In a nuclear reactor internal structure, generation and promotion of
object-downstream separating vortices are suppressed so that coolant
uniformly flows in a reactor core and pressure loss of flow of the
coolant is reduced so that the flow of the coolant is stabilized. A lower
connecting plate 30 arranged in a lower plenum 8 comprises a ring portion
31 in which an arcuate portion 32 and a cut-off portion are alternately
formed. An outer peripheral portion of the ring portion 31 is asymmetric
relative to a flow direction of main flow and also asymmetric relative to
a separating flow generation direction.