2018 | AFC ACCOMPLISHMENTS 170 hindered past development. In FY17, a new model based on bubble surface force equilibrium was proposed, and implemented without great success due to limitations imposed by the underlying MOOSE mechanical solvers as well as incomplete formulations of the swelling model itself.These hurdles were overcome through a complete fleshing out of the swelling model, including approximations where necessary (Matthews, Los Alamos National Laboratory (LANL). In addition, the fully consistent tangent operator for creep, swelling, and creep with swelling was derived to aid in convergence (Prakash, LANL).The creep formulation was implemented into MOOSE for the benefit of all users (Matthews, LANL), and was shown to significantly improve convergence and reduce run times by an order of magnitude in problems with large creep strain (e.g. metallic fuel, cladding). Full implementation of the swelling model is still underway as the last numerical problems are being overcome, although initial results show favorable comparisons with fission gas release data. Zirconium redistribution in metallic fuels has historically relied on a simplified pseudo-binary phase diagram to calculate the individual phase contribution to the total diffusivity.Approximations such as reduced number of phases and horizontal temperature transitions bake in potentially unphysical behaviors into the species redistribution solver.This year, we implemented an extensive set of phase information lookup tables formulated directly via the free energy formulation of U-Zr and U-Pu-Zr fuel (Hirschhorn, LANL/University of Florida (UF).This enables direct use of physically representative phase fractions and compositions, removing an unnecessary layer of simplification via the simplified phase diagram. Using a newly derived zirconium dependent fission rate formulation that includes burnup dependent parameters (Wilkerson, LANL), a new calibration is currently underway to try and bridge the gap between differing data sets (Stevens, LANL).