That means that each MW/hr of production only makes 18 kg/hr, so a 900MW/hr nuclear plant only makes 8 tons of aluminum a hour. Its insane.
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
There aren't many places where electricity can be produced close to that. Iceland is one and it's unsurprisingly the world's major bauxite importer and aluminium exporter. Wholesale electricity there goes for around $42/MWh [1].
OK, the smelters can do a bit better than the average wholesale rate, but not much - Iceland's electricity supply is not highly variable like solar or wind.
So the rest of the expenses of the process - mining, shipping half way around the world twice, capital costs - are all basically free compared to the electricity cost.
[0] https://markets.businessinsider.com/commodities/aluminum-pri...
[1] https://www.datacenterdynamics.com/en/opinions/time-for-an-i...
Overall, recycled aluminum requires only about 5% of the energy that primary aluminum requires.