The mpg of reactors (or rather the burnup) is measured by the energy extracted from the primary fuel. In practice that is not limited by neutron flux, but by ac

The mpg of reactors (or rather the burnup) is measured by the energy extracted from the primary fuel. In practice that is not limited by neutron flux, but by accumulation of gaseous fission products inside the fuel or the cladding. They do not get any better when you supply external neutron flux. Other, secondary concerns such as the fact that you need to leave the fuel in the ractor for a long time inside the reactor, which leaves a lot of time for corrosion to take place might be reduced. But in the end it is rather costly to produce neutrons with non-nuclear means on the outside.