Sentence:  “I think it calls on us here in the U.S., naturally, not to stop building nuclear power plants but to <WSD>put the brakes on</WSD> right now until we understand the ramifications of what's happened in Japan,” Senator Joseph I. Lieberman . . . said. 
Meaning of the word marked with <WSD> tags:  To cease to perform one's current activity or to decrease one's level of activity.