Ideal gas and multiphase flow through porous soil surrounding a nuclear power generating station after a core meltdown accident are analyzed with dimensionless variables. Results are applicable to many soil conditions. The extent of radioactive fluid penetration into the soil, predicted using methods originally developed by Morrison, indicates that years are required for activity to reach the ground surface if silty clays are present but only ∼ 10 h if a path of sand is included. Source pressure decay caused by mass flow into the soil is small if the normal conditions of a large containment shell volume and low soil permeability exist.