Question

why isotope of uranium can undergo fission easily

Answer 1

U235 is the fissionable isotope of Uranium. Natural Uranium contains only about 0.7 percent U235, which is enough to produce fission only with a good moderator such as graphite or heavy water. In light water reactors the Uranium has to be enriched to about 4 percent U 235. Also the artifficial isotope 233U.