Development of a fuel safety research program for metallic fast reactor fuels

Metallic fuel for Sodium Fast Reactor (SFR) applications is a mature technology with an extensive database developed over the irradiations of more than 130,000 pins at the Experimental Breeder Reactor II (EBR-II) and the Fast Flux Test Facility (FFTF), with fuel utilization at or above 20 at%. Expert PIRT studies have concluded that the knowledge base supporting metal fuels is sufficient for designing and licensing an SFR today - within the conservative envelope of the existing database. Still, departure from the existing database has become important with desires for higher burnups, larger core geometries, fuel design simplifications or enhancements, etc. With the recent restart of the Transient Reactor Test (TREAT) Facility, a fuel safety research and development (R&D) program has been developed to support expansion of metal fuel performance limits and to establish limits for novel fuel designs. The purpose of this program is to understand and establish fuel performance limits under anticipated off-normal, design basis, and beyond design basis accident conditions. Three R&D focus areas have been developed to include engineering-scale thermomechanical behavior, fission product transport and source term (in-core behaviors), and microstructural and chemical evolution. Transient testing is at the center of a complete fuel safety R&D program. Multiple capabilities are under development that will support testing a variety of SFR fuels under a range of conditions including prototypic flowing sodium, adapted from the very successful approach used historically at the TREAT Facility. This paper will review historical experimental findings with a focus on fuel performance limits and failure modes and mechanisms. This will lead to a review of recognized experimental needs and plans from historical programs and reviews. Finally, the current activities of the modern fuel safety R&D for metallic fast reactor fuels will be overviewed with preliminary findings from ongoing experimental work and plans for new transient irradiation testing in the TREAT Facility.