Radionuclide monitoring is aimed to perform the complementary monitoring of environmental radioactivity, for purposes ranging from nuclear safety to detection of nuclear tests. The gamma-gamma coincidence technique in gamma ray spectrometry is a powerful method that enhances the sensitivity to nuclides of interest by suppressing the background from naturally occurring radioactivity. The aim of this project is to evaluate the use of modern, state-of-the-art segmented high-resolution High Purity Germanium (HPGe) detectors and apply the gamma-gamma coincidence technique to improve the sensitivity in radionuclide monitoring. It becomes essential to optimize the geometry to enhanced sensitivity before procuring a detector. Therefore, Monte Carlo simulation software, Geant4, will be employed to conduct simulations for better optimization of the detector design.