Irradiation assisted cracking of RCCA rodlets and experimental techniques for their improvement

The tips of the rodlets of rod cluster control assemblies (RCCAS) are exposed to extremely high neutron fluence in the fuel assemblies. Since a high neutron fluence causes degradation of the cladding tube material (Type 304 stainless steel) and swelling of the absorber (Agin-Cd Alloy), on-site rodlet outer diameter measurements and post-irradiation examination of typical rodlet tips were carried out for a PWR plant (2-Loop, 500 MWe, Fuel Type 14 x 14) after 10 cycles. From the examinations, an increase in the outer diameter of the rodlets induced by absorber swelling and an intergranular crack at a tip of a rodlet were observed. The mechanism of the intergranular crack of the cladding tube was investigated and it was concluded that it was irradiation assisted cracking (IAC) caused by an decrease in hoop strain due to the swelling of the absorber and a decrease in elongation due to neutron irradiation. For experimental techniques for the improvement of IAC, tensile tests of the irradiated material and swelling mandrel tests in which a layer of Ag-In-Cd alloy is used between the inner surface of the cladding and the Al2O3 pellets of the mandrel are suitable. (authors)