Liquid releases by Nuclear Power Plants (NPP) are composed of a mixture of radioactive and non-radioactive substances. When organisms are exposed to mixtures of contaminants the resultant toxicity can be enhanced, or reduced, due to interactions. In order to identify potential interactions between substances released by NPP, two substances representative of such effluents (in term of toxicity and of quantity) were selected for studies: Tritiated water (HTO) and copper (Cu). Effects of this binary mixture were studied on the unicellular green algae Chlamydomonas reinhardtii. HTO, when examined along, was not very toxic to C. reinhardtii. The most sensitive and early effect of HTO was an increase in oxidative stress at concentrations of 40 kBq mL-1 (0.13 μGy h-1). Algae exposure to the binary mixture HTO/Cu induced interactive effects on oxidative stress. Reactive Oxygen Species production was higher from exposure to the mixture of contaminants than the addition of the effect from each substance individually. This interaction was explained by an enhanced copper uptake by the alge when in the presence of HTO. The observed supraadditive effect could also be due to direct toxic interactions, especially on the antioxidant system. To conclude, this study showed that the effects of a mixture of radioactive and nonradioactive substances can be greater than what would be predicted based on mere addition of individual effects. Even thought this binary mixture is just a small part of NPP effluents, the study showed that potential interactions should be considered when determining ecological risks too aquatic ecosystems from NPP effluents.