The US Sandia National Laboratories (SNL) has updated the Maccs code to better aid the Nuclear Regulatory Commission (NRC) and the global nuclear industry in assessing the consequences of nuclear accidents. The Maccs code can also evaluate the potential health and environmental risks posed by advanced nuclear reactors and small modular nuclear reactors.
One of the chief improvements to the Maccs code is that it can now model nuclear accident consequences much closer to the reactor building. Previously, Maccs was not recommended for modelling consequences closer than five football fields from the reactor. The improved Maccs can model consequences much closer to the reactor building as well as options to improve the model’s results out to 1000 miles.
“Advanced reactors and small modular reactors are expected to be smaller, therefore leading to an expectation of lower radioactive releases in the case of an accident,” said Jenn Leute, a Sandia nuclear engineer who is working on the Maccs updates. “With this expectation that releases will be lower, we want to ensure we can model much closer to the point of release, as understanding consequences closer in will become more important. This will allow for decision-makers to make the most informed decisions about new plants and mitigate these very unlikely consequences.”
Maccs is used by NRC and the nuclear industry world-wide to assess the environmental impact for new and existing NPPs, Leute said. The system is also used for assessing the risks of licensing new reactors, especially next-generation nuclear reactors, and informing the industry and regulators about decisions on upgrading power plants.
“We, essentially, look at what happens outside the building in the case of an accident at a nuclear power plant, and we model how any radioactive material released moves through the atmosphere and environment,” Leute said. “We look at where the radioactive material goes and any type of radiological health effects and economic effects.”
Many new advanced reactors are fundamentally different from existing light-water reactor designs and NRC understands it would be inefficient to review these reactors against existing criteria for light-water reactors, explained John Fulton, manager for Sandia’s department that supports the commission’s Maccs modeling efforts.. The agency is developing new regulations to support a risk-informed, technology-inclusive regulatory framework for advanced reactor licensing, he added. The commission is investigating how Maccs and other tools can help set risk-informed safety criteria based on specific consequences, such as the radioactive dose a person is projected to receive within a certain area during and after an accident.
To help NRC with these more targeted assessments, the Sandia team has been improving Maccs’ atmospheric models, economic models and the ability to model close-in since 2016. Maccs can determine the health effects of a potential accident on the public. These include the radioactive dose a population within a certain area of the proposed or existing reactor would receive, the risk of acute radiation poisoning and what the dose would mean for the population’s lifetime risk of cancer.
Maccs can make assessments based on an assumed population density and average weather conditions, if the location for a proposed reactor is not finalised. It can also do these consequence assessments for a very specific location using actual weather and population data for that location, Leute noted.
One significant improvement from the five-year effort involves modelling the health effects of an accident. Maccs can now utilise a new model of how particles move in the air, said Clayton, who has been heavily involved in these improvements. “HYSPLIT is the gold-standard model for 4D atmospheric transport — so up-down, left-right, forward-back and through time,” Fulton said. “It’s highly detailed and provides the ability to accurately model larger distances. It also allows us to look at fine meteorological details like sea breezes or the valley flow effect. With small modular and advanced reactors, those localised effects are expected to have greater influence on accident impacts. Having HYSPLIT as part of Maccs is a massive step forward in being able to capture these fine-resolution atmospheric flows which will be critical to addressing environmental impacts.”
Another way Maccs can aid decision-making by NRC and other regulators is by determining the economic impacts of a potential accident. Before 2021, Maccs would add up the costs for evacuating and relocating people within a certain area, as well as the cost of removing affected crops and cleaning up the affected land and water.
Now, Maccs can estimate the Gross Domestic Product-based losses caused by a potential accident, either a general area or for a specific location, and include that in the assessment. This improvement was led by Sasha Outkin, a Sandia systems analyst.
Over the past few years, the team has also added a tool to turn Maccs’ simulations into movies or images to better visualise and share the results, Clayton said. These visualisation tools ensure decision-makers have a full picture of the economic and environmental consequences of a highly unlikely accident at a NPP. This should allow a more complete comparison of potential new plants’ benefits and their potential risks, and for refined planning and mitigation strategies for existing plants.
Photo: Jenn Leute, left, and Dan Clayton, two Sandia National Laboratories nuclear engineers, stroll outside of a nuclear facility (Photo by Randy Montoya)
