Hydrogen is the major flammable gas observed in the dome space of each million-gallon radioactive waste storage tank at the U.S. Department of Energy Hanford Site. Semiempirical rate equations are derived to estimate hydrogen generation based on chemical reactions, radiolysis of water and organic compounds, and corrosion. The rate equations account for tank waste composition, temperature, radiation dose rate, and liquid fraction. Numerical parameters are established by the analysis of gas generation kinetic data from actual waste samples, literature data, and waste characterization and field surveillance data. The model improvement includes development of refined water radiolysis equations, accounting of total alpha radiation contribution to both water and organic radiolysis, new parameterization on the rate equations of organic thermolysis and radiolysis with extra tank waste gas generation test data, and revised corrosion rate equations. A comparison of the generation rates observed in the field with the rates calculated for 28 tanks shows agreement within a factor of 3. The model serves as a useful tool to evaluate flammable gas issues to support Hanford operations.