corium
A lavalike mixture of fissile material created in a nuclear reactor's core during a nuclear meltdown.
uncountable
Mexico was given $100,000 to do a series of experiments in which small amounts (10 to 20 grams) of stainless steel and corium (a mixture of 55 percent iron, 10 percent zirconium, and 35 percent uranium dioxide) will be melted and then flooded with water in an arc-melting chamber.
Where it comes from
From core + -ium. Apparently coined in 1974 by Martin Peehs, a German nuclear engineer.
Adapted from the Wiktionary entry corium (CC BY-SA 4.0).