Fallout across US: danger lost in dose translation

  • # Fukuchima Fallout across the USA.
  • # Calculating dose (in µSv) from Bq/L measurements.
  • # Why comparing doses can be deceptive.

# Fallout (traces of I-131 and sometimes Cs-137) from Japan’s Fukushima Daiichi nuclear power plant have been confirmed detected in the atmospherein:  Hawaii, California, Colorado, Nevada, Massachusetts, Pennsylvania, Washington, North Carolina, South Carolina, Florida. (In Belgium the measurements are so low they are merely on the edge of the detectable.)

Yesterday UC Berkeley measured radioactivity in the AIR at about 3 Bq/L for Cs-137, 0.1 Bq/L for Te-132 and about 3 Bq/L for I-131.

See here for their measurements: http://www.nuc.berkeley.edu/AirSamplingResults

Readings dropped for rainwater.  You can check ‘m here: http://www.nuc.berkeley.edu/RainWaterSampling

# Because of the calculated equivalent dose depends on the distance, I haven’t been able to really figure out how to arrive at a reasonable equivalent dose when presented with an activity reading in Becquerel per liter.  Until now: Found an answer on the ‘Berkeley Radiological Air and Water Dose Calculation‘ page [http://www.nuc.berkeley.edu/node/1897]

So I want to get an equivlent dose rate for the 3 Bq/L of I-131 observed in rainwater (as seen last couple days, after a spike of 20 Bq/L on 3/24).  What does that correspond with?  http://www.nuc.berkeley.edu/node/1897 explains the process for calculating this.

For 3 Bq/L of I-131 in rainwater

Dose Conversion Factor Calculation: I-131 in Water

First, to calculate the Dose Conversion Factor (DCF) for I-131 in water, we take the definition of the 500 µSv limit in 730 liters (L) of water consumed by the reference man in one year to calculate the dose per liter:

(500 µSv) / (730 L) = 0.6849 µSv/L

Next, we use the ALI (Annual Limit on Intake) of I-131 in water of 0.001 µCi/L given in table 2 [ http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/appb/Iodine-131.html ] to calculate the total ‘Dose Conversion Factor’ for I-131 in water:

(0.6849 µSv/L)/(0.001 µCi/L) = 684.9 µSv/µCi.

The activity UC Berkeley and many others report for water is in Becquerel/liter (Bq/L). The conversion between Bq and uCi is 1 µCi = 37,000 Bq.  So the DCF in units of µSv/Bq is:

(684.9 µSv/µCi) * (1 µCi/37,000 Bq) = 0.01851 µSv/Bq  = the dose conversion factor (DCF) for I-131 in water

So depending on the dose units (µSv or millirems) and activity units (µCi or Becquerels) that you prefer, there are four ways of expressing the Dose Conversion Factor for I-131 in water:

Units DCF
millirem/µCi 68.49
µSv/µCi 684.9
millirem/Bq 0.001851
µSv/Bq 0.01851

Now let’s calculate the dose for the specific case: 3 Bq/l (measured across the US the last several days) .   Using the DCF for µSv/Bq, we can easily calculate the total effective dose of I-131 per liter of water:

(3 Bq/L) * (0.01851 µSv/Bq) = 0.0555 µSv/L

The dose from a typical flight from San Francisco to Washington DC and back is approximately 50 µSv.  In order to determine the number of liters one would have to drink to receive this same dose, the dose received for the roundtrip flight is divided by the effective dose per liter:

(50 µSv)/(0.0555 µSv/L) = 900 liters

# So if you were to drink that over a year, you’d have to drink about 2.4 Liters of this 3Bq/L I-131 contaminated water every day, and by the end of he year you would have received the same dose as flying coast to coast in the US.  Okay.   I may want to melt some snow and drink it, after all.

Now, 40 km from the Daiichi NNP, there have been water samples that tested 2,000 Bq/L for I-131 in pond water.  2000 Bq/L * 0.01851 µSv/Bq = 37.02 µSv/L, which would only require drinking 1.35 Liter to get the same dose as for such a continental flight.  Or, another way of putting that is: if you drank just 1 liter of this terribly contaminated water, you would get almost 10 times the normal background radiation in a day.  I you were to drink a mere 14 liters with this much I-131 in it, you’d already be at your ‘annual limit on intake’ for I-131-contaminated water (due to its dangers specifically for the Thyroid).

Some tissue-specific dangers are clearly effectively downplayed by this dose-approach though.  500µSv is not that big of an extra dose to receive in a year, but it is the limit for I-131 in water for good reasons: it accumulates in the Thyroid gland and when you get more than that in a year, there’s a good chance it will lead to health problems, such as Thyroid cancer.  Ten round trip flights SFO-DC won’t do that.

The ‘Low Level Radiation Campaign’ makes a really good point in that regard when they wrote  Friday 25th March 2011 in An open letter to George Monbiot: “When nuclear apologists speak the language of dose they speak the language of deceit.”

Advertisements
This entry was posted in Politics. Bookmark the permalink.

Thank you for commenting. Your comment won't show until approved. Sometimes that can take awhile. - mvb

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s