Noise Applications in Light Water Reactors with Traveling Perturbations

Doctoral thesis, 2012

Neutron noise induced by perturbations traveling with the coolant of light water reactors (LWRs) is investigated. Different methods to simulate the effect of propagating perturbations are considered. The studies are performed in both open- and closed-loop systems and summarized in three chapters. In the first chapter, the space-dependence of the neutron noise due to propagating perturbations calculated in one-group theory and one dimension in a pressurized water reactor (PWR) is investigated. A full analytical solution, obtained by the use of Green's function technique, is analyzed for different frequencies and different system sizes. An interesting new interference effect between the point-kinetic and space-dependent components of the induced noise is discovered and interpreted in physical terms. A similar investigation is performed in two-group theory for four reactor systems with different neutron spectra. The goal is to investigate the dependence of the properties of the induced neutron noise on the neutron spectrum. The presence of the fluctuations of several cross sections is also analyzed and resulted in qualitatively and quantitatively new characteristics of the induced noise. Further, a simple numerical Monte Carlo-based model to simulate the boiling process in a boiling water reactor (BWR) heated channel, is constructed. The output of the model is then used to estimate the local component of the neutron noise induced by density fluctuations in the coolant numerically convoluting it with proper transfer functions. In the second chapter, a four-heated channel reduced order model (ROM), accounting for the first three neutronic modes, is constructed to study both global and regional instabilities. Some additional modifications compared with the earlier-developed models are performed to improve the consistency of the model. It is shown that the ROM is capable to reproduce the main features of core-wide instabilities. Moreover, it is proven that the inclusion of both azimuthal modes brings some importance for the correct identification of stability boundaries. The ROM is also extended to simulate the effect of local instabilities, such as the Forsmark-1 instability event of 1996/1997. A good qualitative agreement with real measurements is found. In the last chapter, a number of the applications of the noise diagnostics based on the foregoing calculations are discussed. The case when the neutronic response of the reactor is affected by a non-white driving force (propagating perturbation) is studied. It is also investigated how the accuracy of the determination of the so-called decay ratio (DR) of the system, based on the assumption of a white noise driving force, deteriorates with deviations from the white noise character of the driving force. Furthermore, the earlier developed ROM is applied to analyze what stability indicators other than the DR can be used to describe the stability of the system. As a candidate, the coupling reactivity coefficients are chosen and their dependence on the DR is investigated. It is shown that such a dependence deviates form the conventional one, presumably caused by the inherent inertia of the system. Finally, two techniques, one based on the break-frequency of auto power spectral density (APSD) of the neutron noise and another on the transit times of propagating void fluctuations are discussed for reconstructing the axial void profile from the Monte-Carlo simulated neutron noise. It is shown that both methods provide promising results.