Nuclear Fission and Fusion

Question 1

When do we use Einstein’s mass-energy equation?

Answer 1

When we need to calculate the energy given out in a nuclear reaction.

Question 2

Define binding energy.

Answer 2

The energy needed to separate all the nucleons in a nucleus. This is equivalent to the mas defect.

Question 3

Define binding energy per nucleon.

Answer 3

The energy needed to separate all the nucleons in a nucleus, divided by the number of nucleons in that nucleus.

Question 4

How much binding energy is 1u of mass equal to?

Answer 4

931 MeV

Question 5

What does the binding energy per nucleon against nucleon number graph look like?

Answer 5

A curve that starts pretty much vertical, reaches it’s peak at about fifty, then decreases slowly.

Question 6

On the binding energy per nucleon against nucleon number graph, where do the most stable nuclei reside?

Answer 6

Around the maximum point of the graph.

Question 7

Where on the binding energy per nucleon against nucleon number graph does fusion occur?

Answer 7

From left to right, up until the maximum point.

Question 8

Where on the binding energy per nucleon against nucleon number graph does fission occur?

Answer 8

From right to left, up until the maximum point.

Question 9

Which releases more energy - fission or fusion?

Answer 9

Fusion.

Question 10

How can we use a binding energy per nucleon against nucleon number graph to estimate the energy released in a fission or fusion reaction?

Answer 10

Use the graph to read off the average increase in binding energy per nucleon, and multiply this by the number of nucleons in the single product/reactant.

Question 11

How can we use E=mc^(2) to determine binding energy?

Answer 11

Use the mass defect of the individual components of the nucleus and the actual mass of the nucleus.

Question 12

Describe the process of induced nuclear

fission.

Answer 12

Large nuclei (at least 83 protons) are unstable and we can encourage these nuclei to split into two, more stable nuclei by firing a low energy nucleon (thermal neutron) into it. This releases energy. The larger the nucleus, the more likely it will fission spontaneously. This limits the number of possible elements to exist.

Question 13

Describe and explain the process of nuclear

chain reaction.

Answer 13

A fission reaction usually emits more than one thermal neutron, though we only put one into the reaction. These multiple neutrons induce a fission reaction in more and more atoms - increasing the rate of reaction.

Question 14

Describe the basic construction of a fission

reactor.

Answer 14

Fuel rods surrounded by a moderator are kept in a concrete case. They have control rods between them, and are connected to a tank of water that they heat. Cool water is fed into the bottom of the tank, and the steam leaves a the top of the tank to turn a generator. A pump is included to keep the moderator flowing.

Question 15

Explain the role of the fuel rods.

Answer 15

To hold the fuel for the fission reaction, such as uranium-235.

Question 16

Explain the role of the control rods.

Answer 16

They absorb neutrons to control the chain reaction and keep it going at a steady rate. In an emergency, the control rods can be released fully to stop the reaction as quickly as possible.

Question 17

Explain the role of the moderator.

Answer 17

To slow down the neutrons released by reactions so that they can cause further fission. It can also absorb neutrons as it reaches higher temperatures to decrease the chance of a meltdown.

Question 18

What are control rods usually made from?

Answer 18

Boron.

Question 19

Explain the role of the coolant.

Answer 19

To remove the heat produced by the fission reactions, allowing it to heat up water, turning it to steam to power electricity-generating turbines.

Question 20

Give an example of a peaceful use of nuclear fission.

Answer 20

Power station.

Question 21

Give an example of a destructive use of nuclear fission.

Answer 21

Nuclear weapons (e.g. atomic bomb).

Question 22

Describe the environmental effects of nuclear

waste

Answer 22

Nuclear waste is often unstable, and therefore very radioactive. When storing this waste, it can affect water sources and soil, which can then damage living organisms. So, we have to seal them in extremely thick concrete containers.

Question 23

Describe the process of nuclear fusion.

Answer 23

Two light nuclei combine to create a larger, more stable nucleus. They can only fuse if they have enough energy to overcome the electrostatic repulsive force between them, and get close enough for the strong interaction to bind them.

Question 24

Describe the conditions in the core of stars that make fusion possible.

Answer 24

The temperature is so high that atoms don’t exist (10^7K). The electrons are stripped away leaving positively charged nuclei and free electrons. This is called plasma. The huge amounts of energy released in the fusions keeps the temperature high enough for further fusions to occur.