Diffusion Study on Measuring and Mitigating the Interactions between Neodymium and Steel Cladding Materials

One of the challenges facing the development of ultra-high burnup fast reactors, such as the Travelling Wave Reactor, is Fuel-Cladding Chemical Interactions (FCCI) between lanthanide fission products and steel cladding materials. At higher burnup, the fission product inventory increases dramatically, which in turn increases the chemical activity of each element and exacerbates the challenges associated with FCCI. This interaction forms a brittle intermetallic within the cladding which may limit fuel performance and durability. To better understand the mechanisms of lanthanide interactions with various steel alloys and the methods by which those interactions may be prevented, diffusion couples composed of neodymium, various steel alloys, and various refractory metal diffusion barriers were annealed for 17.5 to 56 days at temperatures from 550°C to 700°C and analyzed. In total, 204 diffusion couples including 476 interfaces were annealed and analyzed, approximately half of which successfully bonded. Elemental profiles acquired by Electron Probe MicroAnalysis (EPMA) were used to characterize the four intermetallic diffusion zones formed between steel alloys and neodymium, including the effects of alloying elements on growth rates. In addition, diffusion coefficients for potential diffusion barrier materials, which ranged from 3E-20 m^(2)/s to 6E-19 m^(2)/sec, were determined to ascertain their effectiveness in preventing lanthanide/steel interaction. Of the liner materials tested, tungsten, molybdenum, and zirconium interacted the least with neodymium. Simulations of the thermal and neutronic effects of these liner materials on reactor performance were also performed to provide a more complete perspective on their implementation. These simulations demonstrated that minimization of deleterious effects from the addition of a liner depends primarily on minimizing liner thickness, not the material used