A pressure-tube nuclear reactor comprising a) a pressure vessel for containing a pressurized heavy water moderator at a first pressure that is between 5 and 35 Mpa and; b) a plurality of fuel channels for a coolant fluid at a second pressure that is greater than the first pressure and fluidly connected at inlet ends thereof to a coolant supply conduit, the fuel channels adapted to receive nuclear fuel bundles and to be mounted within the pressure vessel and surrounded by the moderator, and outlet ends thereof being fluidly connected to a coolant outlet conduit to enable the coolant fluid to circulate from the coolant supply conduit through the fuel channels to the coolant outlet conduit, the plurality of fuel channels maintaining separation between the coolant fluid circulating within the fuel channels and the moderator; and c) a coolant inlet plenum, the inlet ends of each fuel channel being coupled to the coolant inlet plenum to receive coolant fluid and the coolant inlet plenum having a least one inlet port for receiving the coolant fluid; and a coolant outlet plenum, the outlet ends of each fuel channel being coupled to the outlet plenum to discharge the coolant fluid into the coolant outlet plenum, wherein the inlet plenum is separated from the moderator by a fueling tubesheet and the inlet ends of the fuel channels are coupled to the fueling tubesheet, the pressure vessel defines the inlet plenum, the outlet plenum and the fueling tubesheet are provided within the pressure vessel, an external surface of the outlet plenum provides an inside surface of the inlet plenum, the pressure vessel is adapted to withstand the first pressure, and the fueling tubesheet and the fuel channels are adapted to withstand the difference between the first and second pressures. a) a pressure vessel for containing a pressurized heavy water moderator at a first pressure that is between 5 and 35 Mpa and; b) a plurality of fuel channels for a coolant fluid at a second pressure that is greater than the first pressure and fluidly connected at inlet ends thereof to a coolant supply conduit, the fuel channels adapted to receive nuclear fuel bundles and to be mounted within the pressure vessel and surrounded by the moderator, and outlet ends thereof being fluidly connected to a coolant outlet conduit to enable the coolant fluid to circulate from the coolant supply conduit through the fuel channels to the coolant outlet conduit, the plurality of fuel channels maintaining separation between the coolant fluid circulating within the fuel channels and the moderator; and c) a coolant inlet plenum, the inlet ends of each fuel channel being coupled to the coolant inlet plenum to receive coolant fluid and the coolant inlet plenum having a least one inlet port for receiving the coolant fluid; and a coolant outlet plenum, the outlet ends of each fuel channel being coupled to the outlet plenum to discharge the coolant fluid into the coolant outlet plenum, wherein the inlet plenum is separated from the moderator by a fueling tubesheet and the inlet ends of the fuel channels are coupled to the fueling tubesheet, the pressure vessel defines the inlet plenum, the outlet plenum and the fueling tubesheet are provided within the pressure vessel, an external surface of the outlet plenum provides an inside surface of the inlet plenum, the pressure vessel is adapted to withstand the first pressure, and the fueling tubesheet and the fuel channels are adapted to withstand the difference between the first and second pressures.