Structured analysis of density of powder-pressed fuel pellets for zero power salt reactors

• A multiscale analysis of fuel density was performed for a zero power reactor. • Density measurements of different composition of NaCl – UCl 4 salts mixtures at room temperature. • Analytical techniques for density measurement and understanding of complex formation. • Salt-density and fissile density data have provided. In a zero power reactor loaded with salt fuel, with very low burnup and heat rate, the thermal energy released during operation is so small that the fuel remains solid at room temperature, during low power operation. The considered Zero Power Salt Reactor (ZPSR) is in the form of salt (chloride) fast reactor with high energy neutron flux during operation. Density is one of the critical thermo-physical properties of any reactor. Determining the density of the salt system (NaCl-UCl 4 ) is very important to evaluate salt fuel-reactivity and behaviour of the core. A multi-scale approach is followed to perform the analytical methodology. The analytical methods of measuring the densities of salt components involving volume and weigh determination, pores volume measurements in the salt fuel by Scanning Electron Microscopy (SEM) at the µm scale and amorphization ratio or any defects along with density quantification by X-Ray Diffraction (XRD) for nm features. Fuel density of the pellet (at the mm − µm scale) was first estimated on a geometrical basis considering its weight and volume. The measurement of the densities of salt material was then carried out tracking pores and cracks using 2D (SEM), surface investigation techniques. For the nanoscale level, grains analysis was performed using X-ray diffraction (XRD), revealing the defects, vacancies and swelled part of the grains. In addition, XRD reveals formation of phases including Na 2 UCl 6 complex. The NaCl − UCl 4 compositions studied in this paper correspond to the important points on the phase diagram. Densities were measured by changing compositions along with the identification of the Na 2 UCl 6 complex phase. Results obtained were in good agreement with the ideal mixed phase (heterogeneous) density model, thereby establishing that SEM and XRD are suitable techniques to measure the densities of salt fuels. This work provides salt density data (matter and fissile density) which can be used to correct the reactivity of the fuel in the ZPSR core.