One of the biggest experimental programs helping to improve nuclear installation safety is PHEBUS, an international cooperative research program that provides data for validating computer codes dedicated to the analysis of severe accidents and their calculation results. In the European Union, HORIZON 2020 EURATOM project MUSA (Management and Uncertainties of Severe Accidents) is in progress. Modelling of the PHEBUS FPT-1 test within the frame of this project is provided by project partners using different severe accident codes. Uncertainty quantification of the received results is provided using different methods and statistical tools. The Lithuanian Energy Institute (LEI) is taking part in the MUSA project and this uncertainty exercise. LEI is using the RELAP/SCDAPSIM severe accident code together with GRS methodology and SUSA statistical tool to evaluate the uncertainties of modelling results of PHEBUS FPT-1 test. In this article, uncertainty quantification of the PHEBUS FPT-1 test modelling results provided by LEI in the frame of MUSA project are presented. Provided uncertainty analysis for total hydrogen generation showed that upper and lower uncertainty limits are bounding the experimental data. At the end of the calculation, the upper and lower uncertainty limits of calculations are within the band of experimental uncertainties. Uncertainty analysis for Cs/I release fraction showed that uncertainty limits are bounding experimental data until the middle of the heat-up phase, but at the end of the experiment, the calculated upper uncertainty limit is lower than the experimental data. Results of sensitivity analysis showed that the thermal conductivity of the ZrO2 layer of the shroud has the dominant influence on hydrogen generation and Cs/I release fraction calculation results. Other important parameters are changing at the different experimental phases. In the future, it is planned to update the RELAP/SCDAPSIM model for the PHEBUS FPT-1 test using the results of the provided sensitivity analysis. This will allow a better agreement with experimental data.