New Source Term Model for the RESRAD-OFFSITE Code Version 3 (NUREG/CR-7127)

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Publication Information

Manuscript Completed: May 2013
Date Published: December 2013

Prepared by:
C. Yu, E. K. Gnanapragasam, J. J. Cheng, S. Kamboj, and S. Y. Chen

Argonne National Laboratory
9700 South Cass Avenue Argonne, IL 60439

Ralph Cady, NRC Project Manager

NRC Job Codes N6731 and V6360

Prepared for:

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

This report documents the new source term model developed and implemented in Version 3 of the RESRAD-OFFSITE code. This new source term model includes: (1) "first order release with transport" option, in which the release of the radionuclide is proportional to the inventory in the primary contamination and the user-specified leach rate is the proportionality constant, (2) "equilibrium desorption release" option, in which the user specifies the distribution coefficient which quantifies the partitioning of the radionuclide between the solid and aqueous phases, and (3) "uniform release" option, in which the radionuclides are released from a constant fraction of the initially contaminated material during each time interval and the user specifies the duration over which the radionuclides are released. The source term model implemented in RESRADOFFSITE Version 2 computes the release of radionuclides from the contaminated zone, as well as the concentration and distribution of the radionuclides within the contaminated zone. The model calculates the atmospheric release of particulates due to resuspension and that of volatiles due to diffusion and evapotranspiration, the surface water release due to erosion by runoff water, and the groundwater release due to leaching by infiltrating water. The release rates are used by the code to compute the transport of contaminants and exposure at offsite locations, whereas the concentration and distribution of radionuclides within the contaminated zone are used to compute the direct external exposure from the primary contamination, as well as any exposure from onsite activities. The newly developed source term models for RESRADOFFSITE code Version 3 also consider these releases and distribution in the primary contaminated zone. The new release mechanisms are not limited to treating soil materials as the source (primary contamination). Furthermore, the new release mechanisms are also not limited to uncontainerized sources because the new source release models assume that the releases are from a distinct phase of source material; once released into surrounding soils, they are transported in the soil in the region of primary contamination. The new source release models implemented in RESRAD-OFFSITE code Version 3 are compared with DUST-MS code and the GoldSim software. The results of comparison are presented and discussed in the report. A detailed description of how to conduct sensitivity and probabilistic analyses using RESRAD-OFFSITE code is presented, and the results of using sensitivity and probabilistic analyses on the new source release input parameters are discussed. The revisions and additions to Version 3.1 included a time-delay feature for the source release mechanism. This time-delay feature provides the ability to model scenarios where the release of radionuclides is delayed and/or distributed over time. The "uniform release" option of Version 3.0 beta is now part of the time-distributed "equilibrium desorption release" option. The time–distributed, first– order, rate-controlled release is an additional option available in Version 3.1. Another new feature in Version 3.1 facilitates the calculation of area factors for small areas of elevated activity. This area factor calculation was performed using the probabilistic analysis feature of the RESRAD-OFFSITE code.