Rachel Maddow explains the mechanics of what happens when a nuclear reactor has a meltdown, and talks with David Lochbaum, a nuclear engineer and director of the Nuclear Safety Project of the Union of Concerned Scientists, about how experts are reading signs from the compromised Japanese nuclear reactors to understand how close they are to meltdown.

This video is from MSNBC's The Rachel Maddow Show, broadcast March 14, 2011.

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