This paper presents the structural design and supporting analysis for the Microreactor Applications Research Validation and Evaluation (MARVEL) Primary Coolant System using the ASME Boiler and Pressure Vessel Code Section III, Division 5 rules. MARVEL is a liquid-metal-cooled microreactor intended to provide experimental capabilities for the rapid testing and development of microreactor technologies. The PCS utilizes high-temperature sodium-potassium liquid metal as the primary coolant and operates at a design temperature of 570?°C, necessitating the consideration of creep-related failure mechanisms. The PCS is constructed from 316H stainless steel for the base metal and 16-8-2 stainless steel for the weldments. The design approach incorporates the current base Code rules along with ASME Code Cases N-924, N-861, and N-862 to address primary load, ratcheting, and creep-fatigue evaluations, respectively. The reactor?s operation involves complex thermal and mechanical interactions due to natural convective flow and differential thermal expansion between components. The paper discusses the structural engineering challenges encountered, such as managing thermal stresses in the distribution plenum and guard vessel, and outlines the strategies implemented to meet the ASME Code requirements, including design modifications and operational constraints.
