If I have a reactor that needs to operate at 300 kW thermal power for twenty years, how much fissionable material (U-235) do I need? I calculated that this will require 1.89E14 J, or 1.18E27 MeV. 931 MeV = 1u, so I have 1.27E24 u. This is .002 kg, which seems really small. Am I doing something wrong? I'm really just looking for a simple, ballpark analysis here. Thanks!