Nuclear fuel assembly

A nuclear fuel assembly has a plurality of fuel rods each containing a multiplicity of fuel pellets. The fuel rods include a plurality of first fuel rods and a plurality of second fuel rods. Each of the first fuel rods contains a burnable poison over substantially the entire axial length thereof. The first fuel rod also has a greater means enrichment in the upper axial region thereof than that in the lower axial region thereof. Each of the second fuel rods contains no burnable poison and has a uniform enrichment distribution over substantially the entire axial length thereof. In each cross-section of the fuel assembly perpendicular to the axis thereof, the outer peripheral portion has a greater mean enrichment than the central portion. The first fuel rods are disposed in the peripheral portion except the outermost peripheral portion of the cross-section of the fuel assembly.

BACKGROUND OF THE INVENTION 
The present invention relates to a nuclear fuel assembly and, more 
particularly, to a nuclear fuel assembly suitable for use for a boiling 
water reactor. 
The reactor core of a boiling water reactor is charged with a plurality of 
nuclear fuel assemblies each of which is constituted by a channel box, 
lower tie plate, upper tie plate and a multiplicity of fuel rods. The fuel 
rods arranged in the form of a bundle are held at their upper and lower 
ends by the upper and lower tie plates. The bundle of the fuel rods is 
disposed in a channel box secured to the upper tie plate. Each fuel rod is 
charged with a multiplicity of fuel pellets (UO.sub.2 pellets). In each 
fuel assembly, there are several fuel rods having UO.sub.2 pellets which 
contain gadolinea as a burnable poison. There also are two water rods 
disposed in the central region of the nuclear fuel assembly. 
In general, a boiling water reactor exhibits a void distribution in the 
vertical or axial direction. Due to the variation of void reactivity along 
the axis, the boiling water reactor shows such a power distribution that 
the peak of the power is shifted to the lower side along the axis. 
In order to attain a flat axial power distribution by obviating the axially 
downward shifting of the power peaking, it has been proposed to use a fuel 
assembly having different degrees of enrichment at the upper and lower 
regions thereof. One of such a fuel assembly is disclosed in the 
specification of the U.S. Pat. No. 4,229,258. In this fuel assembly, some 
of the fuel rods arranged in the peripheral region thereof have different 
degrees of enrichment at their upper and lower regions. More specifically, 
the upper region of each of such fuel rod has an enrichment which is about 
15% higher than that in the lower region thereof. 
In recent years, various studies have been made for the development of fuel 
assemblies suitable for higher burn-up, i.e., fuel assemblies which can be 
burnt up to a high degree. Such a fuel is obtained by arranging fuel rods 
rich in fissile material, i.e. fuel rods having high enrichment, in the 
vicinity of the channel box having a high density of thermal neutron flux. 
Japanese Patent Laid-Open No. 26292/1983 discloses a fuel assembly which 
can be burnt up to a high degree in accordance with the theory disclosed 
in U.S. Pat. No. 4,229,258. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to provide a fuel assembly which 
is improved such that the difference in the power level between different 
cross-sections along the axis is minimized so as to flatten the power 
distribution in the axial direction of the fuel assembly. 
Another object of the invention is to provide a fuel assembly having a 
simple construction constituted by a fewer number of kinds of the fuel 
rod. 
Still another object of the invention is to provide a fuel assembly capable 
of improving the fuel economy. 
To these ends, according to the invention, there is provided a fuel 
assembly having first fuel rods each containing a burnable poison over the 
substantial region in the axial direction thereof, and second fuel rods 
containing no burnable poison, each of said first fuel rods having such an 
enrichment distribution that the mean enrichment over the most part of the 
upper region thereof is higher than the mean enrichment over the most part 
of the lower region thereof, while each of said second fuel rods has a 
substantially uniform enrichment distribution over the entire axial region 
thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention has been attained as a result of a minute study on 
the characteristics of the known fuel assembly disclosed in Japanese 
Patent Laid-Open No. 29878/1983, as will be understood from the following 
description. 
First of all, an explanation will be made with reference to the fuel 
assembly shown in U.S. Pat. No. 4,229,258 which provides a base for the 
fuel assembly shown in Japanese Patent Laid-Open No. 29878/1983. 
The fuel assembly of the U.S. Patent mentioned above exhibits a uniform 
power distribution along the axis thereof. In the fuel assembly used in a 
boiling water reactor, however, voids are generated within the channel 
box, whereas no void is generated outside the channel box. Therefore, in 
FIG. 4 attached to the U.S. Patent mentioned above, a non-uniform density 
distribution of water (moderator) appears in a vertical section which 
contains a corner of the channel box facing a control rod and another 
corner which is on the diagonal line passing the first-mentioned corner. 
More specifically, the water density is lower in the inside of the channel 
box than in the outside of the channel box. Therefore, the density of the 
thermal neutron flux .phi. exhibits such a distribution in the section 
containing above-mentioned two corners of the channel box that the density 
is low at the mid portion of the section and high outside the channel box, 
as will be seen from FIG. 1. 
The power of each of the fuel rods constituting the fuel assembly is given 
by the following formula: 
EQU P=.phi..multidot..sigma..sub.F .multidot.N (1) 
where, .phi. represents the thermal neutron flux density at the position of 
the fuel rod, .sigma..sub.F represents the fission cross-sectional area, 
and N represents the atomic number density of the fissile material in the 
fuel rod. 
In the known fuel assemblies such as that shown in U.S. Pat. No. 4,229,258, 
the atomic number density N (which is in proportion to enrichment e) of 
the fissile material in the fuel rods disposed in the peripheral region of 
the fuel assembly where the thermal neutron flux density .phi. is high is 
selected to be small as compared with that of the fuel rods in the central 
region of the fuel assembly as shown in FIG. 2, in order to flatten the 
power distribution of each fuel rod (referred to as local power 
distribution, hereinunder) thereby minimizing the local power peaking 
which is the ratio between the maximum power of the fuel rod and the mean 
power of the fuel assembly. For instance, in the fuel assembly shown in 
the above-mentioned U.S. Patent, the fuel rods adjacent the channel box 
have a mean enrichment of uranium 235 which is about 25 to 50% lower than 
that in the fuel rods in the central region of the fuel assembly. 
When a reactor core is charged with new fuel assemblies, the excess 
reactivity in the reactor core is so large as can never be controlled by 
the control rods solely. In order to suppress the excess reactivity in the 
beginning period of the burning, several fuel rods in the fuel assembly 
are made to contain gadolinea as a burnable poison as mentioned before. 
This burnable poison has an extremely large neutron absorption 
cross-sectional area, so that it is decreased more rapidly than the 
uranium 235 as the time elapses. Thus, the burnable poison is extinguished 
completely after a certain period of time so that the reactivity is not 
affected by the burnable poison in the later period of burning. 
FIG. 3 illustrates how the reactivity is suppressed by the use of 
gadolinea. More specifically, the full-line curve shows the infinite 
multiplication factor of the fuel assembly which contains gadolinea, while 
the broken-line curve shows the infinite multiplication factor of the fuel 
assembly which does not contain gadolinea. Thus, the reactivity 
suppressing effect produced by the gadolinea is shown as the difference 
between the values on both curves. 
When the fuel assembly contains the gadolinea, the infinite multiplication 
factor is linearly increased in accordance with the increment of the 
burn-up degree and, after exhibiting a peak at exposure around 10 GWd/t at 
which the gadolinea is burnt out, decreases linearly in accordance with 
the increase in of exposure, as shown by the full-line curve in FIG. 3. 
The period in which the infinite multiplication factor increased up to the 
peak will be referred to as "earlier burning period", while the period 
after the peak will be referred to as "later burning period". 
The fuel assembly shown in Japanese Patent Laid-Open No. 29878/1983 can 
provide a higher exposure without imparing the flat axial power 
distribution proposed by the above-mentioned U.S. Patent. Namely, the fuel 
assembly can be burnt for a longer period of time. 
FIG. 4 shows the cross-section of the fuel assembly as shown in Japanese 
Patent Laid-Open No. 29873/1983. This fuel assembly 23 is constituted by 
fuel rods 1 to 6, and G.sub.1 and G.sub.2. These fuel rods 1 to 6 and 
G.sub.1, G.sub.2 exhibit enrichment distributions and gadolinea density 
distributions as shown in FIG. 5. It will be seen that the fuel rods 
G.sub.1 and G.sub.2 have uniform gadolinea density distribution along the 
axis thereof. Both fuel rods G.sub.1 and G.sub.2 have an equal gadolinea 
density Gd.sub.0. The fuel rods 1 to 6 do not contain gadolinea. Each of 
the fuel rods 1 to 6 and G.sub.1, G.sub.2 has a clad tube charged with 
pellets of UO.sub.2 as the fissile material. The fuel rods 1 to 6 and 
G.sub.1, G.sub.2 have enrichments e.sub.1 to e.sub.6 as shown in FIG. 5. 
These enrichment are selected to meet the condition of e.sub.1 &gt;e.sub.2 
&gt;e.sub.3 &gt; e.sub.4 &gt;e.sub.5 &gt;e.sub.6. In this fuel assembly 23, fuel rods 
having mean enrichments greater than the mean enrichment of the fuel 
assembly are disposed in a large number in the peripheral region, whereas 
fuel rods having a mean enrichment lower than that of the fuel assembly 
are disposed in the central region of the fuel assembly. 
In this fuel assembly 23, the power of the fuel rods in the peripheral 
region is increased so that the power is increased locally in the 
peripheral region of the fuel assembly throughout the period of operation 
of the nuclear reactor. The infinite multiplication factor of the fuel 
assembly 23 is increased substantially in linear proportion to the 
increase of the local power in the peripheral region. Therefore, the 
increase in the infinite multiplication factor of the fuel assembly can be 
maximized by increasing the local power peaking. The maximum value of the 
local power peaking is restricted by the thermal condition of the fuel 
rod, so that the increase in the infinite multiplication factor is 
materially limited. For attaining a further increase in the infinite 
multiplication factor, it is necessary to increase the local powers of the 
fuel rods in the peripheral region of the fuel rods. This, however, must 
be done equally over all fuel rods in the peripheral region. 
Thus, the local power distribution and enrichment distribution which will 
maximize the increase in the reactivity are determined on condition that 
the mean enrichment and the maximum value of the local power peaking in 
the fuel assembly 23 are given. 
FIG. 7 shows an example of the optimum local power distribution in the 
peripheral region, particularly in the outermost peripheral region, of the 
fuel assembly when the local power peaking factor is 1.30. In this Figure, 
each square represents each fuel rod. 
The outer peripheral region of the fuel assembly 23 includes both the fuel 
rods having high enrichment intended for attaining high local power 
peaking and fuel rods having two axial regions of different enrichments 
intended for suppressing the axial power peaking. In this fuel assembly 
23, the axial power peaking is suppressed such as to allow a corresponding 
increase in the local power in the peripheral region, thus attaining a 
greater reactivity gain. 
This fuel assembly 23, however, proved the following disadvantage. Namely, 
since the fuel rods 2 and 3 having axial regions of different enrichments 
are disposed in the peripheral region as shown in FIG. 4, the local power 
distribution is deviated from the optimum power distribution shown in FIG. 
7 either in the upper region or the lower region of the fuel assembly 23. 
This makes it impossible to attain the local power distribution for 
maximizing the reactivity both in the upper and lower regions of the fuel 
assembly. FIGS. 8 and 9 show, respectively, the local power distributions 
at the upper and lower regions of the fuel assembly 23 in the beginning of 
burn-up. These local power distributions cannot provide a uniform power 
peaking of, for example, 1.30 over the peripheral region, particularly in 
the outermost region, of the fuel assembly, unlike the local power 
distribution shown in FIG. 7. In addition, a difference of local power 
distribution is produced in the outermost region between the upper and 
lower regions of the fuel assembly 23. 
As a result of an intense study on the characteristics of the fuel assembly 
23, the present inventors have found that the above-explained problems of 
the known fuel assemblies can be obviated by providing a difference in the 
enrichment between the upper and lower regions of each fuel rod which 
contain a burnable poison over almost entire axial region thereof. 
More specifically, the power P of the fuel rod is proportional to the 
product of the enrichment e and the thermal neutron flux density .phi.. 
Namely, the condition of P=e.multidot..phi. is met. Therefore, if a fuel 
rod having a large thermal neutron flux density .phi. or a large power P 
is made to have such an enrichment distribution as being high in the upper 
region than in the lower region, the change in the power caused by a given 
change in the enrichment e becomes excessively large, resulting in a large 
difference in the power between the upper and lower regions of the fuel 
rod. 
In contrast to the above, in the case of a fuel rod which contains 
gadolinea over almost entire axial region thereof, the thermal neutron 
flux density .phi. is small due to the presence of gadolinea, so that only 
a small change in the power P is caused by a given change in the 
enrichment e. Thus, the fuel rod containing gadolinea over almost entire 
axial region thereof exhibits a comparatively small change in the axial 
power distribution. For this reason, when the difference in the enrichment 
between the upper and lower regions is provided in a fuel rod which 
contains gadolinea over almost entire axial region thereof, the local 
power distributions in the upper and lower regions of the fuel rod are 
substantially equalized. 
The present invention is based upon the discovery explained hereinbefore. 
The invention will be fully understood from the following description of 
the preferred embodiments. 
FIG. 10 shows a preferred embodiment of the fuel assembly in accordance 
with the invention. The fuel assembly 30 has a channel box 10, a lower tie 
plate 11, an upper tie plate 12, spacers 15 and fuel rods 16. The fuel 
rods 16 are held at their lower and upper ends by the lower tie plate 11 
and the upper tie plate 12. A plurality of the spacers 15 are arranged in 
the axial direction such as to maintain predetermined gaps between 
adjacent fuel rods. The channel box 10 is secured to the upper tie plate 
12 and surrounds the bundle of the fuel rods 16 held by the spacers 15. A 
channel fastener 13 is secured to the upper tie plate 12. 
FIG. 11 shows the detail of the fuel rod 16. The fuel rod 16 has a clad 
tube 20 charged with a multiplicity of fuel pellets 21 and closed at its 
upper and lower ends by means of upper and lower end plugs 17 and 18. The 
fuel pellets 21 are pressed by a spring 22 disposed in a gas plenum 
defined in the clad tube 20. 
FIG. 12 is a sectional view of the fuel assembly taken along the line 
XII--XII of FIG. 10. Fuel rods 16 are arranged in a lattice-like form 
within the channel box 10. Two water rods 14 are disposed in the central 
region of the channel box 10. There are several fuel rods 16 which contain 
gadolinea as a burnable poison. Water gaps are formed between adjacent 
fuel assemblies. These water gaps are adapted to receive control rods 19. 
The fuel rods 16 disposed in the fuel assembly 30 can be sorted into 6 
(six) kinds: namely, fuel rods 31, 32, 33, 34, G.sub.4 and G.sub.5 which 
have enrichment distributions and gadolinea distributions as shown in FIG. 
13. These fuel rods 31 to 34, G.sub.4 and G.sub.5 are disposed within the 
channel box 10 in a pattern shown in FIG. 12. The fuel rods 31 to 34 
contain fuel pellets 21 of UO.sub.2 as the nuclear fuel. These fuel 
pellets contain uranium 235 as the fissile material. These fuel rods 31 to 
34 do not contain gadolinea as the burnable poison. On the other hand, the 
fuel rods G.sub.3 and G.sub.4 have UO.sub.2 fuel pellets which contain 
gadolinea together with the uranium 235. The enrichments e.sub.1 to 
e.sub.4 in the fuel rods shown in FIG. 13 are determined to meet the 
conditions of e.sub.1 &gt;e.sub.2 &gt;e.sub.3 &gt; e.sub.4. The fuel rods 31 to 34 
and G.sub.4 have a uniform enrichment over the entire axial length 
thereof. The upper region of the fuel rod G.sub.4 above the level of 11/24 
of the effective length of fuel as measured from the bottom of the 
effective length of fuel uniformly contains gadolinea, while the lower 
region below the abovementioned level does not contain gadolinea. The term 
"effective length of fuel" means the length or region of the fuel rod 
charged with the nuclear fuel material, i.e., the fuel pellets. The fuel 
rod G.sub.3 contains gadolinea uniformly over the entire axial length 
thereof. The fuel rods G.sub.3 and G.sub.4 have an equal density of 
gadolinea. In the fuel rod G.sub.3, the upper region above the level of 
11/24 of the entire length as measured from the bottom of the fuel has a 
higher enrichment that the lower region below the above-mentioned level. 
Namely, the fuel rod G.sub.3 has upper and lower regions having different 
enrichments, but the enrichment is uniform in each of the upper and lower 
regions. 
The fuel assembly 30 having the fuel rods G.sub.3 and G.sub.4 naturally 
have two regions: namely, an upper region above the level of 11/24 of the 
effective fuel length as measured from the bottom of the effective fuel 
length and a lower region below the above-mentioned level. The mean 
enrichment in a plane perpendicular to the axis of the fuel assembly 
within the upper region is greater than that in a plane perpendicular to 
the axis of the fuel assembly within the lower region thereof. In 
addition, the amount of gadolinea contained by the upper region of the 
fuel assembly 30 is greater than that contained by the lower region of the 
same. It is to be understood also that the upper region of the fuel 
assembly 30 has a greater infinite multiplication factor than the lower 
region. 
Thus, the amount of gadolinea contained by the upper region is greater than 
that contained by the lower region. This axial gadolinea distributions 
serves to provide a smaller infinite multiplication factor than in the 
lower region of the fuel assembly 30. On the other hand, the upper region 
of the fuel assembly 30 has a greater mean enrichment than the lower 
region of the same. This enrichment distribution serves to provide a 
greater infinite multiplication factor in the upper region than in the 
lower region of the fuel assembly. In the fuel assembly of the invention, 
the mean enrichment in the upper region is selected to be large enough to 
compensate for any reduction in the infinite multiplication factor due to 
the presence of the gadolinea in the upper region, so that the fuel 
assembly as a whole exhibits a greater infinite multiplication factor in 
the upper region than in the lower region. 
Referring to FIG. 12 showing the fuel assembly 30 in a cross-section 
perpendicular to the axis thereof, two regions are assumed in this 
cross-section of the fuel assembly: namley, a peripheral region outside 
the one-dot-and-dash line L which is an annular region having two layers 
of fuel rods, and a central region inside the one-dot-and-dash line and 
having three and four layers of fuel rods. In the described embodiment of 
the fuel assembly, the mean enrichment in the peripheral portion is 
greater than that in the central region. 
As stated before, in the described embodiment of the fuel assembly, the 
axial enrichment distribution is created by providing an axial enrichment 
distribution in the fuel rods G.sub.3 which contain gadolinea over almost 
the entire axial region thereof and, therefore, the upper and lower 
regions of the fuel rod G.sub.3 has a substantially equal power 
distribution. Consequently, the difference in the local power between the 
peripheral region of the upper region and the peripheral region in the 
lower region is minimized. In fact, the local powers of these peripheral 
regions become substantially equal to each other. This effect is maximized 
because the fuel rods G.sub.3 are disposed in the portion of the 
peripheral region except the outermost portion. Consequently, the 
reactivity is increased and a higher fuel economy is attained. It is to be 
noted also that, while the known fuel assembly mentioned before employs 8 
kinds of fuel rods, the described embodiment of the fuel assembly of the 
invention employs only six kinds of fuel rods, thus remarkably simplifying 
and facilitating the production of the fuel assembly. Furthermore, the 
described embodiment of the fuel ssembly provides the same advantage as 
that offered by the fuel assembly shown in FIG. 4 of Japanese Patent 
Laid-Open No. 26292/1983, i.e., a longer period of burning of the fuel 
assembly, because the mean enrichment is greater in the peripheral region 
than in the central region. The described embodiment of the fuel assembly 
also produces the same effect as that provided by the fuel assembly shown 
in FIG. 4 of U.S. Pat. No. 4,229,258, i.e., a flat or uniform axial power 
distribution of the fuel assembly, because the mean enrichment is higher 
in the upper region than in the lower region of the fuel assembly. This 
effect becomes appreciable after the burning of the gadolinea in the fuel 
assembly 30. This effect eliminates the use of control rods which are to 
be inserted only to small depth, and the power of the nuclear reactor can 
be controlled only by means of control rods which are to be inserted to a 
large depth. Consequently, the control operation for the control rods can 
be remarkably simplified. Preferably, the boundary between the upper and 
lower regions is positioned within the range between 1/3 and 7/12 of the 
fuel effective length as measured from the bottom of the fuel effective 
length. 
In the described embodiment of the fuel assembly, not only the enrichment 
but also the amount of gadolinea is greater in the upper region than in 
the lower region. This in turn produces an effect called "spectrum shift" 
which is stated in lines 7 to 27, page 10 of the specification of U.S. 
patent application No. 548,845 and shown in FIGS. 5 to 7 attached to this 
U.S. patent specification. This spectrum shift effect also contributes to 
an increase in the discharged exposure of fuel burn-up, i.e., to a 
prolongation of period of burning of the fuel. 
Another embodiment of the invention will be described hereinunder with 
reference to FIGS. 14 and 15. The fuel assembly 40 of this embodiment has 
six kinds of fuel rods, i.e., fuel rods 41 to 44, G.sub.5 and G.sub.6, 
which are arranged in a manner shown in FIG. 14 and having enrichments and 
gadolinea densities as shown in FIG. 15. As will be understood from a 
comparison between FIG. 13 and FIG. 15, the fuel rods 41 to 44, G.sub.5 
and G.sub.6 used in this embodiment are similar to the fuel rods 31 to 34, 
G.sub.3 and G.sub.4 of the first embodiment shown in FIG. 13, except that 
they are provided at their one or both ends with layers of natural uranium 
e.sub.5. Usually, the power is not so large at each axial end of the fuel 
rod, so that only a small discharged exposure is attained at such axial 
ends even if these axial ends are charged with enriched uranium. Rather, 
the use of enriched uranium in such axial ends leads to a wasteful use of 
the uranium. From this point of view, the embodiment of the fuel assembly 
shown in FIGS. 14 and 15 employs layers of natural uranium in one or both 
ends of the fuel rods, thus minimizing the wasteful use of the uranium. 
The fuel rods G.sub.5 and G.sub.6 do not have the layer of natural uranium 
e.sub.5 in their upper ends. The enrichment e.sub.4 has a greater content 
of uranium 235 than the natural uranium e.sub.5. These fuel rods G.sub.5 
and G.sub.6 contain gadolinea and, therefore, produces large volume of 
gases during the operation of the nuclear reactor. In this embodiment of 
the fuel assembly, a sufficiently large volume of gas plenum is provided 
on the upper end of each of the fuel rods G.sub.5 and G.sub.6 which are 
devoid of the layers of natural uranium e.sub.5. 
The effective fuel length of the fuel assembly 40 as a whole is equal to 
that of the fuel rods 41 to 44. The length of the region charged with the 
natural uranium is 1/24 of the effective fuel length. The fuel rods 
G.sub.5 and G.sub.6 containing gadolinea are sectioned axially into two 
regions: namely, an upper region above the level 11/24 of the fuel 
effective length as measured from the bottom of the fuel effective length 
and a lower region below the above-mentioned level. The fuel rod G.sub.5 
contains gadolinea uniformly over almost the entire axial region thereof 
except the lower end constituted by the natural uranium e.sub.5. The 
enrichment in the most part of the lower region of the fuel rod G.sub.5 
except the lower end portion having the natural uranium is lower than the 
enrichment in the most part of the upper region thereof. Each of the fuel 
rods 41 to 44 and G.sub.6 has a substantially uniform enrichment 
distribution over the most part of the axial region thereof except the 
portions charged with the natural uranium. The upper region of the fuel 
rod G.sub.6 uniformly contains gadolinea at a density equal to that in the 
fuel rod G.sub.5. 
The fuel assembly 40 of this embodiment is materially identical to the fuel 
assembly 30 of the first embodiment except that the fuel rods are charged 
at their one or both axial ends with natural uranium. The enrichments and 
the gadolinea densities of the fuel rods 41 to 44, G.sub.5 and G.sub.6 are 
shown in the following table. 
TABLE 1 
______________________________________ 
No. of Fuel Rods 
41 42 43 44 G.sub.5 
G.sub.6 
______________________________________ 
Upper Enrichment 4.1 3.8 3.2 2.5 3.8 2.5 
region wt % 
Gadolinea 0 0 0 0 3.5 2.0 
density wt % 
Lower Enrichment 4.1 3.8 3.2 2.5 2.5 2.5 
region wt % 
Gadolinea 0 0 0 0 3.5 0 
density wt % 
______________________________________ 
FIG. 16 shows the local power distribution in the outermost portion of the 
upper region in the fuel assembly 40 in the beginning of burn-up, while 
FIG. 17 shows the local power distribution in the outermost portion of the 
lower region of the fuel assembly 40 in the beginning of burn-up. From 
these Figures, it will be seen that the local power in the outermost 
portion in the upper region is almost equal to that in the outermost 
portion in the lower region. Thus, the fuel assembly 40 of this embodiment 
offers the same advantage as that produced by the fuel assembly 30 of the 
first embodiment. 
FIG. 18 shows still another embodiment of the invention. The fuel assembly 
50 of this embodiment has a construction similar to that of the fuel 
assembly 40 of the second embodiment, except that four water rods disposed 
in the central portion of the fuel assembly 40 is substituted by a single 
large water rod 51. Thus, the fuel assembly 50 of this embodiment produced 
substantially the same effect as that produced by the fuel assembly 40. 
FIG. 19 shows a further embodiment of the invention. The fuel assembly 60 
of this embodiment employs the fuel rods 31, 32, 33 and 34 used in the 
first embodiment explained in connection with FIG. 13. In this embodiment, 
however, these fuel rods are arranged in a manner shown in FIG. 19. It 
will be seen also that the fuel assembly 60 of this embodiment employs a 
fuel rod G.sub.7 in place of the fuel rod G.sub.3 used in the first 
embodiment. The fuel rod G.sub.7 is materially identical to the fuel rod 
G.sub.3 except that its lower region has a mean enrichment e.sub.3 in 
contrast to the fuel rod G.sub.3 which has a mean enrichment e.sub.4 in 
its lower region. The fuel assembly 60 of this embodiment has a uniform 
distribution of gadolinea because it is devoid of the fuel rod G.sub.4 
shown in FIG. 13. This embodiment, therefore, cannot produce the spectrum 
shift effect which is obtained with the fuel assembly 30 of the first 
embodiment. 
Therefore, the fuel assembly 60 of this embodiment produces all the 
advantages produced by the fuel assembly 30 other than the advantage 
derived from the spectrum shift effect. 
FIG. 20 shows a further embodiment of the invention. The fuel assembly 70 
of this embodiment employs fuel rods 31 to 34 shown in FIG. 13 and fuel 
rods 75 and G.sub.8 shown in FIG. 21. These fuel rods are arranged in a 
manner shown in FIG. 20. The fuel rod 75 and G.sub.8 has a greater 
enrichment in the upper region thereof above the level 1/2 of the fuel 
effective length as measured from the bottom of the same than in the lower 
region below the above-mentioned level. The fuel rod 75 does not contain 
gadolinea. The fuel rod G.sub.8 has a greater gadolinea density Gd.sub.2 
in its upper region above the level 1/2 of the fuel effective length as 
measured from the bottom of the same than that Gd.sub.3 in the lower 
region thereof below the above-mentioned level. 
Thus, the fuel assembly 70 of this embodiment employs lower enrichment fuel 
rods which do not contain gadolinea and which have upper and lower regions 
of different enrichments. Therefore, the difference in the local power 
between the upper and lower regions can be reduced as compared with that 
in the known fuel assembly 23, although the difference is larger than that 
in the fuel assembly 30 of the first embodiment. The fuel assembly 70 of 
this embodiment produces effects substantially the same as those produced 
by the fuel assembly 30 except the point mentioned above. 
FIG. 22 shows a further embodiment of the invention. The fuel assembly 80 
of this embodiment employs the aforementioned fuel rods 31 to 34, 75 and 
G.sub.8 arranged in a manner shown in FIG. 22. This fuel assembly is 
similar to the fuel assembly 70 of the preceding embodiment except that 
some of the fuel rods 75 in the central region thereof are substituted by 
the fuel rods 34. The fuel assembly 80 of this embodiment exhibits a 
difference in the local power between the upper and lower regions which is 
reduced as compared with that in the fuel assembly 70 by an amount 
corresponding to the number of reduction of the fuel rods having 
difference of enrichment between their upper and lower regions. 
FIG. 23 shows a still further embodiment of the invention. The fuel 
assembly 90 of this embodiment employs fuel rods 41 to 44 and G.sub.6 
shown in FIG. 15 and fuel rods G.sub.9 shown in FIG. 24. These fuel rods 
are arranged in a manner shown in FIG. 23. The fuel rod G.sub.9 has a 
length smaller than that of the fuel rods 41 to 44 by amount corresponding 
to the length of natural uranium layer e.sub.5 provided in the fuel rods 
41 to 44. It is to be noted also that the fuel rod G.sub.9 has an 
upper-most region of a length within 3/24 of the fuel effective length 
(this equals to effective length of fuel rods 41 to 44). This uppermost 
region is charged with fuel pellets of low enrichment e.sub.4. Thus, the 
fuel rod G.sub.9 has three axial regions besides the lowermost region of 
natural uranium e.sub.5. This fuel assembly 90 has low enrichment at the 
upper ends of the fuel rods G.sub.9 so that the infinite multiplication 
factor in the cold state of the reactor can be suppressed effectively, so 
that a large reactor shut-down margin can be preserved. The axial 
distribution of enrichment in the fuel assembly 90 is created by providing 
a difference in the enrichment between the upper and lower regions of the 
fuel rods which do not contain gadolinea, so that the difference in the 
local power between different axial regions can be suppressed as in the 
case of the fuel assembly 40 explained before. 
FIG. 25 shows a still further embodiment of the invention. The fuel 
assembly 95 of this embodiment employs the fuel rods 41 to 44, G.sub.5 and 
G.sub.6 shown in FIG. 15 and the fuel rods 84 shown in FIG. 26. These fuel 
rods are arranged in a manner shown in FIG. 25. The fuel rod 84 has the 
total length of layer of natural uranium e.sub.5 greater than that in the 
fuel rods 41 to 44. In fact, the length of the layer of natural uranium in 
the fuel rod 84 reaches 1/6 of the fuel effective length of this fuel rod. 
The enrichment in the upper end of the fuel assembly and, hence, the 
reactor shut-down margin is increased also in this case. The difference in 
the local power distribution in the outer peripheral region between 
different cross-sections of the fuel assembly can be reduced provided that 
the fuel rods 84 are disposed in the portion of the cross-section of the 
fuel assembly other than the outer peripheral region and that the length 
of the layer of the natural uranium is less than 1/6 of the effective fuel 
length. Thus, the fuel assembly 95 of this embodiment produces 
substantially the same effect as those produced by the fuel assembly 40 
explained before. 
The increase in the reactor shut-down margin through a reduction of the 
enrichment in the upper end portion of the fuel assembly and the constant 
local power distribution in the outer peripheral portion of the fuel 
assembly over almost the entire axial region of the fuel assembly are 
attainable also by a fuel assembly 100 of a still further embodiment of 
the invention shown in FIG. 27. The fuel assembly 100 employs fuel rods 41 
to 44, G.sub.5 and G.sub.6 shown in FIG. 15 and fuel rods 85 shown in FIG. 
28. These fuel rods are arranged in a manner shown in FIG. 27. The fuel 
rod 84 is provided at its upper and lower ends with layers of natural 
uranium e.sub.5 each having an axial length of 1/24 of the fuel effective 
length thereof. In addition, the fuel rod 84 is provided with a region of 
a small enrichment e.sub.4 (e.sub.4 &lt;e.sub.3) which extends over a length 
of 1/8 of the fuel effective length downwardly from the lower end of the 
upper layer of the natural uranium e.sub.5. The axial enrichment 
distribution of the fuel rod 85 can be regarded as being materially 
constant, provided that the length of the region of reduced enrichment 
e.sub.4 is less than 1/8 of the fuel effective length. By arranging the 
fuel rods 85 in the central region of the cross-section of the fuel 
assembly, it is possible to increase the local power in the outer 
peripheral region of cross-sections of the fuel assembly as a mean and the 
difference in the local power distribution between different axial regions 
can be minimized as in the case of the fuel assembly 40. 
As has been described, according to the invention, it is possible to 
remarkably reduce the difference in the power distribution between 
different cross-sections taken at different positions along the axis of 
the fuel assembly, thereby attaining a higher discharged exposure of the 
fuel assembly and, hence, a higher fuel economy.