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Aging nuclear power plants require more accurate assessments of potential accident
scenarios, including the economic impacts of such an event.

Accurately characterizing the risk of a nuclear accident is a key challenge for nuclear facilities, regulating agencies, and research institutes alike. The MELCOR Accident Consequence Code System (MACCS) was originally developed in the 1970s to simulate the impact of severe accidents at nuclear power plants and nuclear facilities on the surrounding environment. As the only code used by the NRC to support Level-3 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. As computational capabilities evolve, the MACCS development team is continually looking for opportunities to improve the fidelity of its models. MACCS 4.0 was released in June 2020, and included the 4D high-fidelity Lagrangian particle tracking model HYSPLIT developed by NOAA, and a stateof-practice economic model (RDIEM) to better calculate the economic impact of a nuclear power accident.

Technology Advancement

The evolution of the MELCOR Accident Consequence Code System has provided updated capabilities for the environmental and economic assessment of nuclear accident impacts.

In MACCS 4.0, atmospheric transport and dispersion can be calculated with HYSPLIT, a high fidelity Lagrangian particle tracking code created and distributed by the U.S. National Oceanic and Atmospheric Administration (NOAA). The HYSPLIT option provides a high-fidelity alternative to using the traditional Gaussian plume segment model in MACCS.

Also in MACCS 4.0, economic losses can now be estimated with an alternative, state-of-practice model, called RDEIM (Regional Disruption Economic Impact Model), that estimates GDP losses instead of losses based on expected rate-of-return on property investments. RDEIM is based on standard input-output economic theory and estimates losses at both U.S. regional and national levels.


MACCS 4.0 provides the accurate data prediction necessary to maintain the nation’s advanced reactors and solidifies the U.S. as a global leader in nuclear energy.

The MACCS 4.0 update includes higher fidelity models (coupling with HYSPLIT) for atmospheric transport, and a state-ofpractice economic model (RDIEM) to better calculate the economic impact of a nuclear power accident. These MACCS 4.0 updates support the deployment of advanced reactors and the Nation’s strategic goal to remain the global leader in nuclear energy.

MELCOR Accident Consequence Code System MACCS 4.0

Sandia National Laboratories
Publication Date
Apr 1, 2021
Agreement Type