The invention relates to an assembly for a nuclear reactor, particularly for a sodium-cooled fast-neutron reactor RNR-Na, comprising a housing (10) with a longitudinal axis (X), each main face of the housing comprising, in the central part thereof, a spacer plate (2) with an adjacent assembly, comprising a part projecting towards the outside, the assembly also comprising a reinforcing sleeve (3) which is added inside the housing (10) and held therein, arranged next to the plates (2).