Development and Application of Reactor Noise Diagnostics

Sammanfattning: A number of problems in reactor noise diagnostics have been investigated within the framework of the present thesis. The six papers presented cover three relatively different areas, namely the use of analytical calculations of the neutron noise in simple reactor models, some aspects of boiling water reactor (BWR) stability and diagnostics of core barrel motion in pressurized water reactors (PWRs). The noise induced by small vibrations of a strong absorber has been the subject of several previous investigations. For a conventional -function source model, the equations can not be linearized in the traditional manner. Thus, a new source model, which is called the model, was developed. The correct solution has been derived in the model for both 1-D and 2-D reactor models. Recently, several reactor diagnostic problems have occurred which include a control rod partially inserted into the reactor core. In order to study such problems, we have developed an analytically solvable, axially non-homogenous, 2-D reactor model. This model has also been used to study the noise induced by a rod manoeuvring experiment. Comparisons of the noise with the results of different reactor kinetic approximations have yielded information on the validity of the approximations in this relatively realistic model. In case of an instability event in a BWR, the noise may consist of one or several co-existing modes of oscillation and besides the fundamental mode, a regional first azimuthal mode has been observed in e.g. the Swedish BWR Ringhals-1. In order to determine the different stability characteristics of the different modes separately, it is important to be able to decompose the noise into its mode constituents. A separation method based on factorisation of the flux has been attempted previously, but without success. The reason for the failure of the factorisation method is the presence of the local component of the noise and its axial correlation properties. In the paper presented here, we elaborate a modified and successful factorisation technique that takes the noise structure and the approximations performed into account. Due to an unseated fuel element, a local density wave oscillation appeared in the Swedish BWR Forsmark-1 in 1996. It is of great practical importance to locate the responsible element as soon as possible. We have developed a method for the localisation of such a thermohydraulic channel instability. The localisation method was tested on simulated data and then applied to a measurement taken in Forsmark-1 in early 1997. The results demonstrate the applicability and accuracy of the method. The noise present in the Ringhals PWRs has been investigated and in the course of this work, we have developed a new strategy towards the analysis of core barrel vibrations (CBM). This new approach unites the CBM analysis in the time and frequency domains. We have also developed and applied a new model for the determination of vibration properties such as the amplitude, amount of anisotropy and the preferred direction of the vibrations. This method is quantitative and suitable for long-term trend analysis of CBM vibrations.