Researchers replace neutrons with light to develop next-generation reactors

In a dark corner of a mechatronics lab at the Idaho National Laboratory, researcher Ben Baker types commands on a laptop to power up the reactor.

A few feet away, a surrogate nuclear reactor the size of a telephone booth comes to life. The reactor, called ViBRANT (or Visual Benign Reactor as Analog for Nuclear Testing), is controlled by the microreactor automated control system (MACS) — both invented at INL.

Four real-world actuators slowly rotate control drums to regulate the light being reflected back into the illuminated reactor core. On a computer screen, a graph shows the reactor’s power level steadily increasing as the radiation feeds back, causing a chain reaction.

If ViBRANT were a real nuclear reactor, the machinations of the actuators, control drums and reactor core would be hidden behind a containment vessel, and Baker would be shielded by a thick concrete wall.

However, where a real reactor runs on neutrons, ViBRANT uses photons — particles of light emitted from thousands of LEDs — to tell the story of its surrogate nuclear reaction.

ViBRANT is visually stunning. The LEDs increase in intensity as the reactor powers up and can change color as the core’s temperature and neutronics evolve.

But MACS and ViBRANT aren’t just a fancy light show. They are a bridge between computer models and INL’s Microreactor Applications Research Validation and Evaluation (MARVEL) microreactor. MARVEL is an 85-kilowatt, sodium-potassium-cooled microreactor being developed at INL.

Tony Crawford, an INL researcher and MARVEL’s reactivity control system lead, designed and developed the MACS/ViBRANT systems to unite robust physical reactivity control system motion control with complex reactor neutronic/thermal physics models. His goal is to accelerate reactor development and enhance system understanding.

By equipping the ViBRANT system with LED-driven surrogate flux and thermal physics, and the sensors necessary to achieve closed-loop control, Crawford has created a flexible yet high-fidelity hardware-in-the-loop modeling capability.

INL recently announced selections for MARVEL experiments. The chosen companies will work with the lab to understand how microreactors will integrate with data centers, use autonomous control, benefit from next-generation sensors, and power desalination projects.

Baker and other researchers have used MACS/ViBRANT to apply their state-of-the-art, highly detailed reactor physics models to simulate a real nuclear reactor core.

In the future, reactor developers can leverage MACS/ViBRANT’s customizable features to test a wide range of devices and control systems before their microreactors enter the prototype phase. More than that, MACS/ViBRANT gives researchers and developers deep insights into the interactions between complex nuclear physics, computer models and real-world hardware.

MARVEL and MACS/VIBRANT

Along with the LED-driven surrogate core, MACS/ViBRANT uses the same electromechanical devices that will control MARVEL.

“MACS/ViBRANT is a hybrid,” said Crawford. “The actual actuators are the same technology that will be used in the MARVEL reactor.”

“The fuel, the hazardous reflector and absorber materials driving reactor physics are actually replaced by benign materials amenable to light physics,” Crawford continued. “It reduces all the hazards from a real reactor to safe and accessible levels with the promise of accelerating development.”

Accessible and intuitive

This accessibility allows researchers and reactor developers to understand how real-world hardware interacts with reactor physics.

“Ben Franklin once said, ‘Tell me and I forget, teach me and I may remember, involve me and I learn,’” Crawford said.

“By being accessible and as intuitive as watching a TV screen, nearly everyone in the reactor development process — from the modeler to the control system developer to the assembler — can get involved and learn,” Crawford continued. The takeaways can encompass “not only the concepts relevant to their role, but also adjacent roles, significantly enhancing the reactor development effort.”

As a learning tool, the MACS/ViBRANT reactor can be calibrated to act almost identically to the MARVEL reactor, but it also has enough tunability to speed up time frames. Where the MARVEL reactor will take a day to get up to normal operating conditions, the MACS surrogate reactor mimics the same processes in 10 minutes.

During startup, the MACS/ViBRANT control drums toggle out, inserting reactivity into the reactor core. Reactivity and temperature increase. During shutdown, the drums and their associated absorber materials turn in, reducing the reactivity and temperature.

And, like the MARVEL reactor, the base MACS algorithms capture the natural laws of reactor physics that act as a safety system in case the reactor gets too hot. “As the temperature within ViBRANT increases, you can see reactivity go down,” Crawford said. “So, all these basic physics phenomena are captured with this system.”

The ability to tinker

MACS allows users to watch real-world reactor components function — unobstructed by containment vessels and radiation shields. The system has provided engineers with an invaluable tool: the ability to tinker.

MACS and ViBRANT have yielded numerous refinements to hardware and software in MARVEL’s control system.

“In some cases, we’ve had to tune a few of the actual  reactors’ behaviors based on what we saw with this reactor,” Crawford said.

For example, MARVEL’s control drum actuators were redesigned because of MACS/ViBRANT. “We had a surrogate drum that was on a bearing, and the friction had changed,” Crawford said. “It would slip during operation and wouldn’t scram properly. Consequently, we refined the actuator design.”

During an abnormal event, reactor operators would “scram” the reactor, shutting it down quickly. For MARVEL, that means the control drums would turn, causing the absorber materials to face the core to limit the reaction.

Other control system components were fine-tuned based on insights gleaned from the MACS and ViBRANT system.

The system makes it possible to build non-nuclear prototypes to test important reactor systems, then refine those system designs in an iterative process. This so-called “agile” approach is a common theme as INL researchers approach MARVEL’s construction phase. In addition to ViBRANT/MACS, researchers have conducted the Primary Coolant Apparatus Test, which validated the thermal hydraulic performance of MARVEL’s novel liquid metal coolant system.

Making the models work

ViBRANT and MACS leverage the same computer software programs that researchers and reactor developers are using to develop the next generation of nuclear reactors. These models include RELAP, MCNP (Monte Carlo N-Particle) and the Multiphysics Object-Oriented Simulation Environment (MOOSE)  — codes that describe the highly complex interactions between neutrons, reactor materials, coolant flow and other phenomena that take place in a reactor core.

Translating the differential equations that govern neutron behavior into light particle physics required some complex computer coding. Baker developed a series of algorithms that relate ViBRANT’s light physics to the real nuclear reactor models. The ability to mimic the actual reactor-to-control-system relationship significantly increases the integrity of the control systems that are developed using MACS and ViBRANT.

“The models that were used to develop the MARVEL design have actually been used on this system,” Crawford said. “We take the light physics and relate it to what we’re expecting to see with the reactor. So, we essentially calibrate this surrogate reactor to act like the real reactor.”

Similar to how MACS has allowed researchers to fine-tune real-world hardware, MACS and ViBRANT also can help fine-tune the software. That’s because they operate using highly detailed, highly complex nuclear models.

A boon for reactor developers

In the end, MACS and ViBRANT could help developers test and refine control systems for the next generation of advanced reactors.

“Light water reactors have well-established software packages and control schemes because they’ve had decades of development,” Crawford said. “Advanced reactors have not had the same development opportunities. This is a way to interact with the control system and understand your physics and really appreciate the results.”.”

Plus, Crawford and his colleagues designed MACS/ViBRANT to be fine-tuned to different reactors’ capabilities.

“Right now, it’s configured for MARVEL,” Crawford said, “but that’s not to say that you couldn’t scale up for different power levels or different drum configurations or different sensor placements.”

That ability to tune to a wide range of reactor types significantly reduces risk for reactor developers as they embark on designs that have little operating experience.  . That includes artificial intelligence and machine learning capabilities that could someday help make reactor operation autonomous or semi-autonomous.

“You can push the envelope and investigate ways to optimize current operating scenarios and/or discover new operating possibilities,” Crawford said.