MARVEL Microreactor System Thermal-Hydraulic Design and Analysis

The MARVEL microreactor is an 85-kW(thermal), Nak-cooled, thermal-spectrum, single-phase natural circulation system under development at the Idaho National Laboratory to demonstrate autonomous operation, microgrid integration, and passive safety performance. This paper presents the thermal-hydraulic design basis, analytical evaluation, stability assessment, and RELAP5-3D system modeling that collectively establish the reactor’s operating envelope. The primary coolant system consists of four parallel natural circulation loops that provide a predicted steady state NaK mass flow rate of 1.5 kg/s, yielding a core temperature rise of about 70°C at nominal operating temperatures near 500°C. Due to the tightly packed fuel bundle geometry (P/D = 1.056), the core exhibits reduced heat transfer performance, with average Nusselt numbers of about 3.0 at hot full power, necessitating dedicated modeling corrections. Linear stability analyses demonstrate stable natural circulation behavior across laminar, transitional, and turbulent regimes. The RELAP5-3D evaluation model predicts robust steady-state operation with substantial thermal margins. A set of bounding beyond extremely unlikely transients—including unprotected transient overpower, loss of heat sink, loss of flow, and lower plenum gas ingress—confirms the system’s inherently self-limiting response. Even under conservative assumptions, peak clad temperatures remain below the 764°C limit. The results demonstrate that MARVEL maintains large thermal margins, stable natural circulation performance, and robust passive heat removal capability, providing a validated technical foundation for microreactor deployment and future design development.