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Continuing our look at Nuclear Chemistry, Hank takes this episode to talk about Fusion and Fission. What they mean, how they work, their positives, negatives, and dangers. Plus, E=mc2, Mass Defect, and Applications of Fission and Fusion in the real world!
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Transcript Provided by YouTube:
00:00
As I said before chemistry is, like many aspects of your own life, all about a search for stability.
00:05
Last week we talked about radioactive decay
00:08
and how atomic nuclei get rid of various particles in order to become more stable.
00:12
But what is this illusive stability that all things seem to be striving for exactly?
00:16
In nuclear chemistry it simply has to do with keeping the nucleus together.
00:19
If the nucleus is going to break apart, then that’s not going to be something that lasts very long.
00:24
Stability really, is kind of just a way of saying, it can exist.
00:28
And the amount of energy, that holds each proton or neutron in an atom’s nucleus,
00:32
is the same amount that’s released when it’s removed.
00:35
This is known as binding energy and it’s one of the fundamental principles of nuclear chemistry.
00:40
It’s actually what we mean when we talk about nuclear energy.
00:43
Now I’m not going to lie to you, nuclear chemistry is terribly complicated,
00:47
but we have a way of understanding it that, while not exactly simple, is one I’m sure you’ve heard of.
00:51
The binding energy of an atom is calculated with the formula E = mc2.
00:56
Probably the most famous equation in the world,
00:58
and since it was first hit upon by a young patent clerk in 1905 it has become synonymous with scientific genius.
01:04
Part of why it’s so famous, I like to think, is because the logic behind it is so elegantly simple,
01:09
and yet totally counter intuitive, but it’s probably also famous because it’s important.
01:14
It explains one of the most powerful sources of energy known to humanity.
01:18
[Theme Music]
01:28
E = mc2 is formally known as the mass-energy equivalence formula
01:32
and it states that mass is interchangeable with energy.
01:36
OK, there is a lot there, in what I just said.
01:39
To tease it apart consider the nucleus of an atom of oxygen: 8 protons and 8 neutrons.
01:43
Collectively, by the way, these particles are known as nucleons.
01:46
If you were to add up all the individual masses of all 16 nucleons separately
01:50
and then compare that to the total mass of an actual oxygen nucleus,
01:54
you’d find that there’s a difference between the two.
01:56
Specifically the mass of the nucleus, exactly 15.99 atomic mass units,
02:00
is lower than the mass of its individual nucleons put together, in this case 16.13 amu’s.
02:07
That mass went somewhere.
02:09
That “missing mass” in the nucleus, known as its mass defect, is actually present in the form of energy.
02:15
It’s the energy that holds the nucleons together,
02:17
so for example the mass defect for an oxygen atom is negative 2.269 x 10^-28 kilograms.
02:23
To find out how much binding energy that missing mass amounts to, you can use it as the ‘m’ in Einstein’s formula.
02:30
This ingenious little equation relates mass and energy by a simple proportionality constant,
02:34
and thanks to Einstein we know that constant is the square of the speed of light, or c2.
02:39
Solve for ‘E’ and you find that the binding energy in that oxygen nucleus is 2.04 x 10^-11 joules,
02:46
with the negative sign indicating that the energy is being released.
02:49
Now of course 2.04 x 10^-11 is a very small number.
02:53
That might surprise you, but hold the phone, that is just for one single nucleus.
02:57
If we multiply that by Avogadro’s number to find the energy change for a whole mole of oxygen nuclei,
03:02
a mere 16 grams of oxygen, we get an amazing 1.23 x 10^13 joules of energy.
03:10
To produce that energy with coal, you would have to burn 420,000 kilograms, 420 metric tons of coal.
03:19
That energy is what we mean when we talk about nuclear energy,
03:22
the binding energy that’s released when a nucleon is removed from its nucleus.
03:26
Now, to dislodge one of those nucleons and unleash that energy there are two general
03:30
types of nuclear reactions: fission and fusion.
03:33
Fission occurs when a large nucleus splits into two lighter ones.
03:37
Fusion is the opposite when two light nuclei join together to form a heavier one.
03:42
In both cases the products of the reactions are more stable than the starting materials,
03:45
and this is, as always, what drives the reaction.
03:48
This is a graph of the binding energies of various elements compared to their mass numbers.
03:51
Elements with very high binding energies such as iron-56 are very stable and rarely undergo nuclear reactions.
03:58
But elements with lower binding energies can react much more readily.
04:01
If the nucleus is heavier than iron-56 it will tend to break into two or more smaller nuclei; a fission reaction.
04:07
If it’s lighter than iron-56 it will more likely participate in a fusion reaction,
04:12
joining two nuclei together to form a heavier one.
04:14
But the most important thing to notice here is that
04:16
with both fission and fusion stability increases as a result of the reaction.
04:20
Fission is the type of reaction that we use more often
04:22
because it’s the one that we’re better at initiating and controlling, at least so far.
04:26
And whether it’s used in power plants or bombs, the most common fuel for fission is uranium-235.
04:31
There are several ways that it can react,
04:33
but the reaction is almost always triggered by hitting uranium with neutrons from another source.
04:37
When that happens the uranium splits into smaller atoms.
04:40
One such reaction produces krypton-92, yes krypton is a real thing,
04:44
along with barium-141, three free neutrons and lots of energy.
04:49
This energy is released mainly as the kinetic energy of the escaping particles,
04:53
which is immediately transferred to the surroundings as heat.
04:56
Some energy is also released in the form of electro-magnetic radiation such as
04:59
visible light, X-rays, and gamma radiation.
05:01
Nuclear power plants use the energy released by these reactions to convert water to steam,
05:06
which then is passed through turbines spinning a generator, powering cities and stuff.
05:11
Because of the enormous amounts of energy these reactions can release,
05:14
nuclear power plants can potentially produce lots of electricity,
05:18
but there’s also, I think you may have heard, some serious draw backs.
05:20
For one thing, as you know, atoms rarely exist in isolation.
05:23
We write the equation of a fission reaction as it fits just one atom,
05:27
but in reality that one atom is surrounded by many, many more.
05:30
And if one little neutron can trigger the reaction and that reaction liberates three more neutrons,
05:34
well I think you can see where this is going.
05:36
If the reaction isn’t controlled each reaction trigger three more,
05:40
and every reaction releases the same amount of energy, which adds up fast.
05:44
This is pretty much the definition of a chain reaction
05:46
and it is the basis of the remarkable power of the nuclear weapon.
05:49
The same type of reaction occurs in nuclear power plants,
05:52
but those reactions are controlled in several ways to keep them from getting out of hand.
05:56
The fact is these chain reactions have the potential to produce far more heat than the plant can use,
06:01
so much more that the temperature can easily rise to dangerous levels, enough to melt the uranium.
06:06
This is the meltdown that you hear about
06:08
and most reactor cores are immersed in water to disperse the heat and prevent this from happening.
06:12
But that, that’s not enough on its own to control this thing.
06:14
If the chain reaction is allowed to run freely,
06:16
no amount of water can remove the heat fast enough to prevent a meltdown.
06:19
A real way we control nuclear reactions is with control rods.
06:23
They’re made of materials that readily absorb neutrons
06:25
and they’re inserted between the fuel rods of uranium to slow the neutrons down and therefore slow the reaction.
06:31
They can be put in more to slow the reaction more, and lifted out more if you need more heat.
06:35
Now the other sticky wicket of fission reactions is the stuff that’s left behind.
06:39
These reactions not only produce products that are still radioactive,
06:42
they produce tons of them, lots of different troublesome kinds.
06:45
Like we saw last week, uranium undergoes many different types of nuclear decay,
06:49
so not only does each uranium atom produce isotopes of krypton and bromine,
06:52
but that process also produces many other isotopes of other elements.
06:56
And as these various nuclei break down they release more neutrons
06:58
and more unstable products and the process continues for a long time.
07:03
All of these reactions eventually yield stable products
07:05
but they have half-lives ranging from a few years to tens of millions of years.
07:09
The products with shorter half-lives stabilize pretty quickly
07:11
but they release particles and energy like crazy during that time so they’re extra dangerous.
07:16
The ones with longer half-lives decay more slowly, release less energy
07:19
but that means it takes a very, very long time for them to stabilize.
07:23
So long in fact, that for human purposes, it may as well be forever.
07:27
That means they’ll always be an issue in our environment which is why we’re always looking
07:32
for ways to store them, and keep them out of our way.
07:34
Fusion reactions, as you’d expect, are very different from fission.
07:37
For one thing, the energy released in many fusion reactions dwarfs even the huge amount released by fission.
07:43
You might be familiar, for example, with the wonderful work done by our sun.
07:46
The reactions that power the sun are like most fusion reactions,
07:49
in that they involve very small nuclei like isotopes of hydrogen and helium.
07:53
This reaction begins when two atoms of hydrogen,
07:55
accelerated by the sun’s fantastically high temperatures and contained by its high pressures,
08:00
join to form an atom of deuterium, an isotope of hydrogen.
08:04
This fusion of particles releases a positron and some heat energy in the process.
08:08
Then another atom of hydrogen is joined to the deuterium to form helium-3.
08:12
This step also releases a lot of energy in the form of gamma radiation.
08:16
When two atoms of helium-3 are available they join together to form an atom of helium-4,
08:20
as well as two atoms of regular hydrogen which then can be used to begin the process all over again.
08:26
This final step also, as you might imagine, releases a large amount of energy in the form of mostly gamma radiation.
08:32
So this is a chain reaction too, but it’s not a self-perpetuating one like we saw before.
08:36
This reaction requires a total input of 6 atoms of hydrogen but it only produces two,
08:41
in the end the remaining mass being released in the form of helium.
08:44
For this reason more fuel is always needed,
08:46
which is why our sun is going to run out of hydrogen in about 5 and a half billion years.
08:51
We can produce fusion reactions here on Earth too,
08:53
but they’re not very useful for us because we haven’t figured out how to control them.
08:57
They’re super useful if you just want to blow up a big city though,
09:01
just to be clear, depending on your definition of use.
09:04
One reason is, as you can see on the mass-energy graph,
09:07
light nuclei that fuse together undergo a much larger energy change than heavy nuclei that break apart.
09:12
That means their reactions release far more energy than fission reactions do,
09:16
so much more that it’s nearly impossible to contain and therefore use.
09:21
Also, because fusion involves joining nuclei,
09:23
the reaction has to overcome the really strong repulsion that naturally exists between their positive charges.
09:28
For this reason, fusion reactions can only occur when particles collide at very high speeds,
09:33
or under very high pressures.
09:35
At these mind-blowing speeds, the kinetic energy of the particles produces insane temperatures,
09:40
like in the 100 million kelvin range,
09:43
at which point, the material being accelerated actually exists in the form of plasma.
09:47
So not only are those speeds really hard to reach but material at that temperature, how do you control that?
09:53
Which is why we can’t use fusion for things like generating electricity which would be super nice.
09:57
We’ve only found applications for it when we don’t need to control it at all like in nuclear weapons.
10:02
So as you can tell, there is plenty of room for new ideas in nuclear chemistry.
10:06
Fusion would be really great because it would produce a lot of energy
10:09
and you’d just get helium out of the process and helium is awesome!
10:13
How can we use radioactive materials more efficiently?
10:17
Is there a way to achieve the speeds and manage temperatures that come with fusion?
10:21
And how can we do this stuff without blowing our faces off?
10:25
You’ve already taken the first step by learning the basics.
10:27
It’s up to you how far you want to go from here.
10:29
Maybe you’ll write the next totally crazy ingenious and counter intuitive equation that takes us to the next level.
10:35
For now though, thank you for watching this episode of Crash Course Chemistry.
10:38
If you paid attention, you learned how Einstein’s famous formula helps us calculate
10:42
the binding energy of a nucleus from its mass defect.
10:45
You also learned the difference between fission and fusion.
10:47
You saw an example of each one.
10:50
and you learned about their applications in the real world.
10:52
This episode of Crash Course Chemistry was written by Edi González and edited by Blake de Pastino.
10:56
Our chemistry consultant is Dr. Heiko Langner.
10:58
It was filmed, edited and directed by Nicholas Jenkins. Our script supervisor is Caitlin Hofmeister.
11:03
Michael Aranda is our sound designer and our graphics team is Thought Café.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video