The authors have proposed a technique using special metal structures to efficiently convert gamma rays to low-energy electrons, with possible applications such as detoxification of water and hydrogen production using gamma rays from radioactive waste. The present study employed the Monte Carlo N-Particle (MCNP) transport code to understand in detail the mechanisms of low-energy photon and electron generation from gamma rays in water vessels containing various metal structures. The study demonstrated that the amount of low-energy electrons in water generally increases with (a) the Z number of the metal, (b) the volume of the metal, (c) the ability of low-energy electrons to escape from the metal and into the water region, (d) the closeness with adjacent metal plates, and (e) the ability of metal plates to reflect high-energy primary photons to delay their exit from the vessel. Based on these basic understandings, more sophisticated structures were designed and compared in computer simulations. The simulation results indicated that closed-type structures, such as a honeycomb tube, can provide better performance in terms of efficiently generating low-energy electrons in water.