Michael McCarthy: Two disasters that are fundamentally different

Categorisation can sometimes have a dramatic effect on how we perceive the world, and to learn yesterday that the nuclear debacle at Fukushima now officially ranks alongside that of Chernobyl in terms of seriousness appears to be an ominous worsening of the situation in Japan.

The move will certainly have political ramifications, with neighbouring countries perceiving the threat to them as being that much greater. But in reality, as Japanese officials were at pain to stress, nothing fundamental has changed; listing Fukushima as a "level seven" nuclear accident alongside the 1986 disaster in the Ukraine, was merely an overdue recognition of what has happened so far at the tsunami-stricken plant, combined with what may happen in the future.

How do the two incidents compare? Although Fukushima has spilt a lot of radioactivity into the environment, the full amount is still thought to be far below the amount released in the accident at Chernobyl 25 years ago.

Using the common measure of "terabecquerels", it is thought that about 500,000 to 600,000 terabecquerels of radiation has so far been released from the damaged Fukushima reactors, whereas Chernobyl is thought to have released between five and six million terabecquerels, about ten times as much.

Another way of comparing the two incidents would be through the number of people evacuated. At Chernobyl it was 100,000 at once, and the total eventually reached 350,000. At Fukushima something like 70,000 people living within a 12-mile radius of the plant have been evacuated, while 130,000 living up to 20 miles away have been told to leave voluntarily or stay indoors

A third way would be through casualties, although this is not really possible yet. It is universally accepted that the 50 or so heroic workers who died from radiation sickness after being in the front line of fighting the Chernobyl fire were direct casualties.

After that argument begins about how many people have suffered, and estimates of those who may have contracted, or will contract cancer as a result of Chernobyl range from 4,000 to 100,000 plus. At the moment, no deaths can directly be attributed to radiation from Fukushima.

In making the comparison it is important to remember the two incidents were fundamentally different. Chernobyl was a nightmarish "big bang" – an explosion which blew the reactor core and its containment vessel to bits and sent a vast column of radioactive debris into the atmosphere.

Fukushima has been a lengthy partial meltdown of reactors whose temperatures could not be controlled because the tsunami had knocked out their cooling systems. However, the reactor containment vessels are still largely intact, so the radioactive leaks are much less. The problem is that there is more than one reactor involved and it may be some time before they can all be brought back under control.