However, the history of Soviet-era nuclear accidents has prompted some critics to refer to the reactors as "floating Chernobyls-in-waiting" after the meltdown at the plant in Ukraine in 1986. The accident is the only example of a "Level 7" event on the International Nuclear Event Scale – the maximum level that can be reached. The scale was introduced by the International Atomic Energy Agency in 1990. It is a logarithmic scale, similar to the Richter Scale for earthquakes. The fire at Windscale, now Sellafield, in the 1950s would have been classified as a Level 5 event.