An experimental study of the downward penetration of molten UO2 into substrate limestone concrete was conducted. Joule heating was used to melt the UO2. The technique allowed for sustaining of the molten phase for a long time period. The released gases from concrete eliminated the sintering of UO2 and caused the formation of single large interior cavity in the UO2/concrete solidified mixture. Uplifting of the top surface of the mixture and the formation of a mountain-like protrusion, bearing a vent hole on its apex, resulted from significant internal pressure. Except for the bottom surface of the solidified mixture, which was continuous and glassy-like, all the other surfaces were porous. Analyses of selected samples revealed the presence of calcium uranate type compounds. Concrete constituents were well distributed throughout UO2. Magnesium did not participate in compound formation with UO2. Finally, dissolution of the concrete constituents in UO2 resulted in reduction of the internal-heat generation.