Plasma Coolant

Plasma coolant is a substance used to regulate the operating temperature of a starship's warp core. Without it, the system could overheat or fail completely.

Plasma coolant is considered a biohazard and will react violently with organic matter, liquifying it on contact. (Star Trek: First Contact)

A coolant leak has been known to either cause or be the precursor to a warp core breach. (TNG: "Yesterday's Enterprise"; Star Trek Generations)

In 2374, one of USS Voyager's coolant injectors ruptured, causing a plasma coolant leak. (VOY: "Day of Honor")

A Plasma Coolant Leak

Background

Coolant dangers aboard a starship have also been depicted in TOS: "Balance of Terror".

The loss of plasma coolant causing adverse effects to the warp core does not seem to apply to the USS Enterprise-E. In Star Trek: First Contact, when Data breached the primary coolant tank in main engineering, it did not appear to have any adverse affect on the engines, even though the warp core was powered. It is possible that Starfleet engineered a solution to this problem. It can also be hypothesized that since the warp engines were not active during the "First Contact incident", the rupture of one of the two visible plasma coolant tanks was not sufficient to cause a thermal overload of the warp core reaction chamber.