A simple method has been developed for calculation of transient heat losses that occur as a hot fluid produced during a liquid-metal fast breeder reactor hypothetical core-disruptive accident expands through the fission-gas plenum region. The heat-conduction equation of the plenum cladding is formally solved by the Laplace transform for a time-dependent cladding surface temperature, and the resulting solution is numerically evaluated using an integration method based on the trapezoidal rule. The expanding hot fluid may be a two-phase mixture of sodium produced by a fuel-coolant interaction or a two-phase mixture of fuel produced by a severe nuclear excursion. Illustrative calculations have been performed considering a hypothetical fuel-coolant interaction in the Fast Flux Test Facility core.