Sandia extends computer modelling code to advanced reactors

Sandia and the NRC have worked together for decades to advance the understanding of system performance under accident conditions. This research covered areas such as accident progression, combustible gas generation and transport, molten core concrete interaction, fuel coolant interactions and many others. Starting in the 1980s, the NRC directed Sandia to consolidate these capabilities into one software package.

The Melcor code can model a wide array of phenomena including severe accidents that can occur at a nuclear power plant, then estimate the extent of radioactive material release possible due to the accident. Work on the code began after the Three Mile Island accident in 1979.

Since the early 2000s, Melcor has been expanded and updated to support safety assessments for other kinds of nuclear facilities - including research reactors, reactors that produce medical isotopes and US Department of Energy facilities that work with radioactive material - and even fusion reactors.

Currently, the computer code is used to inform the NRC's regulatory decision-making activities, including licensing reviews for new reactors, regarding the risks from very low-likelihood but high-impact accidents. Sandia's code was used to study the accident at Japan's Fukushima Daiichi plant and evaluate the risk-reduction potential of several safety improvements to US nuclear reactors for the NRC.

Next-generation reactor designs

Since 2018, Sandia has expanded the severe accident code to enable the evaluation of the risks of next-generation reactors and impacts to the fuel cycle in general.

"We want to leverage the decades of experience and validated models in the code and extend it to new reactor designs and new applications," said Larry Humphries, the lead code developer. "This code is an ideal tool to apply to new reactor designs where there is a great deal of uncertainty. It has the ability to set sensitivity parameters and determine which variables are critical to produce risk assessment data for regulators."

To demonstrate that Melcor is ready to assist the NRC in reviewing new reactor designs, the team developed models of three published nuclear reactor designs. The three reactor designs were chosen to represent the diversity of next-generation reactors, including a microreactor originally designed by Los Alamos National Laboratory, a high-temperature helium-cooled reactor and a high-temperature molten-fluoride-cooled reactor.

The reactor models include everything from the radionuclides expected to be in the reactor vessel and the building that surrounds it to the coolant pipes and the physical properties of the fluid within. The team then simulated a wide range of potential accidents. They analysed what happens as time progressed to see how much, if any, fission products are released.

Public meetings

The advancements in Melcor have been demonstrated at several recent virtual public meetings. The purpose of those meetings was to show US policymakers, members of the nuclear energy industry, international nuclear energy regulators and members of public interest groups that the NRC has the tools needed to evaluate the safety of new and advanced nuclear reactor designs.

"The code is a repository of decades of knowledge on nuclear accidents, which shows nuclear energy is fundamentally low-risk for society," said David Luxat, manager for Sandia's nuclear reactor severe accident modeling group. "We have been able to leverage this knowledge to enhance the safety and economics of nuclear power plants in the US. Now, Sandia is working to expand on and apply this knowledge to enable the next generation of even safer, more economic nuclear power reactors. This will be critical to combatting climate change and enhancing the energy security of future generations."

"If industry is ever going to be able to build anything new and exciting, we need to be able to do safety and licensing calculations of the next generation designs so that the US regulators can assess them," added Brad Beeny, a Sandia nuclear engineer and code developer. "We're mostly concerned with characterising the radiological hazard that could be posed to the public, should an accident happen. It may not be the most flashy aspect of nuclear energy, but it is one of the most necessary."