Fuel element burnup determination in mixed TRIGA core using reactor calculations

Calculations of fuel element burnup for realistic mixed core conditions in a 250-kW TRIGA Mark II reactor are presented. Two tyres of fuel elements ar e considered: 70% enriched FLIP and 20% enriched standard fuel elements. Tw o calculation models are compared. The first is based oil a one-dimensional two-group diffusion approximation (the TRIGAP computer code), and the seco nd 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 evalua te the influence of the two-dimensional effects on fuel element burnup. The following two-dimensional effects are considered: mixed rings, in-core wat er gaps, vicinity of control rods, and asymmetric core loading patterns.. R elative 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 a ccuracy of the calculation is estimated also by comparing the calculated re sults to the measurements using the reactivity method.