[en] Knowledge of coolant mass flow velocity and power distribution among coolant lines is important for determination of maximum attainable reactor power. This holds especially for operating conditions departing from the stationary case, i.e. transient conditions and natural circulation at power. A mathematical model and first results of measurement procedure are presented. Basic concept of the model is a combination of delay time measurement by means of cross-correlation and an absolute velocity measurement. Signals of gamma detectors in the vicinity of the core are used for measurement of power and correction of dynamic errors. Transfer of signals is carried out by means of optical fibres. (author)