It is an object to easily perform inspection and repair of a reactor cooling system that can cool a reactor for a long time without requiring electric power. An in-vessel heat exchanger disposed in the reactor is fixed to the inner side of an upper lid of a RPV, one of through-pipes piercing through the upper lid is connected to the in-vessel heat exchanger, and the other forms a connection element on the outer side of the upper lid. According to the present invention, it is extremely easy to perform inspection and repair of the reactor cooling system that can cool the reactor for a long period without requiring electric power.
