Patent Number: 
Section: claims

1. A nuclear reactor unit comprising:a containment vessel;a nuclear reactor located in the containment vessel, the nuclear reactor having a reactor vessel that has a reactor vessel wall; anda buffer salt contained in the containment vessel,the buffer salt being in thermal contact with the reactor vessel wall,the nuclear reactor, when running, generating a heat output that produces a first reactor vessel wall temperature,the buffer salt being in a solid state when at a temperature equal to or below the first reactor vessel wall temperature,the nuclear reactor, when external cooling is lost, generating heat that produces a second reactor vessel wall temperature greater than the first reactor vessel wall temperature,the buffer salt absorbing a portion of the decay heat, wherein absorption of the portion of the decay heat raises the temperature of the buffer salt, the buffer salt melting and becoming a liquid buffer salt when at the second reactor wall temperature,the containment vessel maintaining the liquid salt in thermal contact with the reactor vessel wall. 2. The nuclear reactor of claim 1 wherein convective heat transfer in the liquid state is higher than conductive heat transfer in the solid state. 3. The nuclear reactor of claim 1 wherein the buffer salt is a thermal insulator in the solid state and a thermal conductor in the liquid state. 4. The nuclear reactor of claim 1 wherein the liquid buffer salt conducts heat between the reactor vessel and the containment vessel. 5. The nuclear reactor of claim 4 wherein the containment vessel is in thermal contact with an exterior heat absorbing material. 6. The nuclear reactor of claim 5 wherein the exterior heat absorbing material includes water. 7. The nuclear reactor of claim 1 wherein the containment vessel comprises an inner wall and an outer wall, the inner wall being in thermal contact with the reactor vessel wall, the buffer salt being located between the inner wall and the outer wall. 8. The nuclear reactor of claim 1 wherein the nuclear reactor is a molten salt nuclear reactor.