How serious is the Fukushima nuclear leak?

JAPAN'S authorities have said that the ongoing problems at the Fukushima Dai-ichi plant now justify a rating of 7 on the International Nuclear and Radiological Event Scale (INES), the highest possible. But INES can be confusing: it takes into account many different factors, from the level of radiation released to the effects on the environment and how severely safety systems are stretched. Categories 4-7 are classed as “accidents”, 1-3 as mere “incidents”. The scale is subjective, which makes labelling nuclear incidents as much an art as a science; in theory, at least, it is also logarithmic, meaning that a Level 4 “accident” is in some sense ten times worse than a Level 3 “incident”. Although the Fukushima accident is now rated at the same level as the Chernobyl disaster in 1986, in terms of casualties and effects on the environment, it is far less significant. Our chart presents a selection of nuclear incidents and their ranking on the scale.