Name: 【TED-Ed】What are the challenges of nuclear power? - M. V. Ramana and Sajan Saini
Uploaded: 2017-05-11T13:08:30.000Z
Duration: 8 min 7 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

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:

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

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

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,

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

Nuclear power relies on the fission of uranium nuclei

that reproduces this splitting in many more nuclei.

The atomic nucleus is densely packed with protons and neutrons

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.

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

into lighter, radioactive elements called fission products,

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,

more fission results in an even larger third generation, and so on.

this spiraling chain reaction is tamed using control rods

made of elements that capture excess  neutrons and keep their number in check.

a reactor draws power steadily and stably for years.

The neutron-led chain reaction is a potent process driving nuclear power,

in unique demands  on the production of its fuel.

It turns out, most of the neutrons emitted from fission have too much kinetic energy

The fission rate is too low and the chain reaction fizzles out.

The first nuclear reactor built in Chicago used graphite as a moderator

to scatter and slow down  neutrons just enough

to increase their capture by uranium and raise the rate of fission.

Modern reactors commonly use purified water as a moderator,

but the scattered neutrons are still a little too fast.

To compensate  and keep up the chain reaction,

to four to seven times  its natural abundance.

Today, enrichment is often done by passing a gaseous uranium compound

to separate lighter U-235  from heavier U-238.

But the same process can be continued to highly enrich U-235

and create an explosive chain reaction in a bomb.

Methods like centrifuge processing must be carefully regulated

Remember, only a fraction  of the released fission energy

Most of the nuclear power goes into the kinetic energy of the fission products.

Those are captured inside the reactor as heat by a coolant,

This heat is eventually used to drive an electric turbine generator by steam

Water flow is critical  not only to create electricity,

but also to guard against the most dreaded type of reactor accident,

If water flow stops because a pipe carrying it breaks,

the uranium heats up very quickly and melts.

radioactive vapors escape  into the reactor,

a steel and concrete containment building is the last line of defense.

But if the radioactive gas pressure is too high,

containment fails and the gasses escape into the air,

spreading as far  and wide as the wind blows.

The radioactive fission products in these vapors

others take hundreds  of thousands of years.

The greatest challenge  for a nuclear reactor

and keep them from harming humans or the environment.

Containment doesn't stop mattering once the fuel is used up.

In fact, it becomes an even greater storage problem.

and stored in pools of water  that cool the waste