Studies of neutronic coupling by reactor noise analysis in the time domain for facilities of RCN, Petten

Abstract

After a review of existing literature on determination of neutronic coupling from reactor noise, the paper gives a survey of the work on this subject done from 1966 at RCN, Petten, using noise analysis in the time domain. A summary is given of the formulas used for the analysis of the noise. In particular a matrix inversion method is described, by which a matrix of kinetic parameters (“reactor matrix”) and decay constants of the prompt modes can be found from the set of correlation functions. The experimental set-ups are shortly described. Signals from ionisation chambers and from neutron counters have been processed with digital computers. Auto- and corss-correlation functions were determined by direct calculation or by a multi-detector Rossi-α algorithm. Coupling measurements have been performed in the LFR with two-slab core and annular core, in the STEK-Argonaut with two to four coupled thermal cores, and with the various fast-thermal coupled systems STEK-4000, -3000, -2000 and -1000 with a detector in each zone. Experimental results are shown, namely an early determination of coupling reactivity for two weakly coupled cores, the application of the matrix inversion method for various thermal cores, and the attempts to determine the reactor matrix in fast-thermal cores. It was found that the methods of analysis worked well for the various weakly coupled thermal cores. The measurements in the failry strongly coupled core STEK-4000 could be interpreted by a two-point model. The matrix inversion method could be used for the other STEK cores with more pronounced higher mode effects. The two-point analysis did not lead to consistent results, however. Also Avery model calculations have been applied for these cores, but led to results different from the experimental data. It is concluded that the straightforward two-zone Avery model does not apply here and that a two-mode analysis is not sufficient to describe a system like STEK. An extension of the present theory to enable analysis for more nodes (or modes) than detectors is needed.