Bringing a reactor into service

A new reactor passes through three milestones that often get confused. First criticality is the moment the core first sustains a self-sustaining chain reaction: it is a physics test, with no generator yet connected to the grid, and it happens weeks before either of the other two dates.

Grid connection is the generator's first synchronization with the electrical grid: the unit is already delivering electricity, but still under start-up testing — rising-power trials, instrumentation checks — before it runs in commercial mode.

Commercial operation is the date the operator declares the unit in regular service, the one it bills as commercial generation. It usually comes weeks or months after grid connection, while power-ascension testing is completed.

The IAEA's PRIS records all three dates for every unit — first criticality, first grid connection and commercial operation — but the one that separates "under construction" from "operating" in its statistics is grid connection.

Reactor Atlas uses the commercial-operation date to set each unit's in-service year, when the source gives one. If it isn't published, it uses first grid connection; if neither is available, the start-up year Global Energy Monitor publishes; and for a handful of historical units with no published date at all, an estimate based on construction start, flagged as such on the sheet.