NUCLEAR CRISIS: HOW IT HAPPENED / Government radiation data disclosure--too little, too late : National : DAILY YOMIURI ONLINE (The Daily Yomiuri)

However, the March 11 calamity severed power at the Fukushima plant, meaning SPEEDI data could not be transmitted. The government said it did not make forecasts from the system public because "accurate predictions could not be made."

Despite the information blackout on radiation levels, SPEEDI continued to churn out useful data about radiation emissions immediately after the earthquake and tsunami by inputting provisional readings.

The system's estimates on radiation pollution for the afternoon of March 12 show high contamination in areas eerily similar to those the government eventually designated as "planned evacuation areas" in April.

...Commenting on the matter, Hirotada Hirose, professor emeritus of Tokyo Women's Christian University and specialist in risk psychology, said, "In a fast-changing crisis situation, delays in releasing information to try to ensure accuracy often aggravates people's suspicions and unease."