MACCS is a fully integrated, engineering-level computer code developed at Sandia National Laboratories (SNL) for the Nuclear Regulatory Commission (NRC). MACCS simulates the impact of severe accidents at nuclear power plants (NPPs) and other nuclear facilities on the surrounding environment. As the only code used by the NRC to support Level-3 a probabilistic risk assessments (PRAs), MACCS can use a site’s weather data to determine hypothetical land contamination levels, doses to individuals, health effects and risks on populations based on protective action recommendations, and economic losses resulting from a NPP accident. A list of references can be located on the left-hand side of this webpage.

MACCS 4.1 was released on 30 July 2021. Major enhancements for MACCS 4.1 include:

  • Nearfield Modeling: Implemented additional models to enhance MACCS capabilities for nearfield analysis. These additional models include the Ramsdell and Fosmire plume meander model and an updated version of the NRC’s Regulatory Guide 1.145, “Atmospheric Dispersion Models for Potential Accident Consequence Assessments at Nuclear Power Plants,” plume meander model.
  • Projective Peak Dose: Added a new output option for projecting a peak dose over a moving time window.

Additionally, the MACCS Code Suite will be undergoing a modernization effort in the next several years to update the underlying programming languages as well as expanding capabilities to consider new reactor designs.


  • User activates modules only as needed to determine results of interest
  • Activated modules determine input files that are required
  • Some required input files may be used as transmitted with MACCS (without user modification)


The MACCS user community includes both domestic and international users, to include the NRC, DOE, several research organizations, nuclear industry licensees/applicants, and academic institutions. Primary uses of MACCS include performing regulatory cost-benefit analysis, environmental analyses of Severe Accident Mitigation Alternatives (SAMAs) and Severe Accident Mitigation Design Alternatives (SAMDAs), evaluations for emergency planning, Level 3 PRA studies, consequence studies, and other risk-informed activities.



The latest version (4.1) of WinMACCS is available at https://melcor.sandia.gov/md6/ (Instructions).