What percentage of Uranium-235 do we need to have in the fuel to create a self-sustaining nuclear chain reaction in a reactor moderated by heavy water? What is the most probable energy of a neutron...

What percentage of Uranium-235 do we need to have in the fuel to create a self-sustaining nuclear chain reaction in a reactor moderated by heavy water?

What is the most probable energy of a neutron that is released when the nucleus of a uranium atom splits apart?