Fuel rods in three nuclear reactors at the Fukushima plant are likely to have suffered partial meltdown after series of setbacks
The situation at Japan's stricken Fukushima 1 nuclear power plant continued to deteriorate on Monday as engineers fought to save three reactors from meltdown and a second explosion at the site tore the roof off a containment building.
A series of frustrating setbacks during the day saw workers struggle to pump seawater into the reactors in a desperate attempt to cool the overheating nuclear cores.
At one point, emergency cooling at two of the reactors was suspended because the pools from which seawater was being pumped ran dry. Later in the day, a back-up 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, the company that operates the power station, said it believed all three reactors were likely to have suffered partial meltdowns, though this could mean anything from one fuel rod to nearly all of them melting within the cores.
The reactors are at risk of going into meltdown because although they were automatically shut down, the fuel rods continue to give off heat. Primary and back-up power to the cooling systems was knocked out during the massive earthquake and tsunami that struck on Friday.
On Monday evening, Ryohei Shiomi, an official at Japan's Nuclear and Industrial Safety Agency (Nisa), said reactors 1 and 3 appeared stable for the time being, but that reactor two, where fuel rods were most exposed, was still a concern. "Unit 2 now requires all our effort and attention," he said.
The day began with an orange flash and a violent blast that destroyed most of the containment building around reactor 3, with debris falling back inside and onto the structure housing the reactor. The blast was caused by a buildup of hydrogen that was produced when superheated steam in the core reacted with the zirconium alloy cladding that surrounds the reactor's fuel rods.
Tepco said 11 people were injured in the accident, one seriously. A similar explosion blew the top off the reactor 1 building on Saturday morning.
Despite earlier assurances from Tepco that the steel containment vessels surrounding the reactors were undamaged in both explosions, Naoki Kumagai, a Nisa official, said "It's impossible to say whether there has or has not been damage."
Water has to cover the radioactive fuel rods in the nuclear cores completely to prevent them from 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 this reactor.
Speaking about the situation at reactor 2, the government's 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 nuclear reactors – along with boric acid to dampen down radioactivity – is a vastly expensive last resort that effectively writes off the reactors. The plants are usually cooled by highly purified deionised water that does not damage the delicate components inside.
Engineers at the power plant face a difficult balancing act because the seawater being pumped into the reactors immediately boils and the steam is raising the pressure inside. This has to be vented before more water can be pumped in, but this 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 are radioactive isotopes that 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 northwest of the power station recorded radiation levels of 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 immediate vicinity of the power station and in surrounding areas. People 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 with the element and make it harder for the radioactive form to be absorbed.
Under normal conditions, 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 use 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 have emphasised that 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, which released a huge 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, 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.
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