Method of estimating lifetime cancer risk due to chronic radionuclide intake.

A methodology for computing the cancer risk due to chronic radionuclide intake, assuming that cancer risk functions per unit dose as a function of age are known, is presented. In this work, an age-dependent intake function is assumed, the total amount of activity present in the body at any given age is computed, and the annual dose equivalent or effective dose estimated using age-dependent dose conversion factors. In a series of time intervals extending from the age of intake to age 80 y, the radiation-induced cancer mortality is estimated by multiplying the dose in any given year by the cancer risk per unit dose at a given age. By integrating the product of the dose and the risk at each time interval, the overall risk due to various chronic radionuclide intake scenarios over a lifetime is determined. This result is compared to the risk computed using integrated committed dose quantities and to the risk computed from an age-independent risk per unit dose. The example cases of dietary contamination following a nuclear incident and uranium contamination in drinking water are presented. The results show that ignoring the age dependence of the dose-risk relationship underestimates the total lifetime risk by more than 80% for the dose due to ¹³7Cs in milk in a nuclear incident scenario. Furthermore, it is found that if the integrated committed dose quantity is used to evaluate risk, the total risk will be overestimated by almost 50% in the case of chronic uranium ingestion. These results demonstrate the sensitivity of the total lifetime risk to the proper assignment of dose to each time interval and to the use of age-dependent risk coefficients.

Evaluation of total effective dose due to certain environmentally placed naturally occurring radioactive materials using a procedural adaptation of RESRAD code.

Due to a recent upward trend in the price of uranium and subsequent increased interest in uranium mining, accurate modeling of baseline dose from environmental sources of radioactivity is of increasing interest. Residual radioactivity model and code (RESRAD) is a program used to model environmental movement and calculate the dose due to the inhalation, ingestion, and exposure to radioactive materials following a placement. This paper presents a novel use of RESRAD for the calculation of dose from non-enhanced, or ancient, naturally occurring radioactive material (NORM). In order to use RESRAD to calculate the total effective dose (TED) due to ancient NORM, a procedural adaptation was developed to negate the effects of time progressive distribution of radioactive materials. A dose due to United States' average concentrations of uranium, actinium, and thorium series radionuclides was then calculated. For adults exposed in a residential setting and assumed to eat significant amounts of food grown in NORM concentrated areas, the annual dose due to national average NORM concentrations was 0.935 mSv y(-1). A set of environmental dose factors were calculated for simple estimation of dose from uranium, thorium, and actinium series radionuclides for various age groups and exposure scenarios as a function of elemental uranium and thorium activity concentrations in groundwater and soil. The values of these factors for uranium were lowest for an adult exposed in an industrial setting: 0.00476 microSv kg Bq(-1) y(-1) for soil and 0.00596 microSv m(3) Bq(-1) y(-1) for water (assuming a 1:1 234U:238U activity ratio in water). The uranium factors were highest for infants exposed in a residential setting and assumed to ingest food grown onsite: 34.8 microSv kg Bq(-1) y(-1) in soil and 13.0 microSv m(3) Bq(-1) y(-1) in water.