Principal Nuclear Engineer – Lead Reactor Physics (Onsite)

Radiant industries; El Segundo (CA); On-site; USD 180,000 - 260,000; Full time

Benefits offered by this job

Stock plan

Medical coverage

One Medical

Vision plan

Dental plan

Parental leave

Lunch provided

Flexible PTO

Team events

Job summary

Radiant, an El Segundo, CA-based startup, seeks a Principal Nuclear Engineer to lead interdisciplinary nuclear engineering work for Kaleidos. You will own analyses across physics, shielding, and safety, translating results into practical reactor design decisions and regulatory documentation.

The role requires strong leadership, 9+ years in nuclear engineering, and deep expertise in neutron transport and Monte Carlo methods. Onsite work at Radiant’s headquarters in California is required.

Qualifications

• Bachelor’s degree in nuclear engineering or related technical discipline.
• 9+ years of nuclear engineering experience across reactor design, analysis, testing, licensing, safety, or operations.
• Experience leading multidisciplinary teams on complex nuclear projects.
• Experience hiring, mentoring, or technically leading nuclear engineers.
• Deep knowledge of neutron transport theory, Monte Carlo methods, nuclear data, and statistical convergence.
• Experience developing and applying MCNP/OpenMC or comparable transport models.
• Strong understanding of criticality, reactivity coefficients, depletion, power distribution, shutdown margin, and uncertainty analysis.
• Ability translating analysis into reactor design decisions, safety conclusions, or regulatory documentation.
• Familiarity with model verification, configuration control, peer review, and nuclear software quality practices.
• Excellent written and verbal communication skills to explain analyses and uncertainties.

Responsibilities

• Lead an interdisciplinary team across reactor physics, core performance, shielding, dose and activation, thermal-hydraulics, safety analysis, I&C, and operations.
• Establish technical priorities, project plans, resources, and ownership across the nuclear engineering org.
• Serve as technical authority for neutron and radiation transport and Monte Carlo methods.
• Own projects from requirements capture to design iteration, analysis, and final documentation.
• Guide analyses of criticality, reactivity, depletion, power distribution, and shielding performance.
• Translate results into actionable design decisions for core, shielding, controls, and instrumentation.
• Establish rigorous practices for verification, peer review, configuration control, and documentation.
• Collaborate with reactor systems, thermal, mechanical, electrical, test, safety, and regulatory teams.
• Translate standards into analysis plans, calculation packages, and verification activities.
• Support reactor testing and startup with analysis needs, instrumentation requirements, and acceptance criteria.
• Identify risks and mentor engineers through guidance and coaching.

Skills

Neutron transport theory

Monte Carlo methods

Technical leadership

Communication skills

Nuclear data

Education

Bachelor’s degree in nuclear engineering or related field

Master’s or PhD in nuclear engineering (preferred)

Tools

MCNP

OpenMC

Nuclear engineering software quality practices

Job description

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 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 & Duties

• Lead 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 & Skills

• Bachelor’s degree in nuclear engineering or a related technical discipline from an accredited institution.

• 12+ 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 & Skills

• Master’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 Requirements

• Willing to support extended hours when needed to meet critical program milestones.

• Must work 100% onsite at Radiant’s El Segundo headquarters.

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.

• 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 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: [e-mail].

We 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.