Increase in outer diameter of the rod cluster control assembly (RCCA) rodlets in a PWR plant was observed. Visual inspections of RCCA rodlets with large increases in outer diameter using fiber scope and hot cell examinations of some RCCA rodlets lead to the following conclusions. 1) The increase of outer diameter was caused by the swelling of the absorber (Ag-In-Cd alloy). 2) At the RCCA rodlet tip, which has the largest amount of neutron fluence, an axial crack in the cladding tube was observed. 3) It is confirmed that the crack was fully intergranular and initiated at the inner surface of the cladding tube. 4) From the tensile test of cladding tube material, the uniform tensile strain was reduced to about 1.4% at 320 °C due to neutron irradiation. In the slow strain rate tensile test, intergranular cracking was observed. 5) From on-site examination using a fiber scope, cracks were observed only in rodlets which had an outer-diameter change of more than 0.7%. 6) The failure criterion that cracking caused by the hoop stress of a thin wall cylinder occurs when the hoop strain reaches half of the plastic strain of uniform elongation is applicable for RCCA rodlets. 7) From above, it is considered that the axial crack in the cladding tubes of PWR RCCA rodlets was caused mainly by a decrease in elongation due to neutron irradiation and an increase of hoop strain due to the swelling of the absorber. 8) The critical neutron fluence for axial crack initiation is approximately 0.8×1022n/cm2 (E>1 MeV) or 3×1022n/cm2 (E>0.625 eV). The latter is an appropriate index in terms of the actual core management. Prevention of axial cracking in an RCCA rodlet can be obtained if RCCAs are replaced when the total amount of neutron fluence reaches the above critical fluence. 9) Countermeasure, i.e., reducing the outer diameter of the bottom part of absorber or extending the gap between cladding tube and absorber, has been adopted in order to extend the RCCAs' life. Based on a series of experimental results, twice the critical neutron fluence can be achieved by increasing the gaps to 0.1 mm.