The Fukushima nuclear plant in Japan appears to have had a partial meltdown, not "a meltdown".
For an explanation of the distinction see
Meltdown 101: What is a nuclear reactor meltdown? (csmonitor.com)
INES - Scale of nuclear accidents
Scale of nuclear accidents
(The International Nuclear and Radiological Event Scale)
Major release of radioactive material with widespread health and
Significant release of radioactive material likely to require
implementation of planned countermeasures
||Kyshtym, Russia, 1957
Limited release of radioactive material. Severe damage to
Island, US, 1979
Windscale, UK, 1957
Minor release of radioactive material. Fuel melt or damage.
Saint Laurent des Eaux, France, 1980
Exposure in excess of 10 times the statutory annual limit for
workers. Severe contamination but with a low probability of
significant public exposure.
Vandellos, Spain, 1989
Exposure of a member of the public in excess of 10mSv (average
annual dose is 1mSv). Exposure of a worker in excess of the
annual limit. Significant contamination within the facility.
Cadarache, France, 1993
Forsmark, Sweden, 2006
Overexposure of a member of public. Minor safety problems. Low
activity lost or stolen source or device.
Below Scale (Level 0): No safety significance.
Based on information from the IAEA:
IAEA - INES [pdf] with Japan 2011 added.