Photo from the Institute for Science and International Security
The March 11 earthquake and tsunami was enough of a disaster in itself. One of the effects of 10 meter tsunami was to knock out the system that cooled the nuclear facility at Fukushima. The plants automatically shut down with an earthquake - but a tsunami of that size was not anticipated. Without cooling, there has been explosions and some amount of melting and release of radiation.
The presence of radiation has made it difficult to have people working at the plant to fix the problems because of the high levels of radiation around the plant. Based on the sievert level (see below) - there are limits to how much a person should be exposed to in an hour, in a day, in a year.
From the BBC ->"After Tuesday's explosions and fire, radiation dosages of up to 400 millisieverts per hour were recorded at the Fukushima Daiichi site, about 250km north-east of Tokyo. Later, a reading of 0.6 millisieverts (mSv) per hour was recorded at the plant's main gate, the International Atomic Energy Agency (IAEA) said."
Radiation has spread out app. 15 miles. The US advised an evacuation of 50 miles out. France advised it's citizens to leave the country. Tokyo is about 150 miles away. People have been concerned about the wind direction - as far as radiation spread. It has mostly gone out over the ocean so far.
Summary of conditions put together by Greenpeace as of March 18th:
Overall, with possible exception of spent fuel pool of reactor #3, the status of all facilities is very similar to yesterday, which is a bad thing. Major uncertainty relates to amount of radiation already being released to air and sea, to risk of a violent fire in the cladding of the fuel rods as the spent fuel pools are exposed for hours, as well as to the behavior of the reactor cores as water levels remain low.
Good news is that the violent release of radioactivity due to fire or explosion feared yesterday has not happened yet. Power is still not restored to the facility, but some progress has made to bring off site power and more equipment. This means more effective cooling could be established in some days. At least until that happens, the situation remains critical and unpredictable.
Reactors 1-3: water level in reactors low (about half of fuel rods exposed), no grid power, seawater injection apparently ongoing. Fuel rods have certainly damaged and are releasing radioactive substances.
Fire department has brought in 30 more trucks, at least one reported to be a “Super Bomber” able to shoot to a distance of 2 kilometers. Yesterday police trucks were unable to operate close to plant because of high radiation levels, only SDF (Self Defense Force) trucks that can be operated from inside the cabin were used.
Spent fuel pools of units 1&2: Water levels in Unit 1 are decreasing. Steam was reported from unit 2, expected to be boiling.
Spent fuel pool of unit 3: Water in #3 almost depleted, but Tepco hopes some water is left. Fuel rods have certainly damaged, releasing radioactive substances. The reactor buildings are heavily damaged, allowing releases directly to outside air.
Spent fuel pool of unit 4: Water level very unclear.
Spent fuel pools of units 5&6: Temperatures still rising, water left but level unclear.
Worst case scenarios
* The zirconium contained in the fuel rod cladding can react violently with air, if exposed for hours. This fire would release and spread very large amounts of radioactivity high up in the air. Wide disagreement on the probability of this happening.
* A large amount of molten fuel accumulates at the bottom and a nuclear reaction starts. Very low probability and can be prevented if there is any borated water in the pool.
* Reactor boils dry, molten core breaches reactor pressure vessel and comes in contact with the water in the containment, which boils rapidly causing a steam explosion.
* A major risk is an event (e.g. increased release of radioactivity from a spent fuel pool due to overheating) that raises local radiation levels to completely intolerable levels - preventing further work to restore cooling.
Tepco seemed to suggest that encasing the plant in concrete is an option if cooling efforts fail (according to Reuters live feed).
Local wind speed slowed down considerably in the morning but direction remained towards the sea. Winds towards Tokyo are still feared for Sunday.
A more detailed summary can be found @ Reuters
There was 3 Mile Island, there was Chernobyl, and now this. This situation has made it clear that all nuclear plants need to have their systems and possible catastrophic scenarios more thought out. There are many plants of the same type in the US - where people have warned of this and this sort of possible problem and yet - nothing has been done to fix and avert the potential problem.
Clearly more people need to accept that nuclear energy poses very extreme risks to people and the environment.
About that Radiation and Sieverts:
The sievert (symbol: Sv) is the SI derived unit of dose equivalent. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to the physical aspects, which are characterised by the absorbed dose, measured in gray. It is named after Rolf Sievert, a Swedish medical physicist renowned for work on radiation dosage measurement and research into the biological effects of radiation.
Frequently used SI multiples are the millisievert (1 mSv = 10−3 Sv = 0.001 Sv) and microsievert (1 μSv = 10−6 Sv = 0.000001 Sv).
An older unit of the equivalent dose is the rem. In some fields and countries, the rem and millirem (abbreviated mrem) continue to be used along with Sv and mSv, causing confusion. Here are the conversion equivalences:
1 Sv = 1000 mSv (millisieverts) = 1,000,000 μSv (microsieverts) = 100 rem = 100,000 mrem (millirem)
Single dose examples:
Eating one banana: 0.0001 mSv
Sleeping next to a human for 8 hours: 0.0005 mSv
Dental radiography: 0.005 mSv
Average dose to people living within 16 km of Three Mile Island accident: 0.08 mSv; maximum dose: 1 mSv
Mammogram: 3 mSv
Brain CT scan: 0.8–5 mSv
International Commission on Radiological Protection recommended limit for volunteers averting major nuclear escalation: 500 mSv
International Commission on Radiological Protection recommended limit for volunteers rescuing lives or preventing serious injuries: 1000 mSv
Hourly dose examples:
Approximate radiation levels near Chernobyl reactor 4 and its fragments, shortly[clarification needed] after explosion are reported to be 10–300 Sv/hr
Yearly dose examples:
Living near a nuclear power station: 0.0001–0.01 mSv/year
Living near a coal power station: 0.0003 mSv/year
Cosmic radiation (from sky) at sea level: 0.24 mSv/year
Natural radiation in the human body: 0.40 mSv/year
New York-Tokyo flights for airline crew: 9 mSv/year
Total average radiation dose for Americans: 6.2 mSv/year
Current average limit for nuclear workers: 20 mSv/year
Lowest clearly carcinogenic level: 100 mSv/year
Elevated limit for workers during Fukushima emergency: 250 mSv/year
Dose limit examples:
Criterion for relocation after Chernobyl disaster: 350 mSv/lifetime
Public dose limits for exposure from uranium mining or nuclear plants are usually set at 1 mSv/yr above background.
Symptoms of acute radiation (within one day):
0 – 0.25 Sv (0 - 250 mSv): None
0.25 – 1 Sv (250 - 1000 mSv): Some people feel nausea and loss of appetite; bone marrow, lymph nodes, spleen damaged.
1 – 3 Sv (1000 - 3000 mSv): Mild to severe nausea, loss of appetite, infection; more severe bone marrow, lymph node, spleen damage; recovery probable, not assured.
3 – 6 Sv (3000 - 6000 mSv): Severe nausea, loss of appetite; hemorrhaging, infection, diarrhea, peeling of skin, sterility; death if untreated.
6 – 10 Sv (6000 - 10000 mSv): Above symptoms plus central nervous system impairment; death expected.
Above 10 Sv (10000 mSv): Incapacitation and death.