A two-phase level swell will occur in pressurizers, steam generators, and others types of nuclear plant equipment holding large volumes of saturated water in response to a pressure reduction such as would be encountered in a line break event or a relief valve actuation. A simplified, but physically based one-dimensional model especially well suited to plant or system analysis is presented. Marviken test T-11 presents a unique source of data for confirmation of the analysis both in terms of the scale of the test itself and the number and variation of instrument locations.