Japanese engineers fought to save three reactors from meltdown at the stricken Fukushima 1 power station as they battled a series of frustrating setbacks that saw workers struggle to pump seawater into the reactors in a desperate attempt to cool the overheating nuclear cores and make them safe.
The day began with an orange flash and a violent blast that destroyed most of the containment building around reactor 3, causing debris to fall back inside and on to the structure housing the reactor. The blast was caused by a build up of hydrogen that was produced when superheated steam in the core reacted with zirconium alloy cladding that surrounds the reactor's fuel rods.
Tepco, the company that operates the power station, said 11 people were injured in the accident, one seriously. A similar explosion blew the top off the reactor 1 building on Saturday morning.
Early this morning there was another explosion, this time at reactor 2. Early reports suggested that there might have been some damage to the containment vessel after pressure dropped, but there was no immediate word of any damage to the reactor itself, the country's nuclear safety agency said.
Despite earlier assertions from Tepco that the steel containment vessels surrounding the reactors were undamaged in either of the two earlier explosions, Naoki Kumagai, an official at Japan's Nuclear and Industrial Safety Agency (Nisa), said: "It's impossible to say whether there has or has not been damage."
At one point on Sunday emergency cooling at two reactors was suspended because the pools of seawater being pumped in ran dry. Later in the day, a backup pump to a third reactor ran out of fuel, causing water levels to fall so low that the fuel rods were fully exposed.
Officials at Tepco said it believed all three nuclear reactors are likely to have suffered partial meltdowns, though this could mean just one fuel rod or nearly all of them melting within the cores.
The reactors are at risk of going into meltdown because although they had shut down, the fuel rods continue to give off heat. Primary and backup power to the cooling systems was knocked out during the massive earthquake and tsunami that struck on Friday.
Ryohei Shiomi, a Nisa official, said reactors 1 and 3 appeared stable for the time being, but that reactor 2, where fuel rods were most exposed, was still a concern.
Water has to cover completely the radioactive fuel rods in the nuclear cores to prevent overheating, but on Monday afternoon water levels dropped substantially in all three reactors, and at one point fully exposed the fuel rods in reactor 2. A spokesman for Tepco said it could not rule out a meltdown at the reactor.
Speaking about the situation at reactor 2, chief cabinet secretary, Yukio Edano, said: "The pump ran out of fuel, and the process of inserting water took longer than expected, so the fuel rods were exposed from the water for a while."
The decision to pump salty, untreated seawater into the reactors – along with boric acid to dampen down radioactivity – is a vastly expensive last resort that effectively writes-off the nuclear reactors for good. The plants are usually cooled by highly-purified de-ionised water that does not damage delicate components inside. The risk of total meltdown at the plant will fall dramatically over the next few days if engineers can continue to flood the reactors with seawater. The fuel rods will already have lost around 90% of their heat and without further setbacks, the reactors could be cold and rendered safe within a week to 10 days.
But engineers at the power plant face a delicate balancing act because sea water being pumped into the reactors is boiling immediately into steam, which raises the pressure inside them.
This has to be vented off before more water can be pumped in, but doing so releases small amounts of radioactive material into the air.
Nisa has already confirmed that caesium-137 and iodine-131 have been released into the atmosphere. These radioactive substances are produced in the core and can contaminate cooling water if fuel rods get hot enough to melt the cladding that surrounds them.
The release of radioactivity has raised health concerns and wider fears of environmental contamination. Monitoring posts to the north-west of the power station recorded radiation levels at 680 microSieverts per hour on Monday, a dose roughly equivalent to four months of natural background radiation. An American warship, the USS Ronald Reagan, detected low levels of radiation at a distance of 100 miles from the Fukushima plant.
Radiation levels have increased in the vicinity of the power station and nearby areas. Those caught in the evacuation zone around Fukushima were given potassium iodide pills to protect against thyroid cancer. Radioactive iodine is easily absorbed by the thyroid, where it can cause tumours, but the pills saturate the gland and obstruct the radioactive form's absorption. Edano said the release of large amounts of radiation was unlikely.
Under normal conditions, the nuclear reactors produce electricity by using heat from fission reactions in the fuel rods to turn water into steam and drive turbines. Reactors 1 and 2 operate with uranium fuel rods, but reactor 3 uses a mixed oxide fuel, or Mox, which contains plutonium, a highly toxic substance that if released, can linger in the environment for thousands of years. The half life of plutonium is 24,000 years, meaning it takes that long for its radioactivity to drop by half.
Nuclear experts emphasised there are significant differences between the unfolding nuclear crisis at Fukushima and the events leading up to the Chernobyl disaster in 1986. The Chernobyl reactor exploded during a power surge while it was in operation and released a major cloud of radiation because the reactor had no containment structure around it.
At Fukushima, each reactor has shut down and is inside a 20cm-thick steel pressure vessel that is designed to contain a meltdown. The pressure vessels themselves are surrounded by steel-lined, reinforced concrete shells.
"While the material is enclosed in the reactor vessel it is safe, in that it is the same radioactivity that was there in the fuel rods. The issue would come if there is a continued problem to cool down the fuel rods," said Paddy Regan, a nuclear physicist at Surrey University.
He said the worst case scenario would be "that some of the fission fragments and fuel could be widely dispersed if the vessel was to explode. This seems unlikely at present, so the next worst would likely be ongoing venting of the steam which has built up in the reactors."
Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), said it was "unlikely that the accident would develop" like Chernobyl.
"The Japanese authorities are working as hard as they can, under extremely difficult circumstances, to stabilise the nuclear power plants and ensure safety," Amano said in a statement.