As the prestigious scientific journal Nature notes:
Shortly after a massive tsunami struck the Fukushima Daiichi nuclear power plant on 11 March, an unmanned monitoring station on the outskirts of Takasaki, Japan, logged a rise in radiation levels. Within 72 hours, scientists had analysed samples taken from the air and transmitted their analysis to Vienna, Austria — the headquarters of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), an international body set up to monitor nuclear weapons tests.
It was just the start of a flood of data collected about the accident by the CTBTO’s global network of 63 radiation monitoring stations. In the following weeks, the data were shared with governments around the world, but not with academics or the public.
The attempted cover up of the severity of the Fukushima disaster is nothing new. Governments have been covering up nuclear meltdowns for 50 years, and the basic design for nuclear reactors was not chosen for safety, but because it worked on Navy submarines … and produced plutonium for the military.
(Indeed, the government’s response to every crisis appears to be to try to cover it up; and see this.)
Today, Yomiuri Shinbun reports (Google translation) that the U.S. knew within days that Fukushima had melted down:

Links for more info on Fukushima:

http://fukushimaupdate.com/

http://www.huffingtonpost.com/tag/fuk…

http://www.globalresearch.ca/fukushim…

http://www.storyleak.com/scientists-d…

http://ajw.asahi.com/article/0311disa…

http://www.hmbreview.com/news/health-…

http://www.nytimes.com/2012/05/25/wor…

See also:

An Overview of Fukushima and a Call for Meaningful International Collaboration