The economical operation of any nuclear power plant requires an accurate, consistent and permanent supervision of process states and health of major equipments that means performance monitoring, analysis, diagnostics, and optimization. The performance of nuclear power plant depends on many parameters. Any even the smallest improvement leads to a significant energy saving. The actual world tendency cares for all parameters which can enable economical work of nuclear power plant. There is a wide range of problems connected with measurement inaccuracies caused by stochastic and systematic errors and with calculating expected power and efficiency of nuclear power plant. An actual state can be determined by measurement data reconciliation. Data reconciliation serves for getting consistent set of process variables by eliminating random measurement errors. It enables detection of gross errors and thus an early detection of sensor degradations. An expected state can be predicted by a process simulation model. The simulation model is used for detailed calculation of expected or desirable performance of thermal cycle and its major equipments at varying ambient conditions and different steady-state operation modes. Outputs of data reconciliation and process simulation are then used to determine residuals between actual and expected performance of thermal cycle and its equipments thus providing performance engineers and operators (in near-real time) with information about the condition of thermal cycle and major equipments. Processing of residuals enables an early detection of equipment degradations, incorrect adjustment of the process parameters or sensor drifts or failures. It helps to improve operation and predictive maintenance and so reduce plant operation and maintenance costs. Comparison between actual and expected states is based on assumption that it is possible to have a process measurement which is precise but not accurate. Process optimization is a determination of setpoints for nuclear power plant operation or/and flow-sheet structure changes to obtain the maximum of power output and/or thermal efficiency or the maximum/minimum of the other optimization criterion under defined constraints. Some examples are taken from VVER 440 type of nuclear power plant.

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