Re: RESRAD Information

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
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