Crack Initiating Limit for Cladding Tubes of PWR RCCA Rodlets Subject to Absorber Swelling

On-site measurement of the outer diameter of rodlets and hot cell examination of typical rodlet tips were carried out for the rod cluster control assembly (RCCA) of a PWR plant after 10 cycles. The main objectives of the examination are to investigate the extent of degradation of control rods and to determine the cracking mechanism involved since cracks are observed in a number of cladding tubes near the tips. In this study, the crack initiating limit for cladding tubes of PWR RCCA rodlets subject to absorber swelling is investigated. Although the cladding tube material consists of a ductile material, 304 S. S., it should be treated as low ductile material that is intermediate between a ductile and a brittle material because its ductility is reduced by neutron irradiation. The crack initiating strain limit for the low ductile material is investigated and is obtained as follows. e cr =e ue + 1/2e up where e cr : crack initiating strain, e ue : elastic component of uniform strain, e up : plastic component of uniform strain The proposed crack initiating strain limit shows good agreement with the results of on-site examination. Axial cracking in RCCA rodlets can be prevented if RCCAs are replaced when the total amount of neutron fluence reaches approximately 8 x 10 21 n/cm 2 (E>1 MeV). Moreover, it is possible to increase the lifespan of a RCCA by increasing the gap between the cladding tube and the absorber.