Fuel assembly for a boiling water nuclear reactor

A fuel assembly for a boiling water nuclear reactor contains a plurality of vertical fuel rods, which are arranged between a bottom tie plate and a top tie plate in a surrounding vertical casing part. At its lower end the casing part is connected to a bottom part which is provided with an inlet opening for water for conducting water in through the bottom tie plate, through the space between the fuel rods in the vertical casing part and out through the top tie plate. A debris catcher with a low flow resistance to water is provided below or above the bottom tie plate and at a distance from the bottom tie plate.

BACKGROUND OF THE INVENTION 
The present invention relates to a fuel assembly for a boiling water 
nuclear reactor, which fuel assembly comprises a plurality of vertical 
fuel rods arranged between a bottom tie plate and a top tie plate in a 
surrounding casing part, the lower end of which is connected to a bottom 
part which is provided with an inlet opening for water for conducting 
water in through the bottom tie plate, through the space between the fuel 
rods in the vertical casing part, and out through the top tie plate. 
Experience shows that, for example in connection with repair and service of 
a nuclear reactor, debris may enter, which debris then moves with the 
water which circulates through the reactor core. The debris may, for 
example, consist of metal chips (borings) formed in connection with repair 
of, for example, a steam separator, pieces of metal wire, or other foreign 
particles which have entered the system from outside. The debris may give 
rise to abrasion damage, which has serious consequences if the damage 
arises on parts which are particularly easily damaged, such as fuel rods. 
To avoid damage of the above-mentioned kind in pressurized water reactors, 
it is known to provide the bottom tie plates which already exist below the 
fuel rod bundle with a large number of holes to enable each bottom tie 
plate, in addition to each normal function, to function as a debris 
catcher in the form of a strainer and prevent debris from reaching the 
fuel rod bundle with control rod guide tubes and spacers. 
SUMMARY OF THE INVENTION 
According to the present invention, which relates to a fuel assembly for a 
boiling water reactor, a debris catcher is arranged as a separate new unit 
below or above the bottom tie plate and at a distance from the bottom tie 
plate. By arranging the debris catcher in this way, it may be given a very 
low flow resistance and hence constitute a minimum obstacle to the desired 
flow. An advantage, if the debris catcher is located below the bottom tie 
plate, is that the flow, because of the distance to the bottom tie plate, 
has time to even itself out if some part of the debris catcher should 
become clogged. Another advantage in the stated case is that it is 
possible in a simple way to supplement already existing fuel assemblies 
with debris catchers. 
According to a particularly advantageous embodiment of the invention, the 
debris catcher consists of a strainer, parallel to the bottom tie plate, 
preferably in the form of a strainer plate. If the strainer is arranged 
below the bottom tie plate, it has, in addition to the advantages 
mentioned above, the advantage of being capable of being inspected as well 
as being capable of being removed and cleaned without any problems. A 
strainer parallel to the bottom tie plate is preferably arranged with an 
edge zone in the form of a gap towards adjacent parts of the fuel 
assembly, preferably with lower relative flow resistance than in the 
strainer itself and provided with projections extending into the edge 
zone. 
If the strainer is arranged above the bottom tie plate, it is preferably 
formed with holes for end plugs at the bottom of the fuel rods, the holes 
being larger than the cross section of the end plugs so that internal 
zones preferably with lower relative flow resistance than in the rest of 
the strainer exist around the end plugs, and further provided with 
projections which extend into the internal zones. 
According to an additional embodiment, the debris catcher consists of a 
container which is arranged below the bottom tie plate and which has 
limiting walls, parallel to the bottom tie plate and provided with holes, 
and which contains a sparse stack of folded sheets. 
According to other embodiments, the debris catcher is formed as unit common 
to four fuel assemblies. Four fuel assemblies of the same kind are then 
arranged adjacent to each other around a common channel for water and the 
bottom part if formed as a vertical tube with an upper conical portion, 
which is connected to the outwardly-facing bottom portions of the casing 
parts of the fuel assemblies. According to such an embodiment, the debris 
catcher may consist of a funnel-shaped strainer, arranged in the conical 
portion of the bottom part, with the narrower part of the funnel directed 
downwards. In such an embodiment the strainer may cover the entire cross 
section of the tube. In another embodiment of this type the strainer may 
be provided with an opening in the narrower part which takes up a 
considerable part of the cross section of the tube. In this latter case, 
guide vanes for the water for centrifugal effect are suitably arranged in 
the tube to prevent debris from passing through the opening on the 
narrower part of the funnel-shaped strainer. According to another 
embodiment with a common debris catcher for a unit of the kind described 
composed of four fuel assemblies, the debris catcher comprises a number of 
concentric sheet metal cones, arranged in the conical portion of the 
bottom part and provided at the upper end with an outwardly-directed, 
bent-down edge for catching debris. 
The invention will be described in greater detail, by way of example, with 
reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIGS. 1-3, 1 designates a fuel channel with substantially square cross 
section. The fuel channel surrounds, with no significant play, an upper 
square portion of a bottom part 2 with a circular, downwardly-facing inlet 
opening 3 for cooling water and moderator water. In addition to supporting 
the fuel channel 1, the bottom part 2 also supports a supporting plate 4. 
At the lower part the fuel channel 1 has a relatively thick wall portion 
which is fixed to the bottom part 2 and the supporting plate 4 by means of 
a plurality of horizontal bolts, indicated by means of dash-dotted lines 
5. By means of a hollow support member 7 of cruciform cross section, the 
fuel channel 1 is divided into four vertical tubular parts 6 having at 
least substantially square cross section. The support member 7 is welded 
to the four walls 1a, 1b, 1c and 1d of the fuel channel 1 and has four 
hollow wings 8. The central channel formed by the support member is 
designated 32 and is connected at its lower part to an inlet tube 9 for 
moderator water. Each tubular part 6 contains a bundle 25 of twenty-five 
fuel rods 10. The rods are arranged in a symmetrical lattice in five rows 
each containing five rods. Each rod is included in two rows perpendicular 
to each other. Each bundle is arranged with a grid-like bottom tie plate 
11, a grid-like top tie plate 12 and a plurality of spacers 13. A fuel rod 
bundle 25 with a bottom tie plate 11, a top tie 11, a top tie plate 12, 
spacers 13 and a casing part 6 forms a unit which in this application is 
referred to as a fuel assembly, whereas the device comprising four such 
fuel assemblies shown in FIGS. 1-3 is referred to as a composed fuel 
assembly. In the composed fuel assembly the four bottom tie plates 11 are 
supported by the supporting plate 4 and are partially each inserted into a 
corresponding square hole 14 therein. In each fuel assembly at least one 
of the fuel rods is provided with relatively long, threaded end plugs 33 
and 34 of solid cladding material, the lower end plug 33 being passed 
through the bottom tie plate 11 and provided with a nut 15 and the upper 
end plug 34 being passed through the top tie plate 12 and provided with a 
nut 16. In the embodiment shown the centre rod 26 is formed in this way. 
This rod also serves as spacer holder rod. An upper end portion of the 
fuel channel 1 surrounds a cruciform lifting plate 17 with four horizontal 
arms 18, 19, 20 and 21, which extend from a common central portion. At the 
outer end each arm has an arrowhead-like portion 22, which in each 
respective corner of the fuel channel 1 makes contact with the inner wall 
surface of the fuel channel 1. A lifting handle 23 is fixed to the arms 20 
and 21. The lifting plate 17 and the handle 23 together form a steel 
lifting member cast in one piece. The lifting plate 17 is fixed to the 
support member 7 by inserting four vertical bars 28 into respective wings 
8 of the support member 7 and welding them thereto. At the upper end each 
bar 28 has a vertical, bolt-like portion 29 which is passed with a play 
through a corresponding hole in the mid-portion of the lifting plate 17 
and provided with a nut 30. As will be clear from the figures, the fuel 
channel 1 is provided with indentations 31, intermittently arranged in the 
longitudinal direction, against which the support member 7 is welded. 
In accordance with the present invention, a debris catcher 35 in the form 
of a strainer plate is arranged below each bottom tie plate 11 and at a 
distance of at least 0.5 cm from the bottom tie plate in the composed fuel 
assembly shown in FIG. 1. The debris catcher is fixed with a nut 36 to the 
same end plug 33 as that on which the nut 15 is applied. In the 
exemplified case, as will be clear from FIGS. 4 and 5, the strainer plate 
is formed with bars 37 into a grid. Each square in the example has a size 
of 3.times.3 mm. Around the strainer plate, between the same and the 
supporting plate 4, there is arranged an edge zone in the form of a gap 38 
with lower relative flow resistance than in the strainer plate itself. The 
strainer plate is provided with projections 39 extending into the edge 
zone. The strainer plate may be formed in many different ways, for example 
consist of sheet metal from which circular or elongated holes are punched 
out. It may also be formed in layers, suitably in the form of plates, with 
mutually displaced holes. Instead of a strainer plate, a strainer in the 
form of a wire netting may be used. 
In the case illustrated in FIGS. 6 and 7, a debris catcher in the form of a 
strainer plate 40 is arranged above the bottom tie plate 11. The holes 41 
in the strainer plate for the end plugs 33 at the lower part of the fuel 
rods 10 are larger than the cross section of the end plugs so that inner 
zones 42 with a lower relative flow resistance than in the rest of the 
strainer plate are present around the end plugs of the fuel rods. The 
strainer plate is also arranged with an edge zone 43 between the strainer 
plate and the casing part 1 with a lower relative flow resistance. Both in 
the inner zones 42 and in the edge zone 43, the strainer plate is provided 
with projections 44 and 45, respectively. Furthermore, the strainer plate 
is provided with punched-out smaller holes 46. The openings of the bottom 
tie plate for the cooling water are designated 47. The strainer plate may 
be secured to the fuel assembly, for example by being fixed to the bottom 
tie plate 11 with spacers and screws (not shown). 
FIG. 8 shows the lower part of a composed fuel assembly. The upper part 
(not shown) is of the same kind as the upper part of the fuel assembly 
shown in FIG. 1. According to FIG. 8, each debris catcher comprises a 
container 50 which is arranged below the bottom tie plate 11 and which has 
limiting walls 51 and 52, parallel to the bottom tie plate and provided 
with through-holes (not shown), and contains a sparse stack of folded 
sheets 53. The holes in the limiting walls are sufficiently large and the 
stack of sheets sufficiently sparse for the container to provide a low 
flow resistance to the water while simultaneously providing a good capture 
effect. 
FIG. 9 shows the lower part of a composed fuel assembly. The upper part 
(not shown) is of the same kind as the upper part of the fuel assembly 
shown in FIG. 1. According to FIG. 9, the bottom part 2 is formed as a 
vertical tube with an upper conical portion 2a. The debris catcher 
comprises a funnel-shaped strainer, arranged in the conical portion 2a of 
the bottom part, in the form of a strainer plate 55 with the narrower part 
56 of the funnel directed downwards and with an opening 57 in the narrower 
part which takes up a considerable part of the cross section of the tube 
2. Guide vanes 58 for the water, to provide centrifugal effect, are 
provided in the tube to prevent debris from passing through the opening in 
the narrower part of the funnel-shaped strainer plate. A pocket 59 for 
accumulated debris is arranged in the conical portion 2a. 
FIG. 10 also shows the lower part of a composed fuel assembly. The upper 
part (not shown) is of the same kind as the upper part of the fuel 
assembly shown in FIG. 1. Parts 2, 3 and 9 correspond to corresponding 
parts of the fuel assembly according to FIG. 9, and part 2a deviates only 
in that a pocket 59 is missing. The debris catcher comprises a number of 
concentric sheet metal cones 61-65, arranged in the conical portion 2a of 
the bottom part, which have alternately higher (61-63) and lower heights 
(64, 65). To catch debris, each cone has at its upper end an 
outwardly-directed, bent-down edge 61a-65a, which forms a narrow flow 
opening with adjacent sheet metal cones.