Reactor coolant was a hazardous byproduct generated by the warp reactor of some starships in the 22nd century. It produced potentially lethal levels of radiation.
During a simulation in 2153, reactor coolant was shown leaking out of a Malosian cargo shuttle's innards, while Degra and Jonathan Archer were in the simulated shuttle. (ENT: "Stratagem")
In 2154, Valdore intentionally exposed Trip Tucker to reactor coolant on the Romulan drone ship in an effort to coerce Malcolm Reed to reestablish its warp matrix. (ENT: "United")