Initial core

In an arrangement of an initial core, the core is loaded with 572 high enrichment fuel assemblies H with 4.2 wt % average enrichment and 300 low enrichment fuel assemblies L with 1.5 wt % average enrichment. The average enrichment of the core is about 3.3 wt %. In this core, only the low enrichment fuel assemblies are loaded into the most outer position of the core and the high enrichment fuel assemblies are loaded into an area other than the most outer position. In this arrangement, the average enrichment of reload fuel assemblies is 3.7 wt %.

BACKGROUND OF THE INVENTION 
This invention is related to an initial core loaded before operation of a 
nuclear reactor. A nuclear reactor requires continuous operation for a 
fixed period without supplying fuel, and the core contains a larger 
quantity of fissionable materials than necessary to maintain the critical 
state of the nuclear reactor. Therefore, if there is no control material 
in the core, a critical excess in reactivity will result. This excessive 
in reactivity is called excess reactivity, and it is important to control 
the excess reactivity properly throughout the operation cycle. As a 
technique to do so, it is well known to mix a burnable poison in the fuel. 
The burnable poison is a neutron absorber that burns gradually during the 
operation cycle, and gadolinia is a typical known burnable poison. 
Next, the suppression effect of the reactivity by the burnable poison will 
be explained by reference to FIG. 3. FIG. 3 shows a relation between the 
infinite multiplication factor of a fuel assembly containing gadolinia as 
a kind of the burnable poison and the exposure. As is shown in FIG. 3, as 
the number of fuel rods with gadolinia (Gd fuel rods) is reduced, the 
infinite multiplication factor at the early stage of the exposure 
increases. On the other hand, if the density of gadolinia (density of Gd) 
is increased, the maximum value of the infinite multiplication factor can 
be suppressed, because the time when the gadolinia will burn out can be 
delayed. Therefore, excess reactivity can be properly controlled by a 
selected combination of the density of the burnable poison and the number 
of the fuel rods with the burnable poison. 
Next, the improvement of the fuel economy of the initial core will be 
explained. Parts of fuel assemblies loaded into the initial core are taken 
out after the first operation cycle (first cycle) and these are exchanged 
for reload fuel assemblies. The fuel assemblies taken out after the first 
cycle have a lower burnup and a lower generated energy than other fuel 
assemblies. Then, to efficiently utilize the fissionable materials, the 
formation of an initial core using a plurality of fuel assemblies that 
have a different uranium enrichment according to the duration of their use 
in the reactor is determined. 
With regard to this initial core, a core, composed of high enrichment fuel 
assemblies with 3.4 wt % average enrichment, middle enrichment in the 
axial direction of fuel assemblies with 2.3 wt % in the axial direction of 
the fuel assemblies and low enrichment fuel assemblies with 1.1 wt % in 
the axial direction of fuel assemblies, has been described in Japanese 
Patent Laid-open print No. 5-249270. It is also described in this 
publication that fuel assemblies with lower average enrichment in the 
axial direction are taken out from the core in an earlier stage and other 
fuel assemblies with a higher average enrichment in the axial direction 
are loaded into the core for a long period to efficiently utilize the 
fissionable materials. 
With regard to a known way of improving the fuel economy of the initial 
core, it has been described that fuel assemblies having a higher average 
enrichment than reload fuel assemblies are loaded in the most outer 
position of the core in Japanese Patent Laid-open print No. 60-13283. 
Another known way to improve the fuel economy of the initial core is 
described in Japanese Patent Laid-open print No. 61-165682. In this 
publication, an increase of exposure which originated from the start-up 
test is compensated by increasing the number of the high enrichment fuel 
assemblies of the initial core to more than the number of reload fuel 
assemblies of the equilibrium core. As a result, the fuel economy of the 
initial core is improved. In this known arrangement, the enrichment of the 
high enrichment fuel assembly is the same as that of the reload fuel 
assembly. 
The nuclear reactor requires operation with a proper control of the 
reactivity for a constant term. Generally, the increase of excess 
reactivity caused by increasing the average enrichment of the initial core 
in the axial direction is reduced by insertion of control rods or by 
mixing burnable poison into the fuel. But, in a core which is provided 
with an increased average enrichment in the axial direction for purposes 
of obtaining a higher burnup, the excess reactivity also increases 
further. When the control rods are inserted to reduce the increase of the 
excess reactivity, with an increase in the number of the inserted control 
rods, the channel peaking factor increases, and the thermal margin 
decreases. Moreover, it is also necessary to repeat an adjustment of the 
quantity of the control rods being inserted to compensate for a large 
change of the excess reactivity in the operation cycle. This reduces the 
availability factor of the nuclear reactor, and it is not desirable from 
the viewpoint of fuel economy. In case of increasing the mixing quantity 
(density) of the burnable poison, while the excess reactivity could be 
suppressed by increasing the mixing quantity (i.e. enrichment) of the 
burnable poison, this causes the thermal conductivity of the fuel pellets 
to decline, producing a problem from the integrity point of view. 
As is mentioned above, an increase of the excess reactivity of the core was 
the main factor for a disturbance in case of achieving a high burnup of 
the initial core. 
When the high burnup of the initial core is designed according to the 
technique described in Japanese Patent Laid-open print No. 60-13283, a 
sufficient effect is not achieved. In this case, because fuel assemblies 
with a high average enrichment in the axial direction are only loaded into 
the most outer position of the core, the number of the fuel assemblies 
with a high average enrichment in the axial direction is limited. 
Consequently, there is a limit to the improvement in the fuel economy to 
be obtained by increasing the average enrichment in the axial direction of 
the initial core. 
Even if the technique described in Japanese Patent Laid-open print No. 
61-165682 is used, a sufficient effect is not achieved. In this 
technology, because the number of the high enrichment fuel assemblies of 
the initial core is at most about 20-30% of the total number of the fuel 
assemblies, there is a limit to the increase of the average enrichment of 
the initial core. Consequently, the effect for fuel improvement is not 
sufficient. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an initial core designed 
for high burnup that can properly control the excess reactivity and can 
improve the fuel economy, without increasing the density of the 
conventional burnable poison. 
For the purpose of achieving the above object, in an initial core of the 
present invention, fuel assemblies having a higher average enrichment in 
the axial direction than reload fuel assemblies are loaded in an area 
other than the most outer position of the core. The most outer position of 
the core means a position where at least one side of the fuel assembly 
faces the outer area of the core, when the fuel assembly is loaded into 
position. 
Preferably, the average enrichment in the axial direction of the fuel 
assemblies having higher average enrichment in the axial direction than 
reload fuel assemblies should be 4.0 wt % or more. 
In accordance with the present invention, by taking into consideration the 
fact that a burnup of the burnable poison strongly depends on the neutron 
spectrum of the circumference of fuel assemblies containing the burnable 
poison, delay of the burnup of the burnable poison can be achieved. That 
is, the burnup of the burnable poison is delayed by making the average 
enrichment in the axial direction of the fuel assemblies loaded into the 
initial core higher than that of reload fuel assemblies. The thermal 
neutron absorption cross section of gadolinium (Gd), which is the 
representative example of the burnable poison, is as large as about 61,000 
barns for .sup.155 Gd and about 240,000 barns for .sup.157 Gd. The burnup 
speed of Gd depends on the magnitude of this cross section. Because this 
cross section of higher energy neutrons is smaller than that of lower 
energy neutrons, the burnup of Gd in the area where high energy neutrons 
exist is delayed. That is, if the average energy of the neutrons in the 
area where the Gd burns is made high, the burnup of Gd can be delayed. 
In a light water reactor, fast neutrons having an average energy of 2 MeV 
are produced by fission. These fast neutrons become thermal neutrons by 
mainly scattering with water, and the following fission is caused. 
Therefore, fission reactions should be increased by increasing the uranium 
enrichment of fuel assemblies to increase the average energy of neutrons 
in the nuclear reactor. A function that delays the burnup of the burnable 
poison by increasing the uranium enrichment of fuel assemblies will be 
explained by referring to FIG. 4 
FIG. 4 shows a relationship between the exposure and the infinite 
multiplication factor in the perpendicular section to the axial direction 
of fuel assemblies containing the burnable poison. Both the present 
invention, (the average enrichment in the axial direction of the fuel 
assembly is 4.2 wt %) having 4.6 wt % average enrichment in the section 
perpendicular to the axial direction and a comparative example of 4.2 wt % 
(the average enrichment in the axial direction of the fuel assembly is 3.7 
wt % equal to a typical reload fuel assembly) having 4.2 wt % average 
enrichment in the same section perpendicular to the axial direction are 
shown in FIG. 4. The density of the burnable poison of both are the same. 
When the exposure increases, the infinite multiplication factor increases 
at first, becomes maximum, and then decreases. In this relationship, the 
time when the burnable poison burns out (burn-out time) is thought to 
correspond to an intersection point between an extended line of the first 
half part having a tendency to increase the infinite multiplication factor 
and another extended line of the latter half part having a tendency to 
decrease the factor. The burn-out time of the present invention and the 
comparative example are shown by point B and point A, respectively, in 
FIG. 4. 
From FIG. 4, it can be seen that the burn-out time can be delayed for the 
term corresponding to the exposure of about 2 GWd/t by increasing the 
average enrichment in the section perpendicular to the axial direction by 
about 0.4 wt %. Therefore, even if the high burnup is designed by 
increasing the average enrichment in the axial direction of the initial 
core, the excess reactivity can be properly suppressed by composing the 
core using fuel assemblies having a higher average enrichment in the axial 
direction than the reload fuel assemblies. 
Furthermore, in accordance with the present invention, the economy of the 
initial core can be improved by loading the fuel assemblies having a 
higher average enrichment in the axial direction than the reload fuel 
assemblies into an area other than the most outer position of the core. 
The reason why the fuel assemblies having a high average enrichment in the 
axial direction are not loaded into the most outer position is because it 
is useless from the point of view of fuel economy to load fuel assemblies 
capable of increasing the exposure into the most outer position where the 
exposure does not increase so much. 
The reason why the average enrichment of fuel assemblies having a higher 
average enrichment in the axial direction than the reload fuel assemblies 
is made 4.0 wt % or more is because it achieves an equal effect to the 
case wherein the density of the gadolinia (gadolinium oxide) is increased 
by 1.0 wt % or more. The density of the gadolinia means the weight percent 
of the gadolinia in the fuel rod containing the gadolinia. The above 
function will be explained by using FIG. 11 as follows. FIG. 11 shows a 
relationship between the exposure for burning out the gadolinia and the 
average enrichment of the fuel assembly. From FIG. 11, it can be seen 
that, as the exposure for burning out the gadolinia is increased, the 
burnup of the gadolinia can be delayed by increasing the gadolinia density 
above conventional density. The maximum of the conventional gadolinia 
density is 7.5 wt % at present. The same effect also can be achieved by 
increasing the average enrichment of the fuel assembly. That is, as shown 
in FIG. 11, the same effect as increasing the gadolinia density 1 wt % is 
achieved by increasing the average enrichment in the axial direction of 
the fuel assembly from 3.7 wt % to 4.0 wt %. Consequently, the burnup of 
the gadolinia can be delayed, and the excess reactivity can be properly 
controlled without increasing the conventional gadolinia density by 
increasing the average enrichment in the axial direction of the fuel 
assembly to 4.0 wt % or more. The upper limit of the average enrichment in 
the axial direction of the fuel assembly is 4.9 wt %, which is the maximum 
at present.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Various embodiments according to the present invention will be explained by 
reference to the drawings. 
FIG. 1 shows a 1/4 cross section of a first embodiment of an initial core 
according to the present invention. FIG. 2 shows a structure of the fuel 
rods of the high enrichment fuel assembly of FIG. 1. 572 high enrichment 
fuel assemblies (hereinafter indicated by high enrichment fuel) H with 4.2 
wt % average enrichment in the axial direction and 300 low enrichment fuel 
assemblies (hereinafter indicated by low enrichment fuel) L with 1.5 wt % 
average enrichment in the axial direction are loaded into the core of FIG. 
1. The average enrichment in the axial direction of the core is about 3.3 
wt %. In this core, only low enrichment fuels are loaded into the most 
outer position and high enrichment fuels are loaded into areas other than 
the most outer position. In this embodiment, the average enrichment in the 
axial direction of reload fuel assemblies (hereinafter indicated by reload 
fuel) is 3.7 wt % which is a typical average enrichment. That is, the 
average enrichment in the axial direction of the high enrichment fuels is 
0.5 wt % higher than that of the reload fuels. 
The high enrichment fuel used for this embodiment is composed of fuel rods 
a1-a7 as shown in FIG. 2, and the number of each fuel rod is also shown in 
FIG. 2. Fuel rods a1-a4 contain uranium fuel and no gadolinia in the whole 
of the fuel effective length. Fuel rod a5 contains uranium fuel only in 
the range of 1/24-15/24 from the bottom of the fuel effective length, and 
contains no gadolinia. Fuel rod a5 is hereinafter referred as a part 
length fuel rod. Fuel rod a6 contains uranium fuel in the whole of the 
fuel effective length, and gadolinia in the range of 1/24-22/24 from the 
bottom of the fuel effective length. Fuel rod a7 contains uranium fuel in 
the whole of the fuel effective length and gadolinia in the range of 
1/24-8/24 from the bottom of the fuel effective length. 
Fuel rods a1-a4, a6 and a7, contain natural uranium (uranium enrichment is 
0.711 wt %) in the bottom end area of 0/24-1/24 from the bottom of the 
fuel effective length and in the top end area of 0/24-2/24 from the top of 
the fuel effective length. Fuel rod a6 contains uranium fuel with 4.4 wt % 
and gadolinia with 7.5 wt % in the range of 1/24-22/24 from the bottom of 
the fuel effective length. Fuel rod a7 contains uranium fuel with 4.4 wt % 
and gadolinia with 7.5 wt % in the range of 1/24-8/24 from the bottom of 
the fuel effective length, and only uranium fuel with 4.4 wt % in the 
range of 8/24-22/24 from the bottom of the fuel effective length. Fuel rod 
a5 contains uranium fuel with 4.9 wt % in the whole of the part length 
fuel rod. 
The low enrichment fuel contains no gadolinia, and the average enrichments 
in the section perpendicular to the axial direction in the area of 
1/24-8/24, 8/24-15/24 and 15/24-22/24 from the bottom of the fuel 
effective length are 1.49, 1.64 and 1.75 wt % respectively. Natural 
uranium is loaded into the top end area and the bottom end area of the 
fuel effective length like the high enrichment fuel. 
The fuel assembly of FIG. 2 is composed of these fuel rods so as to make 
the average enrichments in the section perpendicular to the axial 
direction in the area of 1/24-15/24 and 15/24-22/24 from the bottom of the 
fuel effective length 4.61 and 4.58 wt % respectively. 
FIG. 5 shows a relationship between the excess reactivity and the cycle 
exposure of the initial core shown in FIG. 1. The cycle exposure indicates 
an increased quantity of the average exposure of the core during one 
operation cycle. FIG. 5 shows the first embodiment shown in FIG. 1 and a 
comparative example. In the high enrichment fuel of the comparative 
example of FIG. 5, the average enrichments in the section perpendicular to 
the axial direction in the area of 1/24-8/24, 8/24-15/24 and 15/24-22/24 
from the bottom of the fuel effective length are 4.04, 4.20, and 4.18 wt 
%, respectively. The average enrichment of the high enrichment fuel of 
this comparative example is 3.7 wt % equal to the reload fuel. From FIG. 
5, it is seen that this embodiment of the present invention can suppress 
the excess reactivity of the core more effectively than the comparative 
example. 
FIG. 6 shows a relationship between the exposure and the infinite 
multiplication factor of high enrichment fuel of the first embodiment. The 
first embodiment and the comparative example of which the average 
enrichment in the axial direction of the fuel assembly is 3.7% are shown 
in FIG. 6. From FIG. 6, the burnup of the gadolinia can be delayed about 2 
GWd/t by increasing the average enrichment in the axial direction of the 
fuel assemblies by about 0.4 wt %. In this embodiment, the high enrichment 
fuel contains part length fuel rods. But, even if the high enrichment fuel 
contains no part length fuel rod, the same effect can be achieved by 
increasing the average enrichment in the axial direction of the fuel 
assembly to a value higher than that of the reload fuel. 
The average enrichment in the axial direction of the reload fuel is 
determined by the core size, the lattice structure, the operation cycle 
and so on. In this embodiment, the average enrichment in the axial 
direction of the reload fuel is set to about 3.7 wt % for the condition of 
an electric power of 1,350,000 kW and an operation cycle of 13 months. 
FIG. 9 shows a relationship between the average enrichment in the axial 
direction of the typical reload fuel and the operation cycle. 
From FIG. 9, it is seen that the excess reactivity can be properly 
controlled and the economy of fuel can be improved without increasing the 
density of the conventional burnable poison by setting up the proper 
average enrichment of the reload fuel according to the operation cycle, by 
increasing the average enrichment of the high enrichment fuel to a value 
higher than that of the reload fuel, and by loading the high enrichment 
fuel into an area other than the most outer position of the core. 
Next, a second embodiment of the high enrichment fuel according to the 
present invention will be explained with reference to FIG. 7. In the high 
enrichment fuel of the first embodiment shown in FIG. 2, natural uranium 
was loaded into both of the top end area and the bottom end area. In this 
embodiment, in contrast to the first embodiment, natural uranium is loaded 
into only the top end area of 0/24-1/24 from the top of the fuel effective 
length. That is, in this example, enriched uranium and natural uranium are 
loaded into the area 0/24-23/24 and 23/24-24/24 respectively from the 
bottom of the fuel effective length. Hereinafter the area loaded with 
enriched uranium will be referred to as an enriched uranium area. The 
number of each fuel rod (b1-b7), the gadolinia density and so on are the 
same as in FIG. 2. 
When the high enrichment fuels of this embodiment are loaded into the core 
of FIG. 1, the average enrichment in the axial direction of the core 
increases by about 0.1 wt % by the expansion of the enriched uranium area 
in the axial direction. However, because of the enrichment of both end 
areas, where the neutron importance is small, is increased, the reactivity 
is not changed so much. 
According to this embodiment, because the effective axial length of the 
core becomes longer by the expansion of the enriched uranium area in the 
axial direction, an axial peaking factor (a ratio of the maximum value and 
the average value of axial power) can be reduced by about 6%. Therefore, 
in addition to the effect of the first embodiment, the linear power heat 
generating ratio of the core can be reduced. As a result, the thermal 
margin is increased and the flexibility of the operation is improved. 
Next, a third embodiment of the high enrichment fuel according to the 
present invention will be explained with reference to FIG. 8. In the high 
enrichment fuel of the first embodiment shown in FIG. 2, both the fuel rod 
a6 containing gadolinia in the whole of the enriched uranium area and the 
fuel rod a7 containing gadolinia in a part of the enriched uranium area 
were loaded. In this embodiment, only fuel rod c6 containing gadolinia in 
the whole of the enriched uranium area is loaded. Each enriched uranium 
area of fuel rods c2 and c3 is divided in the position of 8/24 from the 
bottom of the fuel effective length as a boundary, and the average 
enrichment in the section perpendicular to the axial direction of the 
upper part over the boundary is higher than that of the lower part under 
the boundary. In FIG. 8, the signs a-e denote the enrichment of uranium 
fuel and the sign .alpha. denotes the density of gadolinia. The 
relationship among magnitudes of the signs a-e is a&gt;b&gt;c&gt;d&gt;e. The sign e 
corresponds to 0.711 wt % of natural uranium. The magnitude of the sign a 
is 7.5 wt % or less. 
In a boiling water reactor, voids are distributed in the axial direction by 
a boiling of the cooling water inside the core. Therefore, the higher the 
position is, the higher will be the ratio of the void (void fraction). In 
a light water reactor, because the fission reactions are suppressed on the 
condition of a little quantity of cooling water, the power distribution in 
the axial direction usually has a swollen lower part. Accordingly, in this 
embodiment, the power distribution in the axial direction can be flattened 
in addition to the effect of the first embodiment, by increasing the 
average enrichment (i.e. the average enrichment in the section 
perpendicular to the axial direction) of the upper part in the axial 
direction of the high enrichment fuel to a value higher than that of the 
lower part against the above power distribution. As a result, because the 
axial-peaking factor in the core can tee reduced, the thermal margin is 
increased and the flexibility of the operation is improved. 
As for the enrichment of uranium fuel indicated by the signs a-e in FIG. 8, 
it is a necessary condition that the average enrichment in the axial 
direction of a fuel assembly is higher than that of the reload fuel. In 
the range of this condition, it is necessary to adjust the enrichment of 
fuel rods arranged in the most outer region or corner of the fuel assembly 
so that the power of the fuel rods does not become so high. In the case 
wherein a difference of enrichment is provided between the upper and lower 
parts in the axial direction, it is necessary to adjust the difference of 
enrichment to make the power distribution in the axial direction as flat 
as possible when the fuel assemblies are loaded into the core. 
Lastly, a further arrangement of the initial core according to the present 
invention will be explained with reference to FIG. 10. FIG. 10 shows a 1/4 
cross section of the initial core. In this arrangement, while the number 
and the loading position of the low enrichment fuels L with 1.5 wt % 
average enrichment in the axial direction are the same as the arrangement 
of FIG. 1, the high enrichment fuels with 4.2 wt % average enrichment in 
the axial direction are different from the first arrangement. That is, the 
core is loaded with 344 high enrichment fuels H1 that have a lower number 
of fuel rods containing gadolinia (indicated by low Gd) and with 228 high 
enrichment fuels H2 that have higher number of fuel rods containing 
gadolinia (indicated by high Gd). 
Because the power of the core can be suppressed by degrees by loading the 
fuel assemblies that have different number of fuel rods containing 
gadolinia into the core like this arrangement, the channel peaking factor 
can be flattened. Furthermore, because the power of the outer region of 
the core can be increased by loading the high enrichment fuel (low Gd) 
into the outer region, the channel peaking factor can be flattened more 
effectively.