The scale has seven levels. Every next level is meant to classify incidents, the intensity of which is ten times higher than the preceding one – it is similar to the magnitude of earthquakes scale. Naturally, the rating is approximate as one can differently interpret the seriousness and intensity of such events. The INES scale was developed for the civilian nuclear power, although it can be used for extraordinary situations with military equipment to obtain plutonium, nuclear warheads and nuclear submarines.
The IAEA assigned the top seventh level to two accidents: in Fukushima in 2011 and in Chernobyl in 1986. The similarity between the two is in the horrific scale and the consequences. There is no direct similarity, explains Nikolai Kukharkin, an advisor to the director of the Kurchatov Institute:
"The scenarios were different. Chernobyl was a purely reactor accident, its causes were in the reactor. While Fukushima was a natural catastrophe. The important thing there was that there was a tsunami, which poured water on the equipment at the station. The reactors are of different types. There was an overheating. But in Chernobyl the causes were internal related to the incorrect regime of reactor use. There was an experiment underway there and the reactor was put into a different mode. In Fukushima there was a catastrophe, a wave. The power supply was cut off, the active zone started heating up, it was destroyed and radiation was discharged".