An additively manufactured heat exchanger configured to transfer heat between a first fluid and a second fluid, the heat exchanger comprising: an alternating plurality of hot layers and a plurality of cold layers (32, 132, 232, 332, 432), wherein each of the plurality of hot layers provides a flow path through the heat exchanger for the first fluid and is separated from each other of the plurality of hot layers by one of the plurality of cold layers (32, 132, 232, 332, 432) and each of the plurality of cold layers (32, 132, 232, 332, 432) provides a flow path through the heat exchanger for the second fluid, wherein: each of the plurality of hot layers comprises a plurality of hot flow assemblies (40, 140, 240, 340, 440), each of the plurality of hot flow assemblies (40, 140, 240, 340, 440) comprising: an inner channel (42, 142, 242, 342, 442) configured to port flow of the first fluid, wherein the inner channel (42, 142, 242, 342, 442) includes an inner channel wall (44, 144, 244, 344, 442); and a barrier channel (46, 146, 246, 346, 446) coaxially exterior to the inner channel (42, 142, 242, 342, 442) located between the inner channel wall (44, 144, 244, 344, 442) and a barrier channel wall (48, 148, 248, 348, 448), wherein the barrier channel (46, 146, 246, 346, 446) is configured to prevent mixing of the first fluid and the second fluid when one of the inner channel wall (44, 144, 244, 344, 442) and the barrier channel wall (48, 148, 248, 348, 448) ruptures and characterized in that that the heat exchanger is an additively manufactured heat exchanger and the barrier channel (46, 146, 246, 346, 446) contains unprocessed build powder provided by an additive manufacturing process. an alternating plurality of hot layers and a plurality of cold layers (32, 132, 232, 332, 432), wherein each of the plurality of hot layers provides a flow path through the heat exchanger for the first fluid and is separated from each other of the plurality of hot layers by one of the plurality of cold layers (32, 132, 232, 332, 432) and each of the plurality of cold layers (32, 132, 232, 332, 432) provides a flow path through the heat exchanger for the second fluid, wherein: each of the plurality of hot layers comprises a plurality of hot flow assemblies (40, 140, 240, 340, 440), each of the plurality of hot flow assemblies (40, 140, 240, 340, 440) comprising: an inner channel (42, 142, 242, 342, 442) configured to port flow of the first fluid, wherein the inner channel (42, 142, 242, 342, 442) includes an inner channel wall (44, 144, 244, 344, 442); and a barrier channel (46, 146, 246, 346, 446) coaxially exterior to the inner channel (42, 142, 242, 342, 442) located between the inner channel wall (44, 144, 244, 344, 442) and a barrier channel wall (48, 148, 248, 348, 448), wherein the barrier channel (46, 146, 246, 346, 446) is configured to prevent mixing of the first fluid and the second fluid when one of the inner channel wall (44, 144, 244, 344, 442) and the barrier channel wall (48, 148, 248, 348, 448) ruptures and characterized in that that the heat exchanger is an additively manufactured heat exchanger and the barrier channel (46, 146, 246, 346, 446) contains unprocessed build powder provided by an additive manufacturing process. an inner channel (42, 142, 242, 342, 442) configured to port flow of the first fluid, wherein the inner channel (42, 142, 242, 342, 442) includes an inner channel wall (44, 144, 244, 344, 442); and a barrier channel (46, 146, 246, 346, 446) coaxially exterior to the inner channel (42, 142, 242, 342, 442) located between the inner channel wall (44, 144, 244, 344, 442) and a barrier channel wall (48, 148, 248, 348, 448), wherein the barrier channel (46, 146, 246, 346, 446) is configured to prevent mixing of the first fluid and the second fluid when one of the inner channel wall (44, 144, 244, 344, 442) and the barrier channel wall (48, 148, 248, 348, 448) ruptures and characterized in that that the heat exchanger is an additively manufactured heat exchanger and the barrier channel (46, 146, 246, 346, 446) contains unprocessed build powder provided by an additive manufacturing process.