Radioisotope thermophotovoltaic (RTPV) systems convert the heat of decay of radioisotopes into electricity via thermally radiated photons. In this work, we report the performance analysis of the RTPV system incorporating a cubic-shaped thermal cavity including 1.04 kg of Plutonium-238 (238PuO(2)) and nanophotonic emitters facing low bandgap photovoltaic (PV) cells. The performance of RTPV was investigated by varying the nonidealities of nanophotonic emitters and filters and with introducing a weighting factor (W) that quantifies the benefit of each nanophotonic component. Unlike previous studies, we varied the packing factor (PF) defined by the volume ratio between the radioisotope and the overall thermal cavity, then investigated the effects of the PF on the conversion efficiency, optimal configuration, and radiation safety. Our results show that it is crucial to reduce the nonidealities of nanophotonics at low energy photon level (below the PV cell bandgap) to achieve a high conversion efficiency. With the current state-of-the art nanophotonics including 2D tantalum photonic crystal (PhC) emitter and tandem filter which using interference and plasma filter, we show that the RTPV has the potential to achieve similar to 30% of conversion efficiency and surpass the conversion efficiency of conventional radioisotope thermoelectric generators (RTGs) with the same PF (similar to 10%). The increase in the PF from 10% to 70% provide approximately 23% of further increase in RTPV efficiency while limiting the increase in the total radiation dose only by similar to 10%. This work will help the development of high efficiency nanophotonic RTPV converters. (C) 2016 Elsevier Ltd. All rights reserved.