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RE: RESRAD Risk/Dose Question
Eric,
The reason you are getting non-zero risks from ingestion of Sr-90 in
groundwater almost 30 years before you get breakthrough is due to the
different time frames over which RESRAD calculates dose and risk. Using
your example and Version 6.1 of RESRAD, I got a breakthrough in groundwater
between 37 and 38 years.
RESRAD calculates the dose over a full year during which the receptor is
exposed to the radionuclide (Version 6 and higher does a time integration of
concentrations in water over the year during which intake occurs); the dose
factors themselves incorporate a 50-year commitment, so that the dose from
one year of intake is actually incurred over the 50 years subsequent to the
intake (for Sr-90, most of the dose actually occurs in first year following
intake). Therefore, the dose incurred from drinking water starts some time
into the 37th year.
By default, RESRAD assumes a 30-year exposure duration AFTER the time at
which a receptor starts occupancy of the site. This parameter, found in the
"Occupancy" input window, is used only for risk calculations (and not for
the committed dose in the year of intake). Therefore, risks start showing
up as early as year 8 (8+30 = 38). At this point, the receptor would
actually be exposed only during a fraction of the last year of occupancy, so
the risk will be very low. Since risk factors use models similar to those
used in deriving the dose factors, there will also be a commitment from
ingestion of radionuclides that extends over time (I am not sure if the
integration time is 50 years or 70 years in the case of risk factors, since
these are derived for all members of the public, not just adults).
You may view the exposure duration as a way to integrate exposures and risks
over a period longer than one year. If you divide the 30-year risk by 30,
you will essentially have a 30-year moving average of the annual risk. If
you want risks to be calculated on an annual basis, to be consistent with
the dose incurred in any given year, change the exposure duration from 30 to
1.
Ernesto Faillace, Eng.D, CHP
Nuclear Engineer/Health Physicist
Tetra Tech NUS
900 Trail Ridge Rd
Aiken, SC 29803
(803) 649-7963 x303
(803) 642-8454 (fax)
faillacee@ttnus.com
-----Original Message-----
From: Frohmberg, Eric [mailto:Eric.Frohmberg@STATE.ME.US]
Sent: Tuesday, December 04, 2001 10:20 AM
To: 'radsafe@list.vanderbilt.edu'
Subject: RESRAD Risk/Dose Question
Hi All,
Here is a question for you modelers and RESRAD experts. You also may find
it entertaining as it is modeling "proof" that there is no such thing as
zero risk.
For various obscure reasons, I've done a RESRAD run using all default
parameters with the exception of using an initial concentration of 1 pCi/g
of Sr-90 and a Kd of 6 (for all soil zones). When looking at the results I
get an initial dose beginning at year 41. I also find Sr 90 in the well
water at year 41 (I'm fine with this, this all makes sense, I presume, based
on breakthrough times, etc.). But when I use RESRAD to calculate risk, I
start to get risk from drinking well water at year 8! Granted the risks are
"small" (as one would expect with no concentration!) but one could argue
they shouldn't be there at all!
I presume this is due to a rounding issue - RESRAD is calling concentrations
below a certain limit zero - but it is doing this AFTER calculating (and
storing) the risk numbers. Has anyone experienced this same issue? Can
anyone confirm that this is what is happening?
I can email anyone interested the raw data, but it is easy enough to run
yourself. The initial concentration of Sr-90 is 1 pCi/g. Under transport
option, change the Kd in the various zones (contaminated, unsaturated and
saturated) to 6. Leave all the other values as defaults. Run the model and
look at the "standard graphics" for concentration in well water, dose from
drinking water, and risk from drinking water. You'll see the risk appears
before the strontium ends up in the drinking water.
Thanks for your collective experience,
Eric
Eric Frohmberg, Toxicologist
Environmental Toxicology Program
Maine Bureau of Health
11 State House Station
Key Bank Plaza, 8th Floor
Augusta, ME 04355
207-287-8141 (phone)
207-287-9058 (fax)
eric.frohmberg@state.me.us
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