A heat transfer system (102) includes a heat exchanger (100) configured to transfer heat between a first fluid and a second fluid. The heat exchanger includes an outer sleeve (114) and a microchannel unit (116). The outer sleeve has an interior surface (200) defining a cavity (204) and an exterior surface (202) configured to be in direct thermal communication with the first fluid. The microchannel unit is received in the cavity. The microchannel unit has a plurality of microchannel s (230) configured to receive the second fluid therein and be in direct thermal communication with the second fluid. The microchannel unit has an exterior surface (232) in direct thermal communication with the interior surface of the outer sleeve to transfer heat therebetween. At least one of the interior surface of the outer sleeve and the exterior surface of the microchannel unit has grooves (260) defining breach channels (262) for the first fluid or the second fluid should a breach in the outer sleeve or the microchannel unit occur.
