Tokyo Shinbun reports that according to information they obtained through the Japanese equivalent of the FOIA, the Kan administration did talk about wide-area evacuation in the Tokyo Metropolitan areas even before the Atomic Energy Commission's chief handed the worst-case scenario to then-Prime Minister Kan on March 25, 2011.
It was Yukio Edano, then-Chief Cabinet Secretary (and current Minister of Economy, Trade and Industry), who mentioned the wide-area evacuation, while other unnamed ministers were worried about the stock market, in the evening of March 12, 2011 after Reactor 1 blew up.
... Stock market?
From Tokyo Shinbun evening edition (4/13/2012):
東京、茨城の避難検討 福島１号機 水素爆発直後に官邸
Prime Minister's Office was contemplating evacuation of Tokyo and Ibaraki, right after the hydrogen explosion of Fukushima Reactor 1
On March 12, 2011 when a hydrogen explosion took place in the Reactor 1 building at Fukushima I Nuclear Power Plant, a meeting of the nuclear disaster response headquarters was held in the Prime Minister's Official Residence where a wide-area evacuation including Tokyo and Ibaraki Prefecture was being discussed, this newspaper learned on April 13 from the memo by the government officials obtained via the information disclosure request.
The memo is the summary of remarks during the 4th meeting that started at 10:07PM on March 12, 2011. That day, a hydrogen explosion occurred in the reactor building of Reactor 1, and the emergency cooling system of Reactor 3 was becoming unstable, and the crisis was deepening.
In the meeting, Koichiro Genba, then-minister in charge of national strategy [and current Minister of Foreign Affairs] wanted to have "the worst-case scenario", to which then-Prime Minister Naoto Kan responded with worries about whether it was possible to have radiation contamination like in the Chernobyl accident. Mr. Kan accepted the explanation that the reactors at Fukushima I Nuclear Power Plant were of totally different type [from the Chernobyl reactor], but emphasized the importance to secure the Containment Vessels at all cost.
Ministers expressed worries over blackouts, [availability of] foods, and negative effect on the stock market. Then-Chief Cabinet Secretary Yukio Edano said, according to the memo, "We have to start thinking about wide-area evacuation. Including Tokyo, and Ibaraki."
It has been recently discovered that the minutes of the meetings related to the March 11, 2011 disaster, including those of the nuclear disaster response headquarters, haven't been created. Responding to the criticism, the government published the summaries of the meetings last month.
[Edano's] remark on the wide-area evacuation was not included in the meeting summary. It was probably because the intent of his remark couldn't be ascertained when the summary was created.
I wish Tokyo Shinbun published the actual memo in its entirety.
So these ministers were having the meeting on the upper floor of the Official Residence, while in the basement the real information was being transmitted: SPEEDI simulations that showed the potential wide-area contamination. Somehow they didn't reach the upper floor, and the ministers forgot to turn on the teleconferencing system on the dedicated line.
The Kan administration on the first few days of the nuclear accident did the full-on propaganda of "everything was under control", as if all people needed to worry about was the earthquake and tsunami devastated Tohoku. The administration even sent out the female minister in charge of consumer affairs to scold those selfish Tokyo residents who were trying to hoard bottled water and food. "Don't buy them, because they are more needed in the disaster areas!"
That was just after Reactor 3 blew up in a spectacular fashion. Then one day later Reactor 4 blew up. And something happened in Reactor 2. The government didn't warn residents in Tohoku and Kanto about the possible radioactive fallout from the explosions, and people were still very trusting of their government. Chief Cabinet Secretary Edano was being praised for his "hard work".
Radioactive plumes from Fukushima I Nuclear Power Plant reached Tokyo, and 210 becquerels/kg of radioactive iodine was detected from the tap water in Tokyo on March 22. The Tokyo Metropolitan government didn't have enough bottled water to give to the residents with babies and infants.
Ah. Those bad old days. Makes me mad.