Rated power, uprates and derates

Gross power is what the generator measures; net power is what goes out to the grid after subtracting what the plant itself consumes: primary and cooling-circuit pumps, ventilation, lighting. As the definition of nameplate capacity puts it, the output to the grid is always lower than the generator's because the equipment connecting it also uses power. The atlas shows each unit's net power as recorded by the IAEA's PRIS, which also sets a reference power for every reactor: the maximum it can sustain continuously under reference ambient conditions, and the figure the IAEA uses as the basis for the load factor.

An uprate is an authorised increase of that power. In the United States the NRC distinguishes three kinds: those recovering the measurement-uncertainty margin by calculating reactor power with more accurate techniques, under 2%; stretch uprates, up to about 7%, that use existing design margins and need no major modifications; and extended uprates, up to 20%, that require replacing large balance-of-plant equipment such as high-pressure turbines, condensate pumps and motors, generators or transformers. As of January 2022 the NRC had approved 171 uprates, adding about 24,089 MWt, of the order of 8,030 MWe.

A derate is the opposite: the unit runs below its rated power for a specific reason and for a period, which can be an hour or the rest of its life. The usual causes are a cooling-water temperature limit set in the environmental licence, which forces river-cooled plants to cut load in summer; degraded equipment, such as a condenser with fouled tubes or a steam generator with plugged tubes; a fuel restriction; or a request from the grid operator. If the cause is permanent, the unit reports a lower reference power and the derate stops being one: it becomes the new rated power.

Reading the atlas, keep the two apart. The load factor measures how much energy the unit delivered against its reference power: a long derate lowers that factor, while an uprate leaves it unchanged and raises the power. And a unit listed with fewer MWe than its twin does not necessarily perform worse: it may have a different reference power by design, by licence or through a derate already built in.