[en] The Integrated Safety Analysis (ISA) carried out at the Juzbado Fuel Fabrication Plant aims to identify all the potential accident sequences that might occur during the plant's lifetime. During the manufacturing process of green pellets, and particularly on the blending and homogenization stages, where hydrogenated additives are involved, nuclear criticality safety focuses on neutron moderation, which is controlled by limiting the Hydrogen / Uranium atomic ratio or H/U. The ISA has identified potential accident sequences that challenge the H/U safe-limit value, both for uniform and non-uniform over-moderation scenarios. Three accident sequences have been addressed, the so called Less Uranium Powder, Process Improperly Done and More Hydrogenated Additive scenarios. These three sequences lead to exceeding the H/U limit value and so, we analyzed in detail how reactivity behaves during them in order to achieve better understanding of the fabrication process from a criticality safety point of view. In this way, the 'Less Uranium Powder' sequence considers a fixed amount of hydrogenated additive, homogeneously mixed with a decreasing mass of uranium. Although the H/U safe limit value is exceeded, we see that reactivity slumps as uranium mass decreases. Regarding the 'Process Improperly Done' sequence, it is assumed that hydrogenated additive is not homogeneously mixed with uranium, giving place to an over-moderated region inside the equipment. This region is modelled as a spherical shaped region where uranium is increasingly added, reaching a homogeneous mixture at the end of the sequence. Under these conditions, we see that during the process the system remains subcritical, but the reactivity increases from a certain starting value and so, the safety margin is decreased. On the contrary, the sequence 'More Hydrogenated Additive' considers an increasing mass of hydrogenated additive, homogeneously mixed with the uranium. This sequence not only exceeds the H/U limit value, but also makes the system go critical, which allows us to obtain the maximum additive mass ensuring Subcriticality. Finally, according to the ISA methodology, we were compelled to implement IROFS (Items Relied on For Safety) to ensure that the process complies with the performance requirements established by the regulations. One of those IROFS was developed from the conclusions of the aforementioned safety assessment, and consists of a device that acts as a barrier to avoid the occurrence of the 'More Hydrogenated Additive' and 'Process Improperly Done' sequences. This system firstly limits the mass of additive that can be poured on the equipment by means of a passive-engineered control (pre-set volume container) and secondly by avoiding the pouring of a second additive container into the blend through an active-engineered control. Besides, the device implements an alveolar valve, ensuring that the pouring of additive is fractioned over time through a dosage/time preset program. (authors)