A containment dome is the hemispherical section that closes the top of the cylindrical protective structure (containment) surrounding the reactor. VVER-1200 reactor buildings have two shells: the steel-lined inner containment ensures leak-tightness, while the reinforced-concrete outer containment protects against external impacts such as earthquakes and hurricanes. The domes are assembled on site from sections and lifted into place by a heavy crawler crane.
What does the containment do in a reactor building?
Nuclear safety is ensured by a series of successive physical barriers: the fuel pellet itself, the fuel rod cladding, the pressure boundary of the primary circuit and, outermost, the containment. In the event of an accident, the containment is the last barrier preventing radioactive substances from escaping into the environment. The dome is one of the most difficult parts of this structure to fabricate and one of the largest sections lifted in a single piece; once it is in place, the reactor hall is effectively closed.
The difference between the inner and outer containment
| Feature | Inner containment | Outer containment |
|---|---|---|
| Function | Leak-tightness of the reactor compartment; load-bearing structure for pipe penetrations and the polar crane | Shield against external impacts |
| Structure | Steel liner and special concrete, pre-stressed tendon system | Reinforced concrete; at Akkuyu the wall is 1.5 m thick after concreting |
| Assembly | Seven-tier steel structure; the seventh tier is the dome | Metal tier elements followed by concreting |
| Akkuyu example | At Unit 1, elevation +61.7 m after dome concreting, wall 1,200 mm | At Unit 1, dome of 147.6 tonnes, diameter 34.7 m |
| Design load | Internal pressure and leak-tightness | Magnitude 9 earthquake on the MSK-64 scale, 60 m/s hurricane, tsunami up to 10 m and shock wave |
The double containment is one of the Generation III+ features of the VVER-1200 design.
How is the dome fabricated and installed?
- Section fabrication: The dome is manufactured in sections at the factory and reaches the site by road and sea. The inner dome of Akkuyu Unit 2 consists of 15 assembly sections made of special structural steel.
- Pre-assembly: The sections are welded together on special assembly stands next to the reactor building. At Unit 2 this stage took about four months.
- Lifting: The completed dome is lifted in one piece by a heavy crawler crane. The roughly 215-tonne dome of Unit 2, together with its service platforms, made up a lifting load of more than 250 tonnes; a special 66-tonne lifting traverse was used, and the operation was completed in June 2026 in about seven hours by a team of 40.
- Welding and concreting: The dome is welded to the cylindrical body, followed by reinforcement and concrete. The inner dome of Unit 1 required 422 tonnes of reinforcement and more than 3,200 m³ of concrete.
- Outer dome: The outer dome of Unit 1 was set in place in six hours in September 2024. The metal framework is then filled with high-strength concrete; once concreting is complete, the outer containment wall reaches a thickness of 1.5 m.
Why does logistics preparation start months in advance?
The lift itself takes a few hours, but preparing for it takes months. The order in which sections arrive on site must match the pre-assembly schedule; a single missing part halts the work on the assembly stand. The tier elements are not small either: the sixth tier of Unit 1 consisted of 30 blocks, weighed 224 tonnes and had a diameter of 44 m. The road leg for large sections may require an oversize transport permit.
On the crane side, a Liebherr LR 13000 heavy crawler crane was used at Akkuyu. This crane was the third cargo to reach the plant's Eastern Cargo Terminal by sea; a machine like this arrives on site in parts, and its erection is planned as a separate project. Wind speed, ground bearing capacity and load angle on the day of the lift are defined in advance in the lift plan. General methods are described in the entry on heavy lifting at nuclear power plants.
Sequencing matters too. Large components such as the polar crane and the reactor vessel are placed in the building before the dome is closed; after that, equipment enters the building through the transport airlock. Delivery of sections and auxiliary materials to the site is subject to the site access and handover rules of nuclear project transport.
Frequently Asked Questions
Why is the dome hemispherical?
Once the dome is in place, how does large equipment get into the building?
Sources
- Akkuyu Nükleer – The dome of the outer containment of Power Unit No. 1 has been installed
- Akkuyu Nükleer – Reactor building containment completed at Akkuyu NPP Unit 2
- Akkuyu Nükleer – Concreting of internal containment dome completed at Unit 1
- Akkuyu Nükleer – Installation of Unit 1 containment nearing completion