Bottom grid mounted debris trap for a fuel assembly

A trap for catching debris carried by coolant flowing from the bottom nozzle of a fuel assembly to the bottom or lowermost one of the grids of the assembly which support the fuel rods in an organized array therein includes a structure disposed between the bottom nozzle and the bottom grid and generally aligned with the lower end plugs of the fuel rods. The structure forms a multiplicity of hollow cells each being open at opposite ends and defining a central cavity which receives one of the fuel rod lower end plugs while providing for passage of coolant flow therethrough from the bottom nozzle to the bottom grid. Also, the trap includes means in the form of dimples defined in each of the cells for catching debris carried into the cells by the coolant flowing therethrough. The dimples are formed from the wall portions defining the cells and extend into the cavities of the cells. Each dimple has a configuration generally arched outwardly from the wall portion which is oriented in alignment with the direction of coolant flow through the cell. The distance between those dimples formed on oppositely-disposed ones of the wall portions is slightly greater than the diameter of the fuel rod lower end plug disposed in the cavity of each cell. Additionally, the debris trap further includes structure support means in the form of a plurality of hanger straps attached at their lower ends to the structure adjacent the corners thereof and at their upper ends to the bottom grid so as to locate the structure in the spaced relationship below the bottom grid and support the structure adjacent the corners thereof from the bottom grid.

CROSS REFERENCE TO RELATED APPLICATIONS 
Reference is hereby made to the following copending applications dealing 
with related subject matter and assigned to the assignee of the present 
invention: 
1. "Debris Trap For A Pressurized Water Nuclear Reactor" by John F. Wilson 
et al, assigned U.S. Ser. No. 672,040 and filed Nov. 16, 1984. (W.E. 
52,222) 
2. "Fuel Assembly Bottom Nozzle With Integral Debris Trap" by John F. 
Wilson et al, assigned U.S. Ser. No. 672,041 and filed Nov. 16, 1984. 
(W.E. 52,223) 
3. "Wire Mesh Debris Trap For A Fuel Assembly" by William Bryan, assigned 
U.S. Ser. No. 679,511 and filed Dec. 7, 1984. (W.E. 52,287) 
4. "Debris-Retaining Trap For A Fuel Assembly" by John A. Rylatt, assigned 
U.S. Ser. No. 720,109 and filed Apr. 4, 1985. (W.E. 52,484) 
5. "Nuclear Fuel Rod Support Grid With Improved Multiple Dimple 
Arrangement" by John A. Rylatt, assigned U.S. Ser. No. 729,387 and filed 
May 1, 1985. (W.E. 52,505) 
BACKGROUND OF THE INVENTION 1. Field of the Invention 
The present invention relates generally to nuclear reactors and, more 
particularly, is concerned with a trap mounted below the bottom grid and 
above the bottom nozzle of a fuel assembly and adjacent the lower end 
plugs of the fuel rods supported in the fuel assembly for trapping debris 
left in the reactor after assembly, repair and/or replacement operations 
so as to prevent entry and lodging thereof in the fuel assembly bottom 
grid where the debris can cause cladding perforations in the fuel rods and 
other damage to the fuel assembly. 2. Description of the Prior Art 
During manufacture and subsequent installation and repair of components 
comprising a nuclear reactor coolant circulation system, diligent effort 
is made to held assure removal of all debris from the reactor vessel and 
its associated systems which circulate coolant therethrough under various 
operating conditions. Although elaborate procedures are carried out to 
help assure debris removal, experience shows that in spite of the 
safeguards used to effect such removal, some chips and metal particles 
still remain hidden in the system. 
In particular, fuel assembly damage due to debris trapped at the bottom 
grid has been noted in several reactors in recent years. The damage 
consists of fuel rod tube perforations caused by fretting of debris in 
contact with the exterior of the tube. The debris tends to be relatively 
thin rectangular pieces, as opposed to pieces which are spherical in 
shape. Specifically, most of the debris consists of metal turnings which 
were probably left in the primary system after steam generator repair or 
replacement. The debris lodges in the region of the lower most grid within 
the spaces between its "egg-crate" shaped cell walls and the lower end 
portion of the fuel rod tubes. Amost all of the debris is deposited in the 
bottom grid just above the four coolant flow openings in the lower core 
support plate. 
One approach has been to provide longer end plugs on the lower ends of the 
fuel rods which extend upwardly through the bottom part of the lowermost 
grid so that any fretting would occur against the solid end plug and not 
the cladding of the fuel rod tube. Although effective in reducing fuel rod 
cladding perforation, this concept has the disadvantage that the fuel 
stack height is shortened by up to several inches which has adverse 
effects on margins and on achieving longer burnups. 
Several other different approaches have been proposed and tried for 
carrying out removal of debris from nuclear reactors. Many of these 
approaches, involving the use of screens at various places in the reactor 
core, of basins for catching the debris or of removable filters during 
preoperational testing, are discussed in U.S. Pat. No. 4,096,032 to Mayers 
et al. 
While all of the approaches referred to above operate reasonably well and 
generally achieve their objectives under the range of operating conditions 
for which they were designed, they also create several other problems. 
Consequently, a need still exists for a fresh approach to the problem of 
debris capture in nuclear reactors. The new approach must be compatible 
with the existing structure and operation of the components of the 
reactor, be effective throughout the operating cycle of the reactor, and 
at least provides overall benefits which outweigh the costs it adds to the 
reactor. 
SUMMARY OF THE INVENTION 
The present invention provides a debris-catching trap designed to satisfy 
the aforementioned needs. As an alternative to the common approach of the 
first four patent applications cross-referenced above, wherein a debris 
trap is associated in some manner with the bottom nozzle of the fuel 
assembly, the approach of the present invention is to provide the debris 
trap above the bottom nozzle and below the lowermost one of the plurality 
of grids axially spaced along the fuel assembly length. While the common 
approach of the four cross-reference applications is expected to be 
effective, some of the embodiments resulting from such approach are 
relatively expensive and others tend to increase the overall fuel assembly 
pressure drop which can result in a loss in critical heat flux margin and 
increased requirements on fuel assembly hold-down springs. In the 
alternative approach herein, the debris is trapped at the level of the 
solid lower end plugs of the fuel rods instead of at the level of the fuel 
rod cladding. Thus, any vibration induced fretting wear occurs on the end 
plugs and any failures caused by debris impinging the cladding just below 
the bottom grid will be eliminated. The wear on the end plugs would be 
harmless since even the maximum amount of wear conceivably caused by this 
fretting would not result in failure or in any other problems. This 
approach has a high probability of being an effective solution because it 
simulates the normal bottom grid geometry which has been shown to be 
effective in trapping debris. 
Accordingly, the present invention sets forth in a fuel assembly for a 
nuclear reactor including a plurality of nuclear fuel rods, each fuel rod 
having a cladding tube and a lower end plug attached to the tube, at least 
a bottom grid supporting the fuel rods in an organized array and disposed 
in spaced relationship above the lower end plugs of the fuel rods, a 
bottom nozzle disposed in spaced relationship below the bottom grid and 
disposed below the lower end plugs of the fuel rods and coolant flowing 
upwardly through the bottom nozzle and to the bottom grid, a trap for 
catching debris carried by the flowing coolant to substantially prevent 
the same from reaching the bottom grid. The debris trap comprises: (a) a 
structure disposed between the bottom nozzle and the bottom grid and 
generally aligned with the lower end plugs of the fuel rods, the structure 
forming a multiplicity of hollow cells each being open at opposite ends 
and defining a central cavity which receives one of the fuel rod lower end 
plugs while providing for passage of coolant flow therethrough from the 
bottom nozzle to the bottom grid; and (b) means defined in each of the 
cells for catching debris carried into the cells by the coolant flowing 
therethrough. 
More particularly, the trap structure includes interconnected wall portions 
forming each of the cells and defining the central cavity thereof. Also, 
the catching means in each cell is in the form of at least one member 
attached to one wall portion and extending into the cavity of the cell. 
The member is a projection fabricated from the wall portion so as to 
extend into the cell cavity. 
Still further, the trap catching means includes dimples formed on the wall 
portions so as to extend outwardly into the respective cells. Each dimple 
has a configuration generally arched outwardly from the wall portion which 
is oriented in alignment with the direction of coolant flow through the 
cell. The distance between those dimples formed on oppositely-disposed 
ones of the wall portions is slightly greater than the diameter of the 
fuel rod lower end plug disposed in the cavity of each cell. 
Additionally, the debris trap further comprises means attached to the trap 
structure and the bottom grid so as to support the structure from the 
grid. The trap structure support means extends between the structure and 
the bottom grid so as to locate the structure in spaced relationship below 
the bottom grid. More particularly, the structure of the trap is composed 
of a plurality of outer straps interconnected at their opposite ends so as 
to define a perimeter of the structure and a plurality of corners on the 
structure. Further, the structure support means includes a plurality of 
hanger straps attached at their lower ends to the outer straps adjacent 
the corners of the structure and extending upwardly therefrom. The hanger 
straps are also attached at their upper ends to the bottom grid so as to 
locate the structure in spaced relationship below the bottom grid and 
support the structure adjacent the corners thereof from the bottom grid. 
These and other advantages and attainments of the present invention will 
become apparent to those skilled in the art upon a reading of the 
following detailed description when taken in conjunction with the drawings 
wherein there is shown and described an illustrative embodiment of the 
invention.

DETAILED DESCRIPTION OF THE INVENTION 
In the following description, like reference characters designate like or 
corresponding parts throughout the several views of the drawings. Also in 
the following description, it is to be understood that such terms as 
"forward", "rearward", "left", "right", "upwardly", "downwardly", and the 
like are words of convenience and are not to be construed as limiting 
terms. 
In General 
Referring now to the drawings, and particularly to FIG. 1, there is shown 
an elevational view of a nuclear reactor fuel assembly, represented in 
vertically foreshortened form and being generally designated by the 
numeral 10. Basically, the fuel assembly 10 includes a lower end structure 
or bottom nozzle 12 for supporting the assembly on the lower core plate 
(not shown) in the core region of a reactor (not shown), and a number of 
longitudinally extending guide tubes or thimbles 14 which project upwardly 
from the bottom nozzle 12. The assembly 10 further includes a plurality of 
transverse grids 16 axially spaced along the guide thimbles 14 and an 
organized array of elongated fuel rods 18 transversely spaced and 
supported by the grids 16. Also, the assembly 10 has an instrumentation 
tube 20 located in the center thereof and an upper end structure or top 
nozzle 22 attached to the upper ends of the guide thimbles 14. With such 
an arrangment of parts, the fuel assembly 10 forms an integral unit 
capable of being conventionally handles without damaging the assembly 
parts. 
As mentioned above, the fuel rods 18 in the array thereof in the assembly 
10 are held in spaced relationship with one another by the grids 16 spaced 
along the fuel assembly length. Also referring to FIG. 2, each fuel rod 18 
includes an elongated hollow cladding tube 24 containing a stack of 
nuclear fuel pellets 26 therein and having its opposite ends closed by 
upper and lower end plugs 28,30, typically being welded to the tube, to 
provide a hermetically sealed rod. Commonly, a plenum spring 32 is 
disposed between the upper end plug 28 and the top one of the pellets 26 
in the stack thereof to maintain the pellets in a tight, stacked 
relationship within the rod 18. The fuel pellets 26 composed of fissile 
material are responsible for creating the reactive power of the reactor. A 
liquid moderator/coolant such as water, or water containing boron, is 
pumped outwardly through the guide thimbles 14 and along the fuel rods 18 
of the fuel assembly 10 in order to extract heat generated therein for the 
production of useful work. 
To control the fission process, a number of control rods 34 are 
reciprocally movable in the guide thimbles 14 located at predetermined 
positions in the fuel assembly 10. Specifically, the top nozzle 22 has 
operatively associated therewith a rod cluster control mechanism 36 having 
an internally threaded cylindrical member 38 with a plurality of radially 
extending flukes or arms 40. Each arm 40 is interconnected to a control 
rod 34 such that the control mechanism 36 is operable to move the control 
rods 34 vertically in the guide thimbles 14 to thereby control the fission 
process in the fuel assembly 10, all in a well-known manner. 
Debris Trap Mounted Between Botttom Grid and Nozzle 
As mentioned above, fuel assembly damage due to debris trapped at the 
bottom or lowermost one of the grids 16A has been noticed in recent years. 
As seen in FIG. 1, the bottom grid 16A is disposed in spaced relationship 
above the lower end plugs 30 of the fuel rods 18, while the bottom nozzle 
12 is disposed in spaced relationship below the bottom grid 16A and below 
the fuel rod lower end plugs 30. In absence of the present invention, 
coolant carrying debris flows directly upwardly through the bottom nozzle 
12 past the lower end plugs 30 of the fuel rods 18 and to the bottom grid 
16A positioned about the lower portions of the fuel rod cladding tubes 24. 
The bottom grid 16A has a conventional construction (not shown) composed 
of a multiplicity of interleaved straps defining a matrix of hollow cells, 
a majority of which accept the tube 24 of one fuel rod 18 and a minority 
of which accept one guide thimble 14. The cells which receive the fuel 
rods 18 have sets of springs and dimples for contacting and holding the 
fuel rod tubes 24. These features also catch debris reaching the bottom 
grid 16A which in turn over time causes fretting wear and perforation of 
the fuel rod tubes. Therefore, to prevent occurrence of such damage, it is 
highly desirable to catch this debris before it reaches the bottom grid 
16A. 
The basic concept underlying the present invention is to, in effect, extend 
the bottom grid 16A by providing a separate short grid-like extension so 
that the debris will get trapped between the grid-like extension and the 
lower end plugs 30 of the fuel rods 18. It should be noted that the bottom 
grid 16A cannot simply be lowered because it would then lose contact with 
the fuel rods 18 if the fuel rods were to lift early in life when maximum 
clearance exists between the upper end plugs 28 of the fuel rods 18 and 
the top nozzle 22. 
Therefore, the present invention is directed to a debris trap, generally 
indicated by the numeral 42, being supported from the bottom grid 16A and 
located between it and the bottom nozzle 12 at the level of the lower end 
plugs 30 of the fuel rods 18, as illustrated in FIG. 1. The trap 42 is 
positioned around and between the fuel rod lower end plugs 30 and across 
the path of coolant flow from the bottom nozzle 12 to the fuel rods 18 of 
the fuel assembly 10 so as to catch debris, such as small loose parts or 
pieces, from the flowing coolant and thereby substantially prevent it from 
reaching the bottom grid 16A of the assembly. 
As seen in FIGS. 1 and 3 to 5, the debris trap 42 includes a fuel rod 
nonsupport structure 44 being disposed between the bottom nozzle 12 and 
the bottom grid 16A and generally aligned with the lower end plugs 30 of 
the fuel rods 18. The structure 44 is composed of a plurality of inner 
straps 46 aligned with respect to each other in a cross-laced or 
crisscross interlocking arrangement and a plurality of outer straps 48 
interconnected at their opposite ends 50 and with opposite ends 52 of the 
inner straps 46 so as to define the perimeter of the structure 44. The 
inner and outer straps 46,48 together define a multiplicity of hollow 
cells 54, each cell being open at opposite ends and defining a central 
cavity. A majority of the cells 54 each receives one of the fuel rod lower 
end plugs 30 in nonsupporting relationship, while a minority of cells 54 
each receives one guide thimble 14. Also, the cell cavity provides for 
passage of coolant flow therethrough from the bottom nozzle 12 to the 
bottom grid 16A. 
More particularly, the inner straps 46 define interior wall portions 56 in 
oppositely-disposed interconnected pairs so as to form each of a plurality 
of inner ones 54A of the cells 54. Similarly, the outer straps 48 define 
exterior wall portions 58 which together with selected ones of the 
interior wall portions 56 of the inner straps 46 form a plurality of outer 
perimeter ones 54B of the cells 54. 
Also, the debris step 42 includes means defined in each of the hollow cells 
54 for catching debris carried into the cavities of the cells by coolant 
flowing therethrough past the fuel rod lower end plugs 30. The catching 
means are a plurality of projections, preferably, in the form of dimples 
60 defined in each of the cells 54. The dimples 60 are formed by an 
suitable method, such as die punching, from the wall portions 56,58 
defining the cells 54A,54B and extend into the cavities of the cells. The 
pair of dimples 60 formed on each interior wall portion 56 (with the 
exception of the ones which define the cell receiving a guide thimble 14, 
as seen in FIG. 4) extend outwardly in opposite directions therefrom into 
respective adjacent cells 54 sharing the interior wall portion 56. The 
dimple 60 formed on each exterior wall portion 58 extends into the 
respective perimeter cell 54B. Each dimple 60 has a configuration 
generally arched outwardly from the wall portion 56,58 which is oriented 
in alignment with the direction of coolant flow through the cell 54. As 
clearly seen in FIGS. 3 to 5, the distance between those dimples 60 formed 
on oppositely-disposed ones of the wall portions 56,58 is slightly greater 
than the diameter of the fuel rod lower end plug 30 disposed in the cavity 
of each cell 54 so as to thereby be disposed in a nonsupporting 
relationship with the end plug. 
Finally, the debris trap 42 is preferably supported from bottom grid 16A. 
The structure support means is in the form of a plurality of hanger straps 
62 attached at their lower ends 64 to the trap structure 44 adjacent the 
corners 66 thereof. The upper ends 68 of the straps 62 are, in turn, 
connected to the bottom grid 16A so as to locate the structure in spaced 
relationship below the bottom grid 16A and thereby support the structure 
44 adjacent the corners 66 thereof from the bottom grid 16A. 
As mentioned previously, although the debris now trapped adjacent and 
contact with the lower end plugs 30 of the fuel rods 18 still vibrate and 
cause some fretting wear against the solid end plugs 30, it will be 
harmless compared to fretting through the thin-walled Zircaloy cladding 
tubes 24 which occurs without the debris trap 42. The maximum conceivable 
fretting at the end plugs would not result in failure or any problems. 
It is thought that the debris trap of the present invention and many of its 
attendant advantages will be understood from the foregoing description and 
it will be apparent that various changes may be made in the form, 
construction and arrangement thereof without departing from the spirit and 
scope of the invention or sacrificing all of its material advantages, the 
form hereinbefore described being merely a preferred or exemplary 
embodiment thereof.