Corium

Overview
Corium is a hot material produced only in small amounts during Reactor meltdowns. It is ejected from the Reactor and impacts nearby objects, where it forms rock tiles. Because it is associated with meltdown events rather than normal industry, it is encountered as a hazardous byproduct instead of a standard resource.
In the current game, Corium has no direct use beyond its unusual thermal properties. Those properties can be exploited in solid-based cooling setups, but the material cannot be used as a building material, so any such cooling system must be built close to the reactor meltdown site. This makes Corium more of a niche environmental hazard and heat source than a practical construction element.