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In a March 1, 2018 speech before Russia’s Federal Assembly, Russian President Vladimir Putin discussed new strategic weapons being developed to counter United States ballistic missile defenses. Two of these weapons are allegedly nuclear powered: a previously revealed intercontinental-range nuclear torpedo and a cruise missile. As Putin described them:

Russia’s advanced arms are based on the cutting-edge, unique achievements of our scientists, designers, and engineers. One of them is a small-scale, heavy-duty nuclear energy unit that can be installed in a missile like our latest X-101 air-launched missile or the American Tomahawk missile—a similar type but with a range dozens of times longer, dozens—basically an unlimited range. It is a low-flying stealth missile carrying a nuclear warhead, with almost an unlimited range, unpredictable trajectory and ability to bypass interception boundaries. It is invincible against all existing and prospective missile defense and counter-air defense systems.

Defense and nuclear disarmament experts did a double take. “I’m still kind of in shock,” Edward Geist, a Rand Corporation researcher specializing in Russia, told NPR. “My guess is they’re not bluffing, that they’ve flight-tested this thing. But that’s incredible.”

This is not the first time a government has worked on a nuclear-powered strategic weapon. Decades ago, the US developed engines first for a proposed nuclear-powered bomber and then for a hypersonic nuclear cruise missile. The US has also examined nuclear-powered rockets for space flight (that crazy Project Orion thing is a story for another time). These programs were all dropped, not because they didn’t work but because they were deemed impractical.