Principal Nuclear Engineer

About Radiant Radiant is an El Segundo, CA-based startup building the world’s first mass-produced, portable nuclear microreactors. Rising energy demand is straining the U.S. grid and driving up utility costs, emissions, and interconnection backlogs. Radiant decentralizes energy, producing power close to where it’s used. Its first reactor, Kaleidos, is a factory-built, 1-megawatt, fail-safe microreactor that can be transported by land, sea, or air and runs up to 5 years without refueling.

Deployed locally without a decade-long build or permitting fight, Kaleidos provides independent power for critical infrastructure like military bases, data centers, hospitals, and remote sites. Founded in 2020, Radiant is under contract to deliver power to the U.S. Army, Air Force, Navy, and has 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…