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Tuesday, June 28, 2016

Radiation explained

How much radiation is coming from the Fukushima power plant and reaching the US west coast? How much of this radiation is needed to cause cancer? How much exposure will lead to death? This post will help explain radiation terms and how much radiation is bad for the human body. There will be more posts about cancerous fish next week.

First, radiation emitted from a substance is not the same as radiation absorbed by tissue or other matter.

The Becquerel (Bq)

From Wikipedia:
One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second.
The becquerel simply measures how many times the nucleus decays per second. But that doesn't explain what level of becquerels is dangerous to a person or animal.
  • 2 radioactive decays in a second equals 2 Bq. 
  • 1123 decays per second is the same as 1123 Bq. 
  • 1 becquerel is 10 milliSieverts (mSv).

The Sievert (Sv)

From Wikipedia:
The sievert is used both to represent the risk of the effect of external radiation from sources outside the body, and the effect of internal irradiation due to inhaled or ingested radioactive substances. Conventionally the sievert is not used for high dose rates of radiation which produce deterministic effects, which is the severity of acute tissue damage which is certain to happen. These effects are compared to the physical quantity absorbed dose measured by the unit gray (Gy).
The sievert measures the biological effect on tissue in joules per kilogram. There is a "weighing factor" which must be applied to the matter or tissue which absorbs the radiation. This factor is different for different types of tissue. Examples:

0.098 μSv: banana equivalent dose, an illustrative unit of radiation dose representing the measure of radiation from a typical banana[32][a]
0.25 μSv: U.S. limit on effective dose from a single airport security screening[33]
5 to 10 μSv: one set of dental radiographs[34]
80 μSv: average dose to people living within 16 km of Three Mile Island accident[35]
0.4 to 0.6 mSv: two-view mammogram, using weighting factors updated in 2007[36]
1 mSv: The U.S. 10 CFR § 20.1301(a)(1) dose limit for individual members of the public, total effective dose equivalent, per annum[37]
1.5 to 1.7 mSv: annual dose for flight attendants[38]
2 to 7 mSv: barium fluoroscopy, e.g. Barium meal, up to 2 minutes, 4–24 spot images[39]
10 to 30 mSv: single full-body CT scan[40][41]
50 mSv: The U.S. 10 C.F.R. § 20.1201(a)(1)(i) occupational dose limit, total effective dose equivalent, per annum[42]
68 mSv: estimated maximum dose to evacuees who lived closest to the Fukushima I nuclear accidents[43]

And now some fatal doses in sieverts:

4.5 to 6 Sv: fatal acute doses during Goiânia accident
5.1 Sv: fatal acute dose to Harry Daghlian in 1945 criticality accident[45]
10 to 17 Sv: fatal acute doses during Tokaimura nuclear accident. Hiroshi Ouchi who received 17 Sv was kept alive for 83 days after the accident.[46]
21 Sv: fatal acute dose to Louis Slotin in 1946 criticality accident[45]
54 Sv: fatal acute dose to Boris Korchilov in 1961 after a reactor cooling system failed on the Soviet submarine K-19 which required work in the reactor with no shielding[47]

1 sievert is equal to 100 REM, a dose UOM still used in the US. Some more doses from Our Radioactive Ocean:
10 sieverts, aka 10,000 milllisieverts (mSv): usually fatal within a few weeks.

The Gray

Measures energy absorbed by matter in joules per kilogram of matter. The US 'rad' is equivalent to the gray. From Wikipedia:
The gray is conventionally used to express the severity of what are known as "tissue effects" from doses received in acute unintended exposure to high levels of ionizing radiation. These are effects which are certain to happen, as opposed to the uncertain effects of low levels of radiation which have a probability of causing damage. A whole-body acute exposure to 5 gray or more of high-energy radiation usually leads to death within 14 days. This dose represents 375 joules for a 75 kg adult (equivalent to the chemical energy in 20 mg of sugar). For X-rays and gamma rays the gray is numerically the same value when expressed in sieverts, but for alpha particles one gray is equivalent to 20 sieverts, and a radiation weighting factor is applied accordingly.
Radiation found on the US west cost (it's fatal)

Thanks to a map from Our Radioactive Ocean I found 2 samples of Pacific ocean water that measured 2.6 becquerels. Since 1 becquerel is 10 mSv, then 2.6 Bq is 26 mSv. According to the chart above, 2.6 Bq is close to the radiation received from a full body CAT scan. However, that dose is for one second! An hour on a boat in the water would equal an hour of exposure, which would be: (60*60*2.6 Bq) = 9360 Bq * 10 = 93600 mSv, or 93.6 Sv. That's well above the fatal doses in the table above!

Remember, radiation doses are cumulative. Once you absorb enough, cancer begins to grow, or you simply die from radiation poisoning. The danger is in how long you are exposed to the radiation.

More links
  1. Our Radioactive Ocean. Has 10+ sites right now and is monitoring radiation levels. It is part of WHO. An actual map of results since 2011 are here. Click on the map point to see one or more samples for that site, oldest sample first. Very helpful!
  2. Conversion table. Helpful.
  3. A helpful explanation of the radiation units from Radiation Scott.

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