Two Japanese nuclear power stations are struggling to contain damage from a major earthquake and tsunami, in what could become be the worst nuclear incident since the catastrophic Chernobyl accident of 1986.

This morning, the BBC has footage of what appears to be a massive explosion at the Fukushima Daiichi nuclear power station’’s reactor number 1, which since last night has had trouble with cooling problems at its core. According to Kyodo news service, the blast occurred at 3:36 PM local time after a large aftershock shook the plant. Radiation levels on the premises have reached 1050 μSv per hour—about what an average person would receive in a year. The evacuation zone is currently extended to a 20 kilometer radius of the plant.

The explosion follows a difficult night at the power station, as engineers struggled to contain damage from a magnitude 8.9 earthquake that rocked the region on 11 March. The earthquake automatically put the three operating reactor cores at Fukushima Daiichi into shutdown mode, effectively halting the nuclear processes inside the core. Even after the reactor is shut down, however, it continues to require cooling, particularly in the first hours after an incident. Diesel generators initially supplied the necessary cooling water at Fukushima units 1 and 2, but they appear to have failed about an hour after the quake. The International Atomic Energy Agency says it now believes the tsunami that followed the earthquake was responsible.

Without enough fresh cooling water, the pressure in Fukushima 1 and 2 has begun to rise, as the cooling water covering the core boils into steam. The latest estimates from World Nuclear News, an industry publication, is that the pressure inside Fukushima Daiichi unit 1 is now 840 kPa, about double the reference value. Since this morning, reactor operators have been releasing radioactive steam from the containment vessel, while additional back up generators are brought in to restart the cooling pumps. As long as the reactor remains covered in water, a meltdown is unlikely, but if the core becomes exposed, then the temperature will rise until the uranium fuel begins to melt.

The impact of the explosion on the situation remains unclear, but press photographs indicate major damage to the building surrounding Fukushima 1. Kyodo reports that serious damage to the containment vessel is “unlikely” according to safety officials.

Meanwhile, at the nearby Fukushima Daini station, operators noticed an increase in the temperature of the plant’s unit 1 and unit 2 reactors at around 5:30 am local time. Evacuations have begun around Daini in preparation for an emergency release of steam to relieve pressure in those units.

UPDATE: The IAEA confirms explosion at Fukushima 1, says authorities are preparing to distribute iodine tablets (release).

For full coverage of the Fukushima disaster, go to Nature’s news special.