No Power, No Operator, No Problem: Simulating Nuclear Reactors to Explore Next-Generation Nuclear Safety Systems

There is no actual nuclear fuel at the NSTF. Instead, the heat produced by a reactor core is simulated by exposing a large steel plate to powerful electric heaters. The rest of the facility resembles a real reactor vessel cavity more closely. On the other side of the steel plate, pipes filled with filtered tap water run through an insulated cavity.

Hot water naturally rises, so as the reactor core heats the pipes in the cavity, the water begins to circulate through the pipes. The heat is whisked away with the water as it travels around a large, vertical closed loop of pipes and tanks. The water in this type of loop does not interact with the material in the nuclear reactor; it contains no radioactive material or waste.

“Part of the appeal of passive safety systems is their relative simplicity,” said Jasica. ​“But there are open questions for designers about what might happen in edge cases — those rare instances that still need to be accounted for in their designs.”

For example, when the reactor core gets hot enough, the water in the pipes will start to boil as designed. Boiling can cause challenges including pressure waves and fluid loss. It is also a complex and chaotic process to model. Small deviations in the thermal conditions and system configuration can cause dramatically different performance outcomes.

“We are generating large, high-quality datasets under different conditions, from normal operation to accident scenarios, to make sure the nuclear industry has the best possible understanding of how small changes affect decay heat removal,” said Jasica.

NSTF researchers test the system thoroughly, making one change at a time and observing the system’s response. Examples of tests include running the system with blockages in different areas of the pipes, or with different water levels in the tanks. The team has even worked with industry stakeholders to simulate — based on their own reactor plant design and models — an extended, real-world scenario where active cooling is suddenly lost.

Originally built at Argonne in the 1980s, the NSTF has tested a series of reactor technologies and configurations. In 2005, the program began exploring air-based passive safety systems. This work led to a patent by Darius Lisowski, NSTF’s group manager of reactor safety testing and analysis, for a weather cap device that protects sensitive exhaust systems from wind-induced downdrafts. The program shifted to its present water-based configuration in 2018.

“Our facility is always responding to the evolving needs of the nuclear industry,” said Jasica. ​“This type of open science is critical for the advancement and long-term operation of the next generation of nuclear power plants.”