The paper by a Tokyo University researcher that was finally published nearly one year after he took the samples at the front gate of Fukushima I Nuclear Power Plant and in Iitate-mura in Fukushima Prefecture (see my post from yesterday) has this very interesting chart.
It shows the spacial dose of radiation at the front gate of Fukushima I Nuke Plant and the timeline of events at the plant in March. The largest spike seems to be around or after 12AM on March 15, and the label on the spike says "Dry vent at reactor no.2".
On checking the information from March 2011, I find that TEPCO did the dry vent at Reactor 2 at 12AM on March 15, 2011 for a few minutes. At first (on March 20), TEPCO announced they'd done the dry vent sometime on March 16 and 17, but on March 21 retracted that statement and said the dry vent was done at 12AM on March 15. (See Sankei Shinbun article on 3/21/2011.)
It is not clear whether the company informed the residents or the government before the the Reactor 2 dry vent on March 15 which caused a huge spike in radiation, and which may be the biggest contributor to radiation contamination in wide areas in Tohoku and Kanto. From the Sankei article it doesn't look like they gave any prior notice. This is the same company that didn't even tell the plant workers that the vent on Reactor 1 was about to be carried out on March 12.
Dry vent releases different types of radioactive materials compared to wet vent.
The peak that appears to the right side of the largest peak is after the hydrogen explosion of Reactor 4, but not at the time of explosion. I suspect the peak may have been the result of some event on Reactor 2's Suppression Chamber which seems to have taken place about the same time as the Reactor 4 hydrogen explosion. TEPCO says it was not "explosion" that happened at the Reactor 2 Suppression Chamber but still doesn't say what it was.
The second largest peak was when the fire broke out on Reactor 3 on March 16. The white smoke was seen at 8:30AM on March 16, and the radiation level at the front gate of the plant shot up to 10,000 microsieverts/hour (10 millisievert/hour) at 10:40AM (Asahi Shinbun 3/16/2011).
Again, it is just too bad that the researcher had to sit on the data for 11 months for his article to be published in a peer-review magazine.