Nuclear reactor fuel assembly spacer

A spacer for a nuclear reactor fuel assembly includes sheet metal webs which intersect one another on edge and form grid meshes. A leaf spring projects from a sheet metal web in a grid mesh and is assigned to a bearing boss for the force-locking holding of a rod in the grid mesh. The leaf spring is attached to the sheet metal web at a leaf edge extending in the longitudinal direction of the rod. In order to avoid fretting in a nuclear reactor, a perforation is formed in the leaf spring and has a contour that narrows toward the sheet metal web to which the leaf spring is attached.

CROSS-REFERENCE TO RELATED APPLICATION 
This application is a Continuation of International Application Ser. No. 
PCT/DE95/01145, filed Aug. 28, 1995. 
BACKGROUND OF THE INVENTION 
Field of the Invention 
The invention relates to a spacer for a nuclear reactor fuel assembly, 
including sheet metal webs which intersect one another on edge and form 
grid meshes, a bearing boss on a sheet metal web in a grid mesh, and a 
leaf spring which projects from a sheet metal web and is assigned to the 
bearing boss for the force-locking holding of a rod, the leaf spring is 
attached to the sheet metal web in the grid mesh at a leaf edge extending 
in the longitudinal direction of that grid mesh. 
Such a spacer is known from Published European Patent Application 0 080 853 
A3, corresponding to U.S. Pat. No. 4,897,241. The contour of the leaf 
spring of that known spacer narrows toward that end of the leaf spring 
which projects into the grid mesh and through the use thereof the rod 
located in the grid mesh is held force-lockingly on the bearing boss. A 
force-locking connection is one which locks elements together by force 
external to the elements, as opposed to a form-locking connection which 
locks the elements together due to the shape of the elements themselves. 
The surface with which the leaf spring bears on the rod in the grid mesh 
is relatively small, so that there is the risk that, in the reactor core 
of a nuclear reactor through which a coolant flows, the leaf spring will 
experience fatigue very quickly and a lateral knocking of the rod located 
in the grid mesh against the leaf spring, which is so-called "fretting" 
will occur and cause damage to the rod. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a spacer for a 
nuclear reactor fuel assembly and a nuclear reactor fuel assembly having 
the spacer, which overcome the hereinafore-mentioned disadvantages of the 
heretofore-known devices of this general type and which avoid fretting. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a spacer for a nuclear reactor fuel 
assembly, comprising sheet metal webs intersecting one another on edge and 
forming grid meshes; a bearing boss disposed on one of the sheet metal 
webs in one of the grid meshes; and a leaf spring projecting from one of 
the sheet metal webs and being associated with the bearing boss for 
force-lockingly holding a rod, the leaf spring having a leaf edge attached 
to the sheet metal web in the grid mesh, the leaf edge extending in 
longitudinal direction of the grid mesh, and the leaf spring having a 
perforation formed therein with a contour narrowing toward the sheet metal 
web to which the leaf spring is attached. 
Due to the perforation in the leaf spring and the contour of the 
perforation narrowing toward the sheet metal web, a plasticization of the 
material of the leaf spring can be avoided under the operating conditions 
in a nuclear reactor, at least to such an extent that the leaf spring 
always automatically bears constantly on the rod in the grid mesh, even 
when the spacer is in use in a nuclear reactor for long periods of time, 
and consequently "fretting" is ruled out. 
In accordance with another feature of the invention, the leaf spring has an 
outer contour widening toward the sheet metal web to which the leaf spring 
is attached. Through the use of this advantageous development, the 
mechanical load on the leaf spring is kept optimally low at its leaf edge 
attached to the sheet metal web and the leaf spring consequently ages 
particularly slowly. 
In accordance with a further feature of the invention, the leaf spring has 
a rectangular outer contour. This structure provides the aforementioned 
advantage as well as the additional advantage of causing the leaf spring 
to bear on the rod located in the grid mesh over an optimally long length, 
thereby likewise suppressing fretting. 
In accordance with an added feature of the invention, the leaf spring is 
bent at a leaf edge projecting into the grid mesh to form a bending edge 
disposed between the leaf edge and the perforation, and the leaf spring is 
bent relative to the sheet metal web to which the leaf spring is attached. 
In accordance with an additional feature of the invention, the leaf spring 
has a resilient transverse strip disposed between the bending edge and the 
leaf edge projecting into the grid mesh. 
In accordance with yet another feature of the invention, the bearing boss 
is a sheet metal strip having an end edge projecting from the sheet metal 
web and having a spring constant higher than that of the leaf spring. This 
development brings about not only an optimally low flow resistance of the 
spacer in a nuclear reactor fuel assembly, through which a cooling liquid 
flows in the longitudinal direction in a nuclear reactor, but also a 
long-lasting play-free force-locking retention of a rod in the grid mesh. 
This likewise rules out fretting. 
In accordance with yet a further feature of the invention, the sheet metal 
strip forming the bearing boss has a transversely extending bending edge 
at which the sheet metal strip is bent relative to the sheet metal web 
from which the sheet metal strip forming the bearing boss projects. 
In accordance with yet a further feature of the invention, there is 
provided a resilient transverse strip disposed between the end edge of the 
sheet metal strip projecting from the sheet metal web and the bending 
edge. 
With the objects of the invention in view there is also provided a nuclear 
reactor fuel assembly, comprising the spacer as described above. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
spacer for a nuclear reactor fuel assembly and a nuclear reactor fuel 
assembly having the spacer, it is nevertheless not intended to be limited 
to the details shown, since various modifications and structural changes 
may be made therein without departing from the spirit of the invention and 
within the scope and range of equivalents of the claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the figures of the drawings in detail and first, 
particularly, to FIG. 1 thereof, there is seen a nuclear reactor fuel 
assembly for a boiling-water nuclear reactor, which has a fuel assembly 
head 2 and a fuel assembly foot 3. Furthermore, two fuel rods 4 filled 
with nuclear fuel can be seen. Each of the fuel rods 4 is firmly screwed 
at one end to the fuel assembly head 2 and at the other end to the fuel 
assembly foot 3. Each of the two fuel rods 4 is guided in each case 
through a mesh or opening in grid-shaped spacers 5 which are disposed at a 
distance from one another between the fuel assembly head 2 and the fuel 
assembly foot 3 and which are form-lockingly retained or firmly welded to 
the fuel rods 4 on the outside. 
Further fuel rods which are filled with nuclear fuel and of which only a 
single fuel rod 6 is shown in FIG. 1, are disposed parallel to one another 
and parallel to the fuel rods 4 between the fuel assembly head 2 and the 
fuel assembly foot 3. Each fuel rod 6 is guided in each case through a 
grid mesh of the grid-shaped spacer 5 and is retained force-lockingly in 
these grid meshes. Each fuel rod 6 stands loosely on the fuel assembly 
foot 3 and passes loosely through the fuel assembly head 2. 
As is shown in FIG. 2, a grid-shaped spacer 5 has grid meshes 7, each with 
a square cross section. The grid-shaped spacer 5 has sheet metal webs 8 
and 9 which intersect on edge and of which the sheet metal webs 8 of a 
first group are mutually parallel and disposed at right angles to the 
mutually parallel sheet metal webs 9 of a second group. 
In each case two leaf springs 11 project into each grid mesh 7 of the 
spacer 5. The leaf springs 11 are assigned in each case to two bearing 
bosses 12 and 18 in each grid mesh 7 for the force-locking holding of a 
rod, for example a fuel rod 4 or 6, that is not shown in FIG. 2. 
FIG. 3 shows a wall of a grid mesh 7 at a point on a sheet metal web 9 at 
which a leaf spring 11 is located on one side of this sheet metal web 9 in 
a grid mesh 7, and a bearing boss 12 is located on the other side of the 
sheet metal web 9 in another grid mesh 7, as seen in FIG. 2. 
As FIG. 3 further shows, the outer contour of the leaf spring 11 is a 
rectangle. The leaf spring 11 which is cut out from the sheet metal web 9 
is bent onto one side of the sheet metal web 9 into one of the grid meshes 
7 about a (long) leaf edge 13 extending in the longitudinal direction of 
the grid meshes 7. The leaf spring 11 thus projects from the sheet metal 
web 9. On the other side of the sheet metal web 9, a bearing boss 12 is 
bent into another adjacent grid mesh 7 about a bending edge which is 
located in the sheet metal web 9 and which extends in the longitudinal 
direction of the grid meshes 7. This bearing boss 12 is a sheet metal 
strip which is cut out from the sheet metal web 9 and which has a bending 
edge 14 that runs transversely, that is to say in the longitudinal 
direction of the grid meshes 7, and an end edge 20 which likewise runs in 
the longitudinal direction of the grid mesh 7 and with which the sheet 
metal strip projects from the sheet metal web 9. At the bending edge 14, 
the sheet metal strip is inclined relative to the sheet metal web 9, from 
which the bearing boss 12 projects. 
As FIG. 3 further shows, the leaf spring 11 also has a bending edge 15. 
Along this bending edge 15 which runs transversely relative to the leaf 
spring 11, that is to say parallel to the longitudinal direction of the 
grid meshes 7, the leaf spring 11 is inclined relative to the sheet metal 
web 9, to which it is attached and from which it projects. 
Moreover, the leaf spring 11 has a perforation 17 with the contour of an 
equal-angled trapezium, the mutually parallel bases of which are parallel 
to the longitudinal direction of the grid meshes 7. The contour of the 
perforation 17 narrows in the direction of the bending edge 13, at which 
the leaf spring 11 is attached to the sheet metal web 9. The smaller base 
of the equal-angled trapezium lies along this bending edge 13. The bending 
edge 15 of the leaf spring 11 is located between the perforation 17 and a 
(long) leaf edge 16 of the leaf spring 11. The leaf edge projects into one 
of the grid meshes 7. 
On one of the sheet metal webs 8 of the spacer grid 5 illustrated in FIG. 
2, two bearing bosses 18 are formed on one wall between two other grid 
meshes 7. 
As is shown in FIG. 4, each of these two bearing bosses 18 is composed of a 
sheet metal strip which is cut out from the sheet metal web 8 on one wall 
of a grid mesh 7 in each case and which is bent in each case into one of 
two adjacent grid meshes 7 at a bending edge 19 in the sheet metal 8. The 
bending edge is parallel to the longitudinal direction of the grid meshes 
7. Each of the two bearing bosses 18 has a bending edge 21 in the sheet 
metal strip, between the bending edge 19 and its end edge 20 projecting 
from the sheet metal web 8. The bending edge 21 is parallel to the 
longitudinal direction of the grid meshes 7 and therefore also parallel to 
the bending edges 19 and along the bending edge 21 the sheet metal strip 
of the bearing bosses 18 in each case is bent with the end edge 20 
relative to the sheet metal web 8, to which the sheet metal strip is 
attached. 
The leaf spring 11 according to FIG. 3 forms a bearing surface between the 
bending edge 15 and the end edge 16, for example for a fuel rod 6 that is 
filled with nuclear fuel and is located within the grid mesh 7, into which 
the leaf spring 11 projects. In a similar way, according to FIG. 4, the 
bearing bosses 18 form a bearing surface between the bending edge 21 and 
the end edge 20 which is likewise for a fuel rod that is disposed in the 
adjacent grid mesh 7, is filled with nuclear fuel and extends with its 
longitudinal axis in the longitudinal direction of the grid meshes 7. The 
spring constant of a leaf spring 11 and of a bearing boss 12 or 18 is 
determined by the quotient of a force engaging on the end edge 16 of the 
leaf spring 11 or the edge 20 of the bearing bosses 12 or 18 and the 
spring excursion caused by this force. The force and the spring excursion 
in each case are directed at right angles to that wall of the grid meshes 
to which the leaf spring or the bearing boss is attached. The smaller the 
area of the perforation 17 of a leaf spring 11, the harder this leaf 
spring 11 is, that is to say the higher its spring constant is. Thus, by 
selecting the size of the area of the perforation 17, the spring constant 
of the leaf spring 11 can be lower than the spring constant of the bearing 
boss 12 or 18 assigned to this leaf spring 11 in a grid mesh 7, so that a 
fuel rod disposed in this grid mesh 7 finds a leaf spring 11 which is more 
elastically resilient than the bearing boss 12 or 18 that is assigned to 
this leaf spring 11. It thereby becomes possible for a fuel rod 6 to be 
held force-lockingly at a fixed location in a grid mesh of a grid-shaped 
spacer 5, even in a reactor core in which a liquid coolant constantly 
flows in the longitudinal direction of the fuel rods through a nuclear 
reactor fuel assembly having this fuel rod 6 and this spacer 5. 
In a favorable way, as is shown in FIG. 3, a resilient transverse strip 23 
can be formed out of the leaf spring 11 through the use of two spaced 
transverse slits 30 between the bending edge 15 and the end edge 16 that 
projects from the sheet metal web 9 and is part of the leaf spring 11. In 
a similar way, as is shown in FIG. 4, such a resilient transverse strip 24 
can be formed between the end edge 20 and the bending edge 21 of the 
bearing bosses 18. In the same way, the bearing boss 12 according to FIG. 
3 can also have such a resilient transverse strip. These resilient 
transverse strips form a linear, elastic, flexible bearing point on a rod, 
for example a fuel rod 6, which is disposed in the grid meshes and onto 
which the strips are curved, so that "fretting" is avoided with an even 
greater degree of safety. 
The side surfaces both of the leaf springs and of the bearing bosses of the 
grid-shaped spacer extend in the longitudinal direction of the grid 
meshes. A liquid coolant also flows in this longitudinal direction through 
a nuclear reactor fuel assembly having such a spacer grid in the reactor 
core of a nuclear reactor. Accordingly, the leaf springs 11 and the 
bearing bosses 12 and 18 oppose only little flow resistance to this flow. 
FIG. 5, in which like parts are provided with the same reference symbols as 
in FIG. 3, shows a leaf spring 11 which widens toward the sheet metal web 
8 of a grid-shaped spacer, which is the sheet metal web to which this leaf 
spring 11 is attached. This leaf spring 11 has an equal-angled trapezium 
as an outer contour and a (long) leaf edge 13 in the longitudinal 
direction of the grid meshes. The leaf spring 11 is bent out of the sheet 
metal web 8 about the leaf edge 13 into a grid mesh.