Numerical simulator for noise diagnostic investigations in NPPs

A coupled thermohydraulics and neutron model is used to simulate signals of thermocouples, ex-core and in-core neutron detectors of nuclear power plants (NPP). Noise sources are generated as time functions and the dynamic behavior of the reactor core is modeled by one-dimensional two-group diffusion equations coupled with an axial thermohydraulics model. These equations are solved by numerical methods and the resulting time series are considered as virtual measurements. We show that one can model only a finite set of noise sources with high accuracy by this approach because of the finite nature of numerical methods. The selection of length of space segments is presented and the effect of space aliasing is briefly discussed. An automatic stepsize selection algorithm is introduced which was applied successfully in the simulator. Simulation results are analyzed and compared with real measurements by studying disturbance propagation in the coolant