[en] The uncertainties of calculated integral nuclear parameters such as k_eff values, reaction rates, reactivity worths, spent fuel isotopic concentrations, etc., are most likely caused by the inevitable approximations in cross-section data, description of the geometry, and material compositions. The uncertainties in the integral parameters due to cross section can be assessed using cross-section convariance data produced directly from the uncertainties of the measurements. Uncertainty assessments with covariance data have been continually used in reactor physics relating to the safety margin predictions and/or optimized integral benchmark facility designs for the next nuclear systems