The NuScale plant design includes features such as low power density, high coolant-to-power ratio, extremely reliable passive safety systems, robust containment, and immersion within the reactor building pool (ultimate heat sink). These features cause postulated core damage events to progress more slowly than in large, conventional nuclear reactors.
Slow accident progression provides more time for the short-lived radioactive isotopes to decay. Decay of short-lived radionuclides is therefore more likely to occur within the multiple boundaries incorporated into the NuScale power plant design.
The slow development of potentially damaging events allows greater time for an operator to respond effectively and take mitigating actions.