Fall 2023 - Flipbook - Page 9
In RESRAD-OFFSITE, the contaminants are assumed to be uniformly distributed throughout a soil layer, with an option
to have a clean cover layer overlying the contamination. Release of radionuclides from the waste to underlying
groundwater by infiltrating water can be modeled as either a first order, rate-controlled process or an equilibrium
desorption process. The first-order release model assumes that radionuclide leaching is proportional to the
radionuclide concentration remaining in the waste, while the equilibrium desorption release model assumes that
radionuclide leaching is controlled by the linear equilibrium partitioning between the solid and aqueous phases. The
release options represent release from different types of waste (i.e. ion exchange resin, dewatered sludge, laboratory
equipment) (Yu et al. 2013). Degradation
of a liner or engineered barrier can be
simulated by specifying the delay time for
the release and the duration over which
material becomes available for release.
Up to five homogeneous, partially
saturated zones and one unconfined
saturated zone can be simulated (Yu et al.
2007). In the partially saturated zones,
the code models one-dimensional flow in
the vertical (downward) direction directly
below the waste and considers phenomena such as longitudinal dispersion and
radiological transformations (Yu et al.
2007). When radionuclides enter the
saturated zone, the groundwater
transport model simulates horizontal
downgradient flow and considers
advective transport through the mobile Figure 2. Plot of Carbon-14 concentration in a hypothetical groundwater well over the simulation period from a RESRAD-OFFSITE simulation (note: release delayed by 200 years)
pores in the soil, dispersive transport
in the soil moisture, equilibrium (linear)
adsorption and desorption of nuclides on soil surfaces, diffusion of radionuclides into and out of the immobile pores,
and decay and ingrowth due to radiological transformations (Yu et al. 2007).
Contaminant release to the atmosphere and via surface runoff may occur from a surface soil layer above the waste.
It is assumed that this layer is uniformly mixed by either anthropomorphic or natural processes. Release of
radionuclides to the atmosphere is determined by the concentration in the surface soil and the rate at which dust
is release from the contaminated area. Downwind transport of contaminants is modeled using a Gaussian plume
atmospheric dispersion model that accounts for dry and wet deposition (Yu et al, 2007). Release of radionuclides
as surface runoff is determined by the concentration in the surface soil and the erosion rate. Upon erosion, surface
runoff may be transported to the surface water body by surface processes, and its accumulation is dependent on the
sediment delivery ratio to the surface water body (Yu et al. 2007).
RESRAD-OFFSITE uses the release to the atmosphere, surface runoff, and groundwater to compute accumulation at
hypothetical receptor areas (agricultural areas, one dwelling area, a groundwater well, and a surface water body)
and calculates the dose received by an individual from nine internal and external exposure pathways. Calculation
of deterministic dose from the exposure pathways consider human occupancy within or outside of the contaminated
area, biological accumulation of radionuclides in food goods, inhalation rate, and food and water ingestion rates
(Yu et al. 2007). In addition to deterministic predictions of dose and risk, single parameter sensitivity analyses and
multi-parameter uncertainty analyses can be conducted on many of the input parameters.
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