An integrated block style heat exchanger for use with a heat pipe reactor having a plurality of heat pipes extending from a reactor core, the heat exchanger comprising: a plurality of primary channels each for receiving heat transferred from the core via a corresponding one of the plurality of heat pipes, the plurality of primary channels defined within a block of one or more materials, each primary channel extending in a first direction along a longitudinal axis of the heat exchanger from a first end of the heat exchanger to a second end of the heat exchanger; and a plurality of secondary channels defined within the block, each secondary channel being structured to transmit a flow of the secondary heat transfer medium through the heat exchanger from an inlet to an outlet of the heat exchanger, each secondary channel comprising: a first portion extending from the inlet to adjacent at least one of the primary channels; a second portion extending along, being situated in heat exchanger proximity to, and separated from, at the at least one of the primary channels; and a third portion extending from the second portion to the outlet, wherein each of the first portion and the third portion is disposed at a non-zero angle with respect to the second portion; and wherein the block comprises one or both of: a plurality of plates bonded together, with each plate defining at least a portion of one or more of the plurality of primary channels and/or the plurality of secondary channels, and/or a unitary piece of material formed from an additive manufacturing process; said heat exchanger being characterized in that the second portion of each secondary channel comprises a plurality of separate sub channels spaced around the at least one of the primary channels and extending between the first portion and the third portion of the secondary channel. a plurality of primary channels each for receiving heat transferred from the core via a corresponding one of the plurality of heat pipes, the plurality of primary channels defined within a block of one or more materials, each primary channel extending in a first direction along a longitudinal axis of the heat exchanger from a first end of the heat exchanger to a second end of the heat exchanger; and a plurality of secondary channels defined within the block, each secondary channel being structured to transmit a flow of the secondary heat transfer medium through the heat exchanger from an inlet to an outlet of the heat exchanger, each secondary channel comprising: a first portion extending from the inlet to adjacent at least one of the primary channels; a second portion extending along, being situated in heat exchanger proximity to, and separated from, at the at least one of the primary channels; and a third portion extending from the second portion to the outlet, wherein each of the first portion and the third portion is disposed at a non-zero angle with respect to the second portion; and a first portion extending from the inlet to adjacent at least one of the primary channels; a second portion extending along, being situated in heat exchanger proximity to, and separated from, at the at least one of the primary channels; and a third portion extending from the second portion to the outlet, wherein each of the first portion and the third portion is disposed at a non-zero angle with respect to the second portion; and wherein the block comprises one or both of: a plurality of plates bonded together, with each plate defining at least a portion of one or more of the plurality of primary channels and/or the plurality of secondary channels, and/or a unitary piece of material formed from an additive manufacturing process; said heat exchanger being characterized in that the second portion of each secondary channel comprises a plurality of separate sub channels spaced around the at least one of the primary channels and extending between the first portion and the third portion of the secondary channel. a plurality of plates bonded together, with each plate defining at least a portion of one or more of the plurality of primary channels and/or the plurality of secondary channels, and/or a unitary piece of material formed from an additive manufacturing process; said heat exchanger being characterized in that the second portion of each secondary channel comprises a plurality of separate sub channels spaced around the at least one of the primary channels and extending between the first portion and the third portion of the secondary channel.