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6.3: Unsafe Doses of Radiation

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  • Radiation Leaks at Fukushima Dose Workers. After everyone else was evacuated from the Fukushima Daiichi nuclear power plant following the tsunami disaster, around fifty workers were left behind to cool the spent fuel rods and overheating nuclear reactor. These workers pumped seawater on the reactors in a desperate attempt to bring them back under control, meanwhile exposing the workers to dangerous radiation. 1

    How is radiation exposure measured and what is considered dangerous? Well, there are different methods to measure exposure to ionizing radiation and units to express these measurements. One unit used is “grays per hour” which describes the absorbed dose rate in the air which describes the intensity of gamma radiation in the air. This in turn can be translated to radiation dose, or gray (Gy) which is equal to one joule of radiation energy that is absorbed per kilogram of organ or tissue weight. Before gray was used, the unit rad was; so that 1 Gy = 100 rads. All radiation is not created equal, some forms are more harmful that others so to incorporate that factor (known as the “radiation weighting factor), radiation dose is expressed as equivalent dose in units called sievert (Sv). That makes one gray air dose equal to 0.7 Sv tissue dose. Sievert is the SI unit of radiation dose; SI meaning the International System of units used by the scientific community in order to facilitate communication. Another unit of radiation dose that is sometimes used is rem which is based on the unit rad; 1 Sv is equal to 100 rem.2

    What is considered a dangerous dose of radiation? According to the ICRP (International Commission on Radiological Protection) the recommended annual dose limit for the general public is 1 mSv or 0.001 Sv. Radiation workers however, can be exposed to 50 mSv in any given year but average only 20 mSv per year in a 5 year period. Exposure to 10 Sv will most like result in death within days or weeks and at 1 Sv, a person runs the risk of cancer later in life as well as temporary radiation sickness and a drop in white blood cell count. 2 Sv is high enough to cause immediate radiation sickness which results in skin redness or burns, hair loss and nausea. At 3 Sv, the initial symptoms of radiation sickness can also include vomiting and diarrhea and progress to a period of serious illness including appetite loss, fatigue, fever, gastrointestinal problems, possible seizures or coma and may last from hours to months.3

    There are three ways to reduce radiation exposure; reduce the amount of time exposed to radiation, increase the distance from the source of radiation, and wear protective clothing to shield the radiation. Radiation limits are designed to be “as low as reasonable achievable” or ALARA, and are very conservative as you can see, many times below the level that would cause radiation sickness.4


    1. The standard legal limit for annual radiation dose for radiation workers is 50 mSv but it was raised to 250 mSv so that the 50 workers could continue to cool down the reactors. If radiation reached levels of 400 mSv/hr at Fukishima. How long would it take for an unprotected radiation worker to receive the 250 mSv?

    2. Readings at the border of the nuclear plant site were 6 mSv/hr. How long would it take for a member of the general public to absorb 10 mSv of radiation?
    3. How many mrem is 3 Sv?
    4. The commonly accepted value for increased risk of cancer is 4% per Sv. If a person was exposed to 50 mSv every year for 20 years, what would his increased risk for cancer be?
    5. People are exposed to 350 mrem of background radiation per year so the radiation dose guidelines refer to additional radiation. If you had a CAT scan that delivers a dose of 4.1 mSv, what is your total radiation dose for that year in Sv?
    6. If you lived near a nuclear power plant, what precautions could you take to limit your exposure to radiation in the event of an accident? Answers to Radiation Dose Questions

    1. Biello, David, Workers Battle Fukushima Nuclear Crisis at Personal Risk, Scientific American, March 16, 2011 online 2. McGill Radiation Safety Policy Manual › 3. Radiation dose limit, McGill University, 3. Bai, Nina, How Radiation Affects Health, Scientific American, March 15, 2011 online 4. Biello, David, Workers Battle Fukushima Nuclear Crisis at Personal Risk, Scientific American, March 16, 2011 online

    From ChemPRIME: 19.10: Units of Radiation Dose

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