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Monday, March 28

Fukushima Nuclear Disaster

Japan’s nuclear watchdog has reported that the level of radiation detected near reactor 2 was 1,000 millisieverts an hour (100,000 millirems per hour).

Japan’s nuclear safety officials have since confirmed that the radiation inside Reactor 2 was caused by a partial meltdown of fuel rods.

TEPCO says the radiation in Reactor 2, which was earlier reported at 10 million times the normal, was actually 100,000 the operating level.

Meanwhile, Japan’s nuclear safety agency has dismissed as unreliable a report by Greenpeace that radiation levels of up to 10 microsieverts per hour had been detected 40 km (25 miles) NW of the nuclear plant.

Fukushima NPP. Workers are seen outside the heavily damaged Reactor 4 on March 22, 2011. Photo by TEPCO, via Reuters.

What is a lethal dose of radiation from a single Exposure?

Studies of the 1945 atomic bombing at Hiroshima and Nagasaki show that 100 percent of victims whose bodies were exposed to 600,000 millirems (6,000 mSv) died from radiation. About 50 percent of victims who received 450,000 millirems (4,500 mSv) of radiation also died.

(Note: Rem is a unit of ionizing radiation equal to the amount that produces the same damage to humans as one roentgen of high-voltage x-rays. Source: MIT)

1 rem = 10 mSv (1 Sv = 100 rem)

Background Radiation in millirems per year (mrem/yr)

  • Average background radiation (US): 300
  • Higher altitudes (eg. Denver): 400

“Safe Levels” of Radiation (U.S.)

Limits above natural background radiation levels (average 300 millirems per year) and medical radiation:

  • Occupation Limit: Maximum of 5,000 (the limit for a worker using radiation)
  • Average Natural Background: 300

[Note: Lifetime cumulative exposure should be limited to a person's age multiplied by 1,000 millirems, e.g., a 70-year-old person, 70,000 millirems.]


  • Max single dose for an adult: 3,000
  • Annual total dose: 5,000

Under 18

  • Max single dose for a person aged under 18 years: 300 millirems (whole body equivalent)
  • Annual total exposure: 500

Fetal Exposure

  • Maximum limit for fetal exposure during gestation period: 50 millirems per month above background levels


  • Single Chest X-ray (the whole body equivalent): 2 millirem

Air Travel

  • Coast-to-coast US round trip flight: 12 millirems

Megaquake and Tsunami Death Toll

The latest death released by the authorities put the number of dead at just over 11,000, with about 17,400 people missing.

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