TOKYO—A former nuclear adviser to Japanese Prime Minister Naoto Kan blasted the government's handling of the crisis, and predicted more revelations of radiation threats to the public in the coming months.

In his first media interview since resigning his post in protest in April, Toshiso Kosako, one of the country's leading experts on radiation safety, said Mr. Kan's government has been slow to test for dangers in the sea and to fish, and has understated certain radiation threats...