Unified Technology Readiness Level (TRL) advancement criteria for space nuclear systems

Abstract A unified set of TRL criteria for space nuclear systems, including nuclear electric propulsion (NEP), is presented, reconciling definitions and differences found in NASA, U.S. Department of Defense (U.S. Department of War (DoW) [as of 5 Sept 2025]) (DoD), and U.S. Department of Energy (DOE) guidance documents. The intent is to provide a common framework for consistent TRL assessment across government agencies and industry partners, thereby reducing ambiguity, limiting overstatement of readiness, and supporting programmatic decisions for advancing space nuclear technologies toward flight readiness. Detailed definitions are provided for each level from TRL 1 through TRL 9, with accompanying explanations and examples on the intent and evidence that should be presented to demonstrate advancement. Definitions and explanations emphasize hardware fidelity, software maturity, integration level, environments, lifetime considerations, and validation against analytical predictions and modeling and simulation results.