The 500 metric tons of highly enriched uranium in 20,000 nuclear warheads contains a tremendous amount of energy.
To get a better idea of how much energy is involved, consider the following...
The low enriched uranium downblended from 500 metric tons of HEU could produce enough electricity to power:

for 767 years

for 2 years

for 4 months
The energy from the uranium in 20,000 nuclear warheads is equivalent to:

2.9 billion tons

10.3 billion barrels

62.8 trillion cubic feet