Measurement of kinetic parameters by oscillated experiments

The availability of reliable values of the kinetic parameters (effective delayed neutron fraction β and effective mean neutron generation time Λ ) is important for reactor kinetic analyses and for reactivity monitoring wh en adopting the point kinetic approach [1]. The kinetic parameters may be obtained either by simulations (Monte Carlo or deterministic) of the system or by direct measurements . Several methods have been proposed for the direct experimental determination of the kinetic parameters, e.g. control rod drops and noise analysis approach es, e.g. [2, 3]. In this work, the problem of the determination of kinetic parameters in subcritical systems is tackled . The system is subjected to an oscillated source and the value of the ratio β / Λ is related to a measurable quantity when the system response reaches an equilibrium condition. In the analysis presented, a pure frequency is considered: when analyzing more complicated source periodic behaviours, a Fourier analysis can allow to isolate the contributions of individual frequencies. The phase difference between the oscillating source and the system response is adopted as the experimental observable. The method for the interpretation of the measurements is based on the standard point kinetic model. The response of the system is obtained by localized neutron detectors and, therefore, spatial and spectral effects may also affect the results. This drawback needs some further discussion and will be considered in the future work . The phase shift between the source signal and the system response may be analytically retrieved by Laplace transforming the point kinetic equations in the presence of an oscillated source. An explicit formula relates the phase shift to the ratio β / Λ and to the frequency of the source. Based on this relationship, the ratio β / Λ can be determined. To validate the technique, the experimental data are simulated by a numerical code solving the time-dependent diffusion equations for a source-driven system. The work allows to assess the possibility of the technique and its limits of applicability