MACCS (MELCOR Accident Consequence Code System) 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) 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 brief overview of the code is provided in the MACCS Fact Sheets and provided references.

Currently, the MACCS Development Team is working to couple HYSPLIT with MACCS as an alternative transport and dispersion model. Additionally, a new economic model which considers impacts based on gross domestic product is being developed. Finally, the MACCS Code Suite will be undergoing a modernization effort in the next several years to update the underlying programming language 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)