Principal Thermal-Hydraulics Engineer

About Radiant

Radiant is an El Segundo, CA-based company building the world's only mass-produced, transportable nuclear microreactors. The U.S. grid is under growing strain, with power demand accelerating, driving higher utility costs, rising carbon emissions, and interconnection backlogs. Radiant has a solution, transportable, on-site power. Our first reactor, Kaleidos, is 1 megawatt: small enough to be factory-built, transported by land, sea, or air, and deployed wherever power is needed. 1 MW can power 1000 homes for five years. Ship it back and we refuel for a 20 year operating life. We made energy a product that can be used for critical infrastructure like military bases, data centers, hospitals, and remote sites, as well as the commercial main street businesses that keep America running. Founded in 2020, Radiant is under contract to deliver power to the U.S. Army, Navy and Air Force and has signed pre-order agreements with Equinix, with deployments beginning in 2028.

About the Role

Radiant is seeking a Principal Thermal-Hydraulics Engineer with deep expertise in CFD and thermal FEA to set the technical direction for thermal and fluid simulation of the Kaleidos microreactor. You will lead the most complex and consequential modeling problems for a high-temperature gas reactor, defining the methods, standards, and validation strategy that inform decisions across reactor systems, cooling systems, shielding, and more. As a recognized technical authority, you will mentor engineers, influence program-level decisions, and help drive the milestones that unlock commercialization of the first mass-produced nuclear reactor.

Responsibilities & Duties

• Serve as the technical authority for thermal CFD and thermal FEA modeling of Kaleidos reactor systems, owning the most complex and highest-risk analyses.

• Define modeling strategy, methodology, and best practices for turbulent flow, heat transfer, and multi-physics interactions, and drive their adoption across the team.

• Lead model verification and validation, correlating simulation results with test data and establishing confidence and uncertainty bounds for design and safety decisions.

• Architect automated simulation workflows and tooling that expand capability, accuracy, and efficiency, and set the roadmap for simulation infrastructure.

• Lead the development of reduced-order analysis methods in partnership with design, systems, and test teams.

• Influence reactor, cooling, and shielding design decisions by translating analysis results into clear recommendations for engineering leadership.

• Mentor and develop engineers, set the technical bar through design and analysis reviews, and contribute to hiring.

• Collaborate across disciplines to define analysis requirements, align priorities, and resolve cross-team technical tradeoffs.

Required Qualifications & Skills

• Bachelor’s degree in engineering, physics, or a similar field from an accredited institution; advanced degree (MS or PhD) preferred.

• 10+ years of relevant industry experience, with a track record of technical leadership on complex, high-consequence programs.

• Extensive experience with STAR-CCM+, Ansys Fluent, or other commercial CFD codes, including advanced turbulence and conjugate heat transfer modeling.

• Mastery of finite element and computational fluid dynamics principles and methods, with demonstrated experience validating models against test data.

• Proficiency with CAD software (Siemens NX).

• Experience building automated simulation workflows and scripting tools.

• Demonstrated ability to mentor engineers and communicate complex technical results to cross-functional teams and leadership.

Desired Qualifications & Skills

• Experience with radiation heat transport modeling.

• Knowledge of nuclear reactor design, thermal-hydraulics, or safety analysis.

• Experience with multi-physics coupling, such as thermal-structural or neutronics-thermal.

• Experience with reduced-order modeling or system-level thermal-hydraulic codes.

• Experience with regulatory or safety-basis analysis.

• Driven, self-motivated, and takes ownership of work end to end.

• Prior nuclear experience is not required. Applicants with former or current aerospace or automotive industry experience are encouraged to apply.

Additional Requirements

• Must be willing to work extended hours and weekends as necessary to accomplish our mission.

• Must work 100% onsite at El Segundo HQ.

Benefits and Perks

• Stock: 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.

• Nuclear Engineering Certificate Program: 100% covered educational program with Penn State University, for eligible employees, to rapidly upscale into nuclear.

• 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 Benefits
Radiant compensates people for impact and invests in those who continue to raise the bar. Radiant’s full-time 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.

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.