Patent Number: 
Section: claims

1. A nuclear reactor assembly comprising a bottom nozzle, a top nozzle, a plurality of fuel rods extending axially between the top and bottom nozzles, channels defined in at least the bottom nozzle for passage of coolant past the fuel rods, and each fuel rod having at least a bottom end plug, the nuclear reactor assembly comprising:a plurality of recesses in the bottom nozzle, each recess configured for seating the bottom end plug of one of the plurality of fuel rods, and each recess defining a central axis therethrough;a plurality of first engagement surfaces on the bottom nozzle, one engagement surface for each fuel road;a second engagement surface on the bottom end plug of each fuel rod;the first and second engagement surfaces being configured for engagement with each other for axially retaining each fuel rod within the nuclear reactor assembly; andthe first engagement surface comprising at least one retainer projecting upwardly from the recess for engagement with the second engagement surface of the bottom end plug, wherein the retainer comprises at least one boss member having a stem portion extending axially relative to the central axis from the recess and a flanged portion extending radially outwardly from the stem portion. 2. The nuclear reactor assembly of claim 1 wherein the second engagement surface of the bottom end plug is segmented. 3. The nuclear reactor assembly of claim 1 wherein the boss member is comprised of segmented sections of the stem portion and flanged portions spaced from each other equidistant from the central axis of the recess. 4. The nuclear reactor assembly of claim 1 wherein each second engagement surface is comprised of a cavity for receiving the boss member, the cavity having end portions for engagement with the flanged portion of the boss member. 5. The nuclear reactor assembly of claim 4 wherein one of the first and second engagement surfaces is flexible for enabling movement thereof, in use, during engagement of the first and second engagement surfaces. 6. The nuclear reactor assembly of claim 5 wherein the cavity comprises at least one longitudinal gap therein for enabling flexible movement of the end portions of the cavity. 7. The nuclear reactor assembly of claim 6 wherein the boss member is rigid. 8. The nuclear reactor assembly of claim 5 wherein the boss member comprises a pair of stem and flanged portions spaced from each other for enabling flexible movement of the stem portions. 9. The nuclear reactor assembly of claim 8 wherein the cavity and end portions thereof are rigid. 10. The nuclear reactor assembly of claim 1 further comprising at least one second retainer projecting upwardly from the periphery of the recess for lateral retention, in use, of the fuel rod. 11. The nuclear reactor assembly of claim 10 wherein the second retainer comprises an arm portion and a clip portion. 12. The nuclear reactor assembly of claim 11 further comprising a groove around the exterior of the bottom end plug for receiving the clip portion of the second retainer. 13. The nuclear reactor assembly of claim 12 wherein the arm portion is biased inwardly towards the central axis of the recess for securing the clip portion, in use, in the groove of the bottom end plug. 14. The nuclear reactor assembly of claim 10 further comprising a plurality of second retainers projecting from each recess. 15. The nuclear reactor assembly of claim 1 further comprising a debris deflector positioned in each of a plurality of the channels in the bottom nozzle. 16. The nuclear reactor assembly of claim 15 wherein the debris deflectors comprise ribs.