Principal Nuclear 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 Principal Nuclear Engineer to lead the interdisciplinary nuclear engineering work supporting Kaleidos. This role combines people leadership with hands-on technical ownership across core physics, neutron and radiation transport, shielding, activation and dose, thermal-hydraulics, safety analysis, instrumentation and controls, operations, and regulatory evaluations. You will guide work from requirements and early design trades through analysis, verification and validation, testing, and regulatory documentation. The ideal candidate brings deep expertise in neutron transport and Monte Carlo methods, along with the judgment to translate complex analysis into practical reactor design and operational decisions.Responsibilities & DutiesLead an interdisciplinary team working across reactor physics, core performance, shielding, dose and activation, thermal-hydraulics, safety analysis, I&C, and operations.Establish technical priorities, project plans, resource needs, and clear ownership across the nuclear engineering organization.Serve as a technical authority for neutron and radiation transport and Monte Carlo simulation methods.Own nuclear engineering projects from requirements capture and design iteration through model development, analysis, technical review, and final documentation.Guide analyses of criticality, reactivity control, shutdown margin, power distribution, depletion, temperature feedback, radiation transport, dose, activation, and shielding performance.Translate analytical results into actionable requirements and design decisions affecting the core, shielding, control systems, instrumentation, fluid systems, and structures.Establish technically rigorous practices for model verification and validation, peer review, configuration control, uncertainty evaluation, and documentation.Collaborate with reactor systems, thermal, mechanical, electrical, test, operations, safety, and regulatory teams to resolve multidisciplinary design challenges.Translate nuclear standards and regulatory requirements into analysis plans, design criteria, calculation packages, and verification activities.Support reactor testing and startup by defining analysis needs, instrumentation requirements, acceptance criteria, and methods for comparing predicted and measured performance.Identify technical risks, drive complex problems through closure, and communicate decisions and recommendations to engineering teams and company leadership.Mentor engineers through technical guidance, actionable feedback, and regular coaching.Required Qualifications & SkillsBachelor’s degree in nuclear engineering or a related technical discipline from an accredited institution.9+ years of relevant nuclear engineering experience across reactor design, analysis, testing, licensing, safety, or operations.Demonstrated experience leading complex nuclear engineering projects and multidisciplinary technical teams.Experience hiring, managing, mentoring, or technically leading nuclear engineers.Deep knowledge of neutron transport theory, Monte Carlo simulation methods, nuclear data, and statistical convergence.Professional experience developing and applying MCNP, OpenMC, or comparable neutron and radiation transport models.Strong understanding of criticality, reactivity coefficients, depletion, power distribution, shutdown margin, radiation transport, and uncertainty analysis.Experience translating analysis into reactor design decisions, technical requirements, safety conclusions, or regulatory documentation.Familiarity with model verification and validation, configuration control, peer review, and nuclear software quality practices.Strong written and verbal communication skills, including the ability to explain complex analyses, assumptions, uncertainties, and technical conclusions.Preferred Qualifications & SkillsMaster’s degree or Ph.D. in nuclear engineering or a related discipline.Experience with high-temperature gas-cooled reactors, microreactors, advanced reactors, or first-of-a-kind nuclear systems.Experience with SCALE, Serpent, Shift, Griffin, MOOSE-based applications, or similar analysis tools.Proficiency with Python or another language used for simulation automation and data post-processing.Experience with coupled neutronics and thermal-hydraulics or other multiphysics analyses.Familiarity with DOE authorization, NRC licensing, nuclear codes and standards, or regulatory safety analysis.Experience supporting reactor testing, commissioning, startup, or operational readiness.Additional RequirementsWilling to support extended hours when needed to meet critical program milestones.Must work 100% onsite at Radiant’s El Segundo headquarters.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: $195,713 - $262,500LocationEl Segundo, CAEmployment TypeFull timeLocation TypeOn-siteDepartmentEngineeringNuclear EngineeringCompensation$195,713 – $262,500 • Offers Equity