ON THIS DAY 25  years ago, Reactor 4 at the Chernobyl nuclear power plant in what is now known as northern Ukraine suffered a meltdown.

So what actually happened inside the plant that day?

Nuclear power stations produce electricity by using radioactive elements to heat water rapidly and produce steam, which powers electricity-generating turbines.

The water which is pumped around the reactor’s core keeps it from overheating and control rods are used inside the core to soak up excess neutrons by-produced in the heat-producing nuclear reaction.

Plant operators at Chernobyl were carrying out a safety test on the running reactor which triggered a power surge. This surge prevented the control rods from working properly, meaning the reactor kept heating up until it exploded. The roof of the plant blew off, releasing radioactive materials into the air, and the core was so hot it literally melted down into the ground.

Containment

A small team of local firefighters who arrived on the scene to tackle the blaze in Reactor 4 were not made aware of the radiation leak and suffered high exposure to radiation.

Efforts to extinguish the fire and stop the meltdown involved pulling Soviet pilots back from the war in Afghanistan to drop sand and cement directly onto the reactor. Cement was also poured into the cooling system underneath the core.

Around 600,000 people, including soldiers, firefighters and miners, were involved in the efforts to contain the Chernobyl accident.

A cement sarcophagus was quickly built over the plant to contain it and its chimney, and was initially expected to last about 20 years. World governments recently pledged about €550m for a new containment shell, the construction of which has already started. Experts believe this new covering should prevent further radiation leaks for another 100 years.

Around 150,000 square kilometres in Ukraine, Belarus and Russia were contaminated by the accident.

The nearby city Pripyat, where the plant employees lived, was evacuated the day after the explosions and an exclusion zone was set up surrounding the plant. The zone is laid out according to the pattern of contamination, with part of the zone covering an area in Belarus.

Although residents were initially told their evacuation was temporary, no one has been able to return to live in the city since being forced to leave. Some people have returned to live in other villages initially evacuated after the disaster.

Because of the low level of human activity in the area in the years following the accident, the exclusion zone has become something of a nature preserve, with trees growing among and inside the abandoned buildings in Pripyat.

Although spent nuclear fuel has been removed from Chernobyl’s Reactor 3, spent material remains inside Reactors 1 and 2, and it will be decades before the fuel from Reactor 4 can be safely disposed of, according to the International Atomic Energy Agency (IAEA).

Until the recent Fukushima power plant disaster in Japan, Chernobyl had been the world’s only Level 7 nuclear disaster according to the International Nuclear Event Scale.

Check out some early footage of the stricken plant:

In photos: The abandoned city of Pripyat: