2011/03/30
.Water puddles with high levels of radioactivity detected in the basements of the turbine buildings at three reactors of the crippled Fukushima No. 1 nuclear power plant raise questions about the status of the pressure containers.
The contaminated water was likely due to a melting of fuel rods in the reactor cores.
The prevailing view among experts is that the fuel rods began collapsing within the No. 1, No. 2 and No. 3 reactors between March 12, a day after the Great East Japan Earthquake struck, and March 14. It probably did not take long for the fuel rods to lose their original form.
When fuel rods melt, it can accumulate as a lump at the bottom of the pressure container. If the high temperatures from the lump damage the bottom of the pressure container, some of the melted fuel rods or water contaminated by high levels of radioactivity could have leaked into the surrounding containment vessel.
During the 1979 Three Mile Island nuclear accident in the United States, fuel rods melted and accumulated at the bottom of the core.
According to an analysis after that accident, only about 100 minutes after the reactor core stopped operating due to problems, the fuel rods were exposed when the water level in the core fell.
It took just 30 more minutes for the fuel rods to begin melting.
The melting was not stopped even after water was injected into the core because the fuel rods had become too hot. About 90 minutes later, the melted fuel rods began flowing toward the bottom of the pressure container.
After the cooling mechanism was restored about a dozen hours after the accident started, a catastrophe was avoided and damage leading to a breach in the pressure container was prevented.
However, by that time about 45 percent of the fuel rods in the core had melted.
At the three reactors of the Fukushima plant, after operations were stopped due to the earthquake, water levels in all three reactors began falling from between March 12 and 14. That exposed the fuel rods.
The upper parts of the buildings housing the reactors were also blown away by hydrogen explosions on March 12 at the No. 1 reactor and on March 14 at the No. 3 reactor.
The hydrogen was produced after the metal rods covering the fuel pellets reached high temperatures and created a chemical reaction with steam.
At that time, both the Nuclear and Industrial Safety Agency and Tokyo Electric Power Co., the operator of the Fukushima plant, said there was a possibility of a partial meltdown. However, no detailed explanation was ever given.
Considering what happened at Three Mile Island, there was a high possibility that not only the metal rods, but the fuel itself had already melted at that time.
Fumiya Tanabe conducted an analysis of the Three Mile Island accident when he worked for what was then the Japan Atomic Energy Research Institute. He now heads a research institute dealing with social technology and system safety.
He said about the melting of the fuel, "It would be very evident to specialists who analyze nuclear accidents. There must have been people in the government who also realized what had happened."
On Monday, the Nuclear Safety Commission of Japan acknowledged the possibility that melted fuel rods had leaked through damaged parts at the bottom of the pressure container in the No. 2 reactor and entered the containment vessel.
According to TEPCO officials, water puddles in the basement of the turbine building at the No. 2 reactor had radioactivity levels about 100,000 times what is normal for the reactor core cooling water.
The water contained elements such as iodine-131, cesium-137 and technetium that are only produced when fuel rods begin to collapse.
Water with high levels of radiation was also detected Monday in a trench connected to the turbine building.
Water found at the basements of the turbine buildings of the No. 1 and No. 3 reactors also had radioactivity about 10,000 times what is normally found in the core water.
Workers continue to pump water into the three reactor cores to cool them. If the pressure containers are damaged and breaches exist between the core and containment vessel, water contaminated at high levels of radioactivity could continue to leak outside the containment vessel.
There are various views of where the water is leaking from the pressure container.
Various pipes connect the pressure container to surrounding equipment. Some pipes circulate cooling water and steam when the reactor is operating. When operations are stopped, control rods are inserted through openings at the bottom of the pressure container.
Damage to any of these parts that connect the pressure container to its surroundings may lead to a breach that could allow water to leak out.
However, the failure of the water level in the cores to rise despite the continued pumping has led TEPCO officials to point to the possibility of some breach.
At a Sunday news conference, TEPCO Executive Vice President Sakae Muto said: "There are various parts to the piping connected to the pressure container as well as in the containment vessel that surrounds the pressure container. There are also movable parts in the pipe valves and pumps. There is the possibility of leakage when those parts are exposed to high temperatures and pressures."
The situation in the pressure container of the No. 2 reactor is believed to be the most serious.
After the March 11 earthquake struck, the No. 2 reactor was stopped with the insertion of the control rods. However, a failure of the cooling system likely led to the fuel rods being completely exposed above the water in the core for a time on March 14. An explosion on March 15 is believed to have damaged part of the suppression pool that lies under the containment vessel.
For those reasons, there is the possibility that the situation has progressed beyond simply a leak from the pipes or valves.
The fuel rods are about 4 meters long. If the rods are exposed above the water, the heat from the rods raises the temperature in the core.
If temperatures exceed 1,800 degrees, the metal covering the fuel pellets begins to melt. At temperatures above 2,800 degrees, the pellets themselves begin to melt, releasing large amounts of radioactive materials created from nuclear fission when the reactor was operating. If a nuclear fission reaction should occur, it would emit large amounts of radioactive materials.
"Even though the pressure containers have steel walls about 15 centimeters thick, melted fuel rods reach high temperatures that exceed the melting point (about 1,500 degrees) for steel," said Tadashi Narabayashi, a professor of nuclear reactor engineering at Hokkaido University. "There are some thinner areas at the bottom of the pressure container where the mechanism to operate the control rods and instruments to detect neutron rays are located. If hot nuclear fuel were to flow to that part, there is the possibility of holes being opened."
However, Narabayashi added that because there are no signs of the entire bottom of the pressure container collapsing, what is likely happening is that small amounts of melted fuel are leaking through small holes. Under that situation, continued cooling of the core could bring the situation under control, he said.
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