Core Mechanical Dynamics Experiment in the Phenix Reactor

摘要

The French Phenix reactor was definitively shutdown in 2009 and is now under decommissioning preparatory phase. Before unloading the core, it has been decided to perform one last experimental campaign related to the mechanical dynamic behavior of the core sub-assemblies. It is aimed at providing data for code validation in the particular situation of sub-assembly displacements that would be generated from an initiator located inside the core. The resulting global sub-assembly movement is called core flowering. The context is linked to the four reactor trips that occurred at Phenix in 1989 and 1990 as a consequence of a sudden decrease in the core reactivity signal. A thorough investigation programme followed these events and led to the conclusion that the reactivity transient could be explained by an energy release inside the sub-assembly lattice putting into motion the core sub-assemblies. The origin of the energy release has not been clearly established so far. Some experimental moderated sub-assemblies loaded into the core at that time are suspected to have initiated the events. The experiments performed from June to October 2013 consisted in inserting into one given position a specifically manufactured device able to induce core flowering. The principle of this device is to push apart in a quasi-static way the six neighboring sub-assemblies and then to suddenly release the stress/constraints and let them oscillate freely. The radial displacement of some of the sub-assembly heads is monitored by two means: an ultrasonic system and a device penetrating through the slab and connected to one sub-assembly head. The reactor is in shutdown conditions. Two primary sodium temperatures were tested corresponding to different core compactness. Very accurate measurement of the sub-assembly displacements were recorded and compared to calculation results. The modeling of the flowering mechanism is complex. The fluid-structure interaction plays a significant role and must be taken into account. A calculation tool was developed based on the CEA in-house code CAST3M. It features a finite-element model of the core in which sub-assemblies are represented by beams surrounded by an acoustic fluid. The simplicity of the fluid behavior, combined with an astute homogenization technique, allows to carry out dynamic computations of the full core in a very limited amount of time, while still properly representing the physical coupling (lower vibration frequencies and higher damping). The model also considers frictional impacts of wrapper pads and plasticization of sub-assemblies spikes and pads. This test provided unique data, gathered on an actual reactor core. It will contribute to the validation of mechanical models and bring further elements likely to improve our understanding of the dynamic behavior of sub-assembly lattice in similarly designed Sodium-cooled fast breeder reactors.