On a December afternoon in Chicago during the middle of World War II,

scientists cracked open the nucleus at the center of the uranium atom

and turned nuclear mass into energy over and over again.

They did this by creating for the first time

a chain reaction inside a new engineering marvel:

the nuclear reactor.

Since then, the ability to mine great amounts of energy from uranium nuclei

has led some to bill nuclear power

as a plentiful utopian source of electricity.

A modern nuclear reactor generates enough electricity from one kilogram of fuel

to power an average American household for nearly 34 years.

But rather than dominate the global electricity market,

nuclear power has declined from an all-time high of 18% in 1996

to 11% today.

And it's expected to drop further in the coming decades.

What happened to the great promise of this technology?

It turns out nuclear power faces many hurdles,

including high construction costs

and public opposition.

And behind these problems lie a series of unique engineering challenges.

Nuclear power relies on the fission of uranium nuclei

and a controlled chain reaction

that reproduces this splitting in many more nuclei.

The atomic nucleus is densely packed with protons and neutrons

bound by a powerful nuclear force.

Most uranium atoms have a total of 238 protons and neutrons,

but roughly one in every 140 lacks three neutrons,

and this lighter isotope is less tightly bound.

Compared to its more abundant cousin,

a strike by a neutron easily splits the U-235 nuclei

into lighter, radioactive elements called fission products,

in addition to two to three neutrons,

gamma rays,

and a few neutrinos.

During fission, some nuclear mass transforms into energy.

A fraction of the newfound energy powers the fast-moving neutrons,

and if some of them strike uranium nuclei,

fission results in a second larger generation of neutrons.

If this second generation of neutrons strike more uranium nuclei,