The Chernobyl disaster in then Soviet Ukraine, the worst civil nuclear accident in history, left thousands of people dead or dying and contaminated up to three quarters of Europe.

Here is a timeline:

- Reactor 4 explodes -

- On April 26, 1986, at 1:23 am, the Chernobyl nuclear power station's Reactor Number 4 explodes during an experimental safety check.

The reactor is destroyed and a plume of radioactive contents shoots up to one kilometre (half a mile) into the sky, some falling as debris in the area while the rest is blown by winds as far as western Europe.

Thirty rescue workers and plant staff, receiving abnormally high doses of radiation, are killed at the site either immediately or in the coming weeks.

The radioactive plume drifts first to the north-west, polluting the Soviet republics of Ukraine, Belarus and Russia. It reaches Scandinavian countries before swinging southwards and westwards, dropping contaminated rain on central Europe and the Balkans, Italy, France, Britain and Ireland.

- Silence -

It is only on April 27, a day and a half after the explosion, that Pripyat, a town of 48,000 inhabitants that houses plant workers three kilometres from the station, is evacuated.

Soviet authorities remain silent on the disaster for three days, with the official news agency TASS only reporting an accident on April 28 after the Forsmark nuclear plant in Sweden detects unusually high radiation.

The International Atomic Energy Agency (IAEA) is officially notified of the accident on April 30.

For traditional May 1 celebrations in Kiev, local authorities carry on with parades involving children, even as the wind turns and threatens the Ukrainian capital with radiation.

It is only on May 14 that Soviet leader Mikhail Gorbachev comments publicly on the disaster.

One hundred thousand people are eventually evacuated from an area within 30 kilometres of the plant.

- First sarcophagus -

In November 1986 a 50-metre-high concrete shelter, dubbed a sarcophagus, is completed to prevent further leakage of radiation from tonnes of highly radioactive magma and allow the other three reactors at Chernobyl to continue producing power for Ukraine.

This structure is only intended to be in place for 20-30 years, and in 1993 its lifespan is estimated at seven years.

Between 1986 and 1990 around 600,000 workers known as "liquidators" toil at the site with little or no protective gear to deal with the disaster's aftermath.

In October 1991, Reactor Number 2 is shut down after a fire.

- Chernobyl closed -

In April 1995 Ukraine's new president Leonid Kuchma commits to closing Chernobyl before 2000.

In December a memorandum on the plant's closure in exchange for $2.3 billion in aid is signed in Ottawa by Ukraine, the European Union and the Group of Seven (G7) top industrialised countries.

In November 1996 Kiev shuts down Reactor Number 1.

In September 1998, with the initial sarcophagus over reactor 4 threatening to crumble, reinforcement work begins. The project takes about 10 years and costs $760 million.

In December 2000, after years of international pressure, Ukraine finally shuts down the last reactor.

- Controversial figures -

In September 2005 a controversial UN report estimates the number of proven or projected deaths in the three worst affected countries at up to 4,000. A year later Greenpeace says 100,000 died from radioactive contamination.

In 1998, Ukrainian authorities acknowledge that 12,500 liquidators died.

- New safety arch -

In April 2012, construction of a new 110-metre-high cover is launched with international financing.

In November 2016, the giant 36,000-tonne steel arch begins to slide into place, with the European Bank for Reconstruction and Development, its main sponsor, planning to complete the installation on November 29.

The new structure will include high-tech equipment to continue the clean-up process, and is to be fully operational in late 2017.