Nuclear Energy (Fission and Fusion)

Question 1

When should you treat the neutron and proton as having slightly different masses?

Answer 1

Dealing with fusion or fission

(Calculating mass defects, binding energies or mass difference)

Question 2

Define mass defect (∆m)

Answer 2

Mass lost when nucleons (protons and neutrons) come together to form nucleus

Question 3

Define binding energy

Answer 3

Energy released when nucleons (protons and neutrons) come together to form nucleus
NOTE: This is responsible for the mass defect

Question 4

How is binding energy related to mass defect (∆m)?

Answer 4

Question 5

Define 1 atomic mass unit

Answer 5

Mass of 1/12 of a carbon-12 atom

(1.661x10^-27kg)

Question 6

What’s the next step here?

Answer 6

Multiply by 931.5MeV

Question 7

What’s the next step here?

Answer 7

Use E=mc²

Question 8

Sketch the binding energy per nucleon graph

Answer 8

Question 9

Label the fusion and fission regions

Why are they there

Answer 9

Energy only released if binding energy per nucleon increases

Question 10

Define metastable state in radioactive decay

Answer 10

Long-lived excited state of a nucleus

Eventually the nucleus de-excites emitting γ

Question 11

How many possible decays are there?

Answer 11

2β^- and 3γ

Question 12

What is nuclear fission?

Answer 12

Heavy nucleus splits into two lighter nuclei releasing energy and neutrons

Question 13

What are the 2 main isotopes used as fuel in nuclear reactors?

Answer 13

U-235, U-238

Question 14

How do you calculate the energy released in this fission reaction?

Answer 14

Calculate the mass difference (∆m_diff)

Question 15

How do you calculate if a reaction is possible?

Answer 15

If mass difference is positive reaction is possible

Question 16

What are the main components of the nuclear reactor?

Answer 16

1. Fuel rods - U-235
2. Control rods - Boron
3. Moderator - water or graphite
4. Coolant - CO₂ or water

Question 17

What does the moderator do in a nuclear reactor?

Answer 17

Reduce neutrons’ speeds

(More likely to be absorbed by U-235)

Question 18

What do the control rods do in a nuclear reactor?

Answer 18

Absorb some neutrons

Stop chain reaction occurring

Question 19

What does the coolant do in a nuclear reactor?

Answer 19

Transfer heat from fuel rods to the water that spins the turbines

Question 20

What are the 3 main safety features of the nuclear reactor?

Answer 20

1. Reaction happens inside thick walled concrete vessel
2. Control rods fully inserted to stop chain reaction if meltdown starts
3. Reactor flooded with water to remove thermal energy if meltdown starts

Question 21

In nuclear reactors how are spent fuel rods disposed?

Answer 21

1. Removed remotely from reactor
2. Stored in cooling ponds for up to 1 year
3. Vitrified by mixing with molten glass
4. Sealed in barrels
5. Stored in mountains or deep underground

Question 22

What is nuclear fusion?

Answer 22

Two lighter nuclei are combined to form one heavier nuclei and release energy

Question 23

Why is nuclear fusion so difficult to achieve?

Answer 23

Requires incredibly high temperatures and pressures

1. To ionise the isotopes
2. To bring isotopes close enough to overcome electromagnetic repulsion

Question 24

In fusion how close do the ionised isotopes need to get?

Answer 24

Close enough for the strong force to be larger than electromagnetic

(<1fm)

Question 25

What are the two main fusion isotopes in stars (in their main sequence)

Answer 25

Deuterium → H-2

Tritium → H-3

Question 26

How does binding energy per nucleon relate to the stability of an atom?

Answer 26

The higher the binding energy per nucleon, the greater the stability of the atom, as energy has been taken out of the system and so the atom is at a lower energy level

Question 27

Describe induced fission

Answer 27

• An isotope of uranium 235 is bombarded with slow moving neutrons to form uranium 236
• Uranium 236 is unstable and will undergo fission to split into two lighter nuclei, releasing neutrons and energy
• The neutrons collide with more uranium 235 nuclei and begin a chain reaction

Question 28

How does fusion work?

Answer 28

• For fusion to occur, nuclei must come within 1fm of each other in order for the strong force to overcome the EM repulsion
• The nuclei will be charged and have like charges meaning they repel
• As a result, the nuclei need a high KE to overcome repulsion so high temp and pressure is needed
• This is why fusion mostly happens in stars

Question 29

What must be the case after a decay?

Answer 29

The nuclei must be more stable than the nuclei before so binding energy per nucleon must have increased and mass must’ve decreased