The Fukushima No. 1 nuclear power plant's No. 1 reactor, foreground, No. 2 reactor, No. 3 reactor and No. 4 reactor behind the rising white smoke on March 15 (Tokyo Electric Power Co.)
Work resumed Thursday to cool reactor cores and storage pools containing spent fuel rods at the Fukushima No. 1 nuclear power plant, but two workers were hospitalized because of radiation contamination.
Although the No. 5 and No. 6 reactors have safely stopped operations and cooling mechanisms are functioning using an outside power source, the situation at the other four reactors remains unstable.
Three workers who were laying power cables to the No. 3 reactor had been exposed to 170 to 180 millisieverts of radiation, the Nuclear and Industrial Safety Agency said Thursday.
Contamination was found on the skin of the feet of the two taken to the hospital. They had been working in an area of the plant that was partially soaked.
Work at the Fukushima plant was halted Wednesday when black smoke was observed from the No. 3 reactor.
At 11:30 a.m. Thursday, lights were turned on at the No. 3 reactor's central control room. Lights had already been turned on at the No. 1 reactor's central control room.
Officials of Tokyo Electric Power Co., which operates the Fukushima plant, confirmed around 5 a.m. Thursday that no black smoke was emerging. The order evacuating plant workers issued the previous day was lifted at 7:51 a.m.
All six reactors were connected to an outside power source by Tuesday.
The focus of work now shifts to restoring the cooling systems for the cores of the No. 1 to No. 3 reactors.
Although those three reactors automatically shut down soon after the March 11 Great East Japan Earthquake, there was no power to operate the cooling mechanism because the outside power source was lost and the emergency generators failed.
As a stopgap measure, fire trucks pumped in cold seawater to cool the cores.
Left uncooled, the fuel rods could have heated up and been damaged by decay heat, leading to the release of large amounts of radioactive materials.
With control room lighting re-established, work to restore an outside power source centered on the No. 3 reactor on Tuesday night.
Work is proceeding to prepare for a test run of pumps in the water supply line to inject fresh water to cool the core. A test of the pumps is planned for Thursday at the earliest.
Work also resumed to restore outside power to the No. 1 and No. 2 reactors.
Workers were also trying to fix a pump using an outside power source that had been pumping seawater into the No. 5 reactor, but which stopped Wednesday night.
Figures obtained from instruments indicate that between half to one-third of the approximately 4-meter long fuel rods are exposed, but TEPCO officials do not know what the actual situation is like.
The temperature of the core of the No. 1 reactor at one time reached about 400 degrees, above the design limit of 302 degrees. To cool the core, the amount of seawater being pumped in was increased early Wednesday from 2 cubic meters an hour to 18 cubic meters an hour.
The temperature decreased to 243 degrees as of 1 a.m. Thursday, leading one TEPCO official to say the situation was improving.
However, pressure within the containment vessel that holds the pressure container in the core of the No. 1 reactor increased from about 1.7 atmospheres (atm) at 11 a.m. Tuesday to 3.6 atm at 6 p.m. Wednesday.
The amount of seawater being pumped in was reduced to about 10 cubic meters per hour from 2:30 a.m. Thursday.
At a Wednesday night news conference, Haruki Madarame, chairman of the Nuclear Safety Commission, said: "Personally, I am concerned about the increase in pressure in the No. 1 reactor. We may have to open the vent (to release steam)."
The spent fuel rods kept in storage pools also have decay heat, although not at the same levels as fuel rods in the core. Work continues to spray water from the outside or pump in water through cooling pipes to replenish the water in the storage pools.
Work to spray water had continued at the No. 3 and No. 4 reactors. The buildings housing the two reactors were damaged due to an explosion believed caused by hydrogen as well as fires. However, work has begun to use existing pipes to supply water to the pool at the No. 3 reactor, similar to what has been done for the No. 2 reactor. The same step will also be taken for the No. 4 reactor.
Work for supplying water to the pool in the No. 1 reactor is taking a back seat to the other three reactors because the amount of heat being released is smaller. However, plans call for supplying water to the pool through pipes from Friday.
To continuously and stably cool the core and storage pools will require restoring an outside power source and then switching from emergency seawater cooling to the reactors' built-in cooling system.
As a result of the accident at the Fukushima plant, radioactive materials have been detected in areas of eastern Japan.
The two likely sources of those materials are the storage pools containing the spent fuel rods as well as damage to the reactor core or surrounding parts.
A fire broke out March 15 near the storage pool at the No. 4 reactor. The water level of the pool dropped, exposing the spent fuel rods. That led to the creation of hydrogen that likely eventually exploded.
The radioactive materials released at that time may have been contained within the plant site and continue to give off radiation.
There is also the possibility that radiation came from the core.
TEPCO officials continue to insist that the No. 1 and No. 3 reactors' containment vessels are sound, even after the outer buildings were damaged due to hydrogen explosions. They also believe that there was no major damage to the No. 2 reactor even though an explosion likely occurred in the suppression pool that lies under the containment vessel.
A TEPCO official said if there had been significant damage "the radiation levels would not have been what was measured so far."
However, there is data that points to partial damage of the core.
One is the detection of radioactive cesium in the outer atmosphere. Cesium is a by-product of nuclear fission involving uranium.
Steam was also released into the atmosphere to prevent damage to containment vessels in which pressure had increased. However, because the steam passes through water, there is a low possibility that cesium was emitted through the release of steam.
There are also questions about what part of the core has been damaged.
According to sources, the weakest link within the core is the sealing material that fills in the space between the containment vessel and airlock door that is opened to enter the core for inspections.
TEPCO officials admit it is possible that other small parts that connect various pieces of equipment may have been damaged.
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