A nonlinear reactivity model with thermal-hydraulic feedback is presented for analysis of PWR fuel with various cladding materials. The simplified, batch-wise neutronic model requires batch reactivities and migration areas as predetermined functions of burnup, temperature (fuel and coolant), and any design parameters having neutronic significance (e.g., enrichment or cladding composition). Here, such functions are found by using symbolic regression, a unique approach that finds both a functional form and any model coefficients simultaneously. Core thermal-hydraulics are modeled using single-channel, axially-averaged values. The models were implemented in an open-source, Python-based tool, used here to the analyze fuel with Zr-based cladding having outer, protective layers of either FeCrAl or SiC, two materials proposed for use in next-generation, accident-tolerant fuel. (C) 2016 Elsevier Ltd. All rights reserved.