2017 | AFC ACCOMPLISHMENTS 154 In general, the behavior of the “axial shift” case performs similar to the baselineT179 case in terms of centerline temperature (Figure 1).At the end of the irradiation, the location of maximum zirconium redistribution tends to follow the axial power profile. The plutonium ramp case results in a unique centerline temperature profile due to the change in thermal conductivity as a function of temperature; In general, a decrease in plutonium results in an increase in diffusivity coefficients.As a result, the increasing surface temperature is compensated by the increased thermal conductivity such that temperature profile at the temperature centerline remains flat for nearly two-thirds of the rod.This leads to cooler overall temperatures which in turn slows zirconium redistribution, resulting in less phase disparity throughout the pin. A critical review of previous work, the physics of FCCI can be separated into individual phenomena so that targeted models can be developed for each (3). Through examination of experiments conducted both in- and out-of-reactor, the behavior of lanthanides provides a natural separation of models by tracking their behavior through (1) production and transport in the fuel to the clad, (2) interaction with macroscopic changes in fuel topography including cracking and swelling, and finally (3) inter-diffusion at the fuel-cladding interface. Our collaboration with Ohio State University generated valuable data that we used in our initial modeling of the lanthanide transport.The diffusion coefficient of cerium in liquid sodium cerium was calculated to be on the order of 10-5 cm2/s (4) while the diffusivity in cesium was slightly lower. The experimental results indicated that the solubility of cerium, praseodymium, and neodymium in liquid sodium varied from 1×10-5 to 3×10-5 at.% in the temperature range of 723 to 823 K (5).The time dependence of solubility data showed that the solubility limit was reached within 30 minutes, indicating the dissolution rate is likely high relative to diffusion, thus suggesting lanthanide transport in liquid metals would be diffusion limited.The solubility of lanthanides in liquid cesium varied from 2x10-5 to 9x10-3 at.% in the temperature range of 473 - 723K (5).The dissolution rate was also fast in cesium. We developed a conceptual pore model to better describe the lanthanide transport behavior through the fuel (7, 8).We apply the conceptual model to a single idealized sodium filled pore configuration as well as idealized sodium filled interconnected pore network.The precipitation kinetics in both the fuel and the pore considers a driving force due to the difference between oversaturated dissolved lanthanide concentration and solubility limit. As long as oversaturated conditions occur, then precipitation in isolated or interconnected porosity are possible outcomes.The dissolved lanthanides diffuse to the cold side of the pore, exceeding the solubility limit in the liquid, resulting in precipitation on the cold side of the pore.The mechanism at the fuel-pore interface based on the Ln chemical potential causes isolated pores to act like a lanthanide sink as opposed to a pump. Experimental results of high solubility in liquid cesium also support the assumption that liquid cesium filled pores attracts the lanthanides and act like a sink. In the case of sodium filled interconnected pores, lanthanides are able to diffuse to inner clad surfaces and precipitate in the vicinity due to lower solubility of lanthanides in sodium. Initial results show that lanthanide transport from fuel to the cladding can be dramatically accelerated only when the pores are interconnected or there are sodium filled cracks present. Figure 2 shows the current results of staged Bayesian calibration of diffusion coefficients along with phase transition temperatures for U-Pu-Zr and U-Zr metallic fuels (9). Comparison of coefficients for three U-Zr and two U-Pu-Zr fuels (see table in Fig. 2) provide insight to the effects of plutonium on the fuel’s phase diagram. Findings indicate change in the gamma diffusion region, where the multiplier increases by a factor of four and has a larger uncertainty when no plutonium is present.Transition temperature form beta to gamma phase also appears to be affected, as it increases for U-Pu-Zr.The transition region from delta to beta, however, remains nearly the same meaning the gap between the two regions increases with plutonium content. Implementation of the statistical calibration approach over the previous manually adjusted coefficients has enabled rapid assessment of several thousand model evaluations that