Criticality nears for US MARVEL nuclear reactor after safety approval

The US Department of Energy (DOE) has approved the Preliminary Documented Safety Analysis (PDSA) for the MARVEL microreactor, a critical safety blueprint that enables the project to proceed with its initial nuclear physics experiments at Idaho National Laboratory (INL). This approval marks a transition from design to demonstration, allowing MARVEL to conduct its first “dry criticality” test—a near zero-power experiment to validate reactor physics before full-power operation. The PDSA confirms that the reactor can operate safely in this configuration and sets the stage for final safety documentation and full assembly. MARVEL is a sodium-potassium-cooled microreactor designed to produce 85–100 kilowatts of thermal energy and about 20 kW of electricity. Despite its small size, its regulatory framework is expected to influence future advanced nuclear projects significantly. Notably, MARVEL employs an innovative Reactivity Control System that replaces traditional vertical control rods with rotating drums, enabling precise control of neutron reflection and absorption. This