Nuclear Physics Part 2

Question 1

What is mass defect or mass difference?

Answer 1

The difference in mass between a nucleus and the mass of its constituent nucleons.

Question 2

What is binding energy?

Answer 2

The energy needed to separate all of the nucleons in a nucleus.

Question 3

How do you work out the average binding energy per nucleon?

Answer 3

Binding energy divided by nucleon number.

Question 4

Describe the graph of Average Binding Energy Per Nucleon against Nucleon Number.

Answer 4

• Steep gradient starting at about 1 MeV/2 nucleons and peaking at 8.8 MeV/56 nucleon number
• Steady decrease after peak down to about 7.2 MeV/250 nucleons

Question 5

What is the most stable nucleus?

Answer 5

Iron.

Question 6

What is Fission?

Answer 6

When large nuclei are unstable and split into two smaller nuclei.

Question 7

Why is energy released in Fission?

Answer 7

Because the new, smaller nuclei have a higher average binding energy per nucleon.

Question 8

How can Fission be induced?

Answer 8

By making a neutron enter a Uranium-235 nucleus, causing it to become very unstable.

Question 9

What is a low energy neutron called?

Answer 9

A thermal neutron.

Question 10

What is Fusion?

Answer 10

When two light nuclei combine to create a larger nucleus.

Question 11

Why is a lot of energy released in Fusion?

Answer 11

Because the new, heavier nuclei have a much higher average binding energy per nucleon.

Question 12

Why is it so hard to fuse nuclei together and about how much kinetic energy is needed?

Answer 12

• Because all nuclei are positively charged therefore there will be an electrostatic force of repulsion between them
• About 1 MeV of kinetic energy needed to fuse nuclei together

Question 13

How can you tell if it is energetically favourable for an element to undergo fission or fusion?

Answer 13

• By looking at the graph of average binding energy per nucleon against nucleon number
• Elements to the right of Iron can release energy through nuclear fission, and elements to the left of Iron can release energy through nuclear fusion

Question 14

How can you work out the energy released through fusion or fission?

Answer 14

The change in binding energy gives the energy released.

Question 15

What is another way of finding the energy released during fission or fusion other than using the graph?

Answer 15

Using E = Δmc ^ 2, where Δm is the total difference in mass between the initial and final nuclei (including neutrons).

Question 16

What are the 5 things included in a Nuclear Fission Reactor?

Answer 16

• Control rods
• Concrete case
• Fuel rods
• Moderator
• Coolant

Question 17

What do fuel rods contain and what are they used for?

Answer 17

Fuel rods contain Uranium-235 (sometimes Plutonium-239) which are used as fuel.

Question 18

How are chain reactions used in Nuclear Fission Reactors?

Answer 18

• Fission reactions produce more neutrons which then induce other nuclei to fission
• Neutrons will only cause a chain reaction if they’re slowed down

Question 19

What is an example of a moderator and what is it used for?

Answer 19

• A good moderator is water

- It is used to slow down and/or absorb neutrons

Question 20

How does a moderator slow down neutrons?

Answer 20

Through elastic collisions (kinetic energy is conserved) - all kinetic energy and momentum is transferred from neutron to moderator particle.

Question 21

What is the critical mass in a fission reactor?

Answer 21

The right amount of fuel which allows for fission to continue on it’s own at a steady rate, where one fission follows another.

Question 22

What are control rods used for?

Answer 22

To control the rate of fission.

Question 23

How do control rods control the rate of fission and what are they made of?

Answer 23

They absorb neutrons so the rate of fission is controlled (normally made of Boron).

Question 24

What is coolant used for in a fission reactor?

Answer 24

To remove heat produced by the fission (coolant usually the same water which is used as moderator).

Question 25

What are the three main safety points to keep a Nuclear fission reactor safe?

Answer 25

• Reactor shielding
• Emergency shut-down
• Handling and storing fission waste products

Question 26

What is reactor shielding?

Answer 26

Reactor surrounded by a thick concrete case, which acts as shielding and prevents radiation escaping.

Question 27

How can reactors be shut down in an emergency?

Answer 27

Through the release of control rods into the reactor - they are lowered fully into the reactor which slows the reaction down as quickly as possible.

Question 28

What is the problem with spent fuel rods?

Answer 28

They can release beta and gamma radiation which is highly penetrating.

Question 29

How can spent fuel rods be stored?

Answer 29

initially placed in cooling ponds to cool down, then stored in sealed containers until its activity has fallen sufficiently.

Question 30

What are the two main benefits of nuclear power?

Answer 30

• Doesn’t produce greenhouse gases

- Generates a lot more energy than fossil fuels

Question 31

What are the problems with nuclear power?

Answer 31

• Risk of nuclear disaster

- Waste products and how they’re dealt with safely