Bouygues Travaux Publics handled the construction of the nuclear island for the Olkiluoto EPR reactor in Finland, in addition to the backup and storage buildings. The biggest issue was ensuring the safety of the site. This colossal project has created a veritable fortress, 50 m in diameter, with an external shell made from reinforced concrete, and an internal containment structure made from 1.30 m of pre-stressed concrete lined with a metallic "skin". In total, 144,000 m³ of concrete were poured, much of it going toward the external shells which protect the power station from outside assaults, including the impact of an aeroplane.