Complex sentence: The lake was used for this purpose until the Kyshtym Disaster in 1957, in which the underground vats exploded due to a faulty cooling system and terribly contaminated the entire Mayak area (as well as a large swath of territory to the northeast).
Split into simpler sentences: The lake was used for this purpose until the Kyshtym Disaster in 1957, in which the underground vats exploded due to a faulty cooling system. This incident caused widespread contamination of the entire Mayak area (as well as a large swath of territory to the northeast).