Complex sentence: In the third season finale, Clarke manages to destroy A.L.I.E. and it is revealed the world faces another nuclear disaster because of nuclear reactors around the world that have fallen into disrepair and are irradiating Earth, once again threatening to make it uninhabitable.
Split into simpler sentences: In the third season finale, Clarke manages to destroy A.L.I.E. after it is revealed that the world is facing another nuclear disaster. This is due to nuclear reactors around the world that have fallen into disrepair which are irradiating Earth, once again threatening to make it uninhabitable.