Roadmap to Right-Size Emergency Planning Zones for Microreactors

Microreactors, as a class of advanced nuclear technologies, offer reduced power output, compact footprints, passive safety systems, and inherent modularity, features that fundamentally alter the basis for emergency planning zone (EPZ) sizing. Traditional EPZ frameworks, developed for large light water reactors, may be overly conservative when applied to microreactors. A risk-informed, performance-based approach is required, one that reflects the significantly lower source term, simplified accident progression, and limited off-site radiological impact of these designs. NUREG-0396, Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants (1978), established a two-zone EPZ model: a 10-mile plume exposure zone and a 50-mile ingestion pathway zone. Importantly, it allows for scaling based on a “realistic appraisal of accident consequences” (Sec. IV), thus supporting a graded, risk-informed methodology particularly relevant for microreactors. This approach has precedent. NuScale Power demonstrated through detailed deterministic and probabilistic safety analyses that its design warranted a reduced-size EPZ. NuScale’s use of mechanistic source term modeling and level 3 probabilistic risk assessment showed minimal off-site consequences, supporting the approval of a site-boundary EPZ by the Nuclear Regulatory Commission. Similarly, X-Energy’s Xe-100 high-temperature gas-cooled reactor incorporates robust tristructural isotropic fuel and passive decay heat removal systems, justifying a reduced EPZ through source term and dose consequence modeling consistent with Protective Action Guide (PAG) thresholds. The current paper presents a structured framework for EPZ sizing tailored to microreactors, grounded in NUREG-0396 and supported by regulatory guidance (e.g. RG 1.183, NUREG-0800) and recent licensing precedent. It aims to facilitate scalable, risk-aligned emergency planning for microreactors deployed in remote, off-grid, or specialized defense environments.