Abstract: A nuclear reactor has a pressure vessel containing a core supported by the bed-plate. The bed-plate is arranged for admitting an upward flow of coolant into the core. The coolant, generally pressurized water, collects in an outlet plenum. The core comprises fuel assemblies each having a lower end piece. The bed-plate defines an inner volume containing coolant at a pressure lower than the coolant pressure under said bed-plate; the lower end piece of each of said fuel assemblies has a lower extension having a cross-sectional area smaller than the cross-sectional area of the fuel assembly above the bed-plate, cooperating with said bed-plate to define a passage for said flow of coolant into said core; and said extension is surrounded by an annular chamber communicating with said volume and defined by a downwardly facing external surface of said lower end piece.