3.8 Warp Core Breach
A breach of the reaction vessel triggers automatic safeguards including a cylindrical forcefield and emergency shutdown, and as a last resort the entire warp core is ejected through a hatch in the Engineering Hull, leaving the ship on impulse until a reserve core is installed.
One of the worst imaginable accidents in an antimatter reactor, the breach of the reaction vessel, exposing the ship to antimatter plasma and other high-pressure, high temperature gasses. In such an accident, a number of emergency safeguards automatically activate. First, a cylindrical forcefield is activated, isolating the engine from the rest of Engineering. Second, an emergency shutdown procedure automatically shuts off all fuel feeds and magnetic quenching fields are activated. Should these fail to work, a series of explosive bolts blow off a hatch on the bottom of the Engineering Hull and the entire warp reactor core is ejected into space. This will result in venting much of the Engineering Hull to the vacuum of space and the probable death of a significant number of engineering personnel. Additionally, without the warp core, the ship will be limited to impulse speed, until repair crews can transfer the Reserve Warp Engine Core from its storage bay, and into the normal structural mount. The ejection procedure can prevent the loss of the entire starship in the event of a catastrophic loss of antimatter containment. (Note: This is probably similar to what happened to the USS Yamato in the episode "Contagion", except that the safety procedures failed and the ship blew up.)