Abstract <p>In the paper, the results of analysis of uncertainties in the impact of nuclear data and design parameters on the calculation of the experiment on irradiation of the uranium–plutonium fuel in the experimental assembly KETWS-1 in the fast reactor BN-600 have been presented. Uncertainty of errors in calculations of isotopic composition due to uncertainties in the main process parameters, namely, the initial composition of the irradiated samples, the holding time before radiochemical analysis, and the neutron flux, has been evaluated. Uncertainties in nuclear data (one-group neutron cross sections, branching ratios, and half-lives) have been determined on the basis of the actual versions of the evaluated data libraries. Nuclear data and design parameters uncertainties of calculations of isotopic composition have been evaluated using deterministic and stochastic methods implemented in the SUN and NUCLEX codes. It has been shown that the difference between the calculated and experimental results can be explained by uncertainties in nuclear data and design parameters for almost all nuclides (except ²⁴³Am).</p>