This master thesis focuses on the determination of nuclear fuel burnup for the TRIGA Mark II research reactor at the Jožef Stefan Institute. The reactor has been operating since 1966 and has been playing an important role in developing nuclear technology and safety culture in Slovenia. During this period 300 different fuel elements were used, arranged in 220 core configurations. In order to determine the current isotopic composition of each fuel element we initiated activities to thoroughly document and analyse complete operation history. From the data regarding changes in reactor power, core configurations and measuremets of excess reactivity, we constructed a complete operational model, which could be used as an experimental benchmark for testing and validation of neutron transport and burnup codes.
Complete operational history of the reactor was simulated with the deterministic method, which solves diffusion approximation of the neutron transport equation. Operational history after the reconstruction in 1991 was calculated with stochastic Monte Carlo, which simulates high number of individual neutrons and by sampling calculates the needed information about our system. Clear correspondence is observed from the comparison of final fuel element burnup, calculated with both codes. The discrepancies were 5 %, except for around 20 fuel elements where the discrepancies were above 20 %. The reason for this is that the mentioned fuel elements were mixed together with the old already burned ones after the reconstruction of the reactor in 1991. For the further analysis of the discrepancies we performed a sensibility study of the effects of old fuel elements, and found that they contribute up to 5 %. The rest is because of the diffusion approximation in deterministic calculations. The calculated changes in core reactivity due to burnup, clearly matches the ones measured during reactor operation and the discrepancy between measured and calculated core reactivity for final core configuration in 2017 is only 200 pcm or below 10 %. Furthermore the effect of individual isotopes on core reactivity was studied with the Monte Carlo method, where the largest negative contribution is due to Xe-135 build-up and the only visible positive contribution due to Pu-239. The results for 20 important isotopes were compared with deterministic calculations performed in the past, where clear correspondence between both methods is observed. In the end Monte Carlo method was used to determined isotope distribution inside each type of fuel element, together with changes in neutron spectrum due to burnup, where the thermal peak of burned fuel is lower for around 10 %.