CRACK INITIATING LIMIT FOR CLADDING TUBES OF PWR RCCA RODLETS SUBJECTTO ABSORBER SWELLING

On-site measurement of the outer diameter of rodlets and hot cell exam ination of typical rodlet tips were carried out for the rod cluster co ntrol assembly (RCCA) of a PWR plant after 10 cycles. The main objecti ves of the examination are to investigate the extent of degradation of control rods and to determine the cracking mechanism involved since c racks are observed in a number of cladding tubes near the tips. In thi s study, the crack initiating limit for cladding tubes of PWR RCCA rod lets subject to absorber swelling is investigated. Although the claddi ng tube material consists of a ductile material, 304 S. S., it should be treated as low ductile material that is intermediate between a duct ile and a brittle material because its ductility is reduced by neutron irradiation. The crack initiating strain limit for the low ductile ma terial is investigated and is obtained as follows. epsilon(cr)=epsilon (ue) + 1/2 epsilon(up) where epsilon(cr): crack initiating strain, eps ilon(ue): elastic component of uniform strain, epsilon(up): plastic co mponent of uniform strain The proposed crack initiating strain limit s hows good agreement with the results of on-site examination. Axial cra cking 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 .