MARVEL Microreactor Project Achieves Key Milestones Towards Criticality
The MARVEL (Microreactor Applications Research Validation and Evaluation) microreactor project has achieved several key milestones as researchers prepare for initial startup tests. The primary coolant system (PCS) arrived at the Idaho National Laboratory (INL) in mid-September, completing its fabrication and delivery. This component is crucial for the microreactor to achieve criticality, a stable nuclear chain reaction necessary for full operation. The PCS will be integrated with the reactor's other systems at INL's Transient Reactor Test Facility, with criticality anticipated in December.
The PCS is designed to circulate heat during operation, using a sodium-potassium alloy as a coolant. This alloy remains liquid at room temperature and efficiently transfers heat at high reactor temperatures without requiring pressurization. The MARVEL microreactor employs a two-loop natural circulation system for thermal energy transfer. Its core consists of 36 solid fuel rods, each containing five uranium hydride fuel meats, arranged in a hydrogen-moderated, beryllium-reflected configuration. Fission heats 120 kg of eutectic sodium-potassium (NaK) alloy in the primary loop, which then transfers heat to four secondary loops containing eutectic gallium–indium–tin (eGaInSn) liquid metal via four intermediate heat exchangers (IHXs). Each secondary loop connects to a commercial Stirling engine, generating 5 to 7 kWe, for a total output of up to 20 kWe. The system operates entirely by natural circulation, eliminating the need for pumps. The primary circuit is enclosed in a stainless steel 316H vessel, rated to ASME Section III Division 5 standards. Active reactivity control is managed by rotating beryllium oxide drums clad with neutron-absorbing boron carbide, while a central insurance absorber (CIA) rod provides shutdown capability. The reactor also features passive decay heat removal.
Prior to the PCS delivery, the Idaho Operations Office of the Department of Energy (DOE) approved MARVEL's final documented safety analysis (FDSA). This safety document authorizes a dry criticality configuration, which will generate data to serve as a national benchmark for reactor physics. Dry criticality refers to the critical condition achieved with near-zero power generation, thus not requiring coolant. Abdalla Abou-Jaoude, the MARVEL microreactor lead, expressed satisfaction with both the PCS fabrication and the DOE approval, which facilitates progress toward initial criticality. Doug Gerstner, a nuclear safety engineer at INL, noted that the documented safety analysis represents years of engineering and safety analysis, validating the risk-informed methodology developed by the team. John Jackson, national technical director for the DOE Microreactor Program, acknowledged the dedication of the MARVEL team, INL leadership, and DOE in reaching this milestone.