Complex sentence: High pressure bottles are pretty strong so that they mostly stay unruptured in crashes, but if a failure does occur, there is a kind of violent explosion and here it is important how the bottles fracture and how well the fragments can be contained.
Split into simpler sentences: High pressure bottles are fairly strong so that they stay unruptured in crashes, but if a failure does occur, a violent explosion could result. Therefore it is important how the bottles fracture and how well the fragments can be contained.