August 12, 2016
Covering Chernobyl nuclear reactor Unit 4, destroyed in a catastrophic accident in 1986, the New Safe Confinement (NSC) will soon put an end to the alarming degradation of the concrete and steel sarcophagus installed after the disaster. As of 2017, the majestic airtight, seismic-resistant steel arch, capable of withstanding extreme climate conditions, is due to defy both time, climate hazards and the invisible but still insidious and long-lasting threat of radioactivity.
Nickel-containing stainless steels are an essential component of the new shelter. It has been designed to allow the damaged reactor to be safely dismantled and tons of radioactive debris to be removed after a compulsory hundred-year safety period, as well as protecting technical personnel from harmful radiation exposure during on-site maintenance activity.
Preventing corrosion versus fighting it
Weighing 25,000 tons, 108m high, 162m long and 257m wide, this mammoth structure will be tall enough to enclose the Statue of Liberty and will weigh four times more than the Eiffel Tower.
The NSC comprises 15 primary arch frames, forming a 12m thick three-dimensional tubular lattice that shapes the elegant final arch. Fabricated from four hundred 813mm diameter round carbon-manganese steel tubes of similar quality to that of offshore oil rigs, this lattice creates an “annular space” that is completely wrapped—inside and out—with a stainless steel cladding. Once the arch is in place above the damaged reactor, two cladded metallic walls will be unfolded at each end. They will precisely fit the sarcophagus’ profile, hermetically sealing the building for a century.
Durable and reliable confinement against contaminated dust and radioactive emissions will be achieved by maintaining a permanent slight overpressure while the volume under the arch will be maintained under negative pressure. Dehumidified air will be continually blown into the annular space in order to maintain a constant hygrometric level below 40% that will naturally prevent corrosion of the metallic lattice.
Non-magnetic stainless to prevent dust accumulation
Nickel-containing stainless steels were a natural choice for the inner and outer claddings given their durability, high corrosion-resistance with mechanical strength, and the advantage of being maintenance-free in such an exposed area.
The inside cladding is a Type 304 (UNS S30400) stainless steel, with minimum 8.0% nickel, a non-magnetic grade to prevent the accumulation of contaminated dust that would be hazardous inside the arch when the time comes to dismantle it. Over the entire surface of the vault, 80,000m² of 0.5mm thick stainless sheets are tightly fitted without ribs to avoid collecting dust.
The sheets are fixed on a galvanized steel deck. Air-tightness is ensured by non-PVC tape seals and fire resistant silicone mastic, insensitive to ionizing radiation. A polyester paint provides galvanic corrosion prevention between the sheets and the decking. A bright-annealed highly reflective finish has been used in order to enhance natural brightness inside the arch.
316L to withstand climate hazards
Outer cladding forms an impenetrable barrier against external climate hazards. It is equipped with an insulating complex aimed at meeting the stringent resistance specifications for the most severe climatic conditions.
The top slice of this highly efficient insulating sandwich is covered with 88,000m² of 489mm wide, 0.6mm thick Type 316L (S31603) with minimum 10.0% nickel, stainless steel profiles. The choice of Type 316L grade was decisive to fully meet the requirement of this unprecedented 100-year long (and beyond) exposure to climate hazards.
Roll forming at the foot of the arch
Types 304 and 316L stainless steels were supplied by Aperam and delivered to the worksite by Kalzip in 1,000m long coils. They were processed at the foot of the arch in two in-container mobile production facilities. 316L coils were cold roll-formed into 100m long profiles for the roofing of the arch and 50 to 70m long profiles for the cladding of the walls.
The profiles were then transported by crane into their installation position. Standing seam profiles are positioned and clipped into previously installed Type 304 stainless fasteners ('halters'), the next, profile being immediately brought into position with the larger seam overlapping the smaller one. Fasteners are mounted through the rubber membrane on omega profiles by means of 316L self-tapping screws. Finally, a special seaming machine then crimps all profiles together over their total length. This mounting technique will enable the outer cladding to withstand class 3 tornadoes (severe damage), winds topping 250km per hour and temperatures ranging from -45°C to 45°C.
With this feat of engineering, nickel-containing stainless steels will help protect workers, the local population and the environment for a century or more.