Idaho National Laboratory researchers have built the first microreactor control system in decades, replacing the traditional sliding control rods found in conventional nuclear plants with a set of rotating drums that fine-tune reactor power by turning reflector and absorber materials toward or away from the core. The system was designed for MARVEL, a U.S. Department of Energy microreactor that uses four compact control drums alongside a single backup control rod. The achievement highlights a renewed push to modernize how some U.S. microreactor designs manage reactivity, and it draws on control concepts also explored in compact reactor work for remote power and space applications.
How Rotating Drums Replace Sliding Rods
For decades, commercial reactors have controlled fission by raising or lowering neutron-absorbing rods into the core. MARVEL breaks from that pattern. Its compact reactivity hardware relies on four drums that rotate in place, presenting either a neutron-reflecting surface or an absorbing surface to the reactor core depending on the angle. The geometry matters: because a drum can be positioned at any rotational angle rather than simply up or down, operators gain finer control over how many neutrons sustain the chain reaction at any given moment, while the surrounding reflector structure helps keep the entire assembly small enough to transport.
INL engineer Ben Crawford put the distinction plainly. “The control drum and control rod have the same basic functions. One translates, one rotates,” Crawford said. That rotational motion can avoid some of the vertical channels and drive mechanisms that conventional rods require, which designers say can help shrink the overall reactor package and reduce the number of moving parts that must penetrate shielding. MARVEL still carries a separate control rod as an insurance absorber, a redundant safety layer, but the drums handle routine power adjustments. The design borrows from earlier test-reactor practice, where drum-style mechanisms have long been used to shape neutron flux in experimental cores, and adapts that approach for a power-producing unit that must run reliably in the field.
Space Reactors and the Drum Design Heritage
MARVEL’s drums did not emerge from a blank page. Compact reactor concepts developed for space exploration helped establish the engineering logic behind reflector-based control. The Kilopower space reactor program, a joint effort between Los Alamos National Laboratory and NASA Glenn, documented detailed material trade studies covering fuel selection, reflector materials like beryllium oxide, and heat pipe placement for small fission systems. Kilopower’s baseline concept used a central control rod rather than drums, but its emphasis on tight reflector geometry, passive heat removal, and low-maintenance operation fed directly into the design thinking that microreactor engineers now apply on the ground as they seek to balance compactness with robust shutdown capability.
That lineage is extending further. A NASA Small Business Innovation Research program is developing passive criticality controls for Nuclear Thermal Propulsion, or NTP, with the goal of reducing reliance on active mechanical drives in favor of self-actuating components that respond automatically to temperature or power changes. If successful, passive reflector-based controls could reduce reliance on active mechanical drives aboard spacecraft, where maintenance crews are not an option and where every kilogram of hardware must justify its launch cost. The overlap between space and terrestrial microreactor needs is not coincidental: both demand reactors small enough to transport, simple enough to operate with minimal staff, and safe enough to function without large external power or cooling systems.
Not Every New Reactor Is Ditching Rods
The drum approach suits MARVEL’s scale and mission, but it would be a mistake to assume every advanced reactor in the U.S. pipeline is following the same path. TerraPower’s Natrium plant, a sodium-cooled fast reactor backed by significant federal funding, retains conventional rod-based control. Its licensing material describes rod-based control and shutdown functions, according to documents filed with the Nuclear Regulatory Commission. Gravity scram, in which rods fall into the core by their own weight if power is lost or a trip signal is received, is a proven passive safety feature that larger reactor designs continue to rely on to ensure a rapid shutdown without depending on pumps or motors.
The split reflects a practical reality: control drums are typically discussed for very small cores where reflector-to-fuel geometry can make fine rotational adjustments more influential on reactivity. Larger cores with higher power output still benefit from the deep insertion capability and straightforward mechanical layout of linear rods, especially when many separate shutdown banks must be coordinated. Rather than a wholesale industry migration away from rods, what is happening is a branching of control strategies matched to reactor size and application. Microreactors destined for remote military bases, mining sites, or disaster relief zones gain the most from drum compactness and simplified drive systems. Utility-scale advanced reactors like Natrium, designed to feed hundreds of megawatts into the grid, face different engineering constraints that rod-based systems handle well, including more complex fuel loading patterns and higher total reactivity worth that must be managed over long operating cycles.
Licensing Pipelines Are Catching Up
U.S. regulators are beginning to build experience reviewing a wider range of reactor hardware. Kairos Power’s Hermes test reactor, which uses molten salt as its coolant, received a construction permit from the Nuclear Regulatory Commission on December 14, 2023, after an application submitted on September 29, 2021. That permit was the first modern authorization the agency has issued for a non-water-cooled reactor, a milestone that opened the door for other unconventional designs to follow. The Hermes project is not itself a drum-controlled microreactor, but its progress through the licensing process shows the NRC has issued modern licensing decisions for designs that depart from the light-water reactor template that has dominated U.S. commercial nuclear power for decades.
National laboratories are helping smooth that path. Argonne researchers, for example, have examined microreactor deployment issues ranging from transport logistics to safeguards, providing technical groundwork that regulators can draw on when assessing novel control schemes such as rotating drums. Their analyses highlight how factory fabrication, sealed-core operation, and minimal on-site staffing change the risk profile compared with traditional plants, and why control systems must be designed with both cybersecurity and physical access limits in mind. As more designs like MARVEL move from paper to hardware, that body of work will shape how the NRC and other authorities judge whether drum-based controls, backup rods, and passive features together provide the same or better safety margins as the familiar fleets of large, rod-controlled reactors.
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*This article was researched with the help of AI, with human editors creating the final content.
