SOARCA Process, Step 5: Modeling Health Effects

In this step of the State-of-the-Art Reactor Consequence Analyses (SOARCA) process, the project team modeled the offsite health consequences of a severe accident. For additional detail, see the following topics on this page:

• Modeling Rationale and Approach

Modeling Rationale and Approach

In general, we tend to think of the biological effects of radiation in terms of their effect on living cells. However, for low levels of radiation exposure, the biological effects are so small that they may not be detected. This is because living cells have mechanisms to repair damage caused by radiation, as they do for chemical carcinogens. Nonetheless, living cells may experience a range of biological effects of exposure to low levels of radiation. Specifically, exposed cells can react in one of the following three ways:

• Repair themselves correctly, with no residual damage.
• Die and be replaced through normal biological processes, as millions of body cells do every day.
• Repair themselves incorrectly, resulting in a biophysical change that represents the first stage of what may eventually become cancer.

Thus, in modeling the offsite health effects, the project team used MACCS2 (MELCOR Accident Consequence Code System, version 2) to calculate the exposure to the population, and then determine the resultant health effects using a dose-response model. In doing so, the SOARCA team calculated the risk of two different types of consequences from the analyzed scenarios:

• Early Fatality Risk — The risk of dying from radiation sickness as a result of a large dose of radiation.
• Latent Cancer Fatality Risk — The risk of dying from a cancer that could occur years after exposure to radiation.

The risk of a latent cancer fatality (LCF) from chronic exposure to low doses of radiation is very uncertain. Some experts use the linear no-threshold (LNT) model, which assumes that any dose can increase cancer risk. Other experts believe there is a dose below which no definable cancer risk exists, although these experts often do not specifically define that threshold. To reflect the range of opinions on this subject from domestic and international experts, SOARCA evaluated the following four possible dose models:

• LNT
• Truncation of 10 mrem in 1 year
• Truncation of 620 mrem in 1 year, representing the U.S. average of background and medical exposure
• Truncation of 5 rem in 1 year or 10 rem in a lifetime

The SOARCA project used LCF risk factors from a study, entitled "Biological Effects of Ionizing Radiation (BEIR):Health Effects of Exposure to Low Levels of Ionizing Radiation" (BEIR V), the fifth in series of reports published by the National Academy of Sciences, National Research Council.