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ABOUT RADIANT Radiant is an El Segundo, CA-based startup building the world’s first mass-produced, portable nuclear microreactors. The company’s first reactor, Kaleidos, is a 1-megawatt, fail-safe microreactor that can be transported anywhere power is needed and run for up to 5 years without refueling. Portable nuclear power with rapid-deploy capability can replace similar-sized diesel generators and provide critical asset support for hospitals, data centers, remote sites, and military bases. Radiant’s unique, practical approach to nuclear development leverages modern software engineering to rapidly deliver safe, factory-built microreactors that use existing, well-qualified materials. Founded in 2020, Radiant is on track to test its first reactor at the Idaho National Laboratory this summer, with initial customer deliveries beginning in 2028. ABOUT THE ROLE Radiant is seeking a Principal Turbomachinery Engineer to serve as the Responsible Engineer for the turbomachinery within our nuclear-driven power generation system. This role owns the mechanical design, integrity, and performance of the core rotating assembly at the heart of our 1MW closed-loop Brayton cycle. You will define and drive the mechanical architecture of high-speed rotating hardware operating at extreme temperatures, pressures, and rotational speeds. As a deeply technical individual contributor, you will set direction for rotor dynamics, structures, bearings, seals, and system integration while working closely with electrical, thermal, and manufacturing teams. This is a hands-on, high-ownership role focused on technical excellence, sound engineering judgment, and delivering reliable, production-ready turbomachinery. Responsibilities & Duties: Technical Ownership & System Architecture - Serve as Responsible Engineer for the turbomachinery mechanical system. - Own the mechanical architecture and design strategy for high-speed rotating assemblies (up to ~30 krpm). - Lead rotor dynamics, structural integrity, and life analysis for rotating components and assemblies. - Define and flow down requirements from system-level constraints including thermal, structural, and operational environments. - Make key technical decisions on rotor layout, shafting, bearings, seals, materials, and clearances. - Ensure designs meet performance, reliability, safety, and regulatory requirements. - Establish and uphold standards for mechanical design rigor, analysis quality, and documentation. Mechanical Design & Analysis - Lead detailed design of turbomachinery components including rotors, shafts, housings, and support structures. - Test and validate rotordynamic systems, driving model correlation, risk reduction, and performance verification. - Perform and review structural and thermal analysis to ensure integrity under high-speed and high-temperature operation. - Define tolerances, fits, stack-ups, and alignment strategies for precision rotating assemblies. - Partner with electrical engineering on alternator rotor integration, including electromagnetic and structural coupling. Cross-Functional Integration - Partner closely with turbomachinery aero, motor design, thermal, manufacturing, and test teams. - Ensure seamless integration between turbine, compressor, and alternator subsystems. - Drive clear interface definitions across mechanical, electrical, and thermal boundaries. - Collaborate with manufacturing to ensure designs are producible, inspectable, and scalable. - Engage with suppliers on technically critical components and design decisions. Prototyping, Testing & Validation - Support assembly, integration, and test of turbomachinery hardware. - Define validation strategies including spin testing, thermal testing, and system-level operation. - Lead root cause analysis and drive design iteration for test failures or performance gaps. - Remain hands-on in hardware bring-up, troubleshooting, and refinement. Technical Inf