Simplify the following text for a Grade 12 native English-speaking person.
Original: The Kyshtym disaster was a radiation contamination incident that occurred on 29 September 1957 at Mayak , a nuclear fuel reprocessing plant in Russia ( then a part of the Soviet Union ) .
Simplified: The Kyshtym disaster was a radiation contamination disaster , which happened on 29 September , 1957 , in Mayak , a nuclear plant in Russia ( then a part of the Soviet Union ) .