Cladding Tube Cracking Caused by Absorber Swelling for PWR RCCA Rodlets

Abstract

There are three types of rod cluster control assemblies (RCCAs), 14 × 14, 15 × 15 and 17 × 17, depending on the types of fuel assemblies. An intergranular cracking was observed at the tip of the cladding tubes of the 14 × 14 type RCCA. And investigations were carried out to understand crack initiation mechanism, crack initiation strain limit and life time estimation. Performances of cladding tube crackings for the three types of PWR RCCAs are investigated because they are important core components. The fracture mechanism of the cladding tube was irradiation assisted mechanical cracking (IAMC) caused by an increase in hoop strain due to the swelling of the neutron absorber and a decrease in elongation of the cladding tube due to neutron irradiation. Therefore increases in outer diameters for each type at various plants were measured. And they showed similar results to those of 14 × 14. From the investigation it is concluded that gaps between cladding tubes and neutron absorbers are essential to the integrity of the cladding tubes. Because the gaps of three types depend on similar design base, the critical neutron fluence would be equal for the three types. The outer diameter of the absorber tip was reduced to increase the gap between the cladding tube and the absorber in improved RCCAs. Their rodlets were measured increases in outer diameters. The results show that the improvement has been effective and no increase in outer diameter was observed.