The ELESIM code models a single fuel element in a one-dimensional axisymmetric manner. The constituent subroutines are physically based (rather than empirical) models, and include such phenomena as fuel-to-sheath heat transfer; temperature and porosity dependence of fuel thermal conductivity; burnup-dependent neutron flux depression; burnup-and microstructure-dependent fission product gas release; fuel thermal expansion, swelling, and densification; and stress-, dose-, and temperature-dependent agreement with experimental data.