Chernobyl on HBO and Sky closely follows the nuclear disaster of the Chernobyl Power Plant in northern Soviet Ukraine. On April 26, 1986, a failed safety test coupled with design flaws and human error triggered the largest nuclear disaster in human history. The incident led to two explosions of Chernobyl’s Reactor Four, blowing out the roof of the reactor building and releasing up to five percent of the reactor’s nuclear material into the atmosphere. In the HBO and Sky drama, just moments after the Chernobyl explosion, a blue-white beam of light shot up into the sky from the exposed reactor core.

In the aftermath of the nuclear disaster, at least two eyewitnesses reported seeing a blue flash of light after Chernobyl’s first explosion. Alexander Yuvchenko, who worked at the Chernobyl plant on the night of the explosion, recounted how he “ran out of the building and saw half of the building gone and the reactor emitting a blue glow of ionised air”. A similar story was told by Vladimir Chernousenko, former head of the Ukrainian Academy of Science who coordinated the Chernobyl cleanup efforts. Mr Chernousenko spoke to an eye-witness who claimed to have seen a blue flash of light while fishing some 1,640ft (500m) from the disaster. READ MORE: How many people died in Chernobyl?

Chernobyl disaster: When the reactor exploded, a blue beam or flash of light was seen

Chernobyl: The exposed reactor core burned for 10 days, releasing radioactive smoke

The witness said he heard a loud noise and an explosion, followed by a blue flash of light and a second blast. In Svetlana Alexievich’s book Chernobyl Prayer, Lyudmila Ignatenko who’s firefighter husband fought the fire at Chernobyl, described a glow around the burning power plant. However, the glow may have simply been produced by the fires raging on the Reactor Four building. She said: “I never saw the explosion itself. Only the flames. Everything was kind of glowing. The whole sky. “There were these tall flames. Lots of soot, terrible heat. I was waiting and waiting for him.”

Mrs Ignatenko's husband, firefighter Vasily Ignatenko, was one of the first responders at the site of the nuclear disaster. He died just two weeks later from acute radiation sickness (ARS) in a Moscow hospital. So what exactly was this blue light emitted by the burning Chernobyl reactor? According to Lars-Erik de Geer of the Swedish Defence Research Agency, blue light around nuclear radiation is associated with intense ionisation. Ionisation is the process by which an atom or molecule is excited to a new energy level by acquiring or losing electrons. READ MORE: What are the symptoms of radiation sickness?

The nuclear expert and a team of researchers examined the blue glow seen at Chernobyl in a 2017 paper published in the journal Nuclear Technology.

Criticality accidents emit a blue flash, or rather glow Lars-Erik de Geer, Swedish Defence Research Agency

Dr de Geer wrote in the study: “It is well known that criticality accidents emit a blue flash, or rather glow, which derives from fluorescence of excited oxygen and nitrogen atoms in the air. “It is obvious that the most impressive explosion was the one that ruptured the fuel channels and threw the 2000-ton lid with its hanging fuel channels high up in the central hall from where it fell down and came to rest nearly vertically on the rim of the reactor tank. “With the fuel fully exposed, the air was irradiated, and the typical blue glow was lit.” READ MORE: How to visit Chernobyl power plant site after tragedy

Chernobyl: The exposed reactor core ionised the air around the power plant

Chernobyl: The reactor building is now shielded by the so-called Sarcophagus containment unit