Development of methods for the determination of reactivity from flux measurements in nuclear reactors

Accelerator Driven systems (ADS) have been proposed as safe and reliable nuclear systems for energy production and, especially, for the transmutation of nuclear radioactive waste, thus contributing to a safer and non-proliferating use of nuclear energy for peaceful uses. The ADS can operate at a higher safety level, since, being driven by an external source, is always characterized by a sub-criticality condition and can thus be shut down by simply switching off the source. To ensure that the system always remain at a sub-critical level, an on-line reactivity monitoring is needed during both normal operation and transient conditions. Research programs such as MUSE, FREYA and GUINEVERE were involved in the development of reactivity measurement methods for ADS. At Politecnico di Torino a method that is suitable for both on-line and off-line reactivity monitoring has been developed and tested. The method has been given the name MAρTA (Monitoring Algorithm for Reactivity Transient Analysis). The method is based on an inverse point kinetic approach which reconstructs the reactivity from the system neutron flux or power measurement obtained instantaneously by a localized neutron detector. The adoption of point kinetics for the reactivity reconstruction is due to the simplicity of the mathematical formulation of the inverse approach. However, the accuracy of the reactivity prediction by point kinetics can deteriorate owing to spatial and spectral effects. Furthermore the kinetic parameters are needed through evaluations or independent measurements. The research activity during the Ph.D. program has been devoted to assess the MAρTA method for reactivity predictions using computationally simulated experiments, evaluated by means of time-dependent diffusion calculations. A technique to improve the prediction when strong spatial and spectral effects are present is proposed and tested. A method for the measurement of the ratio between the effective fraction of delayed neutrons and the effective mean prompt generation time through oscillated experiments has also been proposed and tested. The method has also been applied to real experimental cases available from the Kyoto University Critical Assembly (KUCA). Part of the work carried out is included as a contribution to the IAEA Collaboration Research Project on Accelerator-Driven System (ADS). The collaboration with INFN is also acknowledged.