One recent November day, they finished the first stage - lifting onto a frame a semi-circular steel roof weighing 5,000 tonnes that will eventually be pushed on rails over the lethal site. In the dangerous engineering feat, mistakes are out of the question. Were the massive structure to collapse onto the reactor site, it would hurl a cloud of radioactive high dust into the sky.
The last time that happened, during the explosion, fire and meltdown on April 26, 1986, the plant belched out plumes of radioactive contamination that blew far across Western Europe. The reactors were hastily encased in a sarcophagus that has since badly crumbled. The site is now a sea of dirty concrete structures. Sickly looking trees grow in the distance.
To secure Chernobyl, just 110 kilometres from the capital Kiev, for future generations, the European Bank for Reconstruction and Development (EBRD) is overseeing the construction of a second shield. By the end of 2015, the reactor is set to be covered with the already completed concrete and lead shell, and a steel structure spanning 257 metres and weighing 29,000 tonnes. "It is a race against time, and we can't afford any mistakes," said EBRD Director for Nuclear Safety Vince Novak.
The new sarcophagus is due to stay in place for 100 years, and there are plans to remove the destroyed reactor within - although no one is quite sure how this will be done. The new building site lies a few hundred metres from the blackened reactor in an area that was less contaminated, said David Jackson, the British head of security for the project.
Still, radioactive soil up to eight metres deep had to be removed before the team of about 1,500 workers could start the years-long project without the need for heavy protective clothing. Each of the employees from 22 countries carries a radiation detector because, as Jackson said, "it's best to be completely safe."
In all, over 40 countries have joined the 1.5-billion-euro (two-billion-dollar) mega project. Igor Gramotkin, Chernobyl nuclear plant director, drew a parallel between the steel-latticed shelter and the Eiffel Tower, saying Chernobyl could eventually become a tourist attraction.
The Eiffel Tower "was never meant to be around that long, but people still travel to Paris to marvel at it," Gramotkin said. Tourists can already make brief visits to the contaminated area, on government-approved tours from Kiev run by private operators. Ukrainian authorities are constantly thinking of ways to attract investors to the stricken area.
A US firm wants to spend 254 million euros to build a medium-term nuclear waste storage site, said Vladimir Kholosha, head of the administration department responsible for Chernobyl. Despite the disaster, Ukraine remains committed to nuclear energy. Plans in Germany and Japan to abandon nuclear energy after the 2011 Fukushima accident are "their own concern", Gramotkin said. "This is to be completely respected," he said. "But Ukraine is poor in comparison and cannot afford such a move right now." Ukraine, Europe's second-largest country by area, now operates 15 nuclear reactors - and their number is set to triple by 2030.