Question

When some large nuclear plants run at full capacity, they produce about
 1246.8 MW of electrical power. At this rate, what mass of 235U (kg)
 would it consume per year if the efficiency of converting nuclear energy 
to electricity is 33%?

Answer 1

the typical commercial nuclear power plant produces about 1246.8 MW of electric power at about 1/3 efficiency overall. Most people simplify it to a more generic 1000 MW average since there are several designs that are less than 1000 MW as well. The 1/3 efficiency means you have to generate about 3 times the "thermal heat" in the reactor core to get that amount of useful energy out of the steam turbine and generator. The rest is "waste heat" that has to be transferred to some "heat sink" like a river, ocean, or cooling tower.

Typically, the fissioning of one kilogram of Uranium or Plutonium results in 20,000,000 kilowatt hours, enough energy to light Washington, D.C. for a day. If you converted that to a bomb it would be more like 17 kilotons of energy (17,000 tons or 34,000,000 lbs of TNT).