The containment structure is the outermost engineered safety barrier in a nuclear power plant, designed to withstand internal pressurization from a design-basis accident (such as a loss-of-coolant accident) and external hazards (including seismic events, tornado missiles, and aircraft impact) while preventing the release of radioactive fission products to the environment. In conventional large PWRs and BWRs, containment buildings are massive reinforced concrete structures with steel liners, typically 40-60 meters in diameter and over 60 meters tall, representing a significant fraction of total plant construction cost. The containment must maintain its leak-tight integrity throughout any credible accident sequence, functioning as the final line of defense in the defense-in-depth safety philosophy.
SMR designs are fundamentally rethinking the containment paradigm. NuScale's VOYGR modules use a compact steel containment vessel that is vacuum-sealed around each reactor module and submerged in a below-grade pool, dramatically reducing the containment volume and pressure requirements compared to conventional plants. The BWRX-300 from GE-Hitachi uses a steel-plate composite containment that integrates structural concrete with steel plates, reducing construction time and enabling factory fabrication of major sections. Holtec's SMR-300 and other light-water SMR designs similarly pursue reduced containment volumes enabled by lower core decay heat, smaller radioactive source terms, and passive safety systems that limit the energy release during design-basis events.
For advanced non-light-water reactors, the containment design philosophy diverges even further from conventional practice. TRISO-fueled reactors like X-energy's Xe-100 argue that the fuel itself provides a primary containment function, with each TRISO particle acting as a micro-containment system, potentially enabling a confinement structure (rather than a pressure-retaining containment) that reduces construction cost and schedule. TerraPower's Natrium operates at atmospheric pressure with sodium coolant that has a very high boiling point, eliminating the high-pressure scenarios that drive containment design in water-cooled reactors. The NRC's evaluation of containment requirements for advanced reactors is a critical aspect of both ongoing licensing reviews and the development of the Part 53 framework, where risk-informed approaches may allow containment functional requirements to be tailored to each design's specific accident phenomenology rather than applying prescriptive requirements derived from light-water reactor experience.