The specter of meltdown and widespread radiation grows ever-more terrifying at Tokyo Electric Power Co.’s (TEPCO’s) Daiichi nuclear power plant in Fukushima Prefecture. Soon after a devastating magnitude 9.0 earthquake shook northeastern Japan on Friday, March 11, at 2:48 p.m. JST, the government declared an emergency as a precaution. Events have dramatically escalated since then, with four explosions and two fires afflicting four of the plant’s six reactors.

On Friday, about an hour after what is being called “Japan’s most forceful quake,” the Daiichi plant and several nearby power plants—including the Fukushima Daini, Onagawa, and Tokai nuclear plants—were deluged by a tsunami wave as high as 7.3 meters. Japan’s Nuclear and Industrial Safety Agency (NISA) told the nation that though control rods were activated at three reactors operating at the Daiichi and Daini plants, their emergency core cooling systems had suddenly failed, possibly as a result of the flooding. Authorities also reported fires at the nearby Onagawa nuclear power plant.

TEPCO on Friday said that the emergency diesel generators had shut down “due to malfunction, resulting in the complete loss of alternating current for all three units.” As experts from the Union of Concerned Scientists (UCS) explained: “The boiling water reactors at Fukushima are protected by a Reactor Core Isolation Cooling (RCIC) system, which can operate without AC power because it is steam-driven and therefore does not require electric pumps. However, it does require DC power from batteries for its valves and controls to function.”

The group warned: “If battery power is depleted before AC power is restored, however, the RCIC will stop supplying water to the core and the water level in the reactor core could drop. If it drops far enough, the core would overheat and the fuel would become damaged. Ultimately, a ‘meltdown’ could occur: The core could become so hot that it forms a molten mass that melts through the steel reactor vessel. This would release a large amount of radioactivity from the vessel into the containment building that surrounds the vessel.”

Early on Saturday, meanwhile, TEPCO announced that it had lost ability to control pressure building in the Daiichi 1 containment—which resulted from the loss of AC power to the reactors and stopped the cooling system. Soon, it notified authorities it would “implement measures to reduce the pressure of the reactor containment vessel for those units that cannot confirm certain level of water injection by the Reactor Core Isolation Cooling System, in order to fully secure safety.” Pressure venting began at Daiichi 1. Meanwhile, as reports began to surface that irradiated fuel at Daiichi 1 may have been damaged—NHK World broadcast that 9 mm of fuel rods had been exposed and crews were scrambling to raise water levels—authorities extended evacuation radii around both plants to 10 km.

Then, at 3:36 p.m. on Saturday, an explosion at Daiichi 1 shook the facility. TEPCO claimed the blast followed an aftershock. It was a hydrogen explosion, Cabinet Secretary Yukio Edano told the nation, however, stressing that the reactor container was not damaged, but revealing that the concrete containment building had been blown away. He added that TEPCO would fill the reactor container with seawater under Trade Minister Banri Kaieda’s instructions. “By doing this, we will use boric acid to prevent criticality,” he said. “At this point, there has been no major change to the level of radiation leakage outside (from before and after the explosion), so we’d like everyone to respond calmly.”

On Saturday, meanwhile, TEPCO was reportedly battling rising pressure at Daini 1,2, and 4—all of which the company said had retained off-site power.

On Sunday, Japanese authorities rated the deteriorating incident at Daiichi as a 4 under the International Nuclear and Radiological Event Scale—lower than the 1979 Three Mile Island fiasco, which was rated a 5, and Chernobyl, which was rated 7 on the 1 to 7 scale. Nevertheless, they extended the evacuation zone around the nuclear plant to 20 km. An estimated 110,000 people were evacuated.

At 3:23 p.m. on Sunday, Edano held another press conference, saying containment pressure at Daiichi 1 had caused the reactor’s building to explode. No damage had been done to the vessel itself, he said, though he admitted the situation remained dire. At the same time, as was being done at Daiichi 1 and Daiichi 3, TEPCO was preparing to put seawater into Daiichi 2, Edano said.

Then, at 7:30 a.m. on Monday, TEPCO reported that radiation levels at the Daiichi plant had surged. Later that morning, at around 11 a.m., reports said there was another hydrogen explosion at Daiichi 3, but no one could confirm that the reactor blast—the second in two days—had released radioactivity. Authorities confirmed that 11 workers had been injured: four TEPCO employees and four SDF agents. Later that afternoon, the International Atomic Energy Agency (IAEA) confirmed that the blast did not damage the primary containment vessel.

Just an hour later, however, Edano said at a press conference that water levels at the Daiichi 2 reactor had fallen—and its cooling functions had stopped. TEPCO said it was “doing all it can” to keep the fuel rods from melting, including pumping in seawater despite a series of technical issues. But, just as the news emerged that the Daini reactors were successfully in cold shutdown, reports claimed that nuclear fuel rods in the Daiichi 2’s core had been fully exposed—for as long as six hours, as Japanese television broadcaster NHK reported. Authorities could not rule out meltdown of the rods.

Another explosion rocked the Daiichi complex at around 6:15 a.m. on Tuesday—this time, at Daiichi 2. It was more serious: Authorities confirmed that that the blast, which reportedly came from the reactor’s suppression pool, had damaged the Daiichi 2 reactor vessel. Radiation levels surrounding the plant had surged fourfold.  Prime Minister Naoto Kan told the nation that the risk of a radioactive leak was rising, warning people within a 30-km radius to stay indoors.

Then, at about 4:44 p.m. on Tuesday, officials revealed that a fire had erupted at Daiichi 4—a reactor that was not operating at the time of the earthquake, but in which 514 units of spent nuclear fuel were being stored. TEPCO admitted a blast had occurred at the reactor, leaving two holes 8 meters square in a wall of the outer building. The fire had originated at the spent fuel storage pond and reportedly began before Daiichi 2 exploded. Japanese authorities told the IAEA that TEPCO had extinguished the fire.

Edano told reporters just an hour later that radiation levels surged to 400 millisieverts per hour. TEPCO at 6 p.m. evacuated 750 workers from the plant—leaving 50 to brave the fire and radiation. At 8:45 p.m., the government announced that radiation levels at Daiichi 4 were too high to conduct normal work from its control room. Despite Japan’s health and labor ministry raising the radiation exposure limit by a factor of 2.5, to 250 millisieverts (in cases of emergency) just an hour earlier, the remaining 50 workers were evacuated.

Earlier this morning, as radiation levels were dropping and workers were preparing to return to the facility, TEPCO announced that the fuel pond at Daiichi 3—where 6% of  the spent fuel rods contain mixed oxide fuel—may have heated and produced steam. Even so, a government spokesperson said, it was not realistic to think that the plant would “reach criticality.” Meanwhile, a newly formed joint task force consisting of special defense forces and TEPCO employees tried but aborted efforts to spray seawater into Daiichi 3—where a new fire was reported—via helicopter. Officials are also preparing to spray water into Unit 4 from ground positions, and possibly later into Unit 3.

This afternoon, as TEPCO said it recorded the site’s highest levels of radiation at Daiichi 3, Japan announced it would seek direct U.S. military help to quell the crisis. It also called on its own military to help pump water to Daiichi 3 reactor and into the spent-fuel pool at Daiichi 4. At 6:30 p.m., in a turn for the worse, TEPCO revealed that Daiichi 5 and 6 were also overheating—and that now it was struggling to cool all six reactors at the facility.

Sources: POWERnews, NISA, TEPCO, NHK Worldline, Kyodo News, Reuters