Researchers at Sandia National Laboratories have been expanding their severe accident computer modeling program, called Melcor, to work with different reactor geometries, fuel types and coolant systems. This work is intended to assist the U.S. Nuclear Regulatory Commission (NRC) to evaluate the safety of the next generation of commercial nuclear power reactors, fuel cycle facilities and fuel technologies.
Sandia and the NRC have collaborated for decades to advance the understanding of system performance under accident conditions. This research has covered areas such as accident progression, combustible gas generation and transport, molten core concrete interaction, fuel coolant interactions and many other subjects of interest.
The Methods for Estimation of Leakages and Consequences of Releases (Melcor) simulation program can model a wide variety of phenomena including severe accidents that can occur at a nuclear power plant. Melcor can estimate the extent of radioactive material release possible due to the accident. Work on Melcor began following the Three Mile Island accident in 1979.
Since the turn of the century, 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 U.S. Department of Energy facilities that work with radioactive materials. Even fusion reactors can be modeled with the program.
The computer program is used to inform the NRC’s regulator decision-making including licensing reviews for new reactors, regarding the risks from very low-likelihood but high-impact accidents. Sandia’s program was used to study the nuclear disaster in Japan’s Fukushima nuclear power plant in 2011. It has also evaluated the risk-reduction potential of several safety improvements to U.S. nuclear reactors for the NRC.
Since 2018, Sandia has expanded Melcor to enable the evaluation of the risks of next-generation reactors and impacts to the fuel cycle in general.
Larry Humphries is the lead Melcor developer. He said, “We want to leverage the decades of experience and validated models in the code and extend it to new reactor designs and new applications. 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.”
In order 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 because they represented the diversity of next generation reactors including a microreactor originally designed by the 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 fluids in the pipes. Melcor analyzed what happens as time progresses to see how much, if any, fission products are released.
The improvements and enhancements to Melcor have been demonstrated at several recent virtual public meetings. The reason for those meeting was to illustrate for U.S. policy makers, members of the nuclear energy industry, international nuclear energy regulators and members of public interest group that the NRC has the computational tools needed to evaluate the safety of new and advanced reactor designs.
David Luxat is the manager for Sandia's nuclear reactor severe accident modeling group. He said, "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.”
Brad Beeny is a Sandia nuclear engineer and Melcor code developer. He said, “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. We're mostly concerned with characterizing 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.”