A significant amount of radiation has been released to the atmosphere from this site since the beginning of the crisis. Two of the main health hazards from the radioactive gases that have been released are from iodine-131 (I-131) and cesium-137 (Cs-137). One analysis estimated that roughly 20% of the I-131 and up to 50% of the Cs-137 released in the Chernobyl accident was released from Fukushima to the atmosphere within the first few days of the accident.
Very high radiation levels are being detected at some points many kilometers away from Fukushima, outside of the evacuation zone, although there is no clear picture at this point because the locations of the readings are not publicly available and there has not been a systematic survey.
Japan initially ordered residents to evacuate out to 3 km (1.9 miles) around the Fukushima site, with residents out to 10 km (6.2 km) told to stay indoors. By late on March 12, Japan expanded the evacuation zone to 20 km (12 miles) with sheltering to 30 km (19 miles). On March 25, Japanese officials said they were encouraging residents to evacuate out to 30 km.
In contrast, on March 17 the U.S. embassy told US citizens to evacuate out to a radius of 80 km (50 miles) from the site.
As the radiation is carried by winds across the ocean, it spreads out and becomes diluted. While trace amounts have been detected in the US, these amounts have been much lower than the natural background levels of radiation that people are constantly exposed to, and are not a serious health hazard.
The radiation released to the atmosphere at Fukushima came from two main sources. First, when cooling stopped in the reactor cores, the fuel began to heat up and the pressure in the reactor vessels increased. To reduce the pressure, workers vented to the atmosphere some of the radioactive gas that had built up in the vessel and primary containment. There are also reports that the primary containment of Unit 2 and possibly Unit 3 may be damaged; if that is true, that would also allow radioactive gases to escape.
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Second, loss of water in the spent fuel pools led to fuel assemblies being exposed to air, which caused damage to the fuel that then released radioactive gases. While the pools are contained in the reactor buildings, hydrogen explosions in the buildings of Units 1, 3, and 4 created holes in the walls that allowed these gases to escape. And vents were opened in the walls of the Unit 2 reactor building to prevent a buildup of hydrogen that could cause an explosion.
Fortunately, monitoring indicates that deposition of Cs-137 is currently no longer occurring around the site. This is because efforts to cool the reactors and spent fuel pools have been successful enough to eliminate the need for additional venting and to stop further releases from the spent fuel pools. However, as discussed below, additional venting may soon be needed.
It is also important to note that the amount of Cs-137 and other radioactive material that remain in the fuel in both the core and spent fuel pool is much larger than the amount that has already been released. Some of this remaining radioactive material could be released if new problems occur, so this remains a very serious concern.
The other source of radioactive contamination around the plant is from contaminated water. To attempt to cool the reactors and spent fuel pools, many thousands of tons of water were dropped by helicopter or sprayed by hoses at the plants. Some of the runoff water from these efforts has apparently become contaminated and run into the ocean, since radiation has been detected in the coastal waters.
More recently, there is a concern about very highly contaminated water in trenches outside the buildings, especially at Unit 2, which appears to be coming from water that has collected in the lower parts of the reactor turbine buildings. Japanese officials apparently believe this is water that was pumped in to cool the fuel in the reactor that has somehow leaked out into the turbine buildings.
On Monday March 28, press reports said the radiation level of this water from the Unit 2 reactor was 1,000 milli-Sv/hr. This is high enough that an hour-long exposure would give someone a radiation dose sufficient to cause acute radiation syndrome. At an April 2 press conference Japanese officials said that this highly contaminated water is leaking into the ocean.
Less highly radioactive water has also been found in tunnels under the turbine buildings at Units 1 and 3.
This issue is creating new problems for workers at the plant. The volume of radioactive water is so large that they are running out of places to store it. To cut down on the volume of water they need to remove and store, they are trying to reduce the amount of water they pump into the reactors to cool the fuel in the cores. But without that cooling, the fuel in the cores has been heating up. This leads to a buildup of pressure in the reactor that may require additional venting of radioactive gas to the atmosphere. If the heating becomes great enough, it can also lead to additional fuel damage and further release of radioactive gases from the fuel.
There is speculation about the amount of fuel in the reactor cores that may have melted, and given the lack of cooling it may be substantial. But because of the lack of monitoring in the reactor vessels no one really knows the condition of the fuel. The state of the fuel at Three Mile Island was not known for several years after the accident there. Similarly, because of lack of water in some of the spent fuel pools early during the Japanese crisis, people assume that some of the fuel in the pools may have melted, but the status is not known.
see also Annotated Photos of Fukushima
A set of very high-resolution photographs of the Fukushima Dai-Ichi nuclear plant have been posted on the web by cryptome.org. I’ve annotated some of those pictures below to help people know what they are seeing in those photos.
Below are some additional annotated versions of high-resolution photographs of the Fukushima Dai-Ichi nuclear plant that have been posted on the web by cryptome.org.