NISA, the Japanese Nuclear and Industrial Safety Agency has finally raised the Fukushima accident to level 7 (the highest) on the INES scale. Previously, the Chernobyl accident was the only one worldwide ever rated at that level. At the same time NISA stated that the amount of radioactivity released so far was about 10% of the amount released in Chernobyl. Most of the leaked radioactivity is assumed to have come from unit 2, which leaked highly radioactive water into its basement and from there into the sea.
The rating probably does not indicate a recent surge of danger to the public, but reflects a more realistic assessment of the existing data than the previous rating of 5, which would have put it on a level with the Three Mile Island accident in 1979: Only one reactor block suffered a melt down in TMI and the containment remained intact, versus 3 damaged reactor blocks and 4 overheating spent fuel pools in Fukushima and a damaged pressure vessel and containment in at least one unit. According to Gesellschaft für Anlage- und Reaktorsicherheit the revised rating primarily reflects the fact that a lot more than 10,000 terabecquerel (TBq) of Iodine-131 have been released into the atmosphere so far. The release of Iodine-131 and Cesium-137 lies somewhere between 370,000 TBq and 630,000 TBq.
US NRC assessment of Fukushima 1
The March 26 NRC assessment of the situation in the six units at Fukushima one makes for interesting reading. It suggests that water flow inside the base of the reactor pressure vessels is severely restricted by melted fuel and salt buildup. Water injected into unit 1 apparently does not contain any boron to guard against criticality. The seals in the recirculation pumps of units 1, 2 and 3 are assumed to have failed, effectively preventing the reactor cores from being filled high enough to cover all fuel, even if sufficient water could get past the melted fuel and salt crust: If the water rises any higher, it will simply leak out of the pumps. The primary containments of units 2 and 3 are assumed to be damaged. Spent fuel may have been ejected “up to 1 mile” from the spent fuel pools of unit 4 or unit 3 in a hydrogen explosion
- US NRC assessment of Fukushima 1 (2011-03-26)