*Schafetz is associated with Rockland Community College, Suffern,. New York, and Mehta with Fayetteville State University,. Fayetteville, North Carolina.
ANL/CP—72668 DE91 018544
Paper for Environmental Remediation '91 Cleaning up the Environment for the 2.1st Century
September 8-11, 1991 Pasco, Washington
SOILD: A COMPUTER MODEL FOR CALCULATING THE EFFECTIVE DOSE EQUIVALENT FROM EXTERNAL EXPOSURE TO DISTRIBUTED GAMMA SOURCES IN SOIL
S.Y. Chen, D. LePoire, S. Schafetz,* P. Mehta,* and C. Yu
Environmental Assessment and Information Sciences Division Argonne National Laboratory, Argonne, Illinois
symposium sponsored by U.S. Department of Energy The submitted manuscript has been authored by a contractor of the U. S. Government under contract No. W-31-109-ENG-38. Accordingly, the U. S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U. S. Government purposes.
*Schafetz is associated with Rockland Community College, Suffern, New York, and Mehta with Fayetteville State University, Fayetteville, North Carolina.
MASTER DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED
SOILD : A Computer Model for Calculating the Effective Dose Equivalent from External Exposure to Distributed Gamma Sources in Soil* by S.Y. Chen, D. LePoire, S. Schafetz,+ P. Mehta,+ and C. Yu Environmental Assessment And Information Sciences Division Argonne National Laboratory, Argonne, Illinois The SOILD computer model was developed for calculating the effective dose equivalent from external exposure to distributed gamma sources in soil- It is designed to assess external doses under various exposure scenarios that may be encountered in environmental restoration programs. The model's four major functional features address (1) dose versus source depth in soil, (2) shielding of clean cover soil, (3) area of contamination, and (4) nonuniform distribution of sources. The model is also capable of adjusting doses when there are variations in soil densities for both source and cover soils. The model is supported by a data base of approximately 500 radionuclides (Kocher 1977). DOSE VERSUS SOURCE DEPTH Effective dose equivalent responses from distributed gamma sources in soil have been derived by Chen (1991) on the basis of the recommendations of the International Commission on Radiological Protection's (ICRP's) Publication 51 (1987) on external exposures. The Monte Carlo method was used to obtain the responses. The results were expressed in the following empirical formula that correlates the effective dose equivalent (H) to the source photon energy (E), source slab thickness (T), and soil density (p): H(p,T,E) = -5° {l-Exp[-anT - P(nT)1-5]} P
(1)
where Ho is the effective dose equivalent for the infinitely thick source layer, y. is the photon attenuation coefficient of the soil, and a and j3 are the fitted coefficients. The dose response from a gamma-emitting radionuclide was calculated as follows: fSchafetz is associated with Rockland Community College, Suffern, New York, and Mehta with Fayetteville State University, Fayetteville, North Carolina. *Work supported by the U.S. Department of Energy, Office of Environmental Restoration and Waste Management, under Contract W-31-109-ENG-38.
where f; is the gamma yield for photon energy Et of the ith photon emitted by the nuclide. SHIELDING FACTOR OF CLEAN COVER SOIL Covering a contaminated area with clean soil has been used as a means of reducing external exposure. The required thickness of the clean soil layer depends on its ability to shield against gamma radiation. The point-kernel method is used to calculate the soil shielding factor. The calculation is based on the latest buildup factors derived by Trubey (1988). Assuming the concrete buildup factors for soil, the shielding factor for cover soil can be expressed as follows:
whera
Im(x) = f — Bm(z) dz, m = a or s J
(4)
z
where n is the photon attenuation coefficient; B is the buildup factor; T is the medium thickness; and a and s are designations for air and soil, respectively. In this equation, Ta is assumed to be 100 cm for the human receptor above the ground. AREA FACTOR Area factors are used when soil contamination is confined to a limited rather than an infinite area. The point-kernel method and air buildup factors are used to calculate area factors. Area factors in SOILD are calculated on the basis of the dose contributions from the area bounded by the concentric circles on the contaminated soil surface. The area factor for a particular area can be estimated as follows:
where I, is the integral defined in Equation 4, and rs and rj+l are
the outer and inner radii of the concentric circles bounding the area of interest. The derived area factor is energy-dependent and, therefore, varies from one radionuclide to the other. NONUNIFORM SOURCE DISTRIBUTION Two methods are used in SOILD to .address nonuniform source distributions. One method models on the assumption that there are stratified source layers, and that each layer has a uniform source distribution. The other method models on the basis of the following source distribution: S(x) = Soe-«x
(6)
where So is the source density at the top layer, x is the source depth measured from the surface, and a is the coefficient that characterizes the source distribution. The SOILD model has capabilities for calculating doses for a given distribution of the source characterized by the parameter. OTHER FEATURES The SOILD model also has the capability to trace the decay chains of the parent nuclide and to add the contributions from the daughter nuclides to that of the parent. In the current version of SOILD, this capability is based on the assumption that equilibrium has been established among the radionuclides in a chain. Approximately 500 radionuclides are built into the data base of SOILD. In addition, the model has three types of dose equivalent data based on different exposure orientations: rotational (ROT), anteroposterior (AP) , and isotopic (ISO) . The ROT data are used to estimate exposure from contaminated ground, the AP data are used to estimate the exposure of cleanup workers from contaminated walls, and the ISO data are used to estimate the exposure of a resident in a uniformly contaminated building. A set of organ dose data is also provided to facilitate the calculation of dose equivalents for specific organs. These dose equivalent data are also based on the recommendations of ICRP Publication 51 (1987). CODE APPLICABILITY The SOILD model has been developed for application in environmental restoration programs. It can (1) determine the effective dose equivalent from external exposures from various contamination scenarios (2) establish guidelines for external exposures from gamma-emitting radionuclides, and (3) be used to design remedial actions (such as level of cleanup or clean soil coverage). The SOILD model was designed in an IBM/PC environment and is equipped with menu-driven, user-friendly features, including graphic output capabilities.
REFERENCES Chen, S.Y., 1991, Calculation of Effective Dose-Equivalent Responses for External Exposure from Residual Photon Emitters in Soil, Health Physics 60(3):411-426. International Commission on Radiological Protection, 1987, Data for Use in Protection Against External Radiation, Pergamon Press, Oxford. Kocher, D.C., 1977, Nuclear Decay Data for Radionuclides Occurring in Routine Release from a Nuclear Fuel Cycle Facility, ORNL/NUREG/TM-102, Oak Ridge National Laboratory, Oak Ridge, Tenn. Trubey, D.K, 1988, New Gamma-Ray Buildup Factor Data for Point Kernel Calculations: ANS-6.4.3 Standard Reference Data, ORNL/RSIC49, Radiation Shielding Information Center, Oak Ridge National Laboratory, Oak Ridge, Tenn.
DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
