1999 Volume 42 Issue 3 Pages 438-446
The tips of the rodlets of rod cluster control assemblies(RCCAs) are exposed to extremely high neutron fluence in fuel assemblies. A high neutron fluence causes degradation of the control rod materials. In a pressurized water reactor(PWR) plant, increases in the outer diameters of the rodlets, induced by absorber swelling, and intergranular cracks at tips of rodlets were observed. First, mechanisms of cracking by material degradation due to neutron irradiation in nuclear reactors were given their definitions because they were previously not clearly defined. Then it was confirmed that the cause of intergranular cracks of RCCA rodlets was irradiation assisted mechanical cracking(IAMC). The cracks were induced by an increase in hoop strain due to the swelling of the absorber and a decrease in elongation due to neutron irradiation. Furthermore, investigations were carried out on the swelling mandrel test, because the structures and environments of the swelling mandrel test were similar to those of RCCA rodlets. As a result, it was found that the mechanism of the swelling mandrel test was not irradiation assisted stress corrosion cracking(IASCC) but IAMC. Also, intergranular cracks on the cladding tubes of RCCA rodlets could be induced at a very low strain rate.