A nuclear reactor system is disclosed which provides an auxiliary cooling system and improved safety in the event of corrosion failure. The reactor comprises an inner reactor vessel containing a reactor core and a liquid fuel, and an outer containment vessel, with an intermediate space between the vessels being filled with a protective material. The protective material is at least partly in a solid state during normal operation of the reactor, and is liquefied by increasing temperatures within the reactor vessel due to abnormal operating conditions and provides enhanced cooling in the liquid state. The protective material and/or the coolant of a heat exchanger may dilute the liquid fuel during corrosive failure, and may optionally be provided with neutron absorbing material to prevent a fission chain reaction from occurring outside the reactor vessel.