How to transform surface deposition in Bq/m^2 to dose equivalent rate in µS/year

helmut

Dear forum,
DOE and MEXT (Japanese Ministry of Education, Culture, Sports, Science and Technology) have pulished the results of common airborne measurements of the surface deposition of Cs-137 inside the 80 km zone of Fukushima Dai-ichi NPP. A geographic map shows that the most contaminated area stretches out from the NPP to north west measuring approx. 20 km * 7 km. The measured order of magnitude of the Cs-137 Surface Deposition there is SD = 10^7 Bq/m^2.

I have tried to figure out what this contamination means in terms of dose equivalent rate in µSv/year. My approach is as follows: Health Canada have published an External Dose Groundshine Coefficient for Cs-137 of GC = 1.48*10^-15 (Sv/s)/(Bq/m^2).

The product SD*GC leads to a Dose Equivalent Rate DER = SD*GC = 1.480*10^-8 Sv/s = 467,000 µSv/year.

MEXT use an upper limit of radiation dose rate permitted for people who engage in emergency work of 250,000 µSv/year which is obviously much higher than the value poeple should be exposed to in their living area. The Cs-137 half life is 30.2 years. So my conclusion is: The mentioned area has to stay evacuated for several human generations.

This is my very first assessment trial in the field of nuclear radiation and I feel very uncertain if my approach is of any meaning. I would desperately whish there is a massive misconception in my calculation. So I kindly ask an expert to falsify or modify it, or -if inevitable - confirm it.

Kind regards
Helmut

Astronuc

One's approach is a reasonable first step. It indicates a problematic area. It is a relatively high dose rate to which to be exposed. Certainly one would want children and infants in the area.

The groundshine rate for Cs-137 is 5.51e-16 Sv s^-1Bq^-1m², whereas 1.480*10^-8 is for Cs-134 (t_1/2 ~ 2 yrs).

Of course, the cumulative dose over one year is subject to the actual duration someone is exposed to that level of activity. This would be a concern for someone in agriculture who would spend 8-16 hrs/day in a field where such activity is found. Otherwise, one then has to consider shielding, such as dwellings. Likely the surface could be removed and quarantined. The ground could be turned over to reduce exposure.

helmut

Thank you for your quick response! And sorry for my wrong ground shine coefficient value!
The corrected result for the Cs-137 Dose Equivalent Rate now is approx. 174,000 µSv/year. A relief but not a comfortable one.

Kind regards
Helmut

tsutsuji

An easy way to figure out the meaning of theses figures, without need to make calculations, is to compare the Fukushima map with the Chernobyl map.

On http://commons.wikimedia.org/wiki/Fi...on_1996-de.svg you can see that the forbidden zones near Chernobyl are the areas over 1,480,000 Bq/m².

helmut

Thanks! It is now even clearer to me that Japan has become a bit smaller in area. Obviously I have missed something in the media since March. Loss of territorry for more than a decade (?) is worthwhile to mention more than once in the news.

zapperzero

You have not missed anything. TEPCO is making reassuring noises from time to time about ending the cleanup and "allowing people to return to their homes" but they never give a timeline. That's about it.

tsutsuji

See also (or http://www.japantoday.com/category/n...hernobyl-study )