Yeah the gravedrive is cracked not the helium three canisters that hold the helium 3 fuel. Imagine if your engine was cracked and combustion happened leading to the leaking fuel to ignite or (bad damn day) thats what would have happened but on a bigger scale to that ship

Interesting point, Mrs. Whitmore did specifically state that "the ship can't jump right now" which kind of suggests its fully broken. When she goes to escape from the Crimson Fleet too she does tell other people on the ship to quickly escape, which suggests it might not just be a ship full of children, would have added it in if not for the 1 minute limit!