Patent Number: 
Section: claims

1. A nuclear fuel assembly comprising:a top nozzle;a bottom nozzle;a plurality of grids arranged in between the top nozzle and the bottom nozzle at spaced axial elevations between the top nozzle and bottom nozzle, the plurality of grids including a bottom grid disposed closest to the bottom nozzle among the plurality of grids; anda plurality of control rod guide assemblies, at least one of the control rod guide assemblies including:a guide tube having an axial dimension, the guide tube being supported by the plurality of grids and extending axially between the top nozzle and the bottom nozzle, the guide tube having an upper portion having a first external radius, a lower portion having a second external radius less than the first external radius, and a transition portion disposed between the upper portion and the lower portion, wherein the guide tube linearly transitions between the first external radius and the second external radius in the transition portion, and wherein the guide tube is monolithic through the first portion, the second portion and the transition portion; andan external dashpot tube disposed around a portion of the lower portion of the guide tube in an area beginning at the bottom grid and extending toward the top nozzle,wherein the guide tube includes at least one bulge and the external dashpot tube includes at least one bulge corresponding to the at least one bulge in the guide tube. 2. The nuclear fuel assembly of claim 1, wherein the external dashpot tube includes at least one weep hole formed therein. 3. The nuclear fuel assembly of claim 2, wherein the at least one weep hole includes a first weep hole and a second weep hole, and wherein the first weep hole is formed in an upper half of the external dashpot tube and the second weep hole is formed in a lower half of the external dashpot tube. 4. The nuclear fuel assembly of claim 1, wherein the guide tube includes a first bulge formed in its upper half and a second bulge in its lower half and the external dashpot tube includes first and second bulges corresponding to the first and second bulges of the guide tube. 5. The nuclear fuel assembly of claim 4, wherein the external dashpot tube includes at least one weep hole formed therein, and wherein the at least one weep hole is formed between the first and second bulges in the external dashpot tube. 6. The nuclear fuel assembly of claim 1, wherein the lower portion of the guide tube is substantially cylindrical and the external dashpot tube is substantially cylindrical. 7. The nuclear fuel assembly of claim 1, wherein the grids include a lower middle grid disposed second closest to the bottom nozzle among the plurality of grids, and wherein the external dashpot tube begins at the bottom grid and ends before reaching the lower middle grid.