guaranteeing the long son-
vice life of the filter 20.
The pulse filter device 18 can serve as a backup
device for the centrifugal clarifier 12. Even if the
fission product removal timing is delayed and the par-
tides each having a size exceeding the critical particle
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size do flow out of the centrifugal clarifier 12, they
are removed by the pulse filter device 18 and will not
flow into the next process.
The above embodiment is concerned with nuclear fuel
reprocessing. However, the present invention can be
applied to clad processing of reactor cooling water.
Namely, the clarifying apparatus of the invention can
clarify corrosion products produced in the clad processing.
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