Perforated debris catcher for a nuclear fuel assembly

DETAILED DESCRIPTION In accordance with a preferred embodiment of the present invention, there is provided a filter plate for overlying a cast lower tie plate having a grid formed of spaced bosses having holes and webs interconnecting the bosses defining with the bosses flow openings through the grid.
The filter plate includes holes spaced one from the other for registration with the holes through the bosses when the filter plate overlies the grid.
The filter plate also has a plurality of reduced apertures, e.
g.
, reduced diameter apertures, spaced from one another and the holes through the filter plate.
The apertures are significantly smaller in size and area than the holes.
Additionally, the upper edges of the webs interconnecting the bosses are spaced below the upper edges of the bosses such that the filter plate rests only on the upper edges of the bosses and not along the upper edges of the webs.
This frees additional apertures through the filter plate for flow of coolant therethrough.
To secure the filter plate in position on the lower tie plate grid, the fuel rods have either smooth-sided or threaded end plugs.
The fuel rods having threaded end plugs constitute tie rods or part-length fuel rods.
Those threaded end plugs extend through the filter plate holes and are threaded into corresponding female threaded holes of the bosses to secure the fuel bundle to the lower tie plate.
However, the threaded end plugs do not engage the margins of the holes through the filter plate and do not assist in securing the filter plate to the tie plate.
The smooth-sided end plugs of the remaining fuel rods extend through the registering holes of the filter plate and bosses and margins of the filter plate holes are engaged by those end plugs to maintain the filter plate positioned on the lower tie plate grid.
The weight of the fuel rods with the smooth-sided end plugs hold down and position the filter plate against the lower tie plate.
The coolant flows through the lower tie plate and into the flow openings between the bosses and the webs