[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Ra, 25 mrem/y



> The NRC DCGL for Ra-226 is given as:
> Ra-226............................  7.0 E-01 pCi/g
> 
> This is equal to the average natural soil > concentration of Ra-226. But,
we know that we don't 
>get 25 mrem/year from the natural Ra-226. How can this
> be reconciled?

I don't know what the known dose from 0.7 pCi/g of Ra-226 is supposed to be,
but let me argue that the real value might be much higher or, at the very
least, pretty close to 25 mrem.  In other words, it is possible we should be
criticizing the NRC for underestimating the dose and setting too high a
DCGL;-)

DCGL calculations usually factor in the contributions from any decay
products. If we just consider the first decay product of Ra-226, Rn-222, the
CEDE is something like 200 mrem per year.

However, the DandD code ignores the Rn-222 contribution. So lets just
pretend it isn't there and consider the dose due to terrestrial Ra-226 and
the Ra-226 in the body.

My understanding from ballparking the numbers in FGR 12 is that the average
external gamma dose due to  Ra-226 and its daughters at 0.7 pCi/g in soil
(15 cm depth)would be approximately 8 mrem or so.

According to NCRP 94, the annual dose from background levels of radium and
its decay products (primarily Pb-210 - Po-210) in the body would be around
15 mrem to soft tissue. 

Adding these ballpark terrestrial and internal doses, I get 23 mrem per
year. Pretty close to 25.  Needless to say, there might be a serious error
in my reasoning here.

Paul Frame
Professional Training Programs
ORAU
framep@orau.gov
http://www.orau.gov/ptp/ptp.htm
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html