The present invention relates to a nuclear fuel assembly with a substantially
square cross section for a light water reactor comprising a plurality of fuel rods
extending between a top tie plate and a bottom tie plate.
In a nuclear reactor, moderated by means of light water, the fuel
exists in the form of fuel rods, each of which contains a stack of pellets of a
nuclear fuel arranged in a cladding tube, a column of extruded fuel cylinders or
an uninterrupted column of vibration-compacted powdered fuel. The cladding tube
is normally made of a zirconium-base alloy. A fuel bundle comprises a plurality
of fuel rods arranged in parallel with each other in a certain definite, normally
symmetrical pattern, a so-called lattice. The fuel rods are retained at the top
by a top tie plate and at the bottom by a bottom tie plate. To keep the fuel rods
at a distance from each other and prevent them from bending or vibrating when
the reactor is in operation, a plurality of spacers are distributed along the fuel
bundle in the longitudinal direction. A fuel assembly comprises one or more fuel
bundles, each one extending along the main part of the length of the fuel assembly.
Together with a plurality of other fuel assemblies, the fuel assembly
is arranged in a core. The core is immersed in water which serves both as coolant
and as neutron moderator. During operation, the water flows from below and upwards
through the fuel assembly, whereby, in a boiling water light-water reactor, part
of the water is transformed into steam. The percentage of steam increases towards
the top of the fuel assembly. Consequently, the coolant in the lower part of the
fuel assembly consists of water whereas the coolant in the upper part of the fuel
assembly consists both of steam and of water. This difference between the upper
and lower parts gives rise to special problems which must be taken into consideration
when designing the fuel assembly.
This problem can be solved by achieving a flexible fuel assembly
which, in a simple manner, may be given a shape in which the upper part of the
fuel assembly differs from the lower part thereof such that optimum conditions
can be obtained. A fuel assembly for a boiling water reactor with these properties
is shown in PCT/SE95/01478 (Int. Publ. No. WO 96/20483). This fuel assembly comprises
a plurality of fuel units stacked on top of each other, each comprising a plurality
of fuel rods extending between a top tie plate and a bottom tie plate. The fuel
units are surrounded by a common fuel channel with a substantially square cross
section. A fuel assembly of this type may in a simple manner be given a different
design in its upper and lower parts.
Also in a light-water reactor of pressurized-water type, it may be
desirable to design the fuel assemblies such that each fuel assembly comprises
a plurality of fuel units stacked on top of each other. As described above, each
one of the fuel units then comprises a plurality of fuel rods extending between
a top nozzle and a bottom nozzle. A fuel assembly for a pressurized-water reactor,
however, comprises no fuel channel.
One factor which must be taken into consideration when designing
such fuel units with a length of the order of size of 300-1500 millimeters is that
fission gases are formed during nuclear fission. In addition, the column of fuel
pellets expands because of the heat generated in the fuel pellets. To take care
of the fission gases and the thermal expansion of the column of fuel pellets, a
relatively large space, an axial gap, is normally formed above the uppermost fuel
pellet in the cladding tube in known full-length fuel rods, that is, fuel rods
with a length of the order of size of 4 metres. The axial gap has a length of the
order of size of 200-300. The fission gases may thus diffuse to this axial. gap
and the column of fuel pellets may expand into this gap.
Another factor which must be taken into consideration when designing
axial gaps is that the temperature of the cladding tube in this region is lower
than in the rest of the cladding tube since no fuel pellet is arranged in the axial
gap. A problem which may arise as a result of this is that hydrogen formed, inter
alia, by corrosion of the cladding tube, which is of a zirconium-based alloy, and
is taken up thereby, diffuses into this colder region. In the event that the concentration
of hydrogen becomes too high in this region, hydrides are formed in the cladding
material and cause embrittlement thereof. In a serious case, the cladding tube
may burst and fissionable material may enter into the cooling water. The same type
of problem may also arise in the regions between the pellets, that is, where a
lower end of a fuel pellet makes contact with an upper end of an adjacent fuel
pellet, and in the region between two fuel' units stacked on top of each other.
The risk of embrittlement due to too high a concentration of hydrogen increases,
to a certain limit, with the size of the axial gap.
Released fission gas contributes to the temperature in the axial
gap decreasing further. This is due to the fission gas deteriorating the thermal
conductivity of the gas which is present in the axial gap. The same thing applies
to the gas which is present in the gap between the fuel pellets and the cladding
tube, in which case the difference in temperature between the outer surface of
the pellets and the inner surface of the cladding tube increases.
It is known to reduce the release of fission gas in different ways.
One such way is to provide one or more of the fuel pellets with through-holes in
their axial directions. In this way, the temperature in the fuel pellet is lowered
whereby the release of fission gas is reduced and the axial gap may be reduced.
In this case, the axial gap may be limited to the order of size of a few millimetres
in a rod with a length of the order of size of 300 millimetres, up to a few tens
of millimetres for longer rods, to allow the thermal expansion of the column of
fuel pellets. A disadvantage of pellets provided with through-holes is that they
are complicated to manufacture. For that reason, it is desirable to arrange axial
gaps in the fissionable material.
Still another factor which must be taken into consideration when
designing axial gaps in a fuel rod is that local power peaks arise here. The power
peaks arise due to the moderation in this region, where fissionable and neutron-absorbing
material are missing, being very good. This results in the power in the pellets
adjoining the axial gap becoming very high, that is, a power peak arises. The power
peak grows with the size of the axial gap.
The object of the present invention is to provide a fuel assembly
with a plurality of short fuel units with fuel rods formed with axial gaps in the
fissionable material adapted to give rise to small power peaks only.
SUMMARY OF THE INVENTION
The present invention relates to a fuel assembly comprising a plurality
of fuel rods, each having at least one axial gap for fission gases, formed during
operation, and thermal expansion of the nuclear fuel. The features which characterize
this fuel assembly are stated in claim 1.
The fuel rod comprises a cladding and a stack of nuclear fuel pellets
arranged therein. The cladding tube is sealed with a plug at each end, more particularly
with a top plug and a bottom plug. The axial gaps in the fuel rods are arranged
such that, in adjacently arranged fuel rods, they are disposed at axially separated
levels. By avoiding to arrange axial gaps at the same levels in adjacently arranged
fuel rods, the risk of high power peaks is reduced as a consequence of the good
moderation in this region.
To further reduce the power peaks at the axial gaps, in one embodiment
of the invention these gaps are distributed at a plurality of levels within one
and the same fuel rod. In this way, each one of the axial gaps may be made considerably
smaller than if only one gap is arranged in the fuel rod.
To achieve the axial gaps at the desired level in the fuel rod, a
spacer is arranged in the axial gap or gaps. The spacer is designed deformable
in the axial direction. In this way, the column of fuel pellets is allowed, because
of thermal expansion, to be extended into the axial gap or gaps while the spacer
is being deformed. When the spacer has been deformed in the axial direction, it
prevents, by friction against the wall of the cladding tube, axial gaps from arising
in the upper part of the fuel rod also when the fuel pellets decrease in size because
of densification. Alternatively, the spacer may be designed resilient, for example
in the form of a spiral spring with the same function as described above.
By not arranging the axial gaps in traditional manner, that is, above
or below the column with the fissionable material in the fuel rods, the power peaks
between two fuel units stacked on top of each other are reduced. The axial gaps
are achieved by arranging a spacer at an arbitrary level in the column of fissionable
material. To further reduce the power peaks in the upper and lower ends, respectively,
of the fuel rods, that is, between two fuel units stacked on top of each other,
the fuel pellets in these regions may be designed with a smaller diameter than
the other fuel pellets. To avoid annular gaps between the fuel pellet and the cladding
tube, that part of the fuel rod which surrounds the fuel pellet and the cladding
tube is designed with a correspondingly smaller inner diameter which has the same
extent in the axial direction as the fuel pellet. Alternatively, the fuel pellets
in this region may be given a lower enrichment.
The advantage of the invention is that axial gaps comprising the
spacers which may be placed in optional positions are avoided in the upper parts
of the fuel rods. The region without fissionable material formed between two fuel
units stacked on top of each other is thus reduced and hence also the local power
peak which may arise in this region due to too good moderation.
Another advantage is that the necessary axial gap, by means of the
spacers which may be located in optional positions, may be divided into a plurality
of smaller axial gaps whereby the power peaks therein are reduced. At the same
time, the risk of too high a concentration of hydrogen in the axial gaps is reduced.
At least to a certain extent, the spacer contributes to increase
the temperature somewhat in the material surrounding the axial gap in comparison
with the temperature of axial gaps without spacers. The increased temperature is
due to the spacer conducting part of the heat, which is generated in the pellets
facing the axial gap, to the cladding tube. By this increased temperature, the
risk of the hydrogen concentration becoming too high in the axial gaps is further
Still another advantage is that the spacer, even at the time of manufacture
of the fuel rods, may accumulate a certain length tolerance of the pellets column.
This means that the requirement for the length tolerance of the individual fuel
pellets is reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
DESCRIPTION OF THE PREFERRED EMBODIMENTS
- Figure 1 shows in a vertical section a fuel assembly of a boiling water type
with short fuel units.
- Figure 2 shows a section A-A of the fuel assembly in Figure 1.
- Figures 2a and 2b show alternative embodiments of a fuel assembly of the same
type as that shown in Figure 1 in a section corresponding to the section A-A of
the fuel assembly in Figure 1.
- Figure 3 shows in a vertical section a fuel assembly of pressurized-water type
with short fuel units.
- Figure 4 shows a fuel rod for a fuel unit according to Figure 1 or 2 with a
spacer arranged in an axial gap.
- Figure 5a shows two adjacently located fuel rods, each with an axial gap, wherein
the axial gaps are arranged at axially separate levels.
- Figure 5b shows two adjacently located fuel rods, each with two axial gaps,
wherein all the axial gaps are arranged at axially separate levels.
- Figure 6a shows a fuel rod with a plurality of axial gaps distributed along
their axial length.
- Figure 6b shows a detail of Figure 6a, wherein a spacer is arranged in an axial
- Figure 7a shows a spacer in a view from the side.
- Figure 7b shows the spacer according to Figure 7a in a view from above.
- Figure 8 shows a fuel rod with an upper and a lower end pellet with a smaller
diameter than the other fuel pellets and a top plug and a bottom plug, respectively,
which are intended to surround the end pellets plugs.
Figure 1 shows a fuel assembly of a boiling water type comprising
an upper handle 1, a lower end portion 2 and a plurality of fuel units 3 stacked
one above the other. Each fuel unit 3 comprises a plurality of fuel rods 4 arranged
in parallel and in spaced relationship to each other in a given lattice. Further,
each fuel unit 3 comprises a top tie plate 5 and a bottom tie plate 6 for attachment
of the fuel rods 4 in their respective positions in the lattice. The fuel units
3 are stacked on top of each other in the longitudinal direction of the fuel assembly
and they are stacked in such a way that the top tie plate 5 in one fuel unit 3
is facing the bottom tie plate 6 in the next fuel unit 3 in the stack and such
that the fuel rods 4 in all the fuel units 3 are parallel to one another. A fuel
rod 4 contains fuel in the form of a stack of fuel pellets 7b of uranium arranged
in a cladding tube 7a. The cladding tube 7a is suitably made of a zirconium-base
alloy or an alloy which, in addition to zirconium, comprises niobium, iron, tin
and chromium. A coolant is adapted to flow from below and up through the fuel
Figure 2 shows that the fuel assembly is enclosed in a fuel channel
8 with a substantially square cross section. The fuel channel 8 is provided with
a hollow support member 9 of cruciform cross section, which is secured to the four
walls of the fuel channel 8. In the central channel 14 formed of the support member
9, moderator water flows. The fuel channel with support members surround four vertical
channel-formed parts 10, so-called sub-channels, with an at least substantially
square cross section. The four sub-channels each comprises a stack of fuel units
3. Each fuel unit 3 comprises 24 fuel rods 4 arranged in a symmetrical 5x5 lattice.
The fuel assembly in Figure 2 comprises 10x10 fuel rod positions.
By a fuel rod position is meant a position in the lattice. All the fuel rod positions
in the lattice need not be occupied by fuel rods 4. In certain fuel assemblies,
a number of fuel rods 4 are replaced by one or a plurality of water channels.
The introduction of a water channel changes the number of fuel rods 4 but not the
number of fuel rod positions.
Figure 2a shows an alternative embodiment of a fuel assembly according
to the invention. Figure 2a shows a horizontal section through the fuel assembly
which is provided with an internally arranged vertical channel 14a through which
water is conducted in a vertical direction from below and upwards through the
fuel assembly. The channel 14a is surrounded by a tube 9a with a substantially
square cross section. The fuel units 3 are kept in position by being fitted onto
the tube which surrounds the vertical channel 14a.
Figure 2b shows an additional embodiment of a fuel assembly according
to the invention. The figure shows a horizontal section through the fuel assembly
which is provided with two centrally arranged vertical water rods 14b through which
water is conducted from below and upwards through the fuel assembly. The water
rods 14b have a diameter which is somewhat larger than the diameter of the fuel
rods 4 and are formed with a substantially circular cross section. The fuel units
3 are kept in position by being fitted onto the water rods 14b.
Figure 3 shows a pressurized-water fuel assembly of square cross
section. In the same way as the fuel assembly in Figure 1, it comprises a plurality
of fuel units 3 stacked on top of each other. Each fuel unit 3 comprises a plurality
of fuel rods 4 arranged in parallel and in spaced relationship to each other in
a given lattice. Each fuel unit 3 further comprises a top tie plate 5 and a bottom
tie plate 6 for attachment of the fuel rods 4 in their respective positions in
the lattice. The fuel units 3 are stacked on top of each other in the longitudinal
direction of the fuel assembly and they are stacked in such a way that the top
tie plate 5 in one fuel unit 3 is facing the bottom tie plate 6 in the next fuel
unit 3 in the stack, and such that the fuel rods 4 in all the fuel elements 3 are
parallel to each other. A fuel rod 4 contains fissionable material in the form
of a stack of fuel pellets 7b of uranium arranged in a cladding tube 7a. A coolant
is adapted to flow from below and upwards through the fuel assembly. A number of
so-called control rod guide tubes 4b are arranged extending through the whole
fuel assembly. The control rod guide tubes 4b are intended to receive finger-shaped
control rods (not shown) which are inserted into and withdrawn from, respectively,
the guide tubes 4b for the purpose of controlling the power of the nuclear reactor.
The guide tubes extend between a top part 15 and a bottom part 16. The top part
15 is arranged above the uppermost fuel unit 3 in the fuel assembly and the bottom
part 16 is arranged below the lowermost fuel unit 3 in the fuel assembly. The fuel
units 3 are kept in position by being fitted onto the control rod guide tubes
Figure 4 shows a fuel rod 4 for a fuel assembly according to Figure
1 or Figure 3. The fuel rod 4 comprises, as mentioned above, a cladding tube 7a
and a stack of fuel pellets 7b arranged in the cladding tube. At the top, the cladding
tube 7a is sealed with a top plug 17 and at the bottom with a bottom plug 18.
The fuel rod 4 is formed with an inner cavity, an axial gap 19, in which fission
gases may accumulate. The axial gap 19 is also intended to permit thermal expansion
of the column of fuel pellets 7b.
A spacer 20 made of a zirconium-base alloy is arranged in the column
of fuel pellets 7b to achieve the axial gap 19 at the desired level in the fuel
rod. The axial gap 19 is arranged such that at least one fuel pellet 7b is arranged
between the axial gap and either the top plug 17 or the bottom plug 18 of the
fuel rod 4. The spacer 20 is formed as a sleeve with V-shaped skits 21 arranged
in the respective ends. The outer parts of the tongues 22 formed between the slits
are bent in towards the centre of the spacer 20 at an angle of the order of magnitude
of 100°. The spacer 20 is adapted to make contact, by its upper end, with a lower
end of a fuel pellet 7b and, by its lower end, to make contact with an upper end
of a fuel pellet 7b. This design of the spacer 20 permits the spacer to be deformed
in the axial direction when the fuel pellets 7b because of thermal expansion grow
in the axial direction. When the spacer 20 is deformed, it will make contact with
the inner surface of the cladding tube 7a. This means that the pellets column across
such a spacer 20, because of its friction against the cladding tube, also when
the pellets 7b shrink due to densification, is retained in its position. In this
way, axial gaps 19 are prevented from forming between the top plug 17 and the
fuel pellet 7b arranged at the top of the column.
The spacer 20 may, of course, be formed in many different ways. It
may, for example, be provided with an edge, folded towards the centre, without
slits 21. Alternatively, it may be formed as a spiral spring. It may also be suitable
to arrange different types of spacers 20 in different parts of the fuel rod 4,
for example non-deformable spacers 20 in certain axial gaps 15a.
In Figure 4, it is indicated that the pellet 7b arranged at the top
and bottom of the fuel rod 4, as well as the pellets 7b arranged adjacent the spacer
20, are made with through-holes 23. With this embodiment, the maximum temperature
in the fuel pellets 7b may be reduced in the region where power peaks due to good
moderation arise. At the same time, the amount of released fission gas may be reduced
and space for accumulation of released fission gases be created in the pellets
7b. Further, the fuel pellets 7b are provided with cupped upper and lower end surfaces
(see reference numeral 24). Because of the thermal expansion, the fuel pellets
7b grow more in the central, warmer parts than in the outer, colder parts. The
cup shape 24 thus permits thermal expansion to a certain extent before the axial
gap 19 is utilized for this purpose. Because of the hollowed 23 and cup-shaped
24 pellets 7b, a smaller axial gap 19 is sufficient for the thermal expansion
and for accumulation of the released fission gases.
Alternatively, fuel pellets 7b with lower enrichment may be used
adjacent the spacers 20. This has, in principle, the same effect as hollowed pellets
when it comes to limiting power peaks, however, not with regard to reducing the
power at the centre of the fissionable material or accumulating fission gases.
Figure 5a shows two fuel rods 4 arranged adjacent to each other,
each with an axial gap 19. The axial gaps 19 in the two adjacently arranged fuel
rods 4 are arranged at axially separate levels. Arranging axial gaps 19 at axially
separate levels in adjacently located fuel rods 4 results in an equalization of
the power along the fuel rod 4 and a reduced risk of high power peaks as a result
of too good moderation in these regions which lack fissionable material 7b.
In an alternative embodiment, an axial gap 19 is arranged at random
in the fuel rod 4 during the manufacture. It is then suitable to determine in advance
a region within which the location of the axial gap 19 may be varied. The random
location of the axial gap 19 may, for example, be achieved with the aid of a conventional
random number generator.
Figure 5b shows an alternative embodiment of the fuel rod 4 according
to Figure 5a. The axial gap 19 is here divided into two smaller axial gaps 19a
in each fuel rod 4. The axial gaps 19a are arranged at different axial levels in
the respective fuel rods 4.
The fuel rods 4 in Figure 5a and Figure 5b are designed preferably
identical, but when putting these together into a bundle for a fuel assembly, every
other fuel rod 4 is placed upside down.
Figure 6a shows another alternative embodiment of the fuel rod 4.
In this fuel rod 4, the axial gap 19 is divided into four smaller axial gaps 19b
arranged at axially separate levels. In this case, fuel pellets 7b without through-holes
23 may be used. In Figure 6b, a spacer 20 is shown which is adapted to such a
short axial gap 19b.
A fuel unit which has a length of the order of size of 400 millimetres
is provided with a gap which is 20-30 millimetres, alternatively two gaps which
are each of the order of size of 10 millimetres, etc.
Figures 7a and 7b show an alternative embodiment of the spacer 20.
This spacer 20 is formed by punching a sheet and forming the sheet into a sleeve
and folding the tongues 22 inwards towards the centre of the sleeve. The dash-lined
slit 25 shown in the figure indicates where the ends of the sheet meet. By forming
the slit 25 with a hook 25a, the spacer 20 may be given a stable design in the
axial direction. This spacer 20 is simple to manufacture since it is punched out
in a piece of sheet, whereafter it is formed into a sleeve.
Figure 8 shows an embodiment of a fuel rod 4 with an upper and a
lower end pellet 7c with a smaller diameter than that of the other fuel pellets
7b. Because of this arrangement, the power peaks at the axial gap between fissionable
material 7b which is formed between two fuel units stacked on top of each other
may be reduced (see reference numeral 19c in Figures 1 and 3, respectively). To
prevent gaps from arising between the fuel pellet 7c and the top plug 17 and the
bottom plug 18, respectively, the material surrounding the end pellet 7c is made
with a correspondingly smaller inner diameter. In Figure 8, the top plug 17 and
the bottom plug 18, respectively, are provided with a larger thickness of material
in relation to the cladding tube 7a. The material around the end pellet 7c may,
of course, be provided with a correspondingly smaller inner diameter in some other
manner than with the aid of the top plug 17 and the bottom plug 18, respectively;
for example, the cladding tube 7a itself may be designed in this way.