Senior Reactor Systems 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

We are seeking a Senior Reactor Systems Engineer to own the technical integration of a first-of-a-kind reactor. With no operating predecessor to copy and no mature supply chain, you will be the person who holds the whole machine together, making sure hundreds of design decisions across nuclear physics, thermal-hydraulics, controls, structures, materials, fuel, and licensing add up to a reactor that works safely and is delivered to customers on time.

This position goes beyond traditional systems engineering. You will be hands-on technically, deep enough to challenge a thermal model or a pump spec, while serving as the connective tissue of the engineering organization. You will lead through credibility rather than title, aligning people who don't report to you and keeping a fast-moving program coherent. Writing is a core part of the job: specifications, decision memos, and system descriptions are how our team stays aligned.

Responsibilities & Duties

• Translate between reactor physics, thermal-hydraulics, materials, controls, mechanical design, manufacturing, and regulatory teams, surfacing assumptions and tensions that only appear at the boundaries.

• Own libraries of P&IDs, sensors, and subsystem specifications to ensure alignment from design and analysis through reactor operations.

• Identify cross-cutting risks early and drive them to resolution before they become safety or schedule problems.

• Work with safety analysis and licensing teams to keep design decisions, safety cases, and regulatory submissions consistent, and evaluate design changes for their safety and licensing implications.

• Support development of the design basis, safety classification, and technical specifications for regulatory engagement and operator training.

• Support the path from component and subsystem prototypes to integrated testing and commissioning, defining test plans and success criteria and feeding results back into the design.

• Maintain the technical baseline (requirements, configuration, change control, risk registers, and documentation) using lightweight processes scaled to a fast-moving startup.

• Communicate clearly to engineers, leadership, regulators, and customers.

Required Qualifications & Skills

• BS in nuclear, mechanical, aerospace, systems, or a related engineering discipline.

• 6+ years of full-time professional engineering experience, including meaningful time in systems engineering, integration, or technical leadership on complex hardware.

• Demonstrated ability to work effectively across multiple engineering disciplines.

• Track record of leading cross-functional technical efforts without direct authority.

• Strong written and verbal communication, including producing clear technical documents and presenting to varied audiences.

• Excellent organizational and administrative skills: you keep complicated work moving, track open items, and follow through.

• Ability to apply first-principles thinking and make sound decisions amid ambiguity, incomplete data, and changing requirements.

• Background in highly regulated or high-consequence industries (e.g., nuclear, aerospace, defense, advanced energy, medical devices, semiconductor equipment).

Desired Qualifications & Skills

• Experience on first-of-a-kind or early-stage hardware programs, ideally from prototype through qualification.

• Familiarity with nuclear reactor design, safety, and regulatory frameworks (e.g., 10 CFR 50/52/53, NQA-1, GDCs, defense in depth).

• Startup or small-team experience where you built the process rather than just followed it.

Additional Requirements

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

• Eligibility to work in the U.S. and ability to meet export control requirements.

Benefits and Perks

• Stock: Substantial incentive stock plan for all full-time employees.

• Daily catered lunch. Free snacks and drinks.

• Flexible PTO policy. Remote workday allocation.