NUCLEAR REACTOR UNIT AND METHOD FOR COOLING NUCLEAR REACTOR UNIT

Provided is a nuclear reactor unit that can reduce a temperature increase in a reactor core at the occurrence of an abnormality with a simple structure. Included are a reactor core having radioactive fuel and causing the radioactive fuel to cause a nuclear reaction and a nuclear reactor vessel housing the reactor core and hermetically sealing the reactor core. The nuclear reactor vessel includes an inner shroud covering the entire periphery of the reactor core and an outer shroud covering the entire periphery of the inner shroud. A first space formed by the outer shroud and the inner shroud is in a vacuum condition. The inner shroud includes a main body and a communicating part placed in part of the main body and communicating the first space to a second space, which is a space inside the inner shroud, when the reactor core reaches a threshold temperature or higher.