Ultraviolet-ray (UV) decomposition is one of the chemical methods authorized in Japan for decomposition of polychlorinated biphenyls (PCBs). The equipment for the UV decomposition consists of photoreactors and UV lamps. Photoreactors are classified as either an internal irradiating reactor (IIE) or an external irradiating reactor (EIR) depending on the lamp location with respect to the reactor. Both types have advantages and disadvantaves. For example, an IIR has higher UV-utilizing efficiency than an EIR, but it is generally easier to perform maintenance on an EIR than an IIR. The purpose of this study was to determine which types of reactor is better suited for PCB detoxification plants. Here, we focus on the UV-utilizing efficiency, but to our knowledge, no study has been reported to estimate it quantitatively by means of simulation. First, under our initial experimental conditions, the UV utilizing efficiency was 64% for IIR and 2% for EIR. Next, to estimate UV utilizing efficiency for EIR, we calculated the efficiency by a Monte Carlo simulation in consideration of Fresnel's reflection law, UV self-absorption of lamp and thermal loss. The result was 4%, which is approximately the same as the experimental result (2%). For this reason, we believe this code is reliable. Finally, to improve the UV-utilizing efficiency for FIR, we calculated the rates under different conditions: more lamps and reactors inside a vessel. The results indicated that the absorption efficiency for EIR would reach 30% in the case of 9 reactors and 16 lamps inside the vessel. Because of the higher UV-utilizing efficiency and lower power cost for the UV lamp, we decided to choose IIR for a PCB decomposition plant.