Nuclear fuel assembly

The present invention relates to a fuel assembly with a substantially square cross section for a light-water reactor. The light-water reactor comprises a plurality of fuel rods extending between a top tie plate and a bottom tie plate. A fuel rod comprises a cladding tube which surrounds a column with fissionable material. According to one aspect of the invention, at least one fuel rod is provided with an axial gap in the fissionable material. Around this axial gap, the mass of the material is greater than the mass of the material in the main part of the cladding tube.

TECHNICAL FIELD 
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. 
BACKGROUND OF THE INVENTION 
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. 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 to 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 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 into 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 providing a flexible fuel assembly which in a 
simple manner may be given a shape where the upper part of the fuel 
assembly differs from the lower part so that optimum conditions may 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 different designs 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 so that each fuel assembly 
comprises a plurality of fuel units stacked on top of each other. As 
described above, each 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 fuel units 
which are of the order of magnitude of 300-1500 millimeters long is that 
fission gases are formed during nuclear fission. In addition, the column 
of fuel pellets expands because of the heat formed in the fuel pellets. To 
take care of the fission gases and the thermal expansion of the column of 
fuel pellets, normally a relatively large space, an axial gap, is formed 
above the uppermost fuel pellet in the cladding tube in known full-length 
fuel rods, that is, fuel rods of the order of size of 4 metres long. The 
axial gap is of the order of size of 200-300 millimeters long. To this 
axial gap, the fission gases may thus diffuse and the column of fuel 
pellets may expand inwardly here. Thus, the axial gap contains no 
fissionable material. 
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 because no fuel pellet is arranged 
in the axial gap. A problem which may arise as a result thereof is that 
hydrogen, formed, among other things, upon corrosion of the cladding tube, 
which is of a zirconium based alloy, and is absorbed thereby, will diffuse 
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 the cooling water. A 
tendency to the same type of problems may also appear in regions between 
the pellets, that is, where a lower end of a fuel pellet makes contact 
with an upper end of an adjacently located fuel pellet, and in the region 
between two fuel units stacked on top of each other. The risk of 
embrittlement because of too high a concentration of hydrogen increases, 
to a certain limit, with the size of the axial gap. 
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 reduced, whereby the release of fission gas is reduced and the axial 
gap may be reduced. In this case, however, an axial gap of the order of 
size of a few millimeters is needed in a rod with a length of the order of 
size of 300 millimeters, up to a few tens of a millimeter for longer rods, 
to allow the thermal expansion of the column of fuel pellets. 
The object of the present invention is to provide a fuel assembly for a 
light-water reactor with a plurality of short fuel units wherein at least 
one fuel assembly is designed with an axial gap and with means for taking 
care of hydrogen diffusing into this gap, thus preventing the build-up of 
impermissibly high concentrations of hydrogen in the material surrounding 
the axial gap. 
SUMMARY OF THE INVENTION 
The present invention relates to a fuel assembly comprising a fuel rod with 
an axial gap for accumulation of fission gases, formed during operation, 
and thermal expansion of the nuclear fuel. 
The fuel rod comprises a cladding tube and a stack of fuel pellets arranged 
therein, a column with extruded fuel cylinders or an unbroken column of 
vibration-compacted fuel powder. The cladding tube is sealed by a plug at 
either end, more particularly with a top plug and a bottom plug, 
respectively. At the space without fissionable material, the fuel rod is 
provided with a larger material thickness than the remaining part of the 
fuel rod. In an advantageous embodiment of the invention, this material 
thickness is achieved in the top plug. The top plug is then provided with 
a cavity facing the uppermost fuel pellet arranged in the cladding tube. 
The top plug is preferably connected to the cladding tube in such a way 
that its inner diameter corresponds to the inner diameter of the cladding 
tube and is thickened radially outwardly. In this way, the column of fuel 
pellets is allowed to expand upwardly and into the top plug because of 
thermal expansion. 
By forming the fuel rod around the axial gap with a larger thickness, the 
hydrogen-absorbing capacity is increased here. By increased hydrogen 
absorption, the hydrogen concentration is reduced in the space to a 
corresponding extent and hence also the risk of embrittlement in the upper 
part of the fuel rod. 
The advantage of the invention is that the risk of embrittlement due to too 
high hydrogen concentration in the axial gap for thermal expansion and 
fission gases is reduced by an increase of the quantity of material and 
hence the hydrogen-absorbing capacity in this region. 
The increased quantity of material in the upper part of the fuel rod also 
entails improved mechanical properties of the upper part in question since 
it reduces the influence of the stresses in the material which arise 
because of the fission gases collected in this space. 
Another advantage of the invention is that the increased quantity of 
material in the upper part of the fuel rod entails an improved resistance 
to damage to the cladding tube in this region caused by abrasion and 
corrosion and/or erosion. In connection with repair and service of a 
nuclear reactor, foreign matter (or debris), such as metal chips, may 
enter the coolant. The debris is then transferred with the coolant when 
this is circulated through the core. Such debris may in certain cases give 
rise to abrasion damage on the cladding tube. The abrasion damage may in 
certain cases arise on a level with the top tie plate as a result of the 
debris adhering thereto and remaining there. When it is brought into 
vibration by the coolant which flows past, it wears on the cladding tubes 
which surround the fuel pellets. In the worst case, the abrasion damage 
may lead to fissionable material entering the cooling water. Still another 
advantage is that mixing vanes may be arranged in a simple manner at the 
upper end of the fuel rod.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a fuel assembly according to the invention. The fuel assembly 
comprises an upper handle 1, a lower end portion 2 and a plurality of fuel 
units 3 stacked one above the other. Each fuel unit 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 
is facing the bottom tie plate 6 in the next fuel unit in the stack and so 
that the fuel rods 4 in all the fuel units 3 are parallel to one another. 
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 up through the fuel assembly. 
FIG. 2 shows that a 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 the 
support member surrounds 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 
comprises 24 fuel rods 4 arranged in a symmetrical 5.times.5 lattice. 
The fuel assembly in FIG. 2 comprises 10.times.10 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. 
FIG. 2a shows an alternative embodiment of a fuel assembly of the same type 
as that shown in FIG. 1. The fuel assembly is provided with an internal 
vertical channel 14a through which water is conducted 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 threaded onto the tube which surrounds the vertical 
channel 14a. 
FIG. 2b shows an additional embodiment of a fuel assembly of the same type 
as that shown in FIG. 1. The fuel assembly is provided with two central 
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. 
FIG. 3 shows a pressurized-water fuel assembly. In the same way as the fuel 
assembly in FIG. 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 faces 
the bottom tie plate 6 in the next fuel unit 3 in the stack, and so 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 4b extend 
between a top part 15a and a bottom part 16a. The top part 15a is arranged 
above the uppermost fuel unit 3 in the fuel assembly and the bottom part 
16a is arranged below the lowermost fuel unit 3 in the fuel assembly. 
FIG. 4 shows a fuel rod 4 for a fuel assembly according to FIG. 1, 2, 2a or 
2b. The fuel rod 4 comprises, as mentioned above, a cladding tube 7a and a 
stack of fuel pellets 7b arranged therein. The cladding tube 7a is sealed 
at the top by a top plug 15 and at the bottom by a bottom plug 16. In FIG. 
4 a top plug 15 is shown which is adapted to partially surround part of 
the column with fuel pellets 7b. The inner diameter of the top plug 15 
corresponds to the inner diameter of the cladding tube 7a so as to obtain 
an even inner side of the fuel rod 4. The top plug 15 is formed with an 
internal cavity, an axial gap 15a, in which fission gases may accumulate. 
The axial gap 15a is also intended to allow thermal expansion of the 
column of fuel pellets 7a. 
The fuel rod 4 has a material thickness which around the axial gap 15a is 
larger than in the rest of the fuel rod 4 (see reference numeral 15b). In 
FIG. 4, the thicker material 15b around the axial gap 15a is achieved in 
the top plug 15 which, is thickened radially outwardly. In an alternative 
embodiment, the cladding tube 7a may be provide with a larger material 
thickness 15b in the region which is intended to surround an axial gap 
15a. 
To reduce the release of fission gases, the fuel pellets 7b, as shown in 
FIG. 4, may be formed with a through-hole 17. In this way, the maximum 
temperature arising in the central part of the fuel pellets 7a, and hence 
the release of fission gases, is reduced. By providing the fuel pellets 7b 
with holes 17, a fission gap space, distributed in the axial direction, is 
also achieved, whereby the axial gap 15a in the upper end of the rod 4 may 
be reduced to a corresponding extent. The axial extent of the axial gap 
15a is dependent on the length of the fuel rod 4. As an example may be 
mentioned that, for a fuel rod 4 which has a length of the order of 
magnitude of 300 millimeters, the axial extent of the axial gap 15a is of 
the order of magnitude of 2-5 millimeters. 
Further, the fuel pellets 7b are formed with cup-shaped upper and lower end 
surfaces (see reference numeral 18). 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 18 thus allows thermal expansion to a certain 
extent before t he axial gap 15a is utilized for this purpose. 
It is important to form the thicker material 15b around the axial gap 15 
such that it gives rise to as small a pressure drop as possible. The fuel 
rod 4 therefore exhibits a smooth transfer between the outer diameter of 
the cladding tube 7a and the largest diameter of the top plug 15. For the 
same reason, the upper par t of the top plug 15 is provided with rounded 
corners. 
FIG. 5 shows an alternative embodiment of the outer surface of the top plug 
15. This embodiment is intended to give rise to a lower pressure drop in 
comparison with the embodiment shown in FIG. 4. The transition (see 
reference numeral 15c) between the smaller and larger diameters of the 
fuel rod 4 is here made longer than that shown in FIG. 4. In the same way, 
the upper part of the reference numeral 15d is provided with a more 
elongated transition. 
FIGS. 6a and 6b show an embodiment of the top plug 15 where the plug has 
been provided with mixing vanes 19. The mixing vanes 19 are formed as bars 
extending from the outer surface of the top plug 15 in a direction across 
the flow direction of the coolant and in a direction parallel to the top 
plug 15. The mixing vanes 19 may be one or more in number. In an 
advantageous embodiment of the invention, four mixing vanes 19 are 
arranged evenly distributed along the outer surface of the top plug 15. In 
FIGS. 6a and 6b, the mixing vanes 19 are shown straight and with an 
inclination. The inclination may be chosen to be an angle .alpha. in 
relation to the center axis C of the top plug 15 which is of the order of 
magnitude of 15.degree.-40.degree.. Alternatively, the mixing vanes 19 may 
be formed with a curved shape to further increase the mixing of the 
coolant and hence its cooling capacity. Through the otherwise coil-shaped 
appearance of the top plug 15, the velocity of the coolant is increased 
upon passage thereof, thus increasing the power of the mixing vanes 19. 
In FIG. 7 an embodiment is shown in which the top plug 15 is provided with 
flanges 20. The flanges 20 are formed so as to extend out from the outer 
surface of the top plug 15 in a direction across the flow direction of the 
coolant and in a direction parallel to the center axis C of the top plug 
15. This embodiment of the top plug 15 is particularly suitable to arrange 
in the lower part of the fuel assembly where the need of mixing of the 
coolant is smaller than in the upper part of the fuel assembly. The object 
of the flanges 20 is to increase the hydrogen-absorbing ability of the 
material so that the remainder of the top plug 15 may be given a smaller 
material thickness. With this embodiment, the hydrogen-absorbing quantity 
of material may be increased, resulting in a small pressure drop. At the 
same time, the material thickness in the rest of the top plug may be 
reduced to a corresponding extent. 
The mixing vanes 19 and the flanges 20 also provide a larger surface 
transmitting heat to the coolant. 
FIG. 8 shows in a view from above, a top tie plate 21 for retaining the 
upper ends of the fuel rods 4. The top tie plate 21 comprises a plurality 
of flow openings 22 for passage of the flow flowing upwards through the 
fuel assembly. Further, top tie plate 21 comprises openings 23 for 
receiving and positioning pins 24 arranged in the upper part of the top 
plug 15. 
In an advantageous embodiment, the mixing vanes 19 are adapted so as to be 
completely or partially covered by the surfaces of the top tie plate 21 
which are formed across the flow direction between the flow openings 22. 
In this way, the pressure loss caused by the mixing vanes 19 and the 
flanges 20, respectively, is limited.