Abstract: An apparatus for thermally insulating nuclear reactor primary vessels. The apparatus includes a plurality of cylindrical reflective metal plates located within the primary vessel for thermally insulating the side walls of the vessel from the temperatures generated by the reactor. The reflective plates are radially spaced apart and are positioned in an annular inverted U-shaped chamber that traps inert gas around the reflective plates. The plates and the annular chamber insulate the side walls of the primary vessel from the temperatures generated by the reactor.

Abstract: A nuclear reactor of the type including a reactor vessel and a core assembly to be maintained in a fixed position within the vessel is disclosed herein along with a structural arrangement also located in the vessel, for supporting the core assembly in its fixed position. The structural arrangement disclosed utilizes a plurality of components including a grillage of I-beams interconnected to one another by welded joints so as to define a unitary structure capable of supporting the core assembly within the vessel. These components including the I-beams are also mechanically interlocked such that a total failure of the welded joints will result in a limited but readily detectable downward deflection of the unitary structure, thereby indicating such a failure while, at the same time, retaining sufficient structural integrity to maintain the core assembly in a supported position.