* Astronomy

Unit 2 Explosion and Unit 4 Spent Fuel Pool Fire

It was reported earlier today that the explosion at Unit 2 of the Fukushima Daiichi plant damaged the suppression chamber.
Hydrogen gas from the cladding oxidation with steam collected in the suppression pool and ignited.  This scenario differs from those of units 1 and 3 where the explosion occurred outside the primary containment in the upper part of the reactor building. 
This breach of primary containment is certainly more serious than the situation in units 1 and 3.  Seawater is still being pumped in the containment and the reactor vessel.  At this time radioactive releases from unit 2 have been similar to the ones seen from units 1 and 3.
Source

Concerns shift to reactors 2 and 4

The Fukushima crisis has followed an unpredictable path from the beginning.
When the earthquake hit on Friday and three operating reactors shut down as planned, no one expected all the back-up cooling equipment to be knocked out by the tsunami. No one foresaw the spectacular explosions that destroyed outer buildings at the reactors. And, with everyone focusing on the reactors, Tuesday's shift of attention to a cooling pond for spent nuclear fuel came as a complete surprise.
Read more

The Japanese authorities have informed the IAEA that the following radiation dose rates have been observed on site at the main gate of the Fukushima Daiichi Nuclear Power Plant.
At 00:00 UTC on 15 March a dose rate of 11.9 millisieverts (mSv) per hour was observed. Six hours later, at 06:00 UTC on 15 March a dose rate of 0.6 millisieverts (mSv) per hour was observed.
These observations indicate that the level of radioactivity has been decreasing at the site.
As reported earlier, a 400 millisieverts (mSv) per hour radiation dose observed at Fukushima Daiichi occurred between units 3 and 4. This is a high dose-level value, but it is a local value at a single location and at a certain point in time. The IAEA continues to confirm the evolution and value of this dose rate. It should be noted that because of this detected value, non-indispensible staff was evacuated from the plant, in line with the Emergency Response Plan, and that the population around the plant is already evacuated.
About 150 persons from populations around the Daiichi site have received monitoring for radiation levels. The results of measurements on some of these people have been reported and measures to decontaminate 23 of them have been taken. The IAEA will continue to monitor these developments.
Evacuation of the population from the 20 kilometre zone is continuing. The Japanese have asked that residents out to a 30 km radius to take shelter indoors. Japanese authorities have distributed iodine tablets to the evacuation centres but no decision has yet been taken on their administration.

Source International Atomic Energy Agency (IAEA)

Read more

It appears that for the first time, the containment system around one of the Fukushima Daiichi reactors has been breached.
Officials have referred to a possible crack in the suppression chamber of reactor 2 - a large doughnut-shaped structure, also known as the torus, below the reactor housing.
That would allow steam, containing radioactive substances, to escape continuously.
This is the most likely source of the high radioactivity readings seen near the site.
However, an alternative possible source is the fire in reactor 4 building - believed to have started when a pool storing old fuel rods dried up.
When fuel rods reach the end of their useful life and are taken from the reactor, they still contain a lot of radioactive nuclei, which means they get hot.
Read more