Burst behavior of austenitic and ferritic Fe-based alloy tubes has been examined under a simulated large break loss of coolant accident. Specifically, type 304 stainless steel (304SS) and oxidation resistant FeCrAl tubes were studied alongside Zircaloy-2 and Zircaloy-4 that are considered reference fuel cladding materials. Following the burst test, characterization of the cladding materials was carried out to gain insights regarding the integral burst behavior. Given the widespread availability of a comprehensive set of thermo-mechanical data at elevated temperatures for 304SS, a modeling framework was implemented to simulate the various processes that affect burst behavior in this Febased alloy. The most important conclusion is that cladding ballooning due to creep is negligible for Fe-based alloys. Thus, unlike Zr-based alloys, cladding cross-sectional area remains largely unchanged up to the point of burst. Therefore, for a given rod internal pressure, the temperature onset of burst in Fe-based alloys appears to be simply a function of the alloy's ultimate tensile strength, particularly at high rod internal pressures.

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A major challenge in the design of oxide dispersion strengthened (ODS) FeCrAl alloys is the optimization of the fine-scale particle size distribution that provides both beneficial mechanical properties and irradiation resistance. To address this obstacle, the nucleation, growth, and coarsening of the fine-scale (Y,Al,O) nanoprecipitates within an ODS FeCrAl was studied using atom probe tomography (APT) and small-angle neutron scattering (SANS). Mechanically alloyed Fe-10Cr-6.1Al-0.3Zr+Y2O3 wt.% (CrAZY) powders were heated in-situ from 20-1000°C to capture the nucleation and growth the nanoprecipitates using SANS. Furthermore, CrAZY powders were annealed at 1000°C, 1050°C, and 1100°C at ageing times from 15 min to 500 h followed by either APT or magnetic SANS to study the structure, composition, and coarsening kinetics of the nanoprecipitates. In-situ SANS results indicate nanoprecipitate nucleation and growth at low temperatures (200-600°C). APT results indicate compositions corresponding to the YAG stoichiometry with a possible transition towards the YAP phase for larger precipitates after sufficient thermal ageing. However, magnetic SANS results suggest a defective structure for the nanoprecipitates indicated by deviations of the calculated A-ratio from stochiometric (Y,Al,O) phases. Particle coarsening kinetics follow n=6 power law kinetics, but the mechanism cannot be explained through the dislocation pipe diffusion mechanism. The potential effect of precipitate coarsening during pre-and post-consolidation heat treatments on the irradiation resistance of ODS FeCrAl alloys is discussed with respect to sink strength maximization.

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