Note: Descriptions are shown in the official language in which they were submitted.
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FUEL ASSEMBLY FOR NUCLEAR REACTOR
BACKGROUND OF THE INVENTION
The present invention relates in general to a fuel
assembly used for a nuclear reactor, and more particularly
to a fuel assembly which permits an on-load refuelling
operation by dispersing burnable poison at a relatively low
concentration within a substantial portion, or majority of
the fuel pellets in the fuel assembly in order to control
the rate of power increase of a fuel assembly in the initial
stage after on-load refuelling.
In general, a pressure tube type reactor has the
remarkable merit that it permits on-load refuelling, which
means fuel exchange during operation of the nuclear reactor
without shutdown. When on-load refuelling is carrried out,
it generally takes about 2 hours to exchange one fuel
assembly. Therefore, the power of the loaded fuel assembly
and other fuel assemblies adjacent to it increases rapidly
in this on-load refuelling. This rapid power increase of
the fuel assemblies becomes larger as the difference in the
infinite multiplication factor between the loaded fuel
assembly and the spent fuel assembly becomes larger.
When the power of the refuelled fuel assembly and other
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fuel assemblies adjacent to it increases rapidly, -the
thermal expansion of fuel pellets and the cladding tube
brings the pellets into contact with the tube, and exerts
radial pressure on same to result in possible damage of the
cladding tube.
In order to overcome the disadvantage as described
above, a batch process has been employed in which fuel
exchange is carried out during shutdown of the nuclear
reactor, and the rate of power increase of the reactor is
regulated at the time of start-up of the reactor to hold
down the rate of power increase of the fuel assembly. This
method is called as the reactor operation method to keep
Preconditioning Interim Operational Management Recommendation
(PCIOMR). If a fuel exchange is carried out during shutdown
of the reactor by the batch process and the rate of power
increase of the reactor is regulated to hold down the rate
of power increase of the fuel assembly as described above,
the fuel pellet is creep-deformed at its heated, central
portion due to a compressive stress and the cladding tube is
creep-deformed due to a tensile stress. The stress on the
cladding tube is thus decreased and, consequently, integrity
of the fuel assembly is maintained.
However, such a method as described above detracts from
the remarkable effect of on load refuelling which is
inherent to the pressure tube type reactor resulting in not
only a low load factor but also a low fuel utilization
factor.
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SUMMARY OF THE INVENTION
A general object of the present invention is to provide
an improved fuel assembly for a nuclear reactor capable of
on-load refuelling.
Another object of the present invention is to provide a
nuclear reactor fuel assembly which permits the control of
the rate of power increase of the loaded fuel assembly and
other fuel assemblies adjacent to it in an initial stage
after on-load refuelling. A further object of the present
invention is to provide a nuclear reactor fuel assembly
which permits not only the integrity of the fuel, but also
remarkable improvements in the load factor and the fuel
utilization factor.
The present invention provides a nuclear reactor fuel
assembly in which the majority of fuel pellets have a
burnable poison dispersed therein at a relatively low
concentration so that the burnable poison burns out in the
initial stage after on-load refuelling. Due to the burnable
poison, the rate of power increase of the fuel assembly is
controlled, and on-load refuelling can be carried out. The
wording "the ma~ority" of pellets of fuel assembly used
herein means about 50 ~ or more, and preferably about 80 %
or more, of all the fuel pellets within the fuel assembly.
The fuel assembly is a structure of a number of fuel rods
each of which is loaded with a number of fuel pellets.
However, the Euel pellets have different power distributions
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such that, in general, fuel rods positioned outwardly
provides higher power than those positioned inwardly, and in
respect of each fuel rod, fuel pellets positioned near the
middle thereof provide higher power than those at the ends.
If the proportion of fuel pellets to which burnable poison
is added is as small as 50 %, it is advisable to select a
particular region of the fuel pellets where the power
increase must be controlled, and add the burnable poison to
those selected fuel pellets.
As a suitable burnable poison dispersed in the fuel
assembly, gadolinia (Gd203), may be used by, for example,
adding uniformly to uranium dioxide ~UO2) or mixed dioxide
(Pu02 - UO2) which is a nuclear reactor fuel, and then
sintering to produce oxide fuel pellets. In this case, the
concentration of gadolinia is chosen to be about 0.2 % or
less, and preferably as low as 0.05 %. Other burnable
poison such as boron, cadmium, dysprosium, etc. can also be
used in the present invention.
Those skilled in the art will be aware that burnable
poison is used in fuel pellets of the fuel assembly so as to
flatten the power distribution in a reactor core. By the
conventional use of the burnable poison, excess reactivity
and power distribution are restricted to the predetermined,
suitable values. In order to maintain the predetermined and
suitable excess reactivity and power distribution steady,
highly concentra-ted burnable poison, for example about 1 %
of gadolinia, has been added to the fuel assembly locally,
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generally ten per cent of the fuel pellets. By locally
adding such hlgh concentration of burnable polson,the
effects of of the burnable poison are maintained Eor a long
time. The addition of burnable poison to the fuel assembly
in the conventional technique is therefore quite different
from the present invention for controlling the rate of power
increase of the fuel assembly in the initial stage after
on-load refuelling.
According to the present invention, a burnable poison is
dispersed into the majority of the fuel pellets at a low
concentration. The burnable poison dispersed at such low
concentration burns out in the initial stage after on-load
refuelling, and can effectively control the rate of power
increase of the fuel assembly so that the power of the fuel
assembly reaches the prescribed value on the intended day.
BRIEF DESCRIPTION OF THE DRAWINGS
-
Figure 1 is a perspective view of a fuel pellet
containing a burnable poison.
Figure 2 is a descriptive illustration, partly in
section, of a fuel rod incorporating the fuel pellets shown
in Figure 1.
Figure 3 is a perspective view of a nuclear reactor fuel
assembly incorporating the fuel rods shown in Figure 2.
Figure 4 shows a descriptive illustration, in section,
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of the fuel assembly shown in Figure 3.
Figure 5 i.s a graph showing the relation between the
fuel burn-up and the infinite multiplication factor of the
fuel assembly according to an embodiment of the invention.
Figure 6 is a graph showing the relation between the
fuel burn-up and the infinite multiplication factor of the
fuel assembly according to another embodiment of the
invention.
PREFERRED EMBODIMENTS OF THE INVENTION
An embodiment of the present invention will be explained
with xeference to the accompanyinq drawings. The basic
structure of the fuel assembly is considered to be similar
to that of a conventional fuel assembly. Figure 1 shows a
fuel pellet 10 containing burnable poison. The fuel pellet
10 is formed by adding gadolinia (Gd203) to uranium dioxide
and shaping it into a columnar structure, followed by
sintering. In the present invention gadolinia (Gd203) is
added to uranium dioxide, which is a nuclear fuel, in such a
low concentration that its burnable rate changes abruptly in
the initial stage after on~load refuelling.
Such fuel pellets as described above are loaded into a
cladding tube 12 which is made, for example, of zircaloy as
illustrated in Figure 2 and sealed by end caps 14 at the
ends of cladding tube 12 so as to form a fuel rod 16. Fuel
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assembly 20 shown in Figures 3 and ~ is of the cluster type,
being composed of a bundle of twenty-elght fuel rods 16.
The fuel rods 16 are concentrically positioned in three
layers, and are flxed by an upper tie-plate 22 and a lower
tie-plate 24. In Figure 4 of the drawing, reference numeral
28 represents rods for supporting the spacers 26 which are
used to provide structural stability of the fuel rods.
Remarkable features of the present invention, which are
distinctive from the conventional technique, reside in
pellet composition and the state of dispersion of burnable
poison within the fuel assembly to control the rate of power
increase. In the present invention, the concentration of
burnable poison in the pellet is low and, in addition, the
burnable poison is included in the majority of fuel
pellets.
Figure 5 shows the relation between the infinite
multiplication factor and fuel burn-up for the fuel assembly.
In Figure 5, the solid line represents the case using the
fuel assembly of the present invention which contains 0.05 %
of gadolinia (Gd203) uniformly dispersed, and the broken
line the case using the conventional fuel assembly containing
no burnable polson. The infinite multiplication factor
immediately after loading of a fresh Euel assembly is about
0.5. When a fresh fuel assembly containing therein burnable
poison uniformly dispersed is loaded into the reactor core
by on-load refuelling, the power of thè fresh fuel assembly
is held at the low level due to the influence of the neutron
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absorption effect of the burnable poison. Since the
infinite multiplication factor of a fresh fuel assembly is
smaller than that of a spent fuel assembly, the powers of
the exchanged and adjacent fuel assemblies are smaller than
their power prior to exchange.
Since the infinite multiplication factor of a fuel
assembly recovers as the burnable poison burns and, in
general, the rate of burn-up per day of the fuel assembly is
about 20 MWd/t, the infinite multiplication factor of the
fuel assembly after 5 days from loading becomes approximately
equal to that of the spent fuel assembly The powers of
the exchanged fuel assembly and other fuel assemblies
located adjacent to it then regain their values prior to
exchange of fuel assemblies. After about 15 days (about 300
MWd/t) from loading, the burnable poison is extinguished
with the result that the infinite multiplication factor
recovers and becomes about 1.2, which is the prescribed
power state.
The rate of power increase of the loaded fuel assembly
is about 0.1 ~ power/hour in the initial stage after on-load
refuelling, while the rate of power increase of assemblies
adjacent to the exchanged fuel assembly is less than the
above value. Accordingly, the rate of power increase of
every fuel assembly in the reactor core can be made
sufficiently lower than the limiting rate of power increase.
In the embodiment of the invention described above, fuel
pellets containing burnable poison of a constant concentration
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are arranged in a substantially uniform manner within the
fuel assembly, but the concentration of the burnable poison
can be changed in accordance with the location of the Euel
rods within the fuel assembly so that the rate of power
increase of each fuel rod may be adjusted.
The present invention can be combined with a conventional
technique in which burnable poison of high concentration is
used locally in the fuel assembly to flatten the power
distribution. In this case, the rate of power increase of
the fuel assembly can be controlled in a similar way to the
embodiment described above. An example is shown in Figure
6, in which the solid line represents infinite multification
factor for the fuel assembly of the present invention
employing 25 fuel rods containing 0.05% Gd203 and 4 fuel
rods containing 1.0% Gd203, the broken line A for the
conventional fuel assembly with no burnable poison,and the
broken line B for another conventional fuel assembly
employing 3 fuel rods with highly concentrated burnable
poison of 1.0 % Gd203 and 25 fuel rods with no burnable
poison to flatten the power distribution. It will be
ciearly understood from Figure 6 that, by combining the
present invention with the conventional technique, the power
increase of the fuel assembly is controlled in the initial
stage after on-load refuelling, and the power level is
continuously smoothed thereafter.
According to the present invention, since burnable
poison of low concentration is dispersed in the majority of
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fuel pellets in the fuel assembly so that the burnable
poison burns up in the initial stage after on-load
refuelling, the power increase of a loaded fuel assembly and
other fuel assemblies located adjacent to it can be
controlled in the initial stage after on-load refuelling.
The integrity of the fuel assembly is thus assured while at
the same time, the load factor and the fuel utilization
factor can be greatly improved since on-load refuelling is
available.
Though the present invention has been described with
reference to the preferred embodiment thereof, many modifica-
tions and alterations can be made within the spirit of the
invention.
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