Experimental transmission of sample A5 compared to TRIPOLI-4^® results. The left-hand plots show TRIPOLI-4^® simulations performed on a homogenous mixture of UO₂ and Gd₂O₃ powders without microspheres. The middle plots show TRIPOLI-4^® simulations obtained with 814 microspheres of Gd₂O₃ (380 μm) randomly distributed within each pellet. Therefore, 3256 microspheres were generated in the sample, which is composed of a stack of four pellets (Tab. 1). The right-hand plot shows TRIPOLI-4^® simulations performed with particle self-shielded neutron cross-sections.