Complex sentence: In the United States, Nuclear Regulatory Commission rules limit licensees to 60 years to decontaminate an installation to background levels, and long-lived isotopes in reactors may not have decayed to background level in only sixty years.
Split into simpler sentences: In the United States, Nuclear Regulatory Commission rules limit licensees to 60 years to decontaminate an installation to background levels. Long-lived isotopes in reactors may not have decayed to background level in only sixty years.