At 01:23:40 on 26 April 1986, the failure of a routine test at the Chernobyl Nuclear Power Plant in Soviet Ukraine caused reactor number four to explode, releasing a cloud of radioactive particles into the atmosphere. It was the worst nuclear accident the world had ever seen.
Thirty-five years on, the Chernobyl reactor remains a danger, although it has been contained by a multi-billion international effort to close the core off and clean up the surrounding region.
Fires broke out, causing the main release of radioactivity into the environment. Wind carried contaminated particles over Ukraine, Belarus and Russia, as well as parts of Scandinavia and wider Europe. The 50,000 inhabitants of the adjacent town of Pripyat were evacuated, never to return.
The accident destroyed reactor four, killing 30 operators and firemen within three months and causing numerous other deaths in the weeks and months that followed. To this day, it remains the only accident in the history of the civil use of nuclear power when radiation-related fatalities occurred. The precise number of short- and longer-term victims remains heavily disputed.
By 06:35 on 26 April, all fires at the power plant had been extinguished, apart from the fire inside reactor 4, which continued to burn for many days. Some 5,000 tonnes of boron, dolomite, sand, clay and lead were dropped from helicopters in a bid to extinguish the blaze. When the destroyed reactor was later enclosed in a provisional structure – the so-called sarcophagus – these fuel-containing materials were also walled in.
The sarcophagus was built under extremely hazardous conditions and unprecedented time pressure. By November 1986, a steel and concrete shelter was in place to lock away the radioactive substances inside the ruined reactor building and to act as a radiation shield. It was always intended as a temporary measure, with an estimated lifespan of 20-30 years.
The search for a long-term solution started soon after, alongside the massive challenge of cleaning up the accident site. By the end of 1991, the Soviet Union had dissolved and newly independent Ukraine had been left with the Chernobyl legacy. Following a G7 Action Plan to improve nuclear safety in central and eastern Europe, the Nuclear Safety Account was set up at the European Bank for Reconstruction and Development (EBRD) in 1993. Two years later, the scope of the programme was extended to include Chernobyl.
Emergency repairs in 1998 and 1999 prevented the imminent collapse of the sarcophagus, as well as a vent stack that was endangering the adjacent turbine hall over reactor 3, which was still in operation. It was only at the end of 2000 that all nuclear power generation in Chernobyl ceased. The following year saw a landmark decision to build an arch-shaped steel structure, called the New Safe Confinement (NSC), to seal off reactor 4.
In the subsequent years, several tasks were carried out simultaneously. Detailed technical work on the NSC started. The site had to be stabilised and prepared for the construction work. The first project was the construction of a liquid radioactive waste treatment plant (LRTP) to handle some 35,000 cubic metres of low- and intermediate-level liquid waste at the site. Meanwhile, the safe storage of the spent fuel assemblies from reactors 1, 2 and 3 came into focus.
All this has been achieved. The LRTP has been operational since 2014. A new interim storage facility for the treatment and storage of spent fuel has been built and, after successful hot tests, is currently awaiting a permanent licence from the Ukrainian regulator. The NSC, the most visible Chernobyl project, was slid into position in late 2016 and then handed over to the Ukrainian authorities.
Today, the New Safe Confinement dominates the skyline over Chernobyl, as the sarcophagus once did. The steel structure is 108 metres high and 162 metres long, with a span of 257 metres and a lifetime of at least 100 years. It was assembled in two stages in a cleaned area near the accident site and, despite its size and weight of 36,000 tonnes, was pushed 327 metres into position. It is the largest moveable structure ever built.
This is not where the story ends, however. The fact that the NSC has a lifespan of 100 years means that the next phase of work now has to be planned, agreed and implemented. The estimated 200 tonnes of radioactive nuclear fuel inside reactor 4 are now shielded by the New Safe Confinement. However, parts of the sarcophagus are becoming unstable and will have to be removed at some point. Once this is done, work will come closer to the reactor’s interior.