The CHALMEX process represents a solvent extraction–based partitioning strategy developed at Chalmers University of Technology for the recycling of spent nuclear fuel. The process is designed to achieve the quantitative recovery of actinides using dedicated solvent extraction equipment, including mixer-settlers and centrifugal contactors. From a nuclear safety perspective, mixer-settlers are of particular concern, as they may give rise to localized criticality scenarios with potentially severe radiological consequences. The settler compartment is especially critical, as it operates without mechanical agitation and relies exclusively on gravity-driven phase disengagement. Such conditions may lead to phase holdup gradients, stratification effects, and localized accumulation of fissile material (e.g., plutonium-239). Accordingly, the handling of fissile material at any stage of the process must be preceded by rigorous criticality safety assessments, including the determination of the effective neutron multiplication factor (k_eff), to demonstrate compliance with subcriticality requirements. While criticality (k_eff=1) is an intended and controlled condition in nuclear reactors, it must be strictly excluded in reprocessing and separation systems, which are required to remain subcritical under all normal, abnormal, and credible accidental conditions, typically with conservative safety margins (e.g., k_eff< 0.9), in accordance with international criticality safety principles. In this work, a computational methodology is developed to evaluate the effective neutron multiplication factor using Monte Carlo neutron transport simulations. The methodology combines detailed representations of the CHALMEX process stages with physically consistent modelling of phase behavior and actinide distributions within the settler compartment. The computational workflow starts with the determination of the isotopic composition of a spent nuclear fuel pin from a standard PWR 17×17 square-lattice assembly at selected burnup values (35, 40 and 45 GWd/MTU) after 5 years of pool cooling. The resulting nuclide inventories are then used as input for the CHALMEX system, and the actinide content is mapped to the organic phase in the settler compartment consistently with the measured extraction performance (≈99% actinides extracted under lab-scale conditions after three extraction stages). To capture and describe the random spatial distribution of actinides, a Voronoi-tessellation–based stochastic method is employed, and the corresponding effective multiplication factor (k_eff) is evaluated using SERPENT 2.2.1.