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- [Lecturer] An atomic bomb

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and a nuclear power plant

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works on the same basic principle,

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nuclear fission chain reactions.

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But what exactly is this?

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And more importantly,

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if the same thing is
happening inside both a bomb

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and a nuclear reactor,

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

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What's the difference?

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Well, let's find out.

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So what is nuclear fission?

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Well, the word fission means breaking.

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So nuclear fission is a nuclear reaction

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in which a heavy nucleus
breaks into smaller nuclei.

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But how does it break exactly?

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Well, one way is for it
to break spontaneously.

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It can happen all by itself

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without us having to do anything.

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But we usually call that radioactivity,

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or we sometimes also call
it spontaneous fission.

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But when we usually say nuclear fission,

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we're talking about the
ones where we break it

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by specifically bombarding
it with a neutron.

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Think about it, this
nucleus is already unstable.

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Now you add another neutron to it,

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it makes it more unstable,

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kind of like pushing it over the edge

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and then it breaks into smaller nuclei.

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And here when it breaks,

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you also end up getting a few neutrons.

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You get somewhere between

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one to three neutrons usually out.

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So let's take an example.

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If you take Uranium 235,
an isotope of uranium,

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and if you bombarded with a neutron,

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then it can break into
Strontium 94 and Xenon 140.

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

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don't worry about it.

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But my question would be, can we predict

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how many neutrons we'll get over here?

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Well, we can.

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All we have to do,

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just like any nuclear reaction,

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is to keep track of protons and neutrons.

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

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I have 92 protons on the left hand side.

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

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Well, eight plus four is two, so 12.

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So five plus three.

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I get 92 over here.

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But what about the total
number of particles?

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

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Oops, that is 236 on the left hand side.

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But over here, 94 plus 140.

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So I get four.

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Nine plus four is 13.

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So one carry over, I get 234.

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So there are only 234 particles over here,

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which means two particles
must have been released.

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And these must be two neutrons

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because we've already
accounted for all the protons.

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So that's how I know

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that there must be two
neutrons released over here.

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

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For the same reactants,

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you could get completely
different products altogether.

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For example, if we take
another uranium 235

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and bombard it with another neutron,

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look exactly the same reactance,

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but this time you might get
completely different products.

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You might get Barium
141 and say Krypton 92.

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Again, we'll get some amount of neutrons,

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when you pause the video over here

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and try it yourself to figure out

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how many number of neutrons
we should be getting here.

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Alright, again, we can see

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the number of protons is balanced.

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You have 56 plus 36 is 92.

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But how many total particles we have?

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We have 236 here again,

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this time we have one plus 2, 3,

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14 plus nine is 23.

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So you get 233, which means look,

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three particles are missing.

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So this time we'll get three neutrons.

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And just like with the fusion reactions,

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we will see even here,
some energy is released

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and energy is released
usually as kinetic energy

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of the products and the neutrons.

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And because energy is released

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and remember that energy
and mass are equivalent,

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we will find that the mass of the products

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will be smaller than the
mass of the reactants.

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And just by figuring out
the difference in the mass,

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you can figure out how
much energy was released.

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That difference in the mass is basically

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what got released as energy.

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Again, something that we've seen before

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in the nuclear fusion
reactions, very similar.

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Now, can any heavy nucleus
give you fission reactions?

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No, that can't happen.

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The ones that do, we
call them fissile nuclei.

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So uranium 235 is fissile

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because it does undergo fission reaction

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and gives you energy.

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But if you consider
another isotope of uranium,

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which is say Uranium 92, 238,

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turns out it is non-fissile.

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It does not undergo
nuclear fission easily.

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And if you're wondering why certain nuclei

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are fissile and others are not,

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well, it has something to do

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with energy and stability.

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Well, turns out for uranium,

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when it undergoes fission,

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you end up getting more stable products

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and therefore energy is released.

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

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

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

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But a big question now
we could ask ourselves is

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how much energy do we get out of it?

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Well, if you look at a single reaction,

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

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But if you want to get usable amount,

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then we will require lots
and lots of reactions.

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But how do we do that practically?

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Because nuclear fission

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requires you to bombard
a nucleus with neutron.

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So how do we ensure we get lots

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and lots of reactions like this?

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Well, the answer is right in front of us.

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Since each nuclear fission reaction

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gives us a few neutrons,

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if we can ensure that these neutrons

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

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then they will again undergo fission

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and give you more neutrons

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and each cause even more fission reaction.

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Here's the way we can show that.

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So let me just go to the next page.

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Here we go.

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So if you have one neutron that bombards

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with a uranium 235 giving
you energy, fission reaction,

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giving you energy and some neutrons.

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Now if these neutrons could go

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and hit even more of these urine 235,

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then you'll get even more energy

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and this thing can keep on going

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and you can see very quickly

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this will keep increasing.

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

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

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So the amount of fission
happening per second

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would just keep increasing.

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This is what we call a chain reaction.

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Nuclear chain reactions
can be quite devastating.

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You start with very few
reactions per second,

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but very quickly, very
rapidly, that number increases.

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And within a short amount of time,

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you can release tremendous
amount of energy.

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That is the whole idea
behind atomic bombs.

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What makes atomic bombs
so much more devastating

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compared to traditional regular bombs

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is that we are dealing
with nuclear energy,

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which is hoarders of magnitude higher

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than the chemical energy

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that we get from traditional bombs.

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So a small amount of fissile material

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

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That's not it.

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You see, the products of
nuclear fission reactions

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are usually radioactive,

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which means even after
the explosion is done,

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the whole area is contaminated

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with radioactive isotopes now,

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which can further cause
damage for ears to come,

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making that whole area inhabitable.

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So yeah, atomic bombs
are really destructive.

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

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to generate electricity, let's say,

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then we'll get way more energy

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compared to what we get from fossil fuels.

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Because again, there we are
dealing with chemical energy.

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And of course, another advantage
of using nuclear energy

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is that in fossil fuels,

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because you're using combustion reactions,

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there is CO2 that is
released into the atmosphere.

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None of that happens over here.

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But now this brings us
to the original question.

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How do we use chain reactions

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in nuclear power reactors

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to generate electricity?

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

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So what's the big difference?

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Well, the big difference is over here,

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when it comes to bombs,

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we are using uncontrolled chain reaction.

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Whatever we just saw right now,

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it's a about uncontrolled chain reaction.

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But when it comes to power...

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When it comes to nuclear reactors,

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we use controlled chain reactions.

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How do you control chain
reactions, you ask?

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Well, one of the most common ways

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is by absorbing a lot of neutrons.

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So imagine we absorbed a
lot of neutrons like this.

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Then look, by absorbing neutrons,

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you are controlling how many further

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fission reactions are happening.

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This way you can control it,

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

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

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But there's another major difference.

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Remember how we said earlier

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that uranium 238 is non fissile?

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Well, it turns out if
you take a uranium ore

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then most of it is actually uranium 238.

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That means you cannot
directly use a uranium ore

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

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This means we have to
take it through a process

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where we increase the
amount of fissile material.

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And this process is called enrichment.

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And the big difference is

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if you're using a fuel for...

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You're using it for a bomb,

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then we would want a lot of enrichment.

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

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And that makes sense

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because you would want

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as many fission reactions happening

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as possible per second

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so that the whole thing
explodes immediately.

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But when it comes to nuclear reactors,

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nuclear power plants,

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

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That means a single Uranium 235

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is surrounded by a lot
of non-fissile materials.

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That's why you will...

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

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because it's not enriched as much

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as you would need for a bomb.

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So anyways, by using
controlled chain reaction,

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we get energy as the kinetic
energy of these products,

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which is then used to heat up water.

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And then the process is very similar

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to how any other power plant works.

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The heated water produces
high pressure steam

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that turns turbines,

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and that's how you
eventually get electricity.

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And then that hot steam

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is cooled in a cooling tower.

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And in the process a lot
of water vapor is produced

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and that is released over here.

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I'm mentioning this
because I used to think

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that this itself was a nuclear reactor

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and it was producing a lot of smoke,

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radioactive smoke,
which could be dangerous

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because it's going into the atmosphere.

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But none of that 'cause first of all,

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this is just a cooling tower,

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and what is it releasing is water vapor.

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And that water never comes in contact

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00:10:07,200 --> 00:10:09,060
with any of the radioactive material

266
00:10:09,060 --> 00:10:10,050
that you have over here.

267
00:10:10,050 --> 00:10:11,220
So it's not dangerous,

268
00:10:11,220 --> 00:10:15,480
but there will be radioactive
products left over,

269
00:10:15,480 --> 00:10:18,660
radioactive waste inside
the nuclear power plants,

270
00:10:18,660 --> 00:10:21,000
and that needs to be safely disposed.

271
00:10:21,000 --> 00:10:22,950
And that is a challenge that scientists

272
00:10:22,950 --> 00:10:25,683
and engineers are
actively working on today.