Tuesday, March 29, 2011

What do these fallout simulations tell us?

This is one of a few efforts currently underway which attempts to model the quantity and manner of dispersion of  Fukushima fission products.  For arguments sake, let's assume that this simulation is based on verifiable scientific data.  Based on the colored areas above, what amounts of radioactivity are present and what does this mean for us?

First of all, it's important to understand the definition of a Becquerel. The Bq is a unit of radioactive decay, not a measurement of a dose of radiation absorbed by the body (measured in Sieverts).  It is important to understand the distinction.  The definition of the Becquerel is "the activity of a quantity of radioactive material in which one nucleus decays per second".

We can state amounts of Bq per unit mass.  At the peak of last weeks Tokyo's water contamination problem, each kilogram of water yielded 210 Bq.  This literally means that 210 decays are taking place per second in each kg of water. Depending on the isotopes half-life (in this case- Iodine-131), the amount of Bq can decrease rapidly, as was the case with water in Tokyo, which is currently reading below 100 Bq.

Becquerels can also be stated in amounts per unit volume of air.  During the Chernobyl disaster, Stockholm reported a concentration of Iodine 131 in air that averaged 10 Bq per cubic meter.  They calculated that after two days at those levels, the affected people received about 0.014 rem. Thus, if the isotope and radioactivity in Bq is known, we can convert that reading to an approximate absorbed dose.

Based on the scan above, the Cs-137 concentration is as follows:

Red areas -  10 + Becquerel per cubic meter of air
Orange areas - 1 - 10 Becquerel per cubic meter of air
Tan areas - 0.1 - 1 Bq/m3 (0.1 Bq meaning there is a 10% probability of a decay occuring/sec)
Light blue areas - 0.01 - 0.1 Bq/m3 (a 1% to 10% probability of a decay occuring/sec)
Blue areas - 0.001 - 0.01 Bq/m3 ( a 0.1% to 1% probability of a decay occuring/sec)
Dark blue areas  - 0.001 Bq/m3 or less (less than a 0.1% probability of a decay occuring/sec)

It is also important to understand that either you have an atomic nucleus decaying or you don't, and it therefore becomes a probability-based event.

Assuming i am in a 'blue area'.  What dose would i be exposed to after inhaling/ingesting Cs-137 over an hours time?  About 0.000007640681 microsieverts, an absurdly minute amount.

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