Final answer:
Half a pound of Uranium-235 can power 400 average U.S. homes for a year due to the high energy density released from the fission process, where the mass defect between reactants and products is converted to a significant amount of energy.
Step-by-step explanation:
To understand how half a pound of Uranium-235 (U-235) can power 400 average U.S. homes for a year, we must consider the process of nuclear fission and the enormous energy it releases. When U-235 undergoes fission, it splits into smaller nuclei, releasing a tremendous amount of energy; this energy comes from the slight mass difference between the reactants and the products, known as the mass defect. Using the energy density of U-235 which is 17 × 10⁶ kcal/g, we can calculate the total energy released.
Considering an average American home uses 10,000 W of power, the annual energy usage would be about 75 million kilocalories per home. Thus, since half a pound (approximately 227 grams) of U-235 can generate a significant amount of energy when fissioned, it is sufficient to power 400 homes. The exact amount of energy released by half a pound of U-235 can be quantified by considering that the fission of 1 kilogram of uranium-235 releases a massive amount of energy, approximately 2.5 million times more than burning 1 kilogram of coal.
In summary, the high energy density of U-235, coupled with the efficiency of nuclear reactors in harnessing the energy from fission, explains how such a small quantity of this material can meet a large energy demand.