CNN: Q&A: Japan's nuclear crisis

Workers trying to stabilize three overheating reactors at the plant were "asked to withdraw to a safe area," Japan's Chief Cabinet Secretary Yukio Edano said. Authorities later allowed them to return after radiation levels dropped, the Tokyo Electric Power Company said.

A Japanese self defense forces helicopter also aborted its mission to head to the site to drop cooling water over the reactors because of radiation levels, Japanese public broadcaster NHK reported.