Nuclear fission | Nuclear chemistry | High school chemistry | Khan Academy

- [Lecturer] An atomic bomb

and a nuclear power plant

works on the same basic principle,

nuclear fission chain reactions.

But what exactly is this?

And more importantly,

if the same thing is
happening inside both a bomb

and a nuclear reactor,

then why doesn't a nuclear
reactor just explode like a bomb?

What's the difference?

Well, let's find out.

So what is nuclear fission?

Well, the word fission means breaking.

So nuclear fission is a nuclear reaction

in which a heavy nucleus
breaks into smaller nuclei.

But how does it break exactly?

Well, one way is for it
to break spontaneously.

It can happen all by itself

without us having to do anything.

But we usually call that radioactivity,

or we sometimes also call
it spontaneous fission.

But when we usually say nuclear fission,

we're talking about the
ones where we break it

by specifically bombarding
it with a neutron.

Think about it, this
nucleus is already unstable.

Now you add another neutron to it,

it makes it more unstable,

kind of like pushing it over the edge

and then it breaks into smaller nuclei.

And here when it breaks,

you also end up getting a few neutrons.

You get somewhere between

one to three neutrons usually out.

So let's take an example.

If you take Uranium 235,
an isotope of uranium,

and if you bombarded with a neutron,

then it can break into
Strontium 94 and Xenon 140.

We don't have to remember
the numbers or anything,

don't worry about it.

But my question would be, can we predict

how many neutrons we'll get over here?

Well, we can.

All we have to do,

just like any nuclear reaction,

is to keep track of protons and neutrons.

So if I keep track of protons, let's see,

I have 92 protons on the left hand side.

How many protons do I have
on the right hand side?

Well, eight plus four is two, so 12.

So five plus three.

I get 92 over here.

But what about the total
number of particles?

Well, I have 235 plus one that is 230...

Oops, that is 236 on the left hand side.

But over here, 94 plus 140.

So I get four.

Nine plus four is 13.

So one carry over, I get 234.

So there are only 234 particles over here,

which means two particles
must have been released.

And these must be two neutrons

because we've already
accounted for all the protons.

So that's how I know

that there must be two
neutrons released over here.

But you know what's cool about
nuclear fission reactions?

For the same reactants,

you could get completely
different products altogether.

For example, if we take
another uranium 235

and bombard it with another neutron,

look exactly the same reactance,

but this time you might get
completely different products.

You might get Barium
141 and say Krypton 92.

Again, we'll get some amount of neutrons,

when you pause the video over here

and try it yourself to figure out

how many number of neutrons
we should be getting here.

Alright, again, we can see

the number of protons is balanced.

You have 56 plus 36 is 92.

But how many total particles we have?

We have 236 here again,

this time we have one plus 2, 3,

14 plus nine is 23.

So you get 233, which means look,

three particles are missing.

So this time we'll get three neutrons.

And just like with the fusion reactions,

we will see even here,
some energy is released

and energy is released
usually as kinetic energy

of the products and the neutrons.

And because energy is released

and remember that energy
and mass are equivalent,

we will find that the mass of the products

will be smaller than the
mass of the reactants.

And just by figuring out
the difference in the mass,

you can figure out how
much energy was released.

That difference in the mass is basically

what got released as energy.

Again, something that we've seen before

in the nuclear fusion
reactions, very similar.

Now, can any heavy nucleus
give you fission reactions?

No, that can't happen.

The ones that do, we
call them fissile nuclei.

So uranium 235 is fissile

because it does undergo fission reaction

and gives you energy.

But if you consider
another isotope of uranium,

which is say Uranium 92, 238,

turns out it is non-fissile.

It does not undergo
nuclear fission easily.

And if you're wondering why certain nuclei

are fissile and others are not,

well, it has something to do

with energy and stability.

Well, turns out for uranium,

when it undergoes fission,

you end up getting more stable products

and therefore energy is released.

Turns out that's not the
case for Uranium 238,

or at least that's not
very easy to happen.

But of course we'll not
dive too much into it.

But a big question now
we could ask ourselves is

how much energy do we get out of it?

Well, if you look at a single reaction,

of course we'll get a
tiny amount of energy.

But if you want to get usable amount,

then we will require lots
and lots of reactions.

But how do we do that practically?

Because nuclear fission

requires you to bombard
a nucleus with neutron.

So how do we ensure we get lots

and lots of reactions like this?

Well, the answer is right in front of us.

Since each nuclear fission reaction

gives us a few neutrons,

if we can ensure that these neutrons

go and hit other uranium
235 atoms, nuclei, sorry,

then they will again undergo fission

and give you more neutrons

and each cause even more fission reaction.

Here's the way we can show that.

So let me just go to the next page.

Here we go.

So if you have one neutron that bombards

with a uranium 235 giving
you energy, fission reaction,

giving you energy and some neutrons.

Now if these neutrons could go

and hit even more of these urine 235,

then you'll get even more energy

and this thing can keep on going

and you can see very quickly

this will keep increasing.

You'll have one fission,
then you have three fission,

and then you'll have nine
and so on and so forth.

So the amount of fission
happening per second

would just keep increasing.

This is what we call a chain reaction.

Nuclear chain reactions
can be quite devastating.

You start with very few
reactions per second,

but very quickly, very
rapidly, that number increases.

And within a short amount of time,

you can release tremendous
amount of energy.

That is the whole idea
behind atomic bombs.

What makes atomic bombs
so much more devastating

compared to traditional regular bombs

is that we are dealing
with nuclear energy,

which is hoarders of magnitude higher

than the chemical energy

that we get from traditional bombs.

So a small amount of fissile material

can give you a lot of
energy, but that's not it.

That's not it.

You see, the products of
nuclear fission reactions

are usually radioactive,

which means even after
the explosion is done,

the whole area is contaminated

with radioactive isotopes now,

which can further cause
damage for ears to come,

making that whole area inhabitable.

So yeah, atomic bombs
are really destructive.

But on the flip side, if you're using this

to generate electricity, let's say,

then we'll get way more energy

compared to what we get from fossil fuels.

Because again, there we are
dealing with chemical energy.

And of course, another advantage
of using nuclear energy

is that in fossil fuels,

because you're using combustion reactions,

there is CO2 that is
released into the atmosphere.

None of that happens over here.

But now this brings us
to the original question.

How do we use chain reactions

in nuclear power reactors

to generate electricity?

Wouldn't they just explore
just like an atomic bomb?

So what's the big difference?

Well, the big difference is over here,

when it comes to bombs,

we are using uncontrolled chain reaction.

Whatever we just saw right now,

it's a about uncontrolled chain reaction.

But when it comes to power...

When it comes to nuclear reactors,

we use controlled chain reactions.

How do you control chain
reactions, you ask?

Well, one of the most common ways

is by absorbing a lot of neutrons.

So imagine we absorbed a
lot of neutrons like this.

Then look, by absorbing neutrons,

you are controlling how many further

fission reactions are happening.

This way you can control it,

you can ensure that the energy
is released in a steady rate.

And that's how you can get
controlled chain reaction.

But there's another major difference.

Remember how we said earlier

that uranium 238 is non fissile?

Well, it turns out if
you take a uranium ore

then most of it is actually uranium 238.

That means you cannot
directly use a uranium ore

either as a bomb or as a
fuel for nuclear power plant.

This means we have to
take it through a process

where we increase the
amount of fissile material.

And this process is called enrichment.

And the big difference is

if you're using a fuel for...

You're using it for a bomb,

then we would want a lot of enrichment.

In fact, we'd want about 90% enriched.

And that makes sense

because you would want

as many fission reactions happening

as possible per second

so that the whole thing
explodes immediately.

But when it comes to nuclear reactors,

nuclear power plants,

you see we have only about
three to 5% enrichment.

That means a single Uranium 235

is surrounded by a lot
of non-fissile materials.

That's why you will...

That's why the nuclear fuel
will not explode like a bomb

because it's not enriched as much

as you would need for a bomb.

So anyways, by using
controlled chain reaction,

we get energy as the kinetic
energy of these products,

which is then used to heat up water.

And then the process is very similar

to how any other power plant works.

The heated water produces
high pressure steam

that turns turbines,

and that's how you
eventually get electricity.

And then that hot steam

is cooled in a cooling tower.

And in the process a lot
of water vapor is produced

and that is released over here.

I'm mentioning this
because I used to think

that this itself was a nuclear reactor

and it was producing a lot of smoke,

radioactive smoke,
which could be dangerous

because it's going into the atmosphere.

But none of that 'cause first of all,

this is just a cooling tower,

and what is it releasing is water vapor.

And that water never comes in contact

with any of the radioactive material

that you have over here.

So it's not dangerous,

but there will be radioactive
products left over,

radioactive waste inside
the nuclear power plants,

and that needs to be safely disposed.

And that is a challenge that scientists

and engineers are
actively working on today.