Russian sate nuclear corporation Rosatom said in a statement that the second inner steel ring is one of the largest structural components of the reactor building and its installation marks a milestone for the project.
The statement said the installed component weighs 411 tonnes and its diameter exceeds 20 metres.
Once the inner steel framework, comprising three steel rings, is fully complete, workers onsite will be able to proceed with a pressurised leakage test.
The inner containment is part of the hermetic protection system of the reactor building, designed to withstand high external and internal pressures and protect against radiation release.
The Russian Generation III+ VVER-1200 reactor design includes both an internal and external containment. The external contrainment is designed to protect the reactor installation from external influences such as earthquakes, storms, explosions and aircraft impact.
Akkuyu, which will be the first commercial nuclear power station in Turkey, will have four VVER-1200 units, with the first expected to come online in 2023 and a further unit starting every year afterwards.