Senior Nuclear Thermal-Hydraulics Engineer

About RadiantRadiant 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 RoleRadiant is seeking a Senior Nuclear Thermal-Hydraulics Engineer to lead thermal-hydraulics and coupled physics analysis for the Kaleidos microreactor. You will own the modeling that determines how heat, coolant, and reactor core behavior interact under normal and accident conditions — work that directly informs core design, cooling systems, and safety case development for the first mass-produced nuclear reactor. This role is suited to engineers with reactor-specific thermal-hydraulics or coupled multiphysics experience who want to apply that depth to a fast-moving, factory-built reactor program.Responsibilities & DutiesOwn and execute thermal-hydraulics analysis to evaluate design and safety performance of Kaleidos reactor systems under normal and accident conditions.Develop and apply coupled physics models integrating thermal-hydraulics with neutronics and/or structural response.Identify and implement best practices for reactor thermal-hydraulics and systems-code analysis.Build automated workflows to expand simulation capabilities, accuracy, and efficiency.Collaborate across reactor physics, mechanical, and safety teams to support licensing and design trade studies.Required Qualifications & SkillsBachelor's degree in nuclear engineering, mechanical engineering, physics, or similar field from an accredited institution.5+ years of relevant industry or national laboratory experience.Experience with thermal-hydraulics analysis and/or reactor systems codes (e.g., RELAP5, TRACE, CTF/COBRA-TF, SAM, TRANSFORM).Experience with high-temperature (500F+) heat transfer analysis involving conduction, convection, and radiation.Experience communicating and distilling technical information with cross-functional teams.Desired Qualifications & SkillsExperience with coupled multiphysics modeling (e.g., MOOSE, BISON, MARMOT) or neutronics coupling.Experience with commercial CFD/FEA tools (STAR-CCM+, Ansys Fluent) and CAD (Siemens NX).Prior experience at a national laboratory, reactor vendor, or advanced reactor developer.Driven, self-motivated, and takes ownership of work end to end.Additional RequirementsMust be willing to work extended hours and weekends as necessary to accomplish our mission.Must work 100% onsite at El Segundo HQ.Benefits and PerksStock: Substantial incentive stock plan for all full-time employees.Medical: 100% up to base silver level plan for employee + 50% dependents, offers up to Platinum plans.One Medical: Sponsored memberships for employees and their dependents.Vision: 100% top tier plan coverage + 50% for dependents.Dental: 100% top tier plan coverage, orthodontia extra, 50% for dependents.Voluntary life, accident, hospital, critical illness, commuter and FSA/HSA are offered as employee contributed benefits.8-weeks of paid parental leave for all parents. Additional paid pregnancy leave for CA employees.Daily catered lunch. Free snacks and drinks.Flexible PTO policy. Remote workday allocation.Company and team-bonding events, happy hours and in-person camaraderie.Beautiful El Segundo headquarters close to the Pacific Ocean.Total Compensation and BenefitsRadiant compensates people for impact and invests in those who continue to raise the bar. Radiant’s new hire compensation package includes base salary, substantial equity grants, and comprehensive health benefits. Total compensation and level are determined through a rigorous evaluation of interview performance, experience, education, and qualifications, and are designed to support continued growth as scope, responsibility, and impact expand at Radiant.The best of today’s advanced reactor builders don’t wait for job postings. They join the winning team. If that’s you, contact us directly for immediate opportunities: View email address on click.appcast.io are an equal opportunity employer and all qualified applicants will receive consideration for employment without regard to race, color, religion, sex, sexual orientation, gender identity or expression, pregnancy, age, national origin, disability status, genetic information, protected veteran status, or any other characteristic protected by law.This position involves access to technology that is subject to U.S. export controls. Any job offer made will be contingent upon the applicant’s capacity to serve in compliance with U.S. export controls.Compensation Range: $139.5K - $224.7KLocationEl Segundo, CAEmployment TypeFull timeLocation TypeOn-siteDepartmentEngineeringMechanical EngineeringCompensation$139.5K – $224.7K • Offers Equity