Analysis: How bad is the nuclear accident in Japan?

NEW YORK (Reuters) - The Japanese nuclear safety agency rated the damage at a nuclear power plant at Fukushima at a four on a scale of one to seven, which is not quite as bad as the Three Mile Island accident in the United States in 1979, which registered a five. But what does that mean?

The International Atomic Energy Agency -- an inter-governmental organization for scientific co-operation in the nuclear field -- said it uses the scale to communicate to the public in a consistent way the safety significance of nuclear and radiological events.

The International Nuclear and Radiological Event Scale, or INES, ranges from one to seven with the most serious being a seven referred to as a "major accident", while a one is an "anomaly". The scale is designed so the severity of an event is about ten times greater for each increase in level.

The Chernobyl explosion in the Ukraine in 1986, the worst nuclear power accident ever, was rated a seven. That was the only event classified as a major accident in nuclear power history, exploded due to an uncontrolled power surge that damaged the reactor core, releasing a radioactive cloud that blanketed Europe.

The Three Mile Island accident in Pennsylvania was a partial core meltdown in which the metal cladding surrounding the fuel rods started to melt. That metal surrounds the ceramic uranium fuel pellets, which hold most of the radiation and power the reactor.

Nuclear reactors operate at between 550 and 600 degrees F (between 288 and 316 degrees C). The metal on the fuel rods will not melt until temperatures are well above 1000 degrees F. The ceramic uranium pellets themselves won't melt until about 2000 degrees.

About half the reactor core at Three Mile Island melted before operators restored enough cooling water to stop the meltdown. The core holds the uranium fuel rods, which must be cooled by water to prevent overheating.

So what happened at Fukushima?

The blast at the 40-year-old Daiichi 1 reactor came as plant operator Tokyo Electric Power Co (Tepco) worked to reduce pressure from mildly radioactive steam in the core after the total loss of power needed to keep water circulating to prevent the reactor fuel from overheating.

That blast led to fears of a disastrous meltdown at the plant, which automatically shut after the quake, even though the government has insisted that radiation levels were low.

The cause and exact location of the blast still needs to be established, nuclear experts queried about the incident said.

A couple of examples of fours on the INES scale include a fatal overexposure of workers following an incident at a nuclear facility at Tokaimura, Japan in 1999 and the melting of one channel of fuel in the reactor -- though no radiation was released outside the site -- at Saint Laurent des Eaux, France in 1980.

(Reporting by Scott DiSavino in New York, Bernie Woodall in Detroit and Fredrik Dahl in Vienna. Editing by Martin Howell)