Method of installing an external dashpot tube around a control rod guide tube in a nuclear fuel assembly

A method of installing a control rod guide assembly in a nuclear reactor fuel assembly including a top nozzle, a bottom nozzle, and a plurality of grids arranged between the top nozzle and the bottom nozzle, the method comprising:

• providing a guide tube and an external dashpot tube;
• inserting the guide tube into a skeleton of the nuclear reactor fuel assembly to a lower middle grid, the lower middle grid being a grid of the plurality of grids that is the second closest grid to the bottom nozzle;
• after the guide tube is inserted to the lower middle gird, installing the external dashpot tube over the guide tube;
• after the external dashpot tube is installed over the guide tube, inserting the guide tube into the skeleton to the bottom nozzle;
• attaching the guide tube to the skeleton;
• and
• bulging the guide tube onto the external dashpot tube;
• wherein the guide tube comprises 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 and lower portion;
• wherein the guide tube linearly transitions between the first external radius and the second external radius in the transition portion;
• wherein the guide tube is monolithic through the first portion, the second portion, and the transition portion;
• wherein the plurality of grids comprise a bottom grid that is the closest grid to the bottom nozzle;
• and
• wherein, after bulging the guide tube onto the external dashpot tube, the external dashpot tube is disposed around a portion of the guide tube in an area beginning at the bottom grid and extending toward the top nozzle.