Improved Nuclear Accident Code Helps Policy Makers Assess Small Reactor Risks

Newswise – ALBUQUERQUE, NM – Sandia National Laboratories recently updated the Mac code to better help Nuclear Regulatory Commission and the global nuclear industry in assessing the consequences of nuclear accidents. The Maccs code can also assess the potential health and environmental risks posed by advanced nuclear reactors and small modular nuclear reactors.

Small modular reactors range from conventional scaled-down nuclear reactors with modern safety features to totally innovative designs that use different cooling methods and fuel designs. Some could even be built at a central facility and transported to where reliable, carbon-neutral electricity is needed.

One of the major improvements to the Maccs code is that it can now model the consequences of a nuclear accident much closer to the reactor building. Previously, Maccs was not recommended for modeling consequences within five football fields of the reactor. Improved Maccs can now model aftermath starting much closer to the reactor building as well as options to improve model results up to 1,000 miles.

“Advanced reactors and small modular reactors are expected to be smaller, leading to the expectation of lower radioactive releases in the event of an accident,” said Jenn Leute, a nuclear engineer at Sandia who works on upgrades. macs day. “With this expectation that releases will be lower, we want to ensure that we can model much closer to the point of release, as understanding the consequences closer up will become more important. This will allow decision makers to make the most informed decisions regarding new plants and to mitigate these highly unlikely consequences.

Small reactors require refined code

Maccs is used by the Nuclear Regulatory Commission and the nuclear industry worldwide to assess the environmental impact of new and existing nuclear power plants, Leute said. The system is also used to assess the risks associated with licensing new reactors, in particular next-generation nuclear reactors, and to inform industry and regulators of decisions relating to the modernization of existing power plants.

“We basically look at what happens outside the building in the event of an accident at a nuclear power plant, and we model how any released radioactive material moves through the atmosphere and the environment,” Leute said. “We’re looking at where the radioactive material is going and any kind of radiological health effects and economic effects.”

Small modular reactors are expected to occupy up to a tenth or less of the area of ​​a current nuclear power plant, while producing about a quarter to an eighth of the carbon-neutral electricity of a current nuclear power plant, Dan Clayton said. . , another Sandia nuclear engineer involved in the Maccs update. In addition, advanced reactors will be even safer than previous generations of nuclear reactors, given 60 years of experience and technological improvements.

Many of these new advanced reactors are fundamentally different from existing light water reactor designs. The Nuclear Regulatory Commission understands that it would be inefficient to review these reactors against existing criteria for light water reactors. The agency is developing new regulations to support a risk-informed, technology-inclusive regulatory framework for licensing advanced reactors, said John Fulton, head of the Sandia department that supports the commission’s Maccs modeling efforts. The commission is studying how Maccs and other tools can help define risk-informed safety criteria based on specific consequences, such as the radioactive dose a person is expected to receive in a certain area during and after an accident.

To assist the Nuclear Regulatory Commission in these more focused assessments, Sandia’s team has been enhancing Maccs’ atmospheric models, economic models, and close modeling capability since 2016.

New atmospheric model facilitates health impact assessments

One of the ways Maccs can help members of the nuclear industry is to determine the health effects of a potential accident on the public. These effects include the radioactive dose that a population in 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 cancer risk.

Maccs can make those assessments based on assumed population density and average weather conditions, if, for example, the location of a proposed reactor is not finalized, Leute said. Or Maccs can also do these consequence assessments for a very specific location — using actual weather and demographic data for that location, she added.

A significant enhancement to the Sandia team’s five-year effort is modeling the health effects of an accident. Maccs can now use a new model of how particles move through the air, said Clayton, who has been heavily involved in these improvements.

HYSPLIT is the reference model for 4D atmospheric transport – so up-down, left-right, forward-backward, and through time,” Fulton said. “It is very detailed and offers the ability to accurately model greater distances. It also allows us to look at fine weather details like sea breezes or the valley flow effect. With small, modular and advanced reactors, these localized effects are expected to have a greater influence on accident impacts. Having HYSPLIT in Maccs is a big step forward in being able to capture those fine resolution atmospheric flows that will be essential in addressing environmental impacts.

This atmospheric model allows researchers and regulators to examine how radioactive material would move after an accident under very specific weather conditions, or a random representative collection of weather conditions, Clayton said. This is just one example of how Maccs can provide answers, even when there is uncertainty about weather conditions.

Add GDP to refine economic impacts

Another way Maccs can help decision-making at the Nuclear Regulatory Commission and other regulators is to determine the economic impacts of a potential accident.

Prior to last year, Maccs added up the costs of evacuating and relocating people to a certain area, as well as the cost of disposing of affected crops and cleaning up affected land and water, Leute said.

Now Maccs can also estimate losses based on gross domestic product caused by a potential accident – ​​again for a general area of ​​the contiguous United States or for a specific location – and include it in the assessment. This improvement was led by Sasha Outkin, system analyst at Sandia.

Maccs can also use atmospheric transport results and a food chain model to help regulators determine the impacts of farmland contamination after an accident.

In recent years, the team has also added a tool to turn Maccs simulations into movies or images to better visualize and share results, Clayton said.

These visualization tools provide decision makers with a complete picture of the economic and environmental consequences of a highly improbable accident at a nuclear power plant. This should enable a more comprehensive comparison of the potential benefits of new plants from reliable, carbon-neutral electricity and their potential risks, and refine planning and mitigation strategies for existing plants.

“Maccs models what might happen in an accident,” Clayton said. “This helps the NRC determine if the reactor design complies with applicable regulations. By using Maccs for these analyses, the NRC can assess what might happen so that it can make an informed decision whether the reactor will meet regulatory safety requirements. These improvements will support existing analyzes and assessments for new reactors.


Sandia National Laboratories is a multi-mission laboratory operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the National Nuclear Security Administration of the United States Department of Energy. Sandia Labs has significant research and development responsibilities in the areas of nuclear deterrence, global security, defense, energy technologies, and economic competitiveness, with primary facilities in Albuquerque, New Mexico, and in Livermore, California.