Some methods have been developed to detect leakage of CO2 from its desired storage domain, but that is not sufficient to prevent and mitigate a leak. Two techniques have been proposed to prevent the migration of buoyant CO2 from the storage domain by expediting mixing of CO2 with the brine and mitigate risk of its leakage risk. These two methods are injection of CO2 pre-mixed with brine, and injection of CO2 with nanoparticles (NPs). The former has been studied to some extent, however, understanding of the latter is very limited. Unlike the application of NPs in hydrocarbon recovery, its use to enhance safety of CO2 storage is a fairly unexplored topic that can have important benefits for the safety of the storage process. Also, the use of NPs for subsurface application in general is compromised for its cost. We investigate how NPs produced from low-level nuclear waste can be added with injected CO2 to enhance the mixing of CO2 with brine, which can mitigate leakage risk of CO2. We numerically investigate the effect of adding NPs from nuclear waste with the CO2 and show that it enhances the mixing of CO2 with in-situ brine in saline aquifers that mitigates the risk related to buoyancy and high mobility of CO2. Additionally, we examine the effect of reservoir heterogeneity on mixing of CO2 in reservoir brine when it is injected with NPs. The results show that: (i) addition of NPs to CO2 leads to higher mixing, (ii) the discrete shape of CO2 concentration in brine tends to diffuse and become smooth as the heterogeneity of the medium increases, and (iii) the impact of heterogeneity is more pronounced than the fraction of NPs on mixing. (C) 2017 Elsevier Ltd. All rights reserved.