In this paper, the optical properties of lead-thorium (Pb-Th), lead-uranium (Pb-U), and lead-cobalt (Pb-Co) nuclear nanoparticles in a container filled with water are simulated and modeled employing finite element analysis (FEA) for diverse particle sizes. The simulated absorption maxima of electronic excitations of nuclear nanoparticles such as Pb-U are red-shifted from 375 to 380 nm for the first peak, from 595 to 600 nm for the second peak, and from 730 to 740 nm for the third peak with increasing particle sizes from core U: 7 nm and shell Pb: 2 nm to core U: 9 nm and shell Pb: 2 nm. Moreover, the absorption peak of the Pb-Th and Pb-Co nanoparticles is red-shifted by increasing the particle size. The FEA-simulated optical band gap energies of Pb-Th, Pb-U, and Pb-Co nanoparticles were also obtained, and the data decreased with increasing the particle size. FEA-based simulations have disclosed restrictions intended for Pb-Th and Pb-Co nanoparticles size greater than 9 nm and for Pb-U nanoparticles size larger than 11 nm. The simulation method in this research enables the prediction of optical properties and contributes to the understanding and design of Pb-Th, Pb-U, and Pb-Co nanoparticles in the water container before manufacturing and functionalizing them. The work here is of particular interest in the nuclear security domain and in the nondestructive, remote detection of special nuclear materials (SNM) in water-filled cargo containers, whose manual inspection imposes physical and financial challenges.