Complex sentence: The site remained in operation for several years following the fire, but after its closure in 1972, the accident resulted in that area remaining off limits to the present day, primarily due to low levels of plutonium contamination.
Split into simpler sentences: The site remained in operation for several years following the fire. After its closure in 1972, the accident resulted in the area remaining off limits to the present day, primarily due to low levels of plutonium contamination.