Wade Karlsen

Crack Initiation Mechanism in Non-ductile Cracking of Irradiated 304L Stainless Steels under BWR Water Environment

The deformation behavior and initiation mechanisms of intergranular (IG) and transgranular (TG) cracks in irradiated 304L stainless steel were studied by slow-strain-rate tensile tests in inert gas and simulated BWR water environments, followed by fractographic and microstructural examinations. Neutron irradiation was made in test reactors to fluences of up to 6.2x1020 n/cm2 (E>1 MeV). Intergranular cracking occurred in water above a critical neutron fluence of around 1 × 1020 n/cm2, based on the results of the SSRT tests and SEM fractography. That critical fluence is mechanistically supported by irradiated, deformed microstructures exhibiting dislocation channeling at that fluence, while radiation-induced Cr depletion at the grain boundaries was minor. Transgranular cracking of the irradiated material occurred in water below the critical fluence, initiating in the non-uniformly strained surface region of the test bar in the later stages of plastic deformation. The initiation of TG cracking is hypothesized to be related to a high density of deformation twins. Intergranular cracking is proposed to have initiated where localized slip bands terminated at grain boundaries, while TG cracking is inferred to have initiated at deformation twin boundaries. High stress and strain concentrations at grain/twin boundaries would be the common cause of non-ductile crack initiation.

Post-Irradiation Examinations of Irradiation Creep Tested Zircaloy-2

This work stems from a test program studying the in-pile creep behaviour of Zircaloy-2 materials. The work involves the post-irradiation examinations (PIE) of the Zircaloy-2 specimens creep tested under neutron irradiation at the HALDEN Test Reactor. The multiple specimens provided to VTT for PIE were creep tested to various plastic strain levels with some failing during testing. In-pile creep testing in the HALDEN Test Reactor involved final fluence levels of ~3×10 n/cm to 5×10 n/cm (>1 MeV) and temperatures of ~550 to ~650oF (~288 to ~343oC). The PIE of the various specimens included mainly fractography of the failed specimens using scanning electron microscopy (SEM) and detailed transmission electron microscopy (TEM) characterization.

Effect of Hot Cracks on EAC Crack Initiation and Growth in Nickel-Base Alloy Weld Metals

The differences in the EAC susceptibility between different weld geometries and weld metals have been distinguished by the doped steam test method. Pure weld metals of Alloy 182 and 82 are clearly more susceptible to EAC than the pure weld metals of Alloy 152 and 52, which did not show any crack initiation. The dissimilar metal welds (DMW) with diluted microstructures are less susceptible than the pure weld metals of Alloy 182 and 82. No crack initiation/extension from hot cracks was observed in any of the studied weld metals. At the hot crack tips no crack growth was observed in any of the studied samples. This is related to the segregated microstructure of the hot crack tips. In accelerated doped steam tests selective dissolution takes place and metallic Ni or NiO forms a continuous layer in the middle of the cracks surrounded by the Cr-rich oxide layer. Selective dissolution typical for EAC was not observed inside the hot cracks or at their crack tips. EAC initiation occurred in the Alloy 600 base metal of the DMWs and selective dissolution inside the EAC cracks in Alloy 600 was extensive. The results are discussed based on the selective dissolution creep model of EAC.