Abstract: A method and computer tool apparatus for determining the final state parameters of a power plant thermohydraulic system in which the thermohydraulic system is represented as a plurality of nodes having a flow regime map monitoring the thermodynamic system parameters. An equation set is provided including a first plurality of field equations including only one momentum equation, a drift equation for correlating the field equations with their variables, constitutive correlations and drift flux coefficients determined in response to the flow regime map so that there is closure between the field equations and the drift flux equation, and state equations for dependent thermodynamic variables. Final state parameters for the monitored thermohydraulic system are determined by solving the equation set in response to the sensed parameters by a momentum method and pressure search method, using a constant time step in the calculations.