4.4.4 Nuclear fission and fusion

Question 1

What is nuclear fission?

Answer 1

The splitting of large, unstable nuclei to form smaller more stable nuclei (+the emission of spare neutrons).

Question 2

Give an example of a fissionable isotope.

Answer 2

Uranium - 235

Question 3

What usually needs to happen to induce fission?

Answer 3

● The unstable nuclei must absorb a neutron
● Spontaneous fission (where no neutron absorption occurs) is rare

Question 4

Alongside two smaller nuclei, what else is emitted in a fission reaction?

Answer 4

● Two or three neutrons
● Gamma rays
● Energy

Question 5

What form of energy do all fission products have?

Answer 5

Kinetic energy.

Question 6

What takes place during a chain reaction in a nuclear reactor?

Answer 6

● An unstable nucleus absorbs a neutron
● The nucleus undergoes fission and
releases 2 or 3 further neutrons
● These induce more fission, which results in a chain reaction

Question 7

What is the consequence of an uncontrolled chain reaction?

Answer 7

● The rate of fission events becomes to high and results in the production of
too much energy
● This can lead to a nuclear explosion

Question 8

What are the three main components of the core a nuclear reactor?

Answer 8

1. Fuel rods
2. Control rods
3. Moderator

Question 9

What is the role of the moderator in a nuclear reactor?

Answer 9

To slow down the neutrons so they are travelling at speeds which allow them to be absorbed by fissile nuclei and cause fission.

Question 10

How is the chain reaction in a fission reactor kept under control?

Answer 10

● Control rods are positioned in between the fuel rods
● The rate of fission is controlled by moving these rods up and down
● The lower the rods are inserted, the slower the rate of fission

Question 11

What term is used to describe nuclei in which fission can be induced through the absorption of slow neutrons?

Answer 11

Fissile Nuclei

Question 12

What is nuclear fusion?

Answer 12

The joining of two light nuclei to produce a heavier nuclei and release energy.

Question 13

Name two isotopes of hydrogen which are commonly used in nuclear fusion.

Answer 13

Deuterium and Tritium

Question 14

Which releases more energy, nuclear fission or nuclear fusion?

Answer 14

Nuclear fusion.

Question 15

Explain the difficulty of generating energy through nuclear fusion.

Answer 15

Fusion requires very high temperatures which in itself requires large quantities of energy and also requires casing which can withstand them.

Question 16

Explain why nuclear fusion is currently not a viable way to produce energy on Earth.

Answer 16

With current equipment/techniques, the energy required is greater than the energy produced, resulting in a net energy loss.