Commercial production of which energy source became practical only after large amounts of investment by the United States and other countries over the past half century?

The correct answer is nuclear

Nuclear or atomic energy is the energy produced in thermonuclear plants, which use uranium and other elements, as fuel.

The operating principle of a nuclear power plant is the use of heat (thermo) to generate electricity. The heat comes from the fission of the uranium atoms.  

Uranium is a non-renewable mineral resource found in nature, which is also used in the production of radioactive material for use in medicine.

In addition to its use for peaceful purposes, uranium can also be used in the production of weapons, such as the atomic bomb.

As a highly concentrated and high-yielding energy source, several countries use nuclear energy as an energy option. Nuclear plants already account for 16% of the electric energy produced in the world.

More than 90% of nuclear power plants are concentrated in the United States, Europe, Japan and Russia. In April 2018, the Russian government inaugurated the world's first floating nuclear power plant, located in the Arctic Sea.

In some countries like Sweden, Finland and Belgium, nuclear energy already represents more than 40% of the total electricity produced. South Korea, China, India, Argentina and Mexico also have nuclear power plants.

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