A heat exchanger, methods therefor and a nuclear fission reactor system. The heat exchanger comprises a heat exchanger body defining an exit plenum chamber therein shaped for uniform flow of a hot primary heat transfer fluid through the chamber. A plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between the heat transfer members. The flow passages open into the exit plenum chamber. Spacing of the heat transfer members by the predetermined distance evenly distributes flow of the primary heat transfer fluid through the flow passages, across the surfaces of the heat transfer members and into the exit plenum chamber. Each heat transfer member defines a flow channel therethrough for flow of a cooler secondary heat transfer fluid. Heat transfer occurs from the hot primary heat transfer fluid to the cooler secondary heat transfer fluid as the primary heat transfer fluid flows through the chamber and as the secondary heat transfer fluid simultaneously flows through the flow channel.