Intergranular Cracking in Cladding Tube of PWR RCCA Rodlets

Abstract

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 a fiber scope and hot cell examinations of some RCCA rodlets lead to the following conclusions. (1) The increase in outer diameter was caused by the swelling of the absorber (Ag-In-Cd alloy) . (2) At the RCCA rodlet tip, which was exposed to the largest amount of neutron fluence, an axial crack in the cladding tube was observed. (3) 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. (4) From the above, it is considered that the axial cracking 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.