THERMIT is a computer code that solves the time dependent two-fluid thermal-hydraulic equations in three space dimensions along with the fuel pin and clad temperatures. Its purpose is to predict the thermal-hydraulic response of a modeling of a reactor core to situations which do not lead to reconfiguration of the core geometry. Both boiling and pressurized water reactors may be modeled with the code. From a mathematical viewpoint, the code is semi-implicit; hence it is flow Courant limited, but the interphase exchange terms are treated in a fully implicit manner. THERMIT does not contain a steady algorithm but does contain acceleration features which speed up the determination of the steady state.