The goal of the proposed rapid turnaround experiment (RTE) led by the University of Wisconsin, Madison (UW) is to investigate the stability of oxide nanoparticles in ion-irradiated ODS steel manufactured by the novel cold spray process using Atomic Probe Tomography (APT) (also referred to as Local Electrode Atom Probe (LEAP)). The proposed research would provide extended data showing stability of the cold spray produced ODS steel materials with respect to the number density, size, and composition of the oxide nanparticles under heavy ion irradiation as compared to ODS steel cladding tubes produced by conventional methods. For this investigation, ten days of instrument time is requested on the APT and six days on the Focused Ion Beam (FIB) for APT sample preparation at Idaho National Laboratory (INL). During the cold spray process, gas atomized 14YWT powder particles containing oxide nanoparticles are propelled at supersonic velocities through a converging-diverging nozzle system by pre-heated pressurized gas onto a substrate to form a deposit. The particle temperature is low and deposition occurs in solid state. Under an ongoing NEET project, UW has successfully demonstrated a manufacturing route for ODS steel cladding tubes using the cold spray process [1], but APT is required to achieve the fine-scale spatial resolution needed to understand the physical and compositional stability at the nanoscale (< 5nm) under irradiation. The size, size distribution, and the composition of the oxide nanoparticles, which dictate the high temperature strength of ODS steel, as well as a radiation sink strength of the oxide nanoparticles will be evaluated.



The first question this RTE will seek to answer is about the stability of oxide nanoparticles in the gas atomized 14YWT powder during the cold spray process and their reprecipitation during ion-irradiation. It is hypothesized that the high velocity impact of the oxide nanoparticles in the powder particle during the cold spray process leads to oxide particle dissolution in the ferritic steel matrix, analogous to the high energy ball milling process used in the conventional methods of preparing ODS. Subsequent heavy ion irradiation of the supersaturated solid-solution matrix can potentially result in reprecipitation of the oxide nanoparticles. Investigation of this hypothesis requires APT examination of as-deposited ODS steel and its ion-irradiated counterpart.



The second question this RTE will seek to answer is how cold spray manufactured ODS steel behaves under radiation as compared to conventionally manufactured ODS steel with respect to the number density, composition, and size of the oxide nanoparticles.



Both the above scientific questions in our opinion are most conclusively addressed by the state-of-the-art APT facility at the Idaho National Laboratory.



Supporting work (outside RTE proposal): (i) Ion irradiation of cold spray produced ODS steel samples performed at UW’s accelerator facilities using 3.7 MeV Fe+2 ions to a maximum damage level of 110 dpa. (ii) Continued SEM and S/TEM-EDS characterization of oxide nanoparticles in the ODS steels (produced by both cold spray and conventional methods) at the UW’s Materials Science Center to complement the APT work proposed in this RTE.

Implementing advanced nuclear reactor concepts is a major goal for the Department of Energy’s Office of Nuclear Energy (DOE-NE) program. These advanced nuclear reactor concepts require materials that have superior high temperature strength and greater stability against radiation damage. One such material being considered for implementation for structural applications (fuel cladding tubes) in sodium and lead fast reactors is oxide dispersion strengthened (ODS) steels. These steels are ferritic providing low swelling with a fine, uniform dispersion of oxide nanoparticles/clusters throughout the ferritic matrix. These oxide nanoparticles increase high temperature strength by acting as pinning sites for dislocation motion. The interfaces between the oxide nanoparticles and the ferritic matrix act as sinks for radiation-induced defects and increase radiation damage resistance.



This proposed RTE directly supports an on-going Nuclear Energy Enabling Technologies (NEET) Project led by the proposing university along with project collaborators at a university, national laboratories, and an international collaborator for the manufacturing of ODS fuel cladding tubes using an innovative cold spray process.



The present manufacturing approach for ODS cladding tubes involves powder consolidation and multiple extrusion steps which is time consuming, can lead to anisotropic properties, and is not amendable to larger scale manufacturing. The cold spray process is being investigated under our NEET program (and advanced under the proposed RTE) as an alternative to this conventional approach in order to advance the deployment and economic viability of advanced nuclear reactors as the DOE-NE is actively supporting. Our research thus far has been successful in producing high density ODS steel cladding tubes with a fine distribution of oxide nanoparticles using cold spray [1]. Although the presence of oxide nanoparticles has been shown by scanning transmission electron microscopy (STEM), their size and spatial distribution and accurate compositional analysis have yet to be fully confirmed because of their crucial importance to the properties of the ODS steels.



In the proposed RTE, instrument time is requested of a national laboratory for the use of the atom probe tomography (APT) and associated sample preparation for APT on the focused ion beam (FIB) for APT experiments. Our fundamental goal with the APT instrument will be to determine the stability of the oxide nanoparticles in ODS steels produced by the novel cold spray process under radiation. The metrics for stability will be size, size distribution, number density, and compositional changes. Irradiation effects on conventional ODS steels will also be examined via APT technique for baseline data.



In preparation for the proposed RTE work, Fe+2 ion irradiation of the ODS steel cladding tubes produced by the cold spray process and the conventional powder consolidation and extrusion process will be performed at the proposing university to a maximum damage level of 110 dpa. This ion irradiation work is scheduled to be completed well before the commencement of the RTE, if it is awarded. The proposed RTE will provide critical information in regards to the feasibility of the cold spray process for an alternative manufacturing route for ODS steel cladding tubes.