[en] In a first approach the constraint supplied by the solidifying UO₂-shell is evaluated. The mass of the injected sodium is assumed to have a spherical form. Its dimensions are negligible in respect to the extension of the UO₂. Using the Von Karman Pohlhausen method for solving the Fourier equation, the temperature distributions in UO₂ and sodium are determined. The physical properties are taken to be independent of temperature. Once these temperature profiles are obtained, the pressure induced into the heated sodium by the hypothesised mechanical constraint of the rigid shell and the tangential stress produced in this shell, can be calculated. In a second approach then, a liquid-liquid contact between UO₂ and Na is considered. The interface temperature, however, is calculated by means of an adjusted initial temperature of UO₂. Following an idea of Cho and Wright, to the actual temperature of UO₂ is added a value obtained by dividing its latent heat of fusion by its heat capacity. The thermal expansion of the sodium drop is initially delayed by the inertial constraint of the surrounding heavy UO₂. The expansion of the liquid drop of sodium continues up to the moment where the average temperature of the entrapped sodium becomes equal to the homogeneous nucleation temperature. At this instant vaporisation starts and the process goes on described by the formation of a two-phase mixture for the sodium. In this way the interaction of an entrapped sodium drop is calculated as a superheat limited explosion