[jaggederest]

First, a clarification: a kilowatt-hour is not a kilowatt per hour (1 kwh = 3.6 * 10^6 joules). Same goes for megawatt-hours. There's no such thing as a "900Mwh" reactor - it's a 900 megawatt reactor. During an hour, it'll produce 900Mwh* (3.24e+12 joules) and can smelt 16363 kg of aluminum. Behold the power (pun intended) of dimensional analysis.

Secondly, you're not wrong, the way that aluminum is smelted is by melting e.g. bauxite or another aluminum compound, and then electrolyzing the resulting fluid to extract pure aluminum. Usually the same electrodes are used for both operations. It's the very grandest scale of electrochemistry, and the reason that aluminum smelting plants are nearly universally located near cheap and highly available power sources.

* Something close to 900Mwh, anyway, given that reactor nameplate capacity is not always the actual running power or peak possible output, plus an allowance for maintenance. Other power sources have different capacity factors that would result in something below 900Mwh, but a typical fission plant is "up" continuously for our purposes