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Risk Assessment
Risk Assessment and Federal Guidance Programs:  

About CAP-88 and CAP-88 PC

CAP-88

The CAP-88 (Clean Air Act Assessment Package-1988) computer model is a set of computer programs, databases and associated utility programs for estimation of dose and risk from radionuclide emissions to air. CAP-88 is composed of modified versions of AIRDOS-EPA (Mo79) and DARTAB (ORNL5692).

The original CAP-88 program is written in FORTRAN77 and was compiled to run on an IBM 3090 under OS/VS2, using the IBM FORTRAN compiler, at the EPA National Computer Center in Research Triangle Park, NC. The CAP88-PC software, released in 1992, allows the user to complete CAP-88 dose and risk assessment calculations in a personal computer environment.

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Regulatory Context

On October 31, 1989 the Environmental Protection Agency issued final rules for radionuclide emissions to air under 40 CFR Part 61, National Emission Standards for Hazardous Air Pollutants (NESHAPS). CAP-88 was used to generate risk estimates for the risk assessment supporting this rule. The CAP88-PC software has been approved for demonstrating compliance with the requirements in 40 CFR 61.93a for Department of Energy facilities.

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Application Niche

CAP88-PC uses a modified Gaussian plume model to estimate the average dispersion of radionuclides released from up to six sources. The sources may be either elevated stacks, such as a smokestack, or uniform area sources, such as a pile of uranium mill tailings. While up to six stacks or area sources can be modeled, all the sources are modeled as if located at the same point; that is, stacks cannot be located in different areas of a facility. Uniform contamination is assumed for area sources. Plume rise can be calculated assuming either a momentum or buoyancy-driven plume. The same plume rise mechanism (buoyant or momentum) is used for each source. Assessments are done for a circular grid of distances and directions with a radius of 80 kilometers (50 miles) around the facility. Errors arising from these assumptions will have a negligible effect for assessments where the distance to exposed individuals is large compared to the stack height, area or facility size.

The program computes radionuclide concentrations in air, rates of deposition on ground surfaces, concentrations in food and intake rates to people from ingestion of food produced in the assessment area. Estimates of the radionuclide concentrations in produce, leafy vegetables, milk and meat consumed by humans are made by coupling the output of the atmospheric transport models with the U.S. Nuclear Regulatory Commission Regulatory Guide 1.109 terrestrial food chain models.

Dose and risk estimates from CAP88-PC are applicable only to low-level chronic exposures, since the health effects and dosimetric data are based on low-level chronic intakes. CAP88-PC cannot be used for either short-term or high-level radionuclide intakes.

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Strengths and Limitations

CAP88-PC provides the CAP-88 methodology for assessments of both collective populations and maximally-exposed individuals, and allows users to edit some environmental transport variables. The complete set of dose and risk factors, census data, and stability array data used in CAP-88 is provided.

Variation in radionuclide concentrations due to complex terrain cannot be modeled; all assessments assume a flat plain


Validation

EPA has made comparisons between the predictions of annual-average ground-level concentration to actual environmental measurements, and found very good agreement. Environmental monitoring data at five Department of Energy (DOE) sites was compared to AIRDOS-EPA predictions in 1987. A report titled, Comparison of AIRDOS-EPA Prediction of Ground-Level Airborne Radionuclide Concentrations to Measured Values (Be86) summarizes the results. EPA concluded that as often as not, AIRDOS-EPA predictions were within a factor of 2 of actual concentrations.

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References

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