When reactor 4 blew at the Chernobyl power plant in 1986, it caused the most catastrophic nuclear disaster in history. The resulting detonation and subsequent fire sent plumes of radioactive material soaring into the atmosphere, and spread it across much of Northern Europe.

Now a new study is giving fresh insight into the early stages of the disaster and suggests that it didn’t actually start in the way it has frequently been assumed.

It has traditionally been thought that when the nuclear plant experienced a power surge, it resulted in a series of steam explosions that were powerful enough to eject a spout of debris up into the clouds, carrying with it a whole host of radioactive particles. These were blown mainly north over Scandinavia where they then came back down to Earth.

It now seems, however, that these steam explosions were in actual fact preceded by two nuclear explosions. These initial eruptions were so forceful they managed to take debris much higher into the atmosphere, before the steam explosions happened a few seconds later that ruptured the reactor took the radioactive particles to a lower altitude, before it was all swept northwards.

The Swedish team was also to determine this by testing radioactive samples collected four days after the cataclysm in a city called Cherepovets, which is north of Moscow and a long way from the pathway that the deadly plume took across Europe. These samples were found to contain xenon isotopes, which they suggest were the result of a recent nuclear explosion. This is significantly different to the radioactive debris found in Sweden that shows equilibrium xenon isotopes that came from the reactor's core.     

These differences in the isotopes and their origins, coupled with the fact that they were found in different areas, point to two separate explosions. The first nuclear one sent particles higher up, and into a slightly different weather system than the second steam explosion.

 “We believe that thermal neutron mediated nuclear explosions at the bottom of a number of fuel channels in the reactor caused a jet of debris to shoot upwards through the refuelling tubes,” explained the Swedish Defence Research Agency’s Lars-Erik De Geer, who led the study published in Nuclear Technology.

“This jet then rammed the tubes' 350kg plugs, continued through the roof and travelled into the atmosphere to altitudes of 2.5-3km where the weather conditions provided a route to Cherepovets,” De Greer continued. “The steam explosion which ruptured the reactor vessel occurred some 2.7 seconds later.”

Interestingly, eyewitness accounts also support this new theory, as people did see a mysterious blue flash above the reactor before the steam explosion. The more we know about what happened that day, the further it goes towards helping prevent it happening again.