Development of an Advanced Nano-structured Ferritic Alloy for Extreme Radiation Tolerance

S. Maloy, M.B. Toloczko, G.R. Odette, N. Cunningham, D. Hoelzer, T.S. Byun
Los Alamos National Laboratory, US

Keywords: ferritic alloy, nanonuclear, fuels

Summary:

The Fuel Cycle Research and Development program funded under the Department of Energy-Nuclear Energy is investigating methods of transmuting minor actinides in various fuel cycle options. To achieve this goal, new fuels and cladding materials must be developed and tested to high burnup levels (e.g. >20%) requiring cladding to withstand very high doses (greater than 200 dpa) while in contact with the coolant and the fuel. The nano-structured ferritic alloys (NFA) variant of oxide dispersion strengthened (ODS) steels show remarkable irradiation resistance at high dose levels thanks to their abundant interfaces and capability of operating at high temperatures[1]. Thus significant research is underway to develop and optimize properties of NFAs for nuclear applications. One of the main challenges in the development of NFAs is transitioning from small developmental batches (≈ 1 kg) to larger heats (>50 kg). To address this issue, a collaborative effort of Los Alamos National Laboratory, Oak Ridge National Laboratory and the University of California-Santa Barbara acquired a large batch (>50kg) of Fe–14Cr– 3W–0.4Ti–0.2Y gas-atomized alloy powders with low O contents[2]. In an effort to improve nanofeature (NF) homogeneity and possibly reduce ball-milling times, the powders were pre-alloyed with Y during the gas atomization (as opposed to the conventional method of ball milling with Y2O3), However, early work showed that the Y was phase separated in the atomized powders; thus extensive systematic studies were carried out to develop optimal milling and consolidation procedures. This resulted in the production of a new heat of 14YWT called FCRD-NFA1, that has been characterized by small angle neutron scattering, atom probe tomography and high resolution transmission electron microscopy all demonstrating the presence of a high concentration of fine oxide particles (2-4 nm diameter) (see Fig. 1). Initial mechanical testing of FCRD-NFA1included tensile, fracture toughness and creep testing also showed equal or better properties than previously produced alloys such as the reference NFA, MA-957 (see Fig. 2). High dose ion irradiation tests confirmed the remarkable irradiation tolerance and swelling resistance of NFAs. To obtain high dose irradiation data from reactor irradiations, tensile and TEM specimens of previously irradiated oxide dispersion strengthened ferritic steels (MA-957) were EDM-machined from tubes after neutron irradiation to doses up to >100 dpa and at temperatures from 385-750C. Results also show excellent resistance of nanometer-scale oxide particles after this neutron dose at all irradiation temperatures.