Deforming a portion of a nut threaded on a guide tube end plug to lock the nut to a lower end fitting of a nuclear reactor fuel assembly

A method comprising:

• inserting a threaded male shaft of an end plug connected with the lower end of a guide tube of a nuclear reactor fuel assembly through an opening of a nuclear reactor fuel assembly lower end fitting;
• installing a female lock nut onto the inserted threaded male shaft of the end plug by rotating the female lock nut in a tightening direction to secure the nuclear reactor fuel assembly lower end fitting to the lower end of the guide tube, wherein the female lock nut has a deformable locking portion alignable with a recess of the nuclear reactor fuel assembly lower end fitting;
• wherein the installing step further includes aligning by visual inspection the deformable locking portion of the female lock nut with the recess of the nuclear reactor fuel assembly lower end fitting;
• and
• after the installing step, deforming the locking portion of the female lock nut into the recess of the nuclear reactor fuel assembly lower end fitting to lock the female lock nut with the nuclear reactor fuel assembly lower end fitting, wherein prior to the deforming step, the recess of the nuclear reactor fuel assembly lower end fitting is visible after the installing step.