There have been hot periods at Daiichi before. Late summer 2011 through spring 2012 was very hot at the plant. Many events were documented on the webcams, including nuclear fires.
I have a friend who encountered 500 microsieverts an hour in Fukushima Prefecture in the summer of 2012.
The plant seemed a bit cooler in the fall of 2012, although many strange sightings continued.
Now in the summer of 2013 I believe the plant is heating up again. I've posted frequently recently on the increased incident, duration, and volume of steam events. Worryingly, the steam is becoming 'darker' and dirtier-looking.
Fukushima Diary has reported Tepco increased amounts of tritium, cesium, and strontium in ground and sea water samples. For example:
Fukushima Diary reported that '500,000,000 Bq/m3 of Tritium and 1,000,000 Bq/m3 of Sr-90 detected from groundwater east side of reactor2' according to documents from Tepco. http://fukushima-diary.com/2013/06/500000000-bqm3-of-tritium-and-1000000-bqm3-of-sr-90-detected-from-groundwater-east-side-of-reactor2/#comments
The source of Fukushima Dairy's data is Tepco's report on groundwater readings taken from the east side of the turbine building.
500,000,000 Bq/m3 is a lot of tritium.
Is fission occurring in reactor 2? A Daiichi site worker reported to the press that unit 2 was the hottest http://no-border.asia/archives/9257
I don't know whether the nuclear criticalities occurring at Daiichi could result in an explosive chain. My guess is that an explosion is quite unlikely unless fires break out.
Tepco has stated publicly that continuous injections of water are needed at the Daiichi site to prevent eruptions of nuclear fires. 400 tons of water was reported as being injected daily in the spring of 2013.
Sub-criticalities are no doubt occurring in the highly contaminated water saturating the Daiichi site. Water is a good moderator for nuclear reactions. However, sub-criticalities by definition don't lead to explosive chains. They decay away, emitting gamma rays and radionuclides into the atmosphere.
Over time, Daiichi should have cooled if the only problem entailed sub-criticalities in fuel-contaminated water.
However, it appears the site is getting hotter again, despite lack of outright nuclear fires.
The high tritium and strontium readings in groundwater and higher ocean contamination by these same radionuclides provide additional evidence of increased radiation in the environment:
Tritium level in seawater spiked over double within 2 weeks
Increase in Rockfish http://optimalprediction.com/wp/enormous-increase-of-cesium-137-in-white-rockfish/
Fukushima EndGame? http://optimalprediction.com/wp/fukushima-endgame-in-progress/
Is something happening with the unit 2 fuel that we should be aware of?
Watch this scary video from a couple of days ago http://www.youtube.com/watch?v=SqnlLD9e5MY&feature=youtu.be
From my screen shots I know that what you see in the video is NOT FOG.
Below are some screen shots from today. They depict the plant's radiation emission aura.
Please see my list of posts for other recent examples of significant steam events.