Video Transcript
In this video, we’ll learn about
the processes of nuclear fusion and nuclear fission. We’ll learn how these processes
produce energy and how that energy is used to generate electricity in a nuclear
power plant. Let’s start off by exploring
nuclear fusion and nuclear fission. During fusion, two or more lighter
nuclei combine to make a heavier one. This is an example of a fusion
reaction. During this reaction, four
lightweight hydrogen nuclei combine to create a heavier helium nucleus and two
positrons. This reaction is the net reaction
of a series of more complicated reactions that occur in the Sun.
Fission is like the opposite of
fusion. During fission, a heavy nucleus is
split into two or more lighter ones. This reaction is an example of a
fission reaction. In this reaction, a neutron strikes
an atom of uranium-235, causing it to split into lighter barium and krypton
nuclei. This also produces three
neutrons. This is the reaction that’s used to
generate electricity inside nuclear power plants. If you ever have trouble
remembering which one is which, just remember that in fusion, nuclei combine or fuse
together. Both fusion and fission involve
changes to the nucleus of the atoms involved. In fusion, the nuclei are combined,
and in fission, a nucleus is split. This means that both reactions are
going to involve changes in binding energy, which is the energy required to keep the
protons and neutrons in the nucleus bound together.
Because of this, both fusion and
fission release enormous amounts of energy. Both fusion and fission release
millions of times more energy than burning fossil fuels does per kilogram of
starting material. But if we wanted to compare the
two, fusion reactions release about four times more energy than fission reactions do
per kilogram of starting material. Fusion reactions also have other
benefits compared to fission reactions. The fuel for fusion reactions,
hydrogen gas, is much easier to obtain than uranium-235. Fusion also produces much less
waste than fission does, and the waste that is produced is much easier to deal
with. The fusion reaction is also much
easier to control once it started than the fission reaction is.
So if nuclear fusion has all of
these benefits as a source of energy compared to fission, why is it that we use
fission in nuclear power plants? Well, as amazing a source of energy
as fusion could be, it’s unfortunately not a feasible source of energy production
right now. The reason for this has to do with
the fact that during fusion, positively charged nuclei must collide with each other
in order to combine. But as we know, positively charged
things tend to repel each other. So in order for these positively
charged nuclei to combine, they need to be traveling at incredibly high speeds when
they collide with each other. These high-speed collisions can
only happen under certain conditions when the temperature and pressure is very
high. This is why fusion can happen
readily in the Sun, where the temperatures can reach millions of degrees
Celsius.
So fusion isn’t feasible right now
because it’s incredibly difficult to maintain the necessary conditions for fusion to
occur, though we may one day be able to use fusion reactions to generate
electricity, since this is a very active area of research. Scientists are investigating using
plasmas and targeting lasers to maintain the fusion reaction. That’s not to say that we’ve never
created fusion on a large scale. This has already happened in the
context of a thermonuclear bomb. A thermonuclear bomb first uses a
fission reaction to create the necessary temperatures and pressures for the fusion
reaction to occur. Since we can’t use fusion reactions
to generate electricity, let’s see how we can accomplish this with fission
reactions.
Let’s take another look at the
reaction that’s used to generate electricity in nuclear power plants. This reaction begins when a neutron
strikes an atom of uranium-235. This produces barium-141 and
krypton-92, as well as three more neutrons. Now, each of those neutrons could
strike another atom of uranium-235, again causing it to split into barium and
krypton and producing three neutrons. And those neutrons can strike
another atom of uranium and so on. So fission reactions are capable of
creating a self-sustaining chain reaction that can continue as long as there’s
neutrons and atoms of uranium. Maintaining this chain reaction is
a large part of the operation of a nuclear power plant.
If there aren’t enough collisions
occurring between the neutrons and uranium, the reaction will stop, and we won’t be
able to produce energy. But if we have too many collisions,
we run the risk of the reaction becoming uncontrolled. In the context of a nuclear power
plant, an uncontrolled reaction is very bad. It can lead to the meltdown of the
reactor core. But an uncontrolled reaction might
be what we were going for if we wanted to build a bomb. We can control the chain reaction
in a nuclear power plant through neutron moderation and neutron absorption.
Neutron moderation slows neutrons
down to ensure more successful collisions between the neutrons and atoms of
uranium. Neutron absorption does the
opposite. Neutrons are blocked or absorbed by
some material, so it will prevent collisions between the neutrons and uranium. In other words, neutron moderation
increases the number of collisions, and neutron absorption decreases the number of
collisions.
Now, let’s see how all of this ties
in together to generate electricity in a nuclear power plant. Generating energy in a nuclear
power plant starts with the fuel rods. Fuel rods are metal tubes that are
packed with the fuel for the fission reaction, uranium-235. The fuel rods are placed inside a
reaction vessel, which is called the reactor core, or just the reactor. The reactor core is full of
water. The water acts as both a coolant
for the reaction and a neutron moderator, so it slows neutrons down, ensuring more
successful collisions.
The last thing inside the reactor
core is the control rods. The control rods are neutron
absorbers. They are used to control the
reaction in the reactor core. If the reaction is too slow,
control rods can be removed so that fewer neutrons are absorbed and there are more
collisions. If the reaction is occurring too
quickly, more control rods can be inserted into the reactor core so that more
neutrons are absorbed and there are fewer collisions.
The fission reaction produces a lot
of energy, which will heat up the water in the reactor core. The hot water is pumped into a
boiler, where it’s turned into steam, or the steam might be created inside the
reactor core depending on the design of the reactor. The steam is used to spin a
turbine, and the spinning turbine is used to generate electricity in a
generator. From there, the steam is pumped to
a cooling tower, where it cools down so it can be pumped back into the reactor core
to be reused.
The operation of a nuclear power
plant generates waste. The waste is things like tools used
near the reactor core, the clothing of workers who worked near the core, and spent
fuel rods. A nuclear power plant generating
enough electricity to power the homes of about one million people will generate
approximately three cubic meters of waste per year. In comparison, a coal plant
generating enough electricity to power the homes of about one million people will
generate 300,000 tons of ash per year and 6,000,000 tons of carbon dioxide per
year. So the amount of waste produced by
a nuclear power plant is a relatively small volume. But nuclear waste is radioactive,
so it needs to be dealt with carefully.
The first thing that we do to deal
with nuclear waste is to put it in barrels and store it underwater. We do this because fresh nuclear
waste is extremely hot and quite radioactive. After the waste has had a chance to
cool down, it can either be recycled or disposed of. If we recycle the waste, the
uranium is extracted, mixed with fresh uranium, and used to create new fuel
rods. But not all waste can be
recycled. There’s always some small amount,
about four percent, that will need to be disposed of. To dispose of the nuclear waste,
it’s buried in barrels in deep underground caverns.
Now, we’ve covered everything we
need to know about nuclear energy. So let’s practice our knowledge
with a problem.
What is the order of transfer of
the electrical energy generated by nuclear fission from the fuel rods through a
power plant? (A) Fuel rods, generator, reactor,
boiler, turbines. (B) Fuel rods, reactor, boiler,
turbines, generator. (C) Fuel rods, boiler, turbines,
generator, reactor. (D) Fuel rods, generator, boiler,
turbines, reactor. (E) Fuel rods, generator, turbines,
reactor, boiler.
Nuclear fission is a type of
nuclear reaction where the nucleus splits into two or more lighter nuclei. Nuclear fission is the kind of
reaction that’s used to generate electrical energy inside a nuclear power plant. This question is asking us to
determine the order of transfer of the energy generated by nuclear fission through a
power plant, in other words, the parts of the power plant that this energy goes
through before it can be used to power our homes and businesses. And we’re told that this process
starts with the fuel rods.
Fuel rods are metal tubes packed
with the fuel for the fusion reaction, uranium-235. The fuel rods are placed inside the
reactor. The reactor is full of water. The water inside the reactor will
heat up because of the energy given off during the fission reaction. The hot water is turned to steam
inside the boiler. The steam is used to spin a
turbine, and the spinning turbine generates electricity in a generator. So the order that the energy is
transferred through nuclear power plant is the fuel rods, the reactor, the boiler,
the turbine, and finally the generator. This matches answer choice (B).
Now, let’s wrap up this video by
summarizing what we learned. During nuclear fusion, two or more
nuclei combine to make a heavier one. An example is this reaction, where
four hydrogen combine to make helium and two positrons. During nuclear fission, a heavy
nucleus splits to make two or more lighter nuclei. An example of a fission reaction is
this reaction, where a neutron strikes an atom of uranium-235 causing it to split
into barium-141 and krypton-92, which also produces three neutrons.
Fission reactions create a chain
reaction, which can be controlled to generate electricity in a nuclear power
plant. The chain reaction is controlled by
neutron moderation and neutron absorption. Neutron moderation slows neutrons
down to ensure more collisions between neutrons and atoms of uranium. Neutron absorption blocks the
neutrons from colliding with atoms of uranium to decrease the number of
collisions.
