Life Time Estimation for Cladding Tube Cracking Caused by Absorber Swelling of PWR RCCA Rodlets

摘要

Intergranular cracks of cladding tubes had been observed at the tips of the rodlets of PWR rod cluster control assemblies (RCCAs). Because RCCAs are important core components, an investigation was carried out to estimate their service life time.(1) As it is essential to know the effect of slumping of the neutron absorber for the life time estimation, tests on absorber material were carried out. Both the dynamic and static stresses of the absorber are sufficiently small compared with its mechanical characteristics and it is concluded that slumping does not occur.(2) The crack was initiated at the inner surface of the cladding tubes and Sipush et al. obtained an intergranular fracture surface in tensile tests of similar material at a very slow strain rate in an argon gas atmosphere. Therefore the mechanism of the intergranular crack of the cladding tube is not IASCC but irradiation assisted cracking (IAC) caused by an increase in hoop strain due to the swelling of the absorber and a decrease in elongation due to neutron irradiation.(3) The crack initiation limit of cylindrical shells made of low ductile material and subjected to internal pressure is determined in relation to the uniform strain of the material and is in accordance with that of the RCCA rodlets in an actual plant.From the above investigation, the method of estimating the life time and countermeasures for its extension are obtained.