As we know 92U238 is bigger in size so it will not breakup unless the energy of bombarding neutrons is more than 1.2 MeV. Such neutrons are called fast neutrons.
On the other hand, U235 can be easily fissioned by slow neutrons which are called thermal neutrons which possess energy of 0.25MeV. Therefore, 92U238 is not suitable for chain reaction.
