It was only late April that TEPCO admitted to the existence of the "survey map" which plotted the radiation levels on the plant compound, supposedly for the workers to avoid "hot" debris scattered around the reactor buildings and turbine buildings.
But Kyodo News says TEPCO had started sending the map to the US Nuclear Regulatory Commission more than one month before they made the map public. It means TEPCO started sending the map to the NRC in mid to late March, practically right after the accident started on March 11, 2011.
What's more, the Nuclear and Industrial Safety Agency started receiving the map a day after TEPCO had started sending it to the NRC, and the Agency had kept it quiet until TEPCO admitted to the existence.
Kyodo News (2/12/2012):
TEPCO had given the radiation map of the plant more than one month before the company made it public
It was revealed on February 11 that the radiation map of the Fukushima I Nuclear Power Plant compound ("survey map") that TEPCO had made public in late April last year had been provided to the US Nuclear Regulatory Commission (NRC) more than one month prior. According to TEPCO, the survey map was sent as it was updated. The company started to send the report to the Nuclear and Industrial Safety Agency under the Ministry of Economy, Trade and Industry one day after the company had started providing the information to the US agency.
In the Fukushima I Nuke Plant accident, it has been revealed that the SPEEDI data from the Ministry of Education and Science and the dispersion simulation data of radioactive materials by the Japan Meteorological Agency had been provided to the US and the international agency [IAEA, who got the data from JMA] from early on, but were not made public in Japan until much later.
On April 24, 2011, Kyodo News reported that TEPCO admitted to the existence of the "survey map". If I remember right, TEPCO only admitted after the "rumor" had spread, supposedly originated from the workers at the plant, that such a map existed.