The steel container protecting a nuclear reactor at a plant facing a possible meltdown was not damaged in an explosion that injured four workers and destroyed the exterior walls of the plant, a Japanese government spokesman said today.
The blast at the Fukushima Daiichi plant occurred outside the containment vessel and did not damage the nuclear reactor itself, which would cause radioactive material to leak out, a government spokesman Yukio Edano said.
Contrary to initial reports of radiation levels rising around the Fukushima Daiichi plant after the blast, Edano said that radiation is decreasing and that the pressure inside the reactor is also dropping.
"Based on this situation, we are concerned about the nuclear reactor and have decided to fill the reactor with sea water" to further decrease pressure and cool down the reactor, Edano said.
Flooding the containment vessel with sea water mixed with boron instead of fresh water is an unusual measure, according to Robert Alvarez, a senior scholar at the Institute for Policy Studies, who described it as a "Hail Mary pass" but a necessary step to keep the reactor core covered and the containment vessel cool. The restoration of power to the plant remains critical to prevent against the possibility of an outright meltdown.
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After assurances from Japanese officials about the situation, a top U.S. scientist said Japan must come to terms with the severity of the nuclear accident it is facing and work to immediately protect its most vulnerable residents from the damage of radiation exposure -- particularly protecting children against exposure to radioactive iodine.
"Any attempt to make it seem that this is not the worst case imaginable is foolhardy," said Edwin Lyman, a senior scientist with the Union of Concerned Scientists.
An estimated 170,000 people surrounding the Fukushima Daiichi plant have been evacuated, according to the International Atomic Energy Agency. Three evacuees have been exposed to radiation, but have not shown signs of illness, a Fukushima prefectural disaster official confirmed to the Associated Press.
Japanese authorities told the International Atomic Energy Agency that they are preparing to distribute potassium iodide to residents in the area around both the Fukushima Daiichi and nearby Fukushima Daini plants. The U.S. and France have plans in place to distribute doses of potassium iodide to children who live in the vicinity of the plant in the event of a catastrophic radiation release.
If the reactor core melts through the steel vessel that is housing it, Lyman said, the risk Japan faces is a radioactive plume that could disperse tens or even hundreds of miles. "You could have large swaths of areas that will need severe remediation. And a lot of people exposed to radioactivity who will have an increased chance of cancer."
After the nuclear accident at Chernobyl, Lyman said there were over 6,000 cases of childhood thyroid cancers, and it was later determined if the children had taken potassium iodide a few hours before being exposed to the radiation it would block the intake of the radioactive material in the thyroid. "That has been shown to reduce exposure significantly," he said.
U.S. officials are concerned that the threat of a radiation leak has been downplayed and that wind patterns, which could carry a potential release of radioactive material outside a 20-kilometers radius towards Tokyo, have not been sufficiently accounted for.
The U.S. has dispatched a nuclear team from South Korea, but the Japanese have not responded to that offer, according to a U.S. official.
The Fukushima Daiichi plant, located about 170 miles northeast of Tokyo, was one of two run by the Tokyo Electric Power Co. whose cooling systems were damaged in the 8.9 earthquake and tsunami that hit Japan Friday.
At least two reactors at the Daiichi plant and three at the Fukushima Daini plant which located about 10 miles away had damaged cooling systems, the Associated Press reported. Officials declared states of emergency for the five reactors.
Ryohei Shiomi with Japan's Nuclear Safety Commission said earlier that a meltdown was possible at the Fukushima Daiichi plant.
Japanese TV images showed the crumbled remains of one of the plant building's walls with smoke emerging from the site.
An evacuation order was expanded from a 10-kilometer to 20-kilometers radius around the plant.
The incident comes as the level of water used to cool a nuclear reactor damaged in Friday's earthquake and tsunami dropped to an alarming level today, according to the Nuclear and Industrial Safety Agency, heightening fears of a larger nuclear disaster.
As of 11:20 a.m. local time, a part of the "fuel assembly" of fuel rods at the Fukushima Daiichi plant's No. 1 reactor was exposed above water, with a maximum exposure of about 90 centimeters.
If the fuel rods remain exposed, they will be damaged, releasing radioactivity.
"This is extremely serious," said Joseph Cirincione, president of the Ploughshares Fund and an expert on national security and international policy. "The best case at this point would still be the worst incident since Chernobyl.
"We are in uncharted territory," he said. "It is possible that this can be contained and we would have a very bad nuclear contamination event. But if the water levels continue to drop and the rods are exposed further it could lead to a core meltdown. The core would melt through the steel holding structure and plunge in a burning, molten mass into the concrete containment structure. If the structure is sound, it could contain the mass, if it has been structurally damaged, then it, too, could breech and we would have a massive, radioactive release."
About 27,000 liters of water, including water stored for firefighting, was being pumped into the reactor via makeshift pumps and other means in order to raise the water level above the reactor's nuclear fuel, NISA said.
"NISA just confirmed that the fuel may be partially melting," Dr. Tatsujiro Suzuki, vice-chairman of the Japan Atomic Energy Commission told ABC News. "The question is whether the situation is getting worse or not. It is reported that the level of water is declining (bad news) but pressure is also decreasing (good news). So, efforts to contain the event (need water) may be working. It is also stated that the amount of radioactivity is still small so that the general public does not need to be concerned at present."
But the situation at the Fukushima Daiichi reactor No. 1 was reported to be the most dire.
Radiation inside the reactor surged to 1,000 times its normal level after Friday's earthquake knocked out power to its cooling system, and the tsunami floods have hampered efforts to get it restored.
Heat-induced pressure built up inside the crippled reactor after the reactor lost power, automatically shut down and the cooling system failed. The situation at the reactor and the four others with compromised cooling systems prompted officials to declare nuclear emergencies.
Scientists said that even though the No. 1 reactor at the Fukushima Daiichi site, in particular, had stopped producing energy, its fuel continued to generate heat and needed steady levels of coolant to prevent it from overheating and triggering a dangerous cascade of events.
"You have to continue to supply water. If you don't, the fuel will start to overheat and could melt," said Edwin Lyman, a senior staff scientist in the Global Security program at the Union of Concerned Scientists in Washington, D.C.
A meltdown could lead to a breach of the reactor's steel containment vessel and allow radiation to escape into an outer, concrete containment building, or even into the environment.
"Up to 100 percent of the volatile radioactive Cesium-137 content of the pools could go up in flames and smoke, to blow downwind over large distances," said Kevin Kamps, a nuclear waste specialist at Beyond Nuclear, which is an advocacy group that opposes nuclear weapons and power.
"Given the large quantity of irradiated nuclear fuel in the pool, the radioactivity release could be worse than the Chernobyl nuclear reactor catastrophe of 25 years ago."
The Kyodo News Service reported Friday that some radioactive material already may have escaped, citing reports from the Japanese Nuclear Safety Agency that radiation levels outside the plan have been eight times the normal level. Experts said that level of exposure is not dangerous to the general population.
"You've got to take all potential precautions," President Obama told reporters Friday when asked about vulnerability of the Japanese nuclear power facilities. "And I've asked Steve Chu, our energy secretary, to be in close contact with their personnel to provide any assistance that's necessary, but also to make sure that if in fact there have been breaches in the safety system on these nuclear plants, that they're dealt with right away."
Experts say cooling the reactor's core to minimize pressure inside the containment structure is a top priority. Japanese authorities have been trying to connect diesel-powered generators to restore the water pumps inside the reactor but have been hampered by the floods.
"If you have something that generates heat and you don't cool it off or release the steam, you're in trouble," said nuclear consultant Mycle Schneider, who compared the situation to a pressure cooker.
Meanwhile, officials performed a controlled release of some slightly radioactive vapor that has been building up inside the containment structure, the Associated Press reported.
The release would allow harmful material to escape into the environment, but not at levels as great as if there was a massive containment failure, Lyman said.
"It's good they evacuated -- let's put it that way," Lyman said. "All indications are that this is a very serious event."
ABC News' Devin Dwyer and Leezel Tanglao contributed to this report.