The NRC is amending its regulations to renew the AP1000 design certification and incorporate Revision 20 of the AP1000 design control document, updating the certified design to match changes made during the licensing, construction and operation of Vogtle Units 3 and 4.

A certified design is what an applicant references instead of re-arguing the reactor from first principles. Renewing it keeps that shortcut available, and incorporating Revision 20 changes what the shortcut contains.

The agency's own phrase is the useful one: Revision 20 represents a current, construction-validated design. The certified document now reflects a plant somebody built.

A direct final rule is the path used when the agency expects no significant adverse comment. It takes effect on its own date unless such comment arrives, in which case the rule is withdrawn and notice of the withdrawal is published.