Data from a model Passive Residual Heat Removal (PRHR) system operated in the ROSA facility during simulated loss-of-coolant accidents were compared with heat transfer calculations using a one-dimensional steady-state computer model. The calculations agree reasonably well with the data. The PRHR consists of a “C” shaped tube bundle submerged in a large tank filled with water. The calculations show that subcooled nucleate boiling occurred near the heat exchanger inlet. The boiling region was followed by turbulent and then laminar natural convection regions for horizontal and vertical the tubes. Transition boiling, with intermittently occuring patches of steam attached to the tubes, may have occurred near the inlet for the higher heat transfer tests, but did not significantly affect the overall heat transfer process.