Abstract: The present invention generally relates to a nuclear reactor, comprising a reactor core, neutron reflectors surrounding the core on all sides, uranium oxide as reactor fuel and a coolant that is liquid at proces conditions and solid at room temperature. The invention is characterized in that the nuclear reactor comprises a thermal isolation which is impermeable for the coolant, which is of a different material than the coolant and which has a melting point higher than the temperature in the reactor, in such a way that the thermal isolation is surrounding the reactor core and the neutron reflectors on all sides in order to provide a leak-tight containment function. Therefore the phenomenon is being used that the temperature on the outside of the installation is always far below the melting point of the coolant, in such a way that the coolant solidifies on the outside of the installation under all possible circumstances.