Video: Understanding Energy Transfers and Conversions

A nuclear reactor contains radioactive material immersed in water. The water is heated by the radioactive material. Which of the following energy types increases for the water? [A] Chemical energy [B] Electrical energy [C] Nuclear energy [D] Internal energy Which of the following energy types decreases for the radioactive material? [A] Chemical energy [B] Electrical energy [C] Nuclear energy [D] Internal energy

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Video Transcript

A nuclear reactor contains radioactive material immersed in water. The water is heated by the radioactive material. This question has two parts. The first part asks us, which of the following energy types increases for the water? A) Chemical energy, B) Electrical energy, C) Nuclear energy, or D) Internal energy. The second part of the question asks us, which of the following energy types decreases for the radioactive material? A) Chemical energy, B) Electrical energy, C) Nuclear energy, or D) Internal energy.

So in this question, we’re thinking about a nuclear reactor. Inside a nuclear reactor, we generally have rods of radioactive material known as fuel rods. The rods are contained within a tank, and water is pumped through the tank. The radioactive fuel rods give off energy which is absorbed by the water, causing it to heat up. In another part of the reactor, this heat energy is used to evaporate water into steam, which turns turbines to generate electricity. However, in this question, we’re not really concerned with this stuff. We’re only concerned with the energy changes that occur inside the reactor core, where energy is transferred from radioactive material to water.

So, the first part of the question asks us to identify which type of energy increases for the water. We’re told in the question that the water is heated by the radioactive material, but heat energy is not one of the available options. So, let’s look at each option in order. Option A is chemical energy. Chemical energy is a type of energy which is stored in the bonds between atoms and chemical compounds. Examples of chemical energy include the energy that we store in our bodies and the energy that we consume in the form of food.

Chemical energy is transferred in chemical reactions, but the water inside a nuclear reactor doesn’t undergo chemical reactions. The water that goes in is the same as the water that comes out. And the water molecules are not altered in any way. This means that the chemical energy of the water doesn’t change, so we can rule out option A. Option B is electrical energy. Electrical energy manifests as the flow of electrons through a conductor. Even though a nuclear reactor is ultimately used to generate electricity, there shouldn’t be any electrical energy being transferred within the reactor core. So, that means option B is not the correct answer either.

Option C is nuclear energy. Nuclear energy is what holds together protons and neutrons in the nuclei of atoms. Nuclear energy is released when a large nucleus containing many protons and neutrons splits into two or more smaller nuclei. This process is known as nuclear fission. Nuclear fission is a type of nuclear reaction, and it serves as the basis for how we get energy out of the kind of nuclear fuel that we use in nuclear reactors. Nuclear fuel is comprised of large, unstable nuclei like this, which naturally undergo fission. That is, they can split into smaller nuclei, which releases a large amount of energy.

But while there’re definitely nuclear reactions going on inside a nuclear reactor and nuclear energy is being released, the actual water doesn’t undergo any nuclear reactions. Instead, the water molecules, shown here in blue, absorb kinetic energy from the particles that are released during nuclear fission. And kinetic energy on a molecular scale is the same as heat energy. So overall, when nuclear fission takes place within a nuclear reactor, we start off with nuclear energy contained within the unstable nuclei of the nuclear fuel. And this is converted into heat energy contained within the water molecules. So, we know that option C is incorrect as well. Although nuclear energy is crucial to the functioning of a nuclear reactor, the water itself doesn’t actually gain any nuclear energy.

This leaves us with just one option, internal energy. Internal energy refers to the energy contained within an object or a material due to the motion of and interactions between the molecules and atoms that make it up. Because the water in the nuclear reactor is being heated, that means the water molecules are moving around and vibrating more, which means the internal energy of the water is increasing. Another way of thinking about this is that if the heat energy contained within an object increases, that means that its internal energy increases too. So, for the first part of this question, the correct answer is D. When the water in a nuclear reactor is heated by the radioactive material, its internal energy increases.

The second part of this question asks us to identify which type of energy decreases for the radioactive material. Well, we now know that in this part of a nuclear reactor, the key energy transfer that takes place is that nuclear energy contained within the radioactive fuel is released in the form of heat energy, which is absorbed by the water in the reactor. It’s important to remember here that energy is always conserved. So, because we know nuclear energy in the radioactive fuel is being converted into heat energy, then the amount of nuclear energy contained within the radioactive fuel must be decreasing, which tells us that the right answer is C, nuclear energy. But let’s quickly take a look at the other answer options just to make sure.

Option A is chemical energy. Well, we know that the radioactive fuel doesn’t actually undergo any chemical reactions. It only undergoes nuclear reactions. That means that no chemical energy is transferred, so the chemical energy of the radioactive material can’t be decreasing. So, that means A is definitely not right. Option B is electrical energy. Once again, we know that no electrical processes are taking place within the reactor. There’s no current flowing. So, this means electrical energy is not being transferred. So, B is definitely not the correct answer either. Finally, option D is internal energy.

Recall that internal energy refers to the energy contained within a material due to the motion of and interactions between atoms and molecules. This is different to nuclear energy, which is all contained within the nuclei of atoms. The radioactive fuel within the reactor isn’t actually giving away any internal energy, which means its internal energy can’t be decreasing. In fact, if we allow all of the water within the nuclear reactor to heat up, then the radioactive material will heat up as well. Which means its internal energy, if anything, will increase. So, we know that D is not the correct answer either. The correct answer is C. Inside a nuclear reactor, the type of energy which decreases for the radioactive material is nuclear energy.

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