Reactor core breach

If the reactor core is breached from excessive damage, the Chief Engineer may or may not jettison it. If the core jettison fails, you can attempt repairs and at the same time tend to injured personnel. However, once the core is breached, it will most certainly explode over a period of time. Once your reactor core is breached, you can always expect the worst.

If you feel that you have a chance of surviving, you should load personnel on to fighters, shuttles and vehicles then launch these craft. The fewer people on-board the ship, the fewer things you have to worry about. However, don't send all your Systems Engineers away or there will be no one to do repairs or clear radiation. Once you've evacuated important personnel, you may then go about repairing the ship and taking steps to contain any radiation detected.

If the reactor integrity falls below 25%, it may trigger a reactor shutdown causing a system wide shutdown of this component. Don't forget that if the reactor core is badly damaged or destroyed, it will leak radiation throughout the ship. As such, while you can override the Chief Engineers’ decision by taking him off station and restarting the reactor, you risk losing the craft and personnel.