Workers were evacuated and the reactor hall was cordoned off, but none of the staff was exposed to greater than the daily permitted limit of radiation.^[52][53][54]

March 12: triggered by falling water levels and exposed fuel rods, a hydrogen explosion occurred at reactor 1, resulting in the collapse of the concrete outer structure.^{[59][60][61][62][63]} Although the reactor containment itself was confirmed to be intact,^[64][65][66] the hourly radiation from the plant reached 1.015 mSv (0.1015 rem) – an amount equivalent to that allowable for ordinary people in one year.^[67][68]

Residents of the Fukushima area were advised to stay inside, close doors and windows, turn off air conditioning, and to cover their mouths with masks, towels or handkerchiefs as well as not to drink tap water.^[69] By the evening of March 12, the exclusion zone had been extended to 20 kilometres (12 mi) around the plant^[70] and 70,000 to 80,000 people had been evacuated from homes in northern Japan.^[71]

March 14: a second, hydrogen explosion (nearly identical to the first explosion in Unit 1) occurred in the reactor building for Unit 3, with similar effects.^[72]

March 15: a third explosion occurred in the “pressure suppression room” of Unit 2^[73] and is initially said not to have breached the reactor’s inner steel containment vessel,^[74] but later reports indicated that the explosion damaged the steel containment structure of Unit 2 and much larger releases of radiation were expected than previously.^[73] That same day, a fourth explosion damaged the 4th floor area above the reactor and spent fuel pool of the Unit 4 reactor.^[75] Contrary to the TEPCO press release, aerial photos show that most of the outer building was actually destroyed. The fuel rods (both new and spent fuel) of reactor Unit 4, stored in the now exposed spent fuel pool, were reportedly exposed to air – this would have risked the melting of the nuclear fuel. However, later research found the fuel rods had been covered by water all the time.^[76][77]

TEPCO estimated that 70% of the fuel in Unit 1 had melted, and 33% in Unit 2, further suspecting that Unit 3's core might also be damaged.^[78] In November 2011 TEPCO released the report of the Modular Accident Analysis Program (MAAP).^[79] The report showed that the reactor pressure vessel (RPV) in Unit 1 (commonly known as the reactor core) had been damaged during the disaster, and that significant amounts of fuel had fallen into the bottom of the primary containment vessel (PCV) – the erosion of the concrete of the PCV by the molten fuel immediately after the disaster was estimated to have been stopped in approx. 0.7 metres (2 ft 4 in) depth, with the thickness of the containment being 7.6 metres (25 ft). Gas sampling done before the report detected no signs of an ongoing reaction of the fuel with the concrete of the PCV and all the fuel in Unit 1 was estimated to be "well cooled down, including the fuel dropped on the bottom of the reactor". MAAP further showed that fuel in Unit 2 and Unit 3 had melted, however less than Unit 1, and fuel was presumed to be still in the RPV, with no significant amounts of fuel fallen to the bottom of the PCV. The report further suggested that "there is a range in the evaluation results" from "most fuel in the RPV (some fuel in PCV)" in Unit 2 and Unit 3, to "all fuel in the RPV (none fuel fallen to the PCV)". For Unit 2 and Unit 3 it was estimated that the "fuel is cooled sufficiently". The larger damage in Unit 1 was according to the report due to long time that cooling water was not injected in Unit 1, letting much more decay heat accumulate – for about 1 day there was no water injection for Unit 1, while Unit 2 and Unit 3 had only a quarter of a day without water injection. As of December 2013, it was reported that TEPCO estimated for Unit 1 that "the decay heat must have decreased enough, the molten fuel can be assumed to remain in PCV (Primary container vessel)".^[80]