US: MARVEL nuclear reactor closer to achieving ‘criticality’ with key safety approval
The US Department of Energy (DOE) has officially greenlit a critical safety blueprint for the MARVEL microreactor. This clears the path for the project to begin its first nuclear physics experiments at Idaho National Laboratory (INL).
The approval of the Preliminary Documented Safety Analysis (PDSA) marks a definitive shift from design to demonstration.
This document serves as the “safety basis” for MARVEL’s initial “dry criticality” test, which is a near zero-power experiment designed to validate how the reactor’s physics behave before it transitions to full-power operation.
“The PDSA affirms that the reactor can operate safely under this configuration and paves the way for final safety documentation and full assembly of the reactor,” said INL in a press release.
A “blueprint” for the Industry
MARVEL is a sodium-potassium-cooled reactor designed to generate 85–100 kilowatts of thermal energy and roughly 20 kW of electricity. While small in scale, its regulatory impact is expected to be massive.
“This is more than just a regulatory requirement—it’s a blueprint for the future of advanced nuclear,” added Abdalla Abou-Jaoude, the MARVEL microreactor lead at INL.
“By receiving approval for our safety documentation, we are now able to share this template with developers to learn from our process and streamline their own timelines.”
The project’s risk-informed methodology has already begun influencing other high-profile DOE-authorized projects, including Pele, the Molten Chloride Reactor Experiment, and VALKRE.
Precision control with swapping rods for drums
The safety approval follows a significant engineering milestone in how the reactor is controlled.
In a departure from decades of nuclear tradition, INL researchers have successfully tested a Reactivity Control System (RCS) that ditches bulky vertical control rods in favor of high-precision rotating drums.
Standard nuclear plants typically regulate power by sliding rods in and out of the core. MARVEL, however, utilizes four drums positioned on the core’s periphery. The reflective side of the drum bounces neutrons back into the core to increase power, while the absorber side soaks up neutrons to slow or stop the reaction.
“The control drum and control rod have the same basic functions—one translates, one rotates,” explained Anthony Crawford, a researcher in INL’s mechatronics group.
This rotating design allows operators to fine-tune the reactor’s output with extreme precision, a necessity for the compact nature of microreactors.
Identifying potential issues early
The upcoming “dry criticality” experiment will be conducted at INL’s Transient Reactor Test Facility. This phased approach allows researchers to identify potential issues early before moving toward final assembly and fuel loading.
“We’re proud to play a leading role in helping to make microreactors commercially available,” remarked John Jackson, national technical director for the DOE Office of Nuclear Energy’s Microreactor Program.
With the safety documentation secured and the control systems tested, the team is now finalizing the requirements for full reactor assembly, bringing the prospect of portable, carbon-free nuclear power one step closer to reality.
“The next phase includes engagement with DOE-Idaho and project stakeholders to finalize the safety basis for full reactor assembly and fuel loading,” concluded the press release.