Work to construct a confinement structure at the notorious Chernobyl nuclear power plant is nearing completion, 30 years after the catastrophic nuclear incident.
Novarka – a joint venture formed by Vinci and Bouygues – was tasked with building the New Safe Confinement which will shelter the number four reactor at Chernobyl which exploded on 26 April 1986.
The steel shell was built in two halves which form an arch, and is being moved into place over the original sarcophagus structure that was erected in extremely hazardous conditions in the immediate aftermath of the explosion. After the fatal incident, the vicinity of the plant was evacuated due to nuclear contamination and a 30km exclusion zone was declared, which remains to this day.
However, analyses show that less than 5% of the radioactivity contained in unit four was released to the environment during the accident. In the weeks that followed, approximately 5,000 metric tonnes of sand, lead and boric acid was dropped from helicopters which mixed with more than 200t of uranium, forming a lava like mass. These fuel containing materials remain highly radioactive and represent the most significant radiological hazard at the site.
To completely seal reactor four from the environment, the New Safe Confinement – measuring 162m in length with a 257m span – has been designed to contain radioactive materials, protect workers at the site and shield the sarcophagus against severe weather damage. With a design life of 100 years, the structure must withstand temperatures ranging from -43°C to +45°C, a category 3 tornado (one in 1M year possibility), and an earthquake reaching a maximum intensity of 6 on the Mercalli scale (one in 10,000 year possibility).
The 108m high structure has been under construction since 2012 on a clean platform near the existing shelter. “To build the New Safe Confinement directly over the existing structure was heavily discouraged because it is too dangerous,” said Vince Novak, director of nuclear safety at the European Bank for Reconstruction and Development, which is administering funding for the project.
Speaking to New Civil Engineer, Novak explained that the assembly of the structure was undertaken in a “clean” assembly area, 300m away from the reactor. “Tens of thousands of cubic metres of contaminated soil was removed and a clean platform was completed which allowed construction to proceed. This work was done without all of the radiation protective gear, which has been extremely useful,” he added. “It has taken us years and years to create these clean conditions under which they are working right now. With all of the radiation gear, productivity would have probably been one-fifth of what has been achieved so far.”
Initial work on site included excavation of two trenches on either side of the reactor to prepare the ground for the longitudinal beams that serve as the arch foundations. Deep foundation works were undertaken which involved 1m diameter metal piles driven to an average depth of 25m in the assembly area. Reinforcements were then installed and concrete cast for the foundation blocks of the lifting towers which were used to lift segments of the arch into place. To protect the team on site from radiation coming from the ground, the 90,000m2 assembly area was backfilled, using clean filler, to an average height of 1m and then partially covered with concrete slabs.
The next stage of the construction involved assembly of the arch. The first segments of the new confinement structure were assembled on the ground. Construction of the arch began with the upper section. The segments were interconnected with bracing before the cladding was fitted on the central section.
The secondary arch elements were connected to the central section using a hinge system. Using the towers which are designed to lift loads of more than 1,000t, the structure was gradually completed with the addition of its remaining components. The east arch section was skidded to a waiting area in November 2014 to await completion of the second half of the structure. The second half of the arch was then assembled using the same process. Last year, Dutch contractor Mammoet connected the two halves to form the structure which weighs 25,000t, increasing to 36,000t when fully equipped. In order to move and join the arch sections, Mammoet developed and built a skidding system using a Teflon surface. Once the two halves were connected, Novarka commenced work to tighten approximately 1,000 bolts to secure the structure.
As well as the construction of the arch, the team has undertaken foundation works for what will serve as the service area. Concrete continuous flight auger (CFA) piles have been used in this area because the installation technique limits the risk of generating vibrations near the sarcophagus. Construction work has also started on the auxiliary building at the foot of the sarcophagus, which will serve as the future control centre for the dismantling and confinement systems built into the arch.
A ventilation system has also been installed to control the atmosphere inside the arch and prevent risk of corrosion. “You can’t send in painters every year to repaint the structure as you would with a structure like the Eiffel Tower. To ensure the 100-year design life, we had to eliminate the risk of corrosion. The only way to do this is to maintain humidity at below 40%, which is why the project required a very complex ventilation system,” explained Novak. The system will regulate the ambient temperature and humidity conditions inside the confinement structure and limit release to the atmosphere.
Currently, the arch is being fitted with overhead bridge cranes and other equipment to facilitate waste management and the dismantling of the existing shelter and the damaged reactor unit. The arch is expected to be slid 300m into its final position over the reactor later this year. The arch and its inner walls will then be connected to the existing structure, completely encompassing the damaged reactor. Novak admitted that the project schedule is “very ambitious” but confirmed that work is progressing as expected.