sentences:  Today, Hanford is the most contaminated nuclear site in the United States and is the focus of the nation's largest environmental cleanup.The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor which underwent a steam explosion and meltdown on January 3, 1961, killing its three operators.The direct cause was the improper withdrawal of the central control rod, responsible for absorbing neutrons in the reactor core.