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The problem of reactor pressure vessel (RPV) integrity during in-vessel melt retention for VVER type reactor is considered. Comprehensive numerical analysis contains the modeling of full severe accident scenario including thermal and mechanical loads on the RPV. The RPV stress calculation takes into consideration vessel thermal erosion and creep with a long-term strength, and error estimations: uncertainty analysis, sensitivity study. These problems are solved with the presented HEFEST-URAN package including: HEFEST code (2D FEM analysis of melt-structure interaction), HEFEST-M code (2D FEM stress analysis) and Varia code (multivariate calculations with uncertainty/sensitivity statistical assessment). HEFEST-URAN is a subset of the SOCRAT code developed in the Russian Federation. In the paper the HEFEST-URAN methodology and validation are described with some examples. The results of multivariate calculations of IVMR for VVER-440 reactor are presented with sensitivity study of the effects of essential factors defining the RPV thermal loading and long-term strength.