For what it’s worth, Japanese news agency NHK confirmed the following on its English-language site:
The Nuclear and Industrial Safety Agency said that fuel in the reactor partially melted. It’s the first such accident in Japan.
It also mentioned that the Fukushima meltdown has been rated a Level 4 on the International Nuclear and Radiological Event Scale.
There have been a small number of Level 5 events, including the 1957 Windscale Fire in the UK, Three Mile Island, the 1952 Chalk River in Ontario, and the 1987 event in Goiânia, Brazil.
Chernobyl was the only Level 7 so far. That’s as high as the scale goes.
Here’s what NHK said about Level 4. It:
…includes damage to fuel and release of significant quantities of radioactive material within an installation. It’s the same level as a criticality accident at a nuclear fuel processing plant in Tokai Village in Ibaraki Prefecture, south of Fukushima, in 1999.
The agency called the accident very regrettable even though it was triggered by an earthquake.
What NHK failed to note is that a Level 4 event is an “Accident with local consequences.” Here’s what Wikipedia says it entails:
Impact on People and the Environment
*Minor release of radioactive material unlikely to result in implementation of planned countermeasures other than local food controls.
* At least one death from radiation.
Impact on Radiological Barriers and Control
* Fuel melt or damage to fuel resulting in more than 0.1% release of core inventory.
* Release of significant quantities of radioactive material within an installation with a high probability of significant public exposure.
Obviously according to this information, that means a meltdown. The Tokaimura event was at a research reactor and involved uranyl nitrate, which is liquid to begin with.
A level 4 rating means the event is limited to local impact. That’s obviously going to change, no matter what. If the meltdown is not halted before it reaches sufficient hits groundwater, of course, that’s bad. It’s called the “China Syndrome” and it’s hypothetical; it’s never happened and probably wouldn’t happen. It’s beyond the worst-case scenario indicated here.
But for what it’s worth, the China Syndrome means a melted core at 2,000 Kelvins or more hits groundwater and turns it explosively to steam. That means a mushroom cloud and fallout. It’s bad. But as far as I can tell it’s very unlikely to happen here.
There have been lots of Level 4 events, which should either make you feel better or worse. Here’s the list from Wikipedia:
* Sellafield (United Kingdom) – 5 incidents 1955 to 1979
* SL-1 Experimental Power Station (United States) – 1961, reactor reached prompt criticality, killing three operators.
* Saint-Laurent Nuclear Power Plant (France) – 1980, partial core meltdown.
* Buenos Aires (Argentina) – 1983, criticality accident during fuel rod rearrangement killed one operator and injured 2 others.
* Jaslovské Bohunice (Czechoslovakia) – 1977, contamination of reactor building.
* Tokaimura nuclear accident (Japan) – 1999, three inexperienced operators at a reprocessing facility caused a criticality accident; two of them died.
* Fukushima I Nuclear Power Plant (Japan) – 2011, reactor shutdown after the 2011 Sendai earthquake and tsunami, failure of emergency cooling caused an explosion. This rating is likely provisional and subject to change.