- [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.