At 16:30 JST (7:30 GMT) on March 12, there was an explosion at the Fukushima I nuclear power plant. At 17:00 JST (0800 GMT), Fox News Channel reported the explosion and a plume of "white smoke" rising above the plant. At 17:03 JST (08:03 GMT) BBC News reported that Japan's NHK TV reported the explosion as "near" the plant, and that four workers were injured. At 18:43 JST (9:43 GMT) officials had confirmed that an explosion has occurred at the nuclear plant. Chief Cabinet Secretary Yukio Edano indicated -according to a Reuters report of 21:36 JST (7:36 ET), that the concrete outer structure had collapsed as a result of a hydrogen explosion triggered by falling water levels. Edano indicated that the container of the reactor had remained intact and their had been no large leaks of radioactive material. An increase in radiation levels has been confirmed following the explosion. "Fukushima Prefecture says a radiation level of 1,015 microsieverts [sic] per hour has been measured near the Fukushima Number 1 nuclear power station. One hour of exposure to this amount of radiation is equivalent to the permissible amount of radiation an ordinary person receives in one year".

The Guardian reported at 17:35 JST (8:35 GMT) that NHK advising residents of the Fukushima area "to stay inside, close doors and windows and turn off air conditioning. They have also been advised to cover their mouths with masks, towels or handkerchiefs" as well as not to drink tap water. Other news agencies have reported information on a definite explosion at the plant. At 19:07 JST (10:07 GMT) Reuters reported that the exclusion zone has been extended to 20 kilometres (12 mi) around the plant. BBC correspondent Nick Ravenscroft was stopped 60 kilometres (37 mi) from the plant by police.

As a further measure Edano indicated that sea water would be used for cooling and supplied in the next ten hours, after which it would require seawater cooling for around ten days.

Yaroslov Shtrombakh, a Russian nuclear expert, said he does not believe that a Chernobyl-style disaster will occur, citing the differences between the designs of the Chernobyl Nuclear Power Plant and the Fukushima I Nuclear Power Plant. He speculated that any nuclear material released during the incident would likely be confined to the grounds in and around the power plant.