Messa a punto e sviluppo di metodologie e analisi per la valutazione preliminare del fenomeno della “core compaction”

Nuclear reactors in operation have to be maintained in a critical state in order to keep the chain fission process stationary and under control. The safety priority is therefore to keep the reactivity known and under control. Nuclear stability considerations dictate that the geometry of the core be closely controlled at all times. This study focuses on the deformations of core (and restraint system) geometry due to the dynamic perturbations. Since the primary function of the core is to provide the reactivity under control, any modification of its geometry must be predictable and safe. The same constraint must be fulfilled by the core restraint system that must be/remain compatible with the requirements of the interfacing reactor systems. It is therefore essential to set up/develop an overall and reliable methodological approach to be used in designing the core system (all structures and components characterizing the core region) and evaluating its performance under operation and accident condition. Modelling the dynamic behaviour of a LMR core is particularly needed for seismic design purpose and more generally for the study of dynamic solicitations due to internal or external accidents. A special attention should be given to these solicitations that could deform the behaviour of core system and of each fuel assembly.This preliminary study firstly presents a way to simplify the problem, by adopting substructure approach to preliminary and general analyse the dynamic behaviour of a LFR core region. To the aim a simplified numerical modelling was adopted, in which the fuel elements and assemblies were represented as lumped mass distributed on the fuel supporting plate; contact points between adjacent assemblies as well as the thin fluid layers between assemblies were not considered at this stage of the assessment. The obtained preliminary numerical results, for the implemented models, highlighted that the displacements at the core region resulted of about 2 cm. Sensitivity analysis was also carried out to evaluate the influence of the mesh size and element type on the dynamic response of structures analysed. Future further developments are of course necessary to determine with more accuracy the deformations of (sub)assemblies and element/pad cross-section undergoing compaction.