Complex sentence: The explosion is thought to be caused by the ignition of hydrogen that had accumulated near the spent fuel pond, the hydrogen was initially thought to have come from the stored fuel rods, but later, TEPCO believed the hydrogen came from Unit 3.
Split into simpler sentences: The explosion is thought to have been caused by the ignition of hydrogen gas that had accumulated near the spent fuel pond. The hydrogen was initially thought to have come from the stored fuel rods, but TEPCO later said they believed the hydrogen came from Unit 3.