Régime vibratoire des aiguilles et assemblages combustibles du réacteur phénix

Ingénieurs C.E.A., Cadarache

Résumé

The fuel assemblies of high-speed sodium-cooled reactors are an important component which operates under very severe conditions (temperature, sodium flow velocity, etc.). The Phenix fuel assembly is shown in Fig. 1. Due to fuel element or assembly clearances, vibration can be a problem, and this has necessitated a certain number of studies and tests. The tests were carried out with water at ambient temperature on a full-size model combining mechanical structural similarity and Froude hydraulic similarity. The main source of excitation is liable to be situated within the flow inside the assembly and underneath the core cover at the otltlet. Due to the symetrical reactor layout, only one-sixth of the core and cover were featured on the model, together with their various flow regions (Fig. 2). The assembly under vibration test was installed inside a very rigid tube (Figs. 3 and 4) by means of which loads could be applied to the assembly plates. The tube could be set up in three different positions. The other assemblies were simulated by ordinary tubes. Accelerometers and strain gauges were used for the measurements, with provision for recording the signals on magnetic tape and processing them, especially in order to establish the spectral density. After measuring the natural frequencies of the assemblies in air and water with various loads on the plates, flow tests were carried out. The recordings from these tests are now being analysed, but the first results show a very low vibration amplitude, so that there does not appear to be much danger as far as the Phenix fuel assemblies are concerned. Fuel element vibration study. Within the assembly the fuel elements are laid out in a triangular pattern, their spacing being established by means of a spiral wire round each element. As the contact levels were thus indefinite, the method chosen to detect vibration consisted in measuring the EMF produced by the movement of a conductor running along the spiral wire through a magnetic field. This method was tried out on a separate mock-up, which was also used to investigate the natural frequencies of a fuel element. Tests were then carried out on various fuel element bundles, including Phenix, and it was found that the latter were not affected by any measurable vibration. The remaining results are being analysed by auto-correlation methods. The next stage in this vibration research programme will be to establish the effects of the various parameters with which the component designer is liable to be concerned.