A pressure-tube nuclear reactor comprising a. an outer shell vessel for containing a moderator at a first pressure; b. a coolant inlet plenum provided above the moderator; c. a coolant outlet plenum, wherein one of the coolant inlet plenum and coolant outlet plenum is within the other of the coolant inlet plenum and the coolant outlet plenum; d. a plurality of fuel channel assemblies for a coolant fluid at a second, higher pressure and fluidly connected at inlet ends thereof the coolant inlet plenum, the fuel channels mounted within the outer shell vessel and surrounded by the moderator and outlet ends thereof being fluidly connected to the coolant outlet plenum to enable the coolant fluid to circulate from the coolant inlet plenum through the fuel channels to the coolant outlet plenum; e. each fuel channel assembly extending in an axial direction and comprising i. an outer conduit, the outer conduit having an upper end and a lower end axially spaced apart from the upper end; ii. an inner conduit disposed at least partially within the outer conduit, the inner conduit having an upper end and a lower end, wherein one upper end of one of the inner and the outer conduits (172, 174) is in communication with the coolant inlet plenum and the other upper end of the other of the inner and outer conduits (172, 174) is in communication with the coolant outlet plenum, whereby coolant flows from the coolant inlet plenum through said one upper to the lower ends of the inner and outer conduits and through the other upper end to the coolant outlet plenum; iii. a fuel bundle chamber defined between an inner surface of the outer conduit and an outer surface of the inner conduit, the fuel bundle chamber at least partially laterally surrounding the inner conduit and configured to receive at least one nuclear fuel bundle, whereby coolant flow in the fuel bundle chamber passes through the fuel bundle; f. the plurality of fuel channel assemblies maintaining separation between the coolant fluid circulating within the fuel channels and the moderator; and g. a passive moderator cooling circuit extending between a passive inlet positioned toward the top of the outer shell vessel, a passive return positioned toward the bottom of the outer shell vessel and a heat exchanger provided between the passive inlet and the passive return. a. an outer shell vessel for containing a moderator at a first pressure; b. a coolant inlet plenum provided above the moderator; c. a coolant outlet plenum, wherein one of the coolant inlet plenum and coolant outlet plenum is within the other of the coolant inlet plenum and the coolant outlet plenum; d. a plurality of fuel channel assemblies for a coolant fluid at a second, higher pressure and fluidly connected at inlet ends thereof the coolant inlet plenum, the fuel channels mounted within the outer shell vessel and surrounded by the moderator and outlet ends thereof being fluidly connected to the coolant outlet plenum to enable the coolant fluid to circulate from the coolant inlet plenum through the fuel channels to the coolant outlet plenum; e. each fuel channel assembly extending in an axial direction and comprising i. an outer conduit, the outer conduit having an upper end and a lower end axially spaced apart from the upper end; ii. an inner conduit disposed at least partially within the outer conduit, the inner conduit having an upper end and a lower end, wherein one upper end of one of the inner and the outer conduits (172, 174) is in communication with the coolant inlet plenum and the other upper end of the other of the inner and outer conduits (172, 174) is in communication with the coolant outlet plenum, whereby coolant flows from the coolant inlet plenum through said one upper to the lower ends of the inner and outer conduits and through the other upper end to the coolant outlet plenum; iii. a fuel bundle chamber defined between an inner surface of the outer conduit and an outer surface of the inner conduit, the fuel bundle chamber at least partially laterally surrounding the inner conduit and configured to receive at least one nuclear fuel bundle, whereby coolant flow in the fuel bundle chamber passes through the fuel bundle; i. an outer conduit, the outer conduit having an upper end and a lower end axially spaced apart from the upper end; ii. an inner conduit disposed at least partially within the outer conduit, the inner conduit having an upper end and a lower end, wherein one upper end of one of the inner and the outer conduits (172, 174) is in communication with the coolant inlet plenum and the other upper end of the other of the inner and outer conduits (172, 174) is in communication with the coolant outlet plenum, whereby coolant flows from the coolant inlet plenum through said one upper to the lower ends of the inner and outer conduits and through the other upper end to the coolant outlet plenum; iii. a fuel bundle chamber defined between an inner surface of the outer conduit and an outer surface of the inner conduit, the fuel bundle chamber at least partially laterally surrounding the inner conduit and configured to receive at least one nuclear fuel bundle, whereby coolant flow in the fuel bundle chamber passes through the fuel bundle; f. the plurality of fuel channel assemblies maintaining separation between the coolant fluid circulating within the fuel channels and the moderator; and g. a passive moderator cooling circuit extending between a passive inlet positioned toward the top of the outer shell vessel, a passive return positioned toward the bottom of the outer shell vessel and a heat exchanger provided between the passive inlet and the passive return.