What makes a reactor “modular”?
SMRs are smaller (under ~300 MWe) reactors built in factories and shipped as units, betting that repetition and standardization beat the cost overruns of giant one-off plants.
A small modular reactor is exactly what the name says: a nuclear reactor that is small (typically under about 300 MWe) and modular — built in a factory as standardized units and shipped to site, rather than constructed piece by piece as a giant one-off. The bet is that repetition and assembly-line manufacturing can tame the cost overruns and delays that have plagued large nuclear builds.
The designs vary widely, from shrunken conventional light-water reactors to high-temperature gas-cooled and molten-salt concepts. What they share is the industrial logic: standardize the unit, build many, and let learning-by-doing drive the price down over a series rather than betting everything on one enormous project.
Whether that logic holds in practice is the central open question. A first-of-a-kind SMR is still a first-of-a-kind nuclear project, with all the licensing and construction risk that implies. The payoff only shows up once the tenth or fiftieth unit is cheaper than the first — which is why the gap between announced designs and reactors actually running matters so much.