Medical radioisotope production using neutron irradiation via fission reaction requires a sufficient neutron source. The Kartini reactor has been proposed and studied to become a neutron source for radioisotope production under the Subcritical Assembly for 99Mo Production (SAMOP) project, which uses uranyl nitrate solution as the irradiation target. A full-scale experiment involving a liquid fission product is difficult to conduct and requires facility rearrangement to reduce the risk of contamination. Although a small-batch experiment is safer to perform, a pre-experimental assessment is necessary to address the practicality of production and the accompanying problems. The goals of this assessment are (1) to characterize the Kartini reactor irradiation facilities’ flux through experiment and Monte Carlo benchmark simulation, (2) to predict the irradiation product inventory in relation to the variation of uranium concentration and the measured flux, and (3) to predict the irradiated sample gamma spectrum reading using high-purity germanium detector simulation. The irradiation simulation uses natural uranium as a control parameter, which caused the irradiation inventory dominated by actinides from transmutation. The simulation also presents the possibility of instant small-batch 99Mo production using the measured Lazy Susan facilities’ flux from a neutronic perspective. The qualitative assessment of the predicted irradiation inventory and its spectrum reading from different sample concentrations are discussed along with the recommendation and possible action to improve the experiment or future production process.