Water evaporation at wet-cooled nuclear power plants on river banks: Application to the French Rhône river

Nuclear power plants (NPP) installed on a river bank use once-through or recirculating wet cooling systems. While the quantitative assessment of mainly local evaporation of river water by a recirculating cooling system (RC) does not involve any major difficulty, the same cannot be said for the once-through cooling system (OTC) for which evaporation (called forced evaporation) occurs in the downstream aquatic environment. Our study applies an energy-balance temperature model to the Rhône River downstream from Lake Geneva. Our results show the approximate figure of 1.5 m3/MWh of forced evaporation per electricity produced (put forward in 2002 by EPRI and widely used in the literature) is debatable for setting a freshwater balance in a watershed because it does not consider the saltiness of the river downstream. The retention time of water bodies in the downstream freshwater stretch, the cross-section shape and regional weather characteristics appear to be key factors to take into account. In particular, for the three NPP with OTC systems installed along the French Rhône, we find an average forced evaporation between 0.3 and 0.7 m3/MWh of net electricity generated in the freshwater river stretch through to Arles between 1994 and 2015. These values are between three to eight times lower than those obtained for the evaporation generated by the plants with RC system installed along the river stretch.