Neutronic analysis of a U-Mo-Al fuel and europium as burnable poison

This work presents the neutronic analysis of the U-Mo-Al dispersion fuel concerning uranium density increase and shows comparisons relatively to the U₃Si₂-Al fuel. The U₃Si₂-Al uranium density varied from 3.0 to 5.5 g U/cm³ while that of U-Mo-Al fuel varied from 4.0 to 7.5 g U/cm³. The molybdenum mass content in the former case varies from 7% to 10% in mass. Here, it is also proposed the utilization of burnable poison nuclides in the U-Mo-Al fuel meat. Since the fuel is metallic, gadolinium and europium were chosen as candidates to cope with this task. A recently developed cell code at IPEN (HRC) composed of the coupling of the codes HAMMER-TECHNION for the cell analysis, ROLAIDS for the actinide self-shielding calculations and CINDER-2 for the actinide and fission transmutation was employed for the neutronic analyses of U-Mo-Al. The simulated reactor core was similar to the one of RMB (Brazilian Multipurpose Reactor) composed of an array of 5 × 5 positions with 23 fuel elements and 2 aluminum blocks. A second analysis of the europium case employed the SERPENT code in an explicit core modeling. The burnup calculations were performed considering a power of 30 MW during three cycles of RMB and 30 days. The analyses revealed that the molybdenum content has a great impact in the core reactivity due to its high absorption cross section. A value of 7% was found adequate for the molybdenum mass content. The analyses also reveal that europium is a better burnable poison than gadolinium for the core cycle length and power level under consideration. However, for the U-Mo-Al case, k_∞ increases up to a maximum value and decreases afterwards. This is a striking result since the reactivity for the U-Mo-Al fuel does not increase steadily as verified for the U₃Si₂-Al case. Beyond a certain uranium density, the reactivity will decrease making useless the addition of more uranium.