From the article: Experts Aghast Over Russian Claim Of Nuclear-Powered Missile With Unlimited Range
The U.S. designed and built a nuclear-powered missile engine during the height of the Cold War. It never flewThe U.S. designed and built a nuclear-powered missile engine during the height of the Cold War. It never flew.
The U.S. tested similar concepts in the 1960s, but abandoned them over concerns of radioactive contamination. Russia's claim seems so fantastic that some analysts didn't believe initial reports