Concerns about the stability of nuclear reactors at a power plant in Fukushima, Japan, grew Monday, with officials stating that the nuclear fuel rods inside three reactors appeared to be melting.
"Although we cannot directly check it, it's highly likely happening," Japanese Chief Cabinet Secretary Yukio Edano said Monday.
That capped a day of continuing turmoil at the Fukushima Daiichi plant, located in Fukushima Prefecture, along Japan's northeastern coast. Monday saw the second explosion at the plant. The first blast took place Saturday.
And by the end of Monday, a critical situation had developed at a reactor at Unit No. 2 where officials said the water surrounding the nuclear fuel rods had dropped twice, making the rods vulnerable to melting.
Series Of Crises
Emergency systems at the Fukushima Daiichi plant, located in Fukushima Prefecture along Japan's northeastern coast, began to fail Friday in the wake of a magnitude 8.9 earthquake and tsunami that knocked out power at the facility. That crippled the systems in place to cool reactors in case of emergency.
Since then, workers have been scrambling to address one crisis after the other at the plant, and there are questions about their ability to keep the reactors and the radioactive material inside them under control.
Even as emergency crews on Monday focused on attending to the damage at the other two units, the cooling system at Unit No. 2 developed problems of its own.
Late Monday, officials from the Tokyo Electric Power Company said that Unit No. 2's fuel rods were exposed and a partial meltdown may have occurred. Nuclear safety officials are concerned about the condition of the rods.
"It is very likely that they have been damaged by now," said Ryohei Shiomi, an official with the Nuclear and Industrial Safety Agency.
A partial meltdown isn't a technical term — it just means that the nuclear fuel rods have gotten so hot that they have started to melt. If the radioactive fuel rods that make up the reactor core melt completely, it's considered a full meltdown — but that doesn't necessarily mean the radioactive materials have breached the containment structure and escaped. Officials have not indicated that any of the three containment structures have been breached.
Meanwhile, workers are pumping in seawater into all three troubled reactors in an attempt to cool the nuclear cores. At least one Japanese media outlet reported that the pump injecting seawater into the core at Unit No. 2 ran out of diesel fuel. Reports suggest that only one out of five water pumps may currently be functioning. The other four pumps may have been damaged by the explosion Monday morning at the No. 3 reactor, TEPCO told the Kyodo news agency.
Questions About Radiation Exposure
Monday's blast blew the roof off a structure built around the No. 3 reactor but did not harm the reactor itself or its containment vessel, government officials said.
The containment vessel is a heavily reinforced barrier that surrounds the reactor's core and provides another layer of protection against the release of radioactive material.
Because the containment structure remains intact, "we believe that the risk that large amounts of radioactive materials have been dispersed is low," said Edano said.
The Unit No. 3 reactor is one of six reactors at the Fukushima Daichi plant about 150 miles north of Tokyo. A similar explosion occurred on Saturday at Unit No. 1, and did release some radioactive material.
"We expect an increase in radiation levels similar to the one that followed the hydrogen explosion that occurred in the Unit 1 reactor," Edano said.
Officials say radiation has also been released by periodic venting of steam from the cores of reactors 1 and 3. Late Monday, TEPCO announced additional problems at Unit 2 with its steam vent, causing the water levels inside the reactor to drop again.
Source Of Blasts
Some workers at the plant have been exposed to worrisome levels of radiation. But Edamo said the radiation releases so far should not endanger the public.
Both blasts were blamed on explosive hydrogen gas coming from the reactor cores.
Hydrogen can be produced when a reactor's core overheats, damaging the zirconium tubes that hold the nuclear fuel.
"The fact that they had a hydrogen explosion tells you there was a lot more hydrogen available than there should have been," said Dale Klein, the former chairman of the Nuclear Regulatory Commission and an associate director of Energy Institute at the University of Texas at Austin.
"The hydrogen is usually generated when the zirconium cladding (on the fuel rods) interacts with steam and releases hydrogen," Klein told NPR.
If some of the rods are damaged and are falling apart, experts say the radioactive fuel pellets inside them could reach the bottom of the reactor vessel, where they would decay, heat up and melt further. Eventually, Klein says, a "molten mixture" could burn through the containment vessel.
"That containment would not survive very long and would result in a very large release of radioactivity, extremely hazardous and probably result in more evacuation," said Klein.
But Japanese officials have not detailed how many of the rods may be damaged at any of the reactors, and they may not be able to check.
International Response
The events at Fukushima have drawn increasing concern from the international community, including the United Nations' nuclear watchdog, the International Atomic Energy Agency.
"The Japanese authorities are working as hard as they can, under extremely difficult circumstances, to stabilise the nuclear power plants and ensure safety," Yukiya Amano, the head of IAEA, said in a statement to IAEA member states in Vienna.
The U.S. Agency for International Development has dispatched a team to Tokyo, which is currently providing technical assistance to the Japanese. Another government agency, the U.S. Nuclear Regulatory Commission, said it was considering sending its own experts as well.
