Nuclear Energy

Question 1

Relate the energy of an object to its mass

Answer 1

E = mc²

Question 2

When a particle and its corresponding antiparticle collide, what happens, what is produces and what are the products’ energies?

Answer 2

They annihilate
2 gamma photons are produced
Each gamma photon has energy mc²

Question 3

For a spontaneous reaction where no energy is supplied, give the equation for the energy released

Answer 3

Q = Δmc²

where Δm is the total mass before and after the reaction

Question 4

For α decay, how is the energy released distributed between the the products and how is the energy related to the products’ masses?

Answer 4

• The nucleus recoils so the energy released is shared between the α particle and the nucleus
• Due to the conservation of momentum, the energy released is shared between the to particles in inverse proportion to their masses

Question 5

For β decay, how is the energy released distributed between the the products and how is the energy related to the products’ kinetic energies?

Answer 5

-The energy released is shared in variable proportions between the β particle, the nucleus and the neutrino or antineutrino.
-When the β particle has maximum KE, the neutrino or antineutrino has negligible kinetic energy in comparison.
The maximum KE of the β is very slightly less than the energy released in the decay because of the recoil of the nucleus

Question 6

For electron capture, which product carries the energy released in the decay?

Answer 6

The neutrino carries away the energy released

The atom also emits an X-ray photon when the inner-shell vacancy (due to the electron capture) is filled

Question 7

Give the average strength of the strong nuclear force over a distance of about 1x10⁻¹⁵m (1fm)

Answer 7

200N

Question 8

Define the binding energy of a nucleus

Answer 8

The binding energy of the nucleus is the work that must be done to separate a nucleus into its constituent neutrons and a protons

Question 9

Define the mass defect of a nucleus

Answer 9

The mass defect Δm, of a nucleus is the difference between the mass of the separated nucleons and the mass of the nucleus

Question 10

Describe what causes the mass defect of a nucleus

Answer 10

When a nucleus is formed from its constituent nucleons, energy is released because the strong nuclear force does work pulling the nucleons together (binding energy)

Question 11

Give the equation for the mass defect for the nucleus of an isotope X with mass number A and Z number of protons

Answer 11

Mass defect Δm =Zmᴾ + (A-Z)mᴺ - mᴺᵘᶜ

where mᴾ and mᴺ represent the masses of the proton and the neutron respectively

Question 12

Give the equation for the binding energy of a nucleus

Answer 12

Binding energy = Δmc²

Question 13

Give 1Mev in Joules

Answer 13

1.6x10⁻¹³ J

Question 14

Explain the cause of quantum tunnelling for an α particle.

Answer 14

The binding energy of an α-particle is very large (7 MeV per nucleon). The α-particle gains sufficient kinetic energy to give it a small probability of ‘quantum tunnelling’ from the nucleus

Question 15

In a diagram for the energy of a nucleus, give the forces that are responsible for:

i) The ‘coulomb barrier’
ii) The ‘well’ of stability

Answer 15

i) The electrostatic force on the α-particle

ii) The strong nuclear force

Question 16

Define the binding energy per nucleon of a nucleus

Answer 16

The binding energy per nucleon of a nucleus is the average work done per nucleon to remove all the nucleons from a nucleus and is thus a measure of the stability of a nucleus.

Question 17

Describe how the binding energy per nucleon changes as mass number, A, increases

Answer 17

• When A 50, an increase in mass number causes a gradual decrease in binding energy from between 8.5MeV when A=50, to about 6MeV when A=250

Question 18

Give the name of the process that causes the mass number of a nucleus to increase and when is it most stable?

Answer 18

Fusion

Question 19

Give the name of the process that causes the mass number of a nucleus to decrease and when is it most stable?

Answer 19

Fission

Question 20

Define induced fission

Answer 20

When a fissionalable nucleus (e.g. U²³⁵ or Pu²³⁹) is bombarded with neutrons and the nucleus splits into two approximately equal fragments

Question 21

Explain why a chain reaction is possible in a fission reaction

Answer 21

Because fission neutrons are released in a fission event, which are each capable of causing a further fission event as a result of a collision with another fissionable nucleus

Question 22

What is the average energy released by each fission event

Answer 22

200 MeV

Question 23

Why is energy released when a fission event occurs?

Answer 23

Energy is released because the fragments repel each other (due to being +ve charged) with sufficient force to overcome the strong force.
The fragment nuclei and the fission neutrons therefore gain kinetic energy.
The binding energy increases due to the decrease in size of the nucleus and thus the energy released is equal to the change in binding energy

Question 24

Explain the change in binding energy for a fission event

Answer 24

The binding energy changes from about 7.5 - 8.5 MeV because because the size of the nucleus decreases (the mass number A, decreases so the binding energy per nucleon increases).

Question 25

Define fusion

Answer 25

A nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus with the release of energy.

Question 26

Explain why energy is released during fusion

Answer 26

Because the binding energy per nucleon of the product nucleus is greater than of the initial nuclei

Question 27

Give the condition required for nuclear fusion to take place and explain why

Answer 27

The nuclei must collide at high speed so that they overcome the electrostatic repulsion between the two nuclei so they become close enough that they interact through the strong nuclear force

Question 28

Give the fusion reaction between 2 protons

Answer 28

p¹₁ + p¹₁ → H²₁ + β⁰₋₁ + ν̄ₑ

Question 29

Give the name for H²₁

Answer 29

Deuterium

Question 30

Give the fusion reaction between Deuterium and a proton

Answer 30

H²₁ + p¹₁ → He³₂ + β⁰₋₁ + ν̄ₑ

Question 31

Give the fusion reaction between 2 helium nuclei

Answer 31

He³₂ + He³₂ → He⁴₂ + 2p¹₁

Question 32

Give the equation for the energy released during a fusion reaction

Answer 32

Q = Δmc²

where Δm is the difference between the total mass before and after the event

Question 33

What is produced as a result of fusion inside the sun

Answer 33

Solar energy

Question 34

At high temperatures of about 10⁸K, what happens to atoms and what is the name of this state

Answer 34

The atoms are stripped of their electrons

This state is referred to as ‘plasma’

Question 35

How does a nuclear reactor work?

Answer 35

Enriched Uranium rods containing 2-3% U-235 heats up water which heats a steel pipe of water in steam which turns a turbine. The reactor is controlled by control rods which absorbs neutrons from the fission reactions of the Uranium so to keep the number of neutrons in the reactor constant.

Question 36

What does PWR stand for?

Answer 36

Pressurised water reactor

Question 37

What is the use of a moderator in a fission reactor and what is the moderator in a PWR?

Answer 37

The moderator surrounds the fuel rods so that the fission neutrons emitted are slowed down, since without it, they are going too fast to cause further fission
Water is the moderator and coolant in the PWR

Question 38

Define and explain critical mass

Answer 38

The minimum amount of fissile material needed to maintain a nuclear chain reaction
This is because some fission neutrons escape from the fissile material without causing fission and some are absorbed by other nuclei without fission

Question 39

Define fissile material

Answer 39

The material capable of sustaining a nuclear fission chain reaction.
Fissile material can sustain a chain reaction with neutrons of any energy

Question 40

What does AGR stand for?

Answer 40

Advanced Gas-cooler Reactor

Question 41

Give 4 common safety features for nuclear reactors

Answer 41

1) The reactor core is a thick steel vessel designed to withstand high pressure and temperature in the core
It also absorbs all β-radiation as well as some γ-radiation and some neutrons
2) The core is in a building with very thick concrete walls which absorb the neutrons and γ-radiation that escape from the reactor vessel
3) Every reactor has an emergency shut-down system designed to insert the control rods fully into the core to stop fission completely
4) The sealed fuel rods are inserted and removed from the reactor by means of remote handling devices

Question 42

What is radioactive waste categorised into?

Answer 42

• High-level radioactive waste
• Intermediate-level radioactive waste
• Low-level radioactive waste

Question 43

Give an example of high-level radioactive waste and how they are disposed of/stored

Answer 43

• Spent fuel rods from a nuclear power station
• The fuel rods are removed by remote control and stored underwater in cooling ponds for up to a year due to the heat released. They can then be stored in sealed steel containers for centuries or the waste can be vitrified by mixing it with molten glass and storing is as glass blocks in underground caverns

Question 44

Described how intermediate-level radioactive waste is stored

Answer 44

They are sealed in drums that are encased in concrete and stored in specially constructed buildings with walls of reinforced concrete

Question 45

Describe how low-level radioactive waste is disposed

Answer 45

They are sealed in metal drums and buried in large trenches