PURPOSE: To reduce members to be used and positions to be welded and simplify the assembling work by expanding a thimble lower end section inserted through the small-diameter section of a thimble fitting hole with a taper recess, then screwing and fastening a thimble screw to a large-diameter section.

CONSTITUTION: A control rod guide thimble 1 and the lowest support lattice 3 are fixed with a sleeve 2, and the length of the thimble 1 is adjusted by the out-margin of a thimble lower end section 1a. The lower end section 1a is expanded along a taper recess 6c from the lower side of a lower nozzle 4, a thimble screw 5 is screwed and fastened to a screw provided at the large- diameter section 6b of a thimble fitting hole, and it is spot-welded at two positions to stop rotation. The thimble lower end section 1a is pinched by the taper recess 6c and the tapered bearing surface 5a of the screw 5 and an insertion section 5c is inserted, thus the thimble 1 is fixed to the nozzle 4 with sufficient strength. The dispersion adjustment of the whole length of the thimble 1 is not required, adjustment can be performed simply and correctly, the nozzle 4 is invariably fitted into close contact with a fuel rod, and the assembling work is simplified.