Calculations of fuel element burnup for realistic mixed core conditions in a 250-kW TRIGA Mark II reactor are presented. Two types of fuel elements are considered: 70% enriched FLIP and 20% enriched standard fuel elements. Two calculation models are compared. The first is based on a one-dimensional two-group diffusion approximation (the TRIGAP computer code), and the second is based on a two-dimensional four-group diffusion equation (the TRIGLAV computer code). In both cases the unit-cell group constants are generated with the WIMS code. Results of the calculations are intercompared to evaluate the influence of the two-dimensional effects on fuel element burnup. The following two-dimensional effects are considered: mixed rings, in-core water gaps, vicinity of control rods, and asymmetric core loading patterns. Relative differences in fuel element burnup of 10% on average and up to 80% in extreme cases are observed because of the two-dimensional effects. The accuracy of the calculation is estimated also by comparing the calculated results to the measurements using the reactivity method.