U-235 decays with a half life of 704 million years, while Pu-239 has a half life of 24,000 years.

The plutonium by-product is a security risk since it can be used to construct nuclear weapons.

Since a partial meltdown at Three Mile Island in 1979, construction of nuclear power plants in the U. S. has declined sharply, even though no environmental or health effects have been documented.