Estimation of offsite accident consequences is the customary final step in a probabilistic assessment of the risks of severe nuclear reactor accidents. Recently, the Nuclear Regulatory Commission reassessed the risks of severe accidents at five U.S. power reactors (NUREG-1150). Offsite accident consequences for NUREG-1150 source terms were estimated using the MELCOR Accident Consequence Code System (MACCS). Before these calculations were performed, most MACCS input parameters were reviewed, and for each parameter reviewed, a best-estimate value was recommended. This report presents the results of these reviews. Specifically, recommended values and the basis for their selection are presented for MACCS atmospheric and biospheric transport, emergency response, food pathway, and economic input parameters. Dose conversion factors and health effect parameters are not reviewed in this report.

Uncertainty and sensitivity analysis techniques based on Latin hypercube sampling, partial correlation analysis and stepwise regression analysis are used in an investigation with the MACCS model of the food pathways associated with a severe accident at a nuclear power station. The primary purpose of this study is to provide guidance on the variables to be considered in future review work to reduce the uncertainty in the important variables used in the calculation of reactor accident consequences. The effects of 87 imprecise] y-known input variables on the following reactor accident consequences are studied: crop growing season dose, crop long-term dose, milk growing season dose, total food pathways dose, total ingestion pathways dose, total long-term pathways dose, area dependent cost, crop disposal cost, milk disposal cost, condemnation area, crop disposal area and milk disposal area. When the predicted variables are considered collectively, the following input variables were found to be the dominant contributors to uncertainty: fraction of cesium deposition on grain fields that is retained on plant surfaces and transferred directly to grain, maximum allowable ground concentrations of Cs-137 and Sr-90 for production of crops, ground concentrations of Cs-134, Cs-137 and 1-131 at which the disposal of milk will be initiated due to accidents that occur during the growing season, ground concentrations of Cs-134, 1-131 and Sr-90 at which the disposal of crops will be initiated due to accidents that occur during the growing season, rate of depletion of (3-137 and Sr-90 from the root zone, transfer of Sr-90 from soil to legumes, tranfer of Cs-137 from soil to pasture, transfer of cesium from animal feed to meat, and the transfer of cesium, iodine and strontium from animal feed to milk.

A new version of the MACCS code (version 1.5.11.1) has been developed by Sandia National Laboratories under sponsorship of the U.S. Nuclear Regulatory Commission. MACCS was de_ Aoped to support evaluations of the off-site consequences from hypothetical severe accidents at commercial power plants. MACCS is the only current public domain code in the U.S. that embodies all of the following modeling capabilities: (1) weather sampling using a year of recorded weather data; (2) mitigative actions such as evacuation, sheltering, relocation, decontamination, and interdiction; (3) economic costs of mitigative actions; (4) cloudshine, groundshine, and inhalation pathways as well as food and water ingestion; (5) calculation of both individual and societal doses to various organs; and (6) calculation of both acute (nonstochastic) and latent (stochastic) health effects al,d risks of health effects. All of the consequence measures may be generated in the form of a complementary cumulative distribution function (CCDF). The current version implements a revised cancer model consistent with recent reports such as BEIR V and ICRP 60. In addition, a number of error corrections and portability enhancements have been implemented. This report describes only the changes made in creating the new version. Users of the code will need to obtain the code's original documentation, NUREG/CR-4691.

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The SECPOP90 team thanks everyone who helped make this report and the SECPOP90 s o h a r e a reality. In particular we are especially grateful to Brian Lewis, our summer student from Cornell University, who helped make SECPOP90 a more useful program by providing it with many additional features and extensive error checking of all user input; and Donald Mitchell, Sandia National Laboratories, whose assistance with the county-level database effort was invaluable.xi NUREGiCR-6525 of the SECPOP90 software. The third section is the user's guide, which explains how to use the software in a step-by-step manner. The fourth section describes the algorithms and methodologies used to perform the population, land fraction, and economic estimates. The fifth section discusses the software design, development, and verification and validation.Additional appendices describe input and output file formats, the U.S. Bureau of Census CD-ROMs to block census and county census data file conversions, and the complete SECPOP90 Visual Basic code listing. .I Report ConventionsThe following conventions will be used throughout this report: Table 1.1 Report Conventions Em 1 an a ti on Convention ~~ SECPOP90 V2.0 MACCS S i t e Data FileA monospaced font w i l l be used for example data files, operating system messages, code fragments, and the SECPOP90 code listings.A bold monospaced font will be used for input that the user is expected to type in.The return or enter key. The user should press this key where it is indicated in this report.An italicized monospaced font will be used for input parameters that the user is to provide and type in.Text in small caps indicates one of the special keys on the keyboard.A plus sign indicates that the first key should be pressed and held down then the second and subsequent keys pressed.Acronyms will be defined where they are first used. The defining phrase will be capitalized in correspondence to the letters that make up the acronym. The definition will be followed by the acronym in parentheses.