Evaluation of uncertainties associated with best estimates of parameters in the deterministic analysis of a severe accident

•Uncertainty analysis is a part of the realistic calculations of severe accidents.•Best estimate of a parameter is calculated taking into account model form error.•Model, input and numerical uncertainties are components of combined uncertainty. This paper describes an approach to evaluate the uncertainties of the results of deterministic severe accident analysis. The approach was developed in order to follow best practices of best estimate methodology and to fulfill the Russian regulatory requirements NP-001–15 in the area of atomic energy use. In accordance with these requirements, deterministic safety analyses must be accompanied with the evaluation of errors and uncertainties of the obtained results. The application of the proposed approach is demonstrated through the example of a deterministic analysis of a severe LOCA initiated by a large break at a generic nuclear power plant with VVER-1000. Within the given example a best estimate value and associated uncertainty of hydrogen mass generated at in-vessel stage is analyzed.