Note: Descriptions are shown in the official language in which they were submitted.
CA 02512708 2005-07-20
CONTROL METHOD FOR LIGHT WATER AND HELIUM GAS FLOW RATE IN
THE LIQUID ZONE CONTROL SYSTEM OF CANDU REACTOR AND CANDU
REACTOR HAVING IMPROVED TUBE SUPPORT PLATE
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for improving instability in the
output
of a CANDU (Canadian Deuterium Uranium) reactor (pressurized heavy water
reactor) by
controlling flow rates of light water and helium gas fed into a compartment of
a liquid zone
control system (LZCS) of the reactor. The present invention also relates to a
tube support
plate in the compartment for improving the instability in the output of the
reactor.
2. Description of the Background Art
A liquid zone control system is one of reactivity control apparatuses for a
CANDU
reactor and provides the function of smoothing an excess power tilt generated
after
replacement of nuclear fuel, which is implemented by controlling the amount of
light water
(H20) in a compartment thereof. Upon rapid rise of power tilt after the
replacement of
the nuclear fuel, the amount of the light water in the compartment is
increased to reduce
the power tilt. When output is stabilized, the amount of the light water is
decreased
accordingly. Thus, the output is smoothly distributed. In other words, when
the LZCS
is normally operated, the power tilt is decreased according to a rise of the
level of the water,
while the power tilt is increased according to a drop in the level of the
water.
Fig. 1 shows the structure of a compartment 100 of a conventional CANDU
reactor. The compartment 100 of Fig. 1 is shown as an upper outer compartment.
In Fig.
1, black arrows indicate the flow of light water and white arrows indicate the
flow of
helium gas. The compartment 100 is a space between upper and lower bulkheads
10 and
11 that are fixedly installed within a pipe 80. A spreader 20 for causing the
light water
introduced through the bulkhead 10 to run down along the wall of the
compartment is
installed at an upper portion of the compartment 10, and a helium gas
discharge hole 12 is
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formed in the lower bulkhead 11. Further, a double tube 40 for supplying the
helium gas
and discharging the light water penetrates through the upper bulkhead 10 and
is then
connected to the lower bulkhead. An outer tube of the double tube 40 connected
to the
lower bulkhead 11 is formed with a scavenger 41 for taking in and discharging
the light
water, and an inner tube of the double tube 40 is caused to communicate with
the helium
gas discharge hole 12 of the lower bulkhead 11. Moreover, a helium gas
discharge tube
70 is fixed to the spreader 20 while penetrating through the upper bulkhead
10. Double
tubes 50 and 60 for inflow and outflow of light water and helium gas for a
lower
compartment are installed within the compartment 100 while penetrating
therethrough. In
addition, a tube support plate 30 is installed within the compartment to
prevent vibration
caused by the fluids in the double tubes 40, 50 and 60 that have lengths
considerably larger
than diameters thereof. As shown in Fig. 2, the tube support plate 30 takes
the shape of a
disk that can be inserted into and fixed to the pipe 80 and is made of a
porous metal
material through which the light water and helium gas can pass vertically. The
tube
support plate is also formed with a through-hole 31 at the center thereof and
a plurality of
cut-away portions 32 at the periphery thereof so that the light water and the
helium gas can
easily pass through the tube support plate.
In the CANDU reactor constructed as above, the amount of the light water
discharged from a lower portion of the compartment is fixed, while the amount
of the light
water introduced at the upper portion of the compartment is controlled to be
changed in a
predetermined range according to operating conditions. Therefore, if the
amount of the
light water introduced at the upper portion of the compartment is greater than
the amount
of the light water discharged from the lower portion of the compartment, the
level of the
water rises, whereas if the amount of the light water discharged from the
lower portion of
the compartment is greater than the amount of the light water introduced at
the upper
portion of the compartment, the level of the water is lowered. The output of
the reactor is
increased or decreased according to the changes in the level of the water. The
LZCS
measures the changes in the level of the water within the compartment by
measuring the
difference in pressure between the upper and lower portions of the
compartment, and
maintains pressure according to the changes in the level of the water by
causing a certain
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amount of helium gas to be introduced into the lower portion of the
compartment and to be
subsequently discharged from the upper portion of the compartment. If the
level of the
water in the compartment reaches 80%, the LZCS performs control such that a
water level
control logic has priority over an output control logic so that the level of
the water cannot
exceed 80%.
In the conventional CANDU reactor having the compartment structure and the
LZCS control scheme, there is a phenomenon in which cycling or rapid drop
periodically
occurs when the level of the water W in an upper outer compartment rises over
80% after
replacement of nuclear fuel, as shown in Fig. 3. Further, the power tilt P of
the reactor
also exhibits a variation similar to that in the level of the water. Such a
variation is
contrary to a variation in the output under normal control (the output is
decreased upon rise
of the level of the water under the normal control).
To alleviate an instability phenomenon in such a CANDU reactor, the following
temporary methods are employed: a toxic material such as gadolinium (Gd) is
input into
the compartment or an operating scheme of a helium gas compressor is changed.
However, these methods have not fundamentally solved the instability
phenomenon.
Such instability may cause sudden shutdown of the reactor. Further, the input
toxic
material becomes radioactive wastes after it absorbs neutrons, resulting in
increase of the
amount of wastes produced.
SUMMARY OF THE INVENTION
A primary object of the present invention is to examine a cause of an
instability
phenomenon such as rapid rise and drop or periodic cycling of a power tilt and
the level of
water in a CANDU reactor and to provide a method of controlling flow rates of
light water
and helium gas in a liquid zone control system to eliminate such an
instability phenomenon.
According to the present invention to achieve the primary object, there is
provided
a method of controlling flow rates of light water and helium gas passing
through a plurality
of compartments of a CANDU reactor, each of the compartments having a tube
support
member installed therein, comprising the step of controlling the flow rates of
the light
water and the helium gas such that flow rates of the light water and the
helium gas supplied
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to and discharged from a portion of each of the compartments below the tube
support
member per unit time satisfy the following relationship:
(a) QL - (01.). <0, or
(b) QL - (Q:). SQxk
where QL is a flow rate of the light water discharged from the compartment per
unit
time, (Qi.). is a maximum flow rate of the light water that can pass through
the tube
support member per unit time, and Q , is a flow rate of the helium gas
supplied. The
tube support member is normally used by a tube support plate having a
plurality of holes,
but the tube support member can be a tube support grid or an elastic spring.
A secondary object of the present invention is to provide a CANDU reactor that
has an improved tube support plate to alleviate the instability phenomenon
such as rapid
rise and drop or periodic cycling of the power tilt and the level of water.
According to the present invention for achieving the second object, there is
provided a CANDU rector having a plurality of compartments, each of the
compartments
having a tube support plate installed therein, wherein the tube support plate
is formed with
a plurality of through-holes through which light water and helium gas pass.
The plurality
of through holes of the support plate enables to satisfy flow conditions of
light water and
helium gas through the through-holes as follows:
(a) Q., - (O ). <0, or
(b) Q.a - (Qr.). SQxu
where Q. is a flow rate of the light water discharged from the compartment per
unit
time, (Qm ). is a maximum flow rate of the light water that can pass through
the tube
support member per unit time, and Qm is a flow rate of the helium gas
supplied. . The
support plate can be replaced by a support grid or a support elastic spring
that satisfies the
flow conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present invention
will
become apparent from the following description of a preferred embodiment in
conjunction
with the accompanying drawings, in which:
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Fig. 1 is a view showing the structure of a compartment of a liquid zone
control
system in a CANDU reactor, and the flow of light water and helium gas therein;
Fig. 2 is a plan view of a tube support plate of a conventional CANDU reactor;
Fig. 3 is a graph showing variations in the level of the water and the output
when
5 an instability phenomenon occurs in the conventional CANDU reactor;
Fig. 4 is an explanatory view illustrating a cause of the instability of the
level of
the water and the output when a conventional method of controlling flow rates
of light
water and helium gas is employed;
Fig. 5 is a graph showing stable and unstable operation regions according to
the
flow rates of the light water and the helium gas;
Fig. 6 is a plan view of an embodiment of a tube support plate of a CANDU
reactor according to the present invention; and
Fig. 7 is a graph showing stable and unstable operation regions when the tube
support plate shown in Fig. 6 is used.
DETAILED DESCRIPTION OF THE INVENTION
When nuclear fuel is replaced in a CANDU reactor, there is a case where an
output
in a specific region rises according to a reactivity property of the nuclear
fuel. Since an
excess power tilt adversely affects the operation of the reactor, a LZCS
control the
operation so that a smooth output can be obtained. Although the control of the
LZCS is
normally performed in a normal operation, an instability phenomenon in which
the control
cannot be performed normally as shown in Fig. 3 occurs if the level of light
water is very
high to such an extent that the level of the water rises over a tube support
plate (TSP).
If the level of the water is below the tube support plate as shown in Fig. 4
(a), there
is no problem in view of the flow of the light water and the helium gas in a
compartment.
However, if an operation is made in a state where the level of the water is
higher than the
level of the water as shown in Fig. 4 (b), there may be a phenomenon in which
abnormality
occurs in a stream of the light water flowing downward and a stream of the
helium gas
flowing upward. That is, the helium gas is accumulated below the tube support
plate 30
as shown in Fig. 4 (c). This phenomenon depends on flow rates of the light
water and the
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helium gas that pass through the tube support plate per unit time. That is,
the
phenomenon occurs if the amount of the light water that passes through the
tube support
plate per unit time is smaller than the amount of the water that is discharged
below the tube
support plate. The light water that has not passed through the tube support
plate is held
up on the tube support plate, while the helium gas introduced at a lower
portion of the
compartment cannot pass through the tube support plate and is also accumulated
below the
tube support plate, so that the amount of the light water below the tube
support plate is
gradually reduced. The level of the water within the compartment of the CANDU
reactor
is controlled such that it cannot generally exceed over 80% to prevent the
light water from
overflowing toward the outside of the compartment. However, if a phenomenon in
which
the light water is held up on the tube support plate 30 occurs as shown in
Fig. 4 (d), a water
level meter for measuring the level of the water using the difference in
pressure between
upper and lower portions of the compartment outputs a measured value
indicating that the
level of the water is maintained at 80%. Actually, since the helium gas is
accumulated
below the tube support plate 30, the light water is held up on the tube
support plate to such
an extent that the level of the water reaches a spreader. Consequently, the
level of the
water measured by the water level meter is completely different from the
actual level
(position) of the water. The light water existing below the tube support plate
that
corresponds to the size of a space occupied by the helium gas accumulated
below the tube
support plate cannot be effectively used for controlling the reactivity.
Further, a part of
the light water held up on the tube support plate is pushed outside a fuel
boundary and thus
cannot contribute to control of the reactivity. Therefore, the output of the
reactor cannot
be normally controlled, thereby making the reactor unstable.
Referring to Fig. 4 (d), if a portion of the compartment below the tube
support
plate is taken as a control volume, flow rates of inflow and outflow of the
light water and
the helium gas (below the tube support plate satisfy the following equation:
QHe,` Out-Qin ......(1)
where O is a flow rate of a fluid that passes through the compartment per unit
time. The
reason why such an abnormal phenomenon occurs while the level of the water is
controlled
to be maintained at 80% is that the amount Qin of the light water passing
through the tube
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support plate is smaller than the amount Qout of the light water discharged at
the lower
portion of the compartment. That is, the abnormal phenomenon occurs if the
following
equation is satisfied:
QOnt - (Kin )max >0 ...... (2)
Further, if the flow rate of the helium gas supplied is larger than an
insufficient
flow rate of the light water below the tube support plate, the helium gas
passes through the
tube support plate so that the abnormal phenomenon does not occur. If the
maximum
flow rate that can pass through the tube support plate is denoted by (Q;n )max
, the abnormal
phenomenon occurs when the following equation is satisfied:
Q0 - (Qin )max > QHe ...... (3)
As an example, Fig. 5 shows calculation results according to the conditions of
equations 1 and 2 and experiment results of a simulation of the abnormal
phenomenon in
connection with the flow rates of the light water and the helium gas in the
liquid zone
control system of the CANDU reactor. Fig. 5 presents the experiment results
and the
theoretical calculation results showing that stable and unstable operation
regions exist
according to the amount Qout of the light water discharged and the amount QHe
of the
helium gas introduced. These results show that the reactor can be always
operated in the
stable operation region by properly controlling the amount Quut of the light
water
discharged and the amount QHe of the helium gas introduced. When data of the
experiment results are fitted between the stable and unstable operation
regions, an
experimental equation for the boundary (bold dotted line) of the stable
operation region
can be obtained as follows:
QHe =14.237 + 2.34 x ln(Qout - 0.458) (where, Qout > 0.458) ...... (4)
Moreover, a general equation for the flow rate below the tube support plate is
Q He = Q out - Qin , and the conditions of the occurrence of the abnormal
phenomenon should
satisfy both Q0 U, - (QQ, )max > 0 and Qout - (Qin )max >_ QHe
Based on the aforementioned four equations, the stable operation region where
the
abnormal phenomenon occurs can be expressed as the following equation:
Q0 ut - (Qin )max <O or Qout - (Qin )max QHe ...... (5)
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Here, the flow rate of the helium gas is QHe =14.24 + ln(Qout - 0.46). If
equation 5 is expressed in a different manner, the stable operation region can
be expressed
by the following equation:
QHe -14.237 + 2.34 x ln(Qo,u - 0.458) ====== (6)
Furthermore, if the value of the amount (Q;,, ),,,ax of the light water
passing
through the tube support plate becomes sufficiently larger without controlling
the amount
Qout of the light water discharged and the amount He of the helium gas
introduced,
Qout _00m, becomes smaller. Therefore, the probability of occurrence of the
abnormal phenomenon is lowered. Accordingly, the occurrence of the abnormal
phenomenon can be prevented by increasing the areas of portions of the tube
support plate
corresponding to flow passages of the light water and the helium gas such that
the light
water and the helium gas cannot be accumulated. That is, as shown in Fig. 6,
additional
through-holes 33 are formed in the tube support plate 30 or the areas of cut-
away portions
of the through-hole 31 are increased to prevent the occurrence of the abnormal
phenomenon. Fig. 7 shows results in which the stable operation region is
increased when
the areas of the flow passages of the light water and the helium gas are
increased by about
10% by adding the through-holes 33 to the tube support plate of the liquid
zone control
system of the reactor.
According to the present invention, the flow rates of the light water and the
helium
gas are controlled so that a CANDU reactor can be operated in a stable region
where an
unstable phenomenon, which may occur in a liquid zone control system, dose not
occur.
Thus, it is possible to prevent a power tilt, thereby improving the stability
and operatability
of the reactor. That is, sudden rise or drop and periodic cycling of the
output and the level
of the water in a compartment of the CANDU reactor are solved, thereby
eliminating a
feeling of uneasiness of an operator and enhancing the economical efficiency,
stability and
operatability of a nuclear power station.
Further, the reactor can be stably operated without inputting a toxic material
into
the light water so that the amount of radioactive wastes generated can be
reduced.
The embodiment of the present invention described above herein and illustrated
in
the drawings should not be construed as defining the technical spirit of the
present
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invention. The scope of the present invention is limited only by the appended
claims.
Those skilled in the art can make various changes and modifications within the
technical
spirit and scope of the present invention. Therefore, such embodiments and
changes fall
within the scope of the present invention so far as they are obvious to those
skilled in the
art.
