Assessment of the structural-dynamic effects caused by the core compaction

Nuclear reactors have to be maintained in a critical state so as to properly manage the chain fission process. Nuclear stability considerations dictate that the geometry of the core be closely controlled at all times: therefore any modification of it must be predictable, compatible with the requirements of the interfacing reactor systems and safely manageable by implemented control mechanisms. This study deals with the evaluation of the deformation of the core (and restraint system) geometry due to dynamic perturbations occurring in some case and determining core compaction – possibly causing reactivity insertion. To the purpose, a finite element model of the overall reactor system and of the inner vessel component that enclose and surround the core has been implemented. Suitable boundary and initial conditions, such as that one related to the core sub-assemblies mass, the restrictions imposed to the geometrical in-structures connections, etc. have been assumed to numerically investigate the dynamic response of the structures, since confidence was established by sensitivity analyses of size and type of the adopted elements carried out in the framework of PAR 2012. Results of the analyses carried out are presented and discussed, highlighting also how contact and friction forces (due to the contact condition mong internal structures) may influence the behaviour of structure and the evolution of compaction scenario. Results will also represent the input data to adopt for further calculations on core reactivity changes.