Abstract: An upper plenum structure of a cooled pressure vessel for a prismatic very high temperature reactor which secures a space for coolant to supply to a core and also supports an upper reflector located inside a graphite structure on top of the core. The upper plenum structure includes a cavity structure where the coolant goes down in the upper plenum structure, a plurality of upper reflector supports formed with the cavity and supporting the upper reflector located on top thereof, and a plurality of coolant distributing blocks. Each of the coolant distributing blocks is coupled with a bottom portion of a respective one of the upper reflector supports and is located on top of the core in order to distribute the coolant collected in a cavity, formed by the upper reflector support, to the core. The coolant distributing blocks cooperate with the upper reflector supports to define the cavity structure.