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Real plutonium. Real experiments. No nuclear yield. Real important. It’s been nearly 22 years since the United States began its self-imposed moratorium on full-scale nuclear weapons tests, with the last one, Divider, occurring on September 23, 1992. While the moratorium has been strictly adhered to, the nation continues to conduct so-called subcritical tests, intended to help scientists determine the impact that old and aging plutonium will have on the U.S. nuclear stockpile. In the most recent subcritical test, Pollux, a hollow shell of plutonium was forced to implode, raising the plutonium’s density until…um, that was it. Nothing else happened. Unlike a nuclear weapons test, a successful subcritical test ends without even a whimper, much less a nuclear bang. “The device used in Pollux didn’t contain enough plutonium to explode,” explained Mike Furlanetto, the Diagnostic Coordinator for Pollux. “The test device couldn’t reach a critical mass.” 20 1663 August 2014 A critical mass is the minimum amount of nuclear material needed to realize a self-sustaining chain reaction, the process by which huge amounts of nuclear energy can be released. In a subcritical test, the plutonium mass is subcritical, and the plutonium density remains subcritical before, during, and after the test. A self-sustaining chain reaction isn’t possible, and the entire experiment proceeds without generating any nuclear yield. As such, subcritical tests are allowed under the Comprehensive Test Ban Treaty, which bans all nuclear and nuclear test explosions. Why spend time, effort, and millions of dollars to probe what amounts to a nuclear dud? It’s because subcritical tests are currently the best and possibly only way to obtain some of the data needed to validate weapons simulations—the extremely sophisticated supercomputer programs used to assess the weapons in the U.S. nuclear stockpile. In the