I have been told that there is a measurable weight difference between the Uranium that goes into a fission reactor and the spent fuel that comes out. My assumption is that the spent fuel is less dense or lighter, not only because the fission products are lighter than the uranium that they originated from but also by the amount of mass that has been converted to energy.

You're mistaken here.

Uranium 235 + A fast neutron = 235.04 + 1.00 = 236.04 amu

A probable stable byproduct pair is Cerium 140 + Strontium 94 + 2 neutrons.

139.90 + 93.90 + 2.00 + 6 total beta (positron) decays + (< .01 amu)

= 235.8 amu

Thats .1% of the total mass lost by conversion to energy, not much. The lighter
weight described is mostly a change in bulk density with possibly some neutron
loss, not mc2.

Johannes,

You need to understand that you can be wrong. Please accept that or make a fool of yourself elsewhere.

You have to accept this possibility or it's just a reality defying quest to PROVE that you can predict the results of physical reality with your common sense. I just PROVED that you can't.

time for a famous quote:

"The way to get good ideas is to get lots of ideas and throw the bad ones away."