INTERGRANULAR CRACKING IN CLADDING TUBE OF PWR RCCA RODLETS

Increase in outer diameter of the rod cluster control assembly (RCCA) rodlets in a PWR plant was observed. Visual inspections of RCCA rodlet s with large increases in outer diameter using a fiber scope and hot c ell examinations of some RCCA rodlets lead to the following conclusion s. (1) The increase in outer diameter was caused by the swelling of th e absorber (Ag-In-Cd alloy). (2) At the RCCA rodlet tip, which was exp osed to the largest amount of neutron fluence, an axial crack in the c ladding tube was observed. (3) From the tensile test of cladding tube material, the uniform tensile strain was reduced to about 1.4% at 320 degrees C due to neutron irradiation. In the slow-strain-rate tensile test, intergranular cracking was observed. (4) From the above, it is c onsidered that the axial cracking in the cladding tubes of PWR RCCA ro dlets was caused mainly by a decrease in elongation due to neutron irr adiation and an increase of hoop strain due to the swelling of the abs orber.