Nuclear energy has fallen off the radar screen since September, when the Russian-occupied Zaporizhzhia nuclear plant finally shut down. But Ukraine’s reliance on the three nuclear plants still operating—as well as their vulnerability—has never been higher. //

Keeping reactors fueled may pose an even bigger challenge. Ukraine is running short of fresh fuel, which must be swapped in in every 12 months. Meanwhile, spent fuel backing up at the plants is complicating those swaps. //

Ukraine and most other European countries with Russian-design reactors use fuel from the Moscow-based nuclear-energy giant Rosatom. Pittsburgh-based Westinghouse Electric is their only alternative fuel supplier, and demand far outstrips its supply.