Through the use of potential flow theory, an expression for two-dimensional axisymmetric pressure distribution in an “infinite” plenum for a coolant-expulsion process in a single-subassembly accident is obtained. An expression for the temporal distribution of the corresponding spatially averaged pressure at the exit of a fuel subassembly is also obtained. The illustrative example considered indicates that the use of a fixed-pressure boundary condition instead of a time-dependent-pressure boundary condition at the exit (or at the inlet) of a subassembly is not valid for very rapid transients, such as due to molten fuel-coolant interaction or to rapid release of fission gas from simultaneous or near-simultaneous breach of several pins near the top of the core region. Furthermore, the results of sample calculations presented for an FFTF subassembly indicate the possibility that, for a sufficiently rapid transient, the transient pressure in the exit region of the subassembly can fall to the saturation pressure of the sodium and result in local boiling during the expulsion in the exit region of the subassembly. The expression developed here for the transient, spatially averaged pressure distribution in the plenum at the exit of a subassembly could be coupled with the one-dimensional type of analysis of the expulsion process in the fuel subassembly.