Multimedia benchmarking analysis for three risk assessment models: RESRAD, MMSOILS, and MEPAS.

This paper is one in a series that describes results of a benchmarking analysis initiated by the Department of Energy (DOE) and the United States Environmental Protection Agency (EPA). An overview of the study is provided in a companion paper by Laniak et al. presented in this journal issue. The three models used in the study--RESRAD (DOE), MMSOILS (EPA), and MEPAS (DOE)--represent analytically-based tools that are used by the respective agencies for performing human exposure and health risk assessments. Both single media and multimedia benchmarking scenarios were developed and executed. In this paper, the multimedia scenario is examined. That scenario consists of a hypothetical landfill that initially contained uranium-238 and methylene chloride. The multimedia models predict the fate of these contaminants, plus the progeny of uranium-238, through the unsaturated zone, saturated zone, surface water, and atmosphere. Carcinogenic risks are calculated from exposure to the contaminants via multiple pathways. Results of the tests show that differences in model endpoint estimates arise from both differences in the models' mathematical formulations and assumptions related to the implementation of the scenarios.

An overview of a multimedia benchmarking analysis for three risk assessment models: RESRAD, MMSOILS, and MEPAS.

Multimedia modelers from the United States Environmental Protection Agency (EPA) and the United States Department of Energy (DOE) collaborated to conduct a detailed and quantitative benchmarking analysis of three multimedia models. The three models--RESRAD (DOE), MMSOILS (EPA), and MEPAS (DOE)--represent analytically-based tools that are used by the respective agencies for performing human exposure and health risk assessments. The study is performed by individuals who participate directly in the ongoing design, development, and application of the models. Model form and function are compared by applying the models to a series of hypothetical problems, first isolating individual modules (e.g., atmospheric, surface water, groundwater) and then simulating multimedia-based risk resulting from contaminant release from a single source to multiple environmental media. Study results show that the models differ with respect to environmental processes included (i.e., model features) and the mathematical formulation and assumptions related to the implementation of solutions. Depending on the application, numerical estimates resulting from the models may vary over several orders-of-magnitude. On the other hand, two or more differences may offset each other such that model predictions are virtually equal. The conclusion from these results is that multimedia models are complex due to the integration of the many components of a risk assessment and this complexity must be fully appreciated during each step of the modeling process (i.e., model selection, problem conceptualization, model application, and interpretation of results).

Uncertainty of historical measurements of 131I in Hanford-area vegetation.

The Hanford Environmental Dose Reconstruction (HEDR) Project was conducted to estimate the radiation dose that individuals could have received as a result of emissions to the air and water from Hanford Site operations since 1944. The largest doses were to the human thyroid gland from 131I released into the atmosphere from Hanford facilities in the 1945-1947 time period. In support of the dose reconstruction effort, a database of historical environmental radioactivity measurements was constructed. This database includes measurements of total radioactivity for vegetation samples collected from 1945-1948 and counted using a Geiger-Mueller (GM) detector system. Because the factors used at that time to convert the GM counts to 131I activity did not take all parameters into account, and because some parameter values were inaccurate, more accurate conversion factors were developed as part of the HEDR Project. These factors can be used to estimate the actual historical activity levels. This paper summarizes the Monte Carlo uncertainty and sensitivity analysis methods used to assess the uncertainty of the newly reconstructed historical vegetation 131I activities and to identify the parameters that contributed the most uncertainty to these reconstructed activities. Based on the study of two vegetation (sagebrush) samples collected in the mid-1940's, it appears that the true 131I activity of the historical vegetation samples should be within a factor of three of the reconstructed activity. Also, the uncertainty in the parameter Icf (the fraction of the background-corrected GM measurement of a vegetation sample that resulted from 131I) was found to contribute the most uncertainty to the reconstructed 131I activities when the uncertainty in Icf was large.

Using cost/risk procedures to establish recovery criteria following a nuclear reactor accident.

In the event of a major accidental release of radionuclides at a nuclear power plant, large populated areas could become seriously contaminated. Local officials would be responsible for establishing radiation recovery criteria that would permit the evacuated population to return safely to their jobs and homes. The range of acceptable criteria could imply variations in property losses in the billions of dollars. Given the likely public concern over the health consequences and the enormity of the potential property losses, a cost/risk analysis can provide important input to establishing the recovery criteria. This paper describes procedures for conducting a cost/risk analysis of a site radiologically contaminated by a nuclear power plant accident. The procedures are illustrated by analyzing a hypothetically contaminated site, using software developed for determining the property and health effects of major reactor accidents.