Identify which is false from the following statements about nuclear weapons and nuclear power plants.

A nuclear explosion occurs when sub-critical pieces of nuclear materials collide to reach a critical mass.

The enrichment of U-235 in the fuel used for nuclear weapons is much higher than that of the fuel used for nuclear power plants.

Nuclear power plants also produce a significant amount of carbon dioxide while operating especially when generating electricity.

The United States is a large producer of nuclear power and sources a significant amount of its electricity from nuclear energy.