An availability-centric Q-metrics framework for assessing fusion power-plant engineering viability

As fusion development advances toward DEMO and a first-of-a-kind power-plant, engineering operability and availability increasingly dominate overall system performance. Plasma physics gain metrics, such as Q PHYSICS , demonstrate scientific feasibility but do not capture the maintenance, inspection, recovery, and operational constraints inherent to nuclear fusion facilities. This paper introduces an availability-centric engineering metric, Q ENG , defined as the ratio of achievable operational availability to an Engineering Burden Index (EBI). The EBI is formulated as a normalised, dimensionless indicator representing the aggregate engineering and operational load associated with maintenance access, remote handling, inspection requirements, and recovery logistics. Anchored to a DEMO-class fusion plant baseline, Q ENG enables architecture-agnostic comparison of fusion power-plant concepts without reliance on detailed cost modelling or speculative performance assumptions. Application of the framework to representative tokamak, stellarator, and inertial fusion energy concepts shows that projected increases in availability are frequently accompanied by higher engineering burden, limiting overall operability. The results highlight the central role of maintainability-by-design and engineering burden reduction in advancing fusion concepts toward reliable, power-plant relevant operation. The framework naturally extends to the definition of Q GLOBAL , supporting integrated assessment of physics performance and engineering maturity within a common evaluative framework.