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Mr.
Chaney,
I
am a former workshop instructor for the RESRAD Family of Codes, and
while I no longer work for the code developers at ANL, I have followed the
code's recent developments very closely due to my current work involving uses of
the RESRAD code.
I
assume that when you mean dose-based risk values, you are converting the doses
calculated by RESRAD to a risk by using a dose-to-risk conversion factor
(e.g., dose-to-risk conversion factor of
4E-7 latent cancer fatalities per mrem). If that is the case, then
the first reason that the risk values calculated by RESRAD would
be different than the risk-converted dose values is because RESRAD uses FGR
13 numbers to calculate risk directly from external exposure or intakes, rather
than applying a single dose-to-risk
conversion factor. Thus, there will be a significant variation in the
dose/risk ratios for different radionuclides depending on the dominant pathway
and/or affected organ (for intake-dominated exposures).
Also,
and perhaps more importantly, the default exposure duration for dose-based
results is one year (results are presented as mrem/year). However, the
default exposure duration for risk-based calculations is 30 years AFTER the
user-specified time at which the exposure is assumed to start (but the user can
change this default to one year or any other duration of interest). The
reason to use a multi-year exposure duration is to bring the calculations into
closer agreement with the EPA methodology for calculating risks from exposure to
chemical carcinogens. This usually has the apparent effect of increasing
the calculated risk numbers significantly above the values estimated based
on a direct conversion from annual dose to risk using a dose/risk conversion
factor. Note that the increase is not always around 30 times higher, since
RESRAD takes into account decay, ingrowth and transport of radionuclides to
integrate the concentrations in the various media over the exposure duration
period. For long-lived, immobile nuclides with no significant ingrowth
relative to the exposure duration period, the values will be very close to 30
times higher. However, for shorter-lived or very mobile nuclides, the
discrepancy will likely be less than a factor of 30, unless the
concentrations are increasing over the 30-year period, as might be the case
for the groundwater pathway doses immediately following breakthrough to the
saturated zone.
Hope
this helps. Please let me know if you have need me to clarify or expand on
my response or assist you with any other RESRAD-related
issues.
Ernesto Faillace, Eng.D,
CHP
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