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AQA A Level Physics > Nuclear Physics - Mass, Energy, Fusion and Fission > Flashcards

Nuclear Physics - Mass, Energy, Fusion and Fission Flashcards

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1
Q

Mass defect

A

The difference between an atom’s mass and the sum of the masses of its protons and neutrons.

mass defect = mass of constituents - mass of nucleus

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2
Q

Binding energy

A

The amount of energy needed to separate a nucleus into its constituent protons and neutrons.

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3
Q

Why do separated nucleons have greater energy and mass than a nucleus

A

Energy is taken in to break the nucleus into its separated nucleons.
Energy is proportional to mass.
So as energy increases, mass increases

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4
Q

Units for binding energy

A

MeV

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5
Q

How to calculate binding energy

A
  1. Calculate the mass defect.
  2. Convert mass from u to kg
  3. E = mc^2 (gives energy in J)

OR

  1. Calculate mass defect in u
  2. E = mass defect x 931.5 (gives energy in MeV)
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6
Q

Fusion

A

The fusing together of two small nuclei to produce a larger nucleus

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7
Q

Describe the condition needed for two nuclei to fuse and why

A

Both nuclei must have high kinetic energy.
This is because nuclei must be able to get close enough to fuse.
The strong nuclear force which binds nucleons together acts at very short distances, so nuclei must get very close together for strong nuclear effect to occur. Electrostatic repulsion between nuclei must be overcome

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8
Q

Fission

A

The splitting of a large atomic nucleus into smaller nuclei

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9
Q

How is fission induced

A

By firing neutrons at a nucleus

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10
Q

Describe the process of fission

A

When the nucleus is hit by a neutron is splits into 2 or more daughter nuclei
During fission, neutrons are ejected from the nucleus
The neutrons collide with other nuclei which leads to a chain reaction.
This lasts until all material has undergone fission or reaction is stopped by a moderator

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11
Q

What does a higher binding energy per nucleon indicate

A

Higher stability

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12
Q

What is the most stable element and why

A

Iron as it has the highest binding energy per nucleon

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13
Q

why does fission release energy

A

The smaller daughter nuclei have a higher binding energy per nucleon

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14
Q

Why does fusion release energy

A

The larger nucleus has a much higher binding energy per nucleon

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15
Q

Why does fusion only occur at high temperatures

A

A massive amount of kinetic energy is required to overcome the electrostatic force of repulsion between nuclei.

The nuclei must also be able to get close enough to fuse. The strong nuclear force which binds nucleons together acts at very short distances, so nuclei must get very close together for strong nuclear effect to occur.

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16
Q

Which elements undergo fusion and fission

A

fusion - nuclei smaller than iron
fission - elements larger than iron

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17
Q

Why are light nuclei stable

A

Attractive nuclear forces are stronger than repulsive electrostatic force between protons

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18
Q

Why are heavy nuclei stable

A

Electrostatic forces of repulsion between protons is greater than attractive nuclear force.

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19
Q

What is the anomaly in the graph of binding energy per nucleon against nucleon number

A

Helium

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20
Q

What does the knowledge of the physics of nuclear energy allows society to do

A

It allows society to use science to inform decision making.

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21
Q

When does induced nuclear fission occur

A

When a stable nucleus splits into small nuclei due to the absorption of a slow-moving neutron

22
Q

What are neutrons involved in induced fission called

A

Thermal neutrons

23
Q

Properties of thermal neutrons

A

They have low kinetic energy
They are slow moving

24
Q

Why can induced fission only occur through absorption of a SLOW-moving neutron and not fast

A

A fast moving neutron will rebound from the nucleus

25
Q

Thermal neutron

A

A neutron which is in thermal equilibrium with its surroundings

26
Q

temperature of core reactor

A

300K

27
Q

How are chain reaction produced as a result of fission

A

Fission produces 2 daughter nuclei and 2 or 3 neutrons.
The neutrons released go on to collide with more nuclei and cause more fission reactions.

27
Q

Kinetic energy of a thermal neutron =

A

3/2 x Boltzmann constant x thermodynamic temperature

28
Q

Critical mass

A

The minimum mass of fuel required to maintain a steady chain reaction

29
Q

What happens when you use less than the critical mass

A

Reaction eventually stops

30
Q

What happens when you use more than the critical mass

A

Leads to a runaway reaction and eventually an explosion

31
Q

Moderator

A

Slows down the neutrons released in fission reactions to thermal speeds through elastic collisions between the nuclei of the moderator atoms and the fission neutrons which causes the neutrons to lose momentum.

32
Q

What materials are often used for moderators

A

Water as its inexpensive and not very reactive
Graphite

These materials are poor absorbers of neutrons

33
Q

Control rods

A

Absorb neutrons in the reactor in order to control chain reactions.

34
Q

How are control rods used to control chain reactions

A

The height of the rods in the nuclear reactor can be controlled in order to control the rate at which fission reactions occur to control the amount of energy produced

35
Q

Coolant

A

Absorbs the heat released during fission reactions in the core of the reactor

36
Q

What material is often used as the coolant and why

A

Water as it has a high specific heat capacity so can transfer large amounts of thermal energy.

37
Q

What does lowering the rods do to the rate of fission

A

Decreases the rate of fission as more neutrons are absorbed

38
Q

What does raising the rods do to the rate of fission

A

Increases the rate as fewer neutrons are absorbed

39
Q

What is the heat energy absorbed by the coolant used to do

A

Produce steam which is used to power electricity-generating turbines.

40
Q

What materials are control rods made from

A

Boron and Cadmium as they must be non-fissionable materials that absorb excess neutrons without decaying themselves

41
Q

How are waste products from nuclear power stations stored

A

In water tanks or sealed underground

42
Q

Why do waste products from nuclear power stations need to be stored safely

A

To prevent damage to people and environment now and later

43
Q

What fuel is used in nuclear reactors and why

A

Enriched uranium
This is U-238 enriched with U-235 as its the isotope which undergoes fission.
U-238 absorbs fission neutrons which help control rate of fission reactions

44
Q

What measures are in place to reduce the worker’s exposure to radiation

A

Fuel rods are handled remotely
Nuclear reactor is surrounded by a very thick lead or concrete shielding so radiation doesnt escape.
Control rods are lowered fully into reactor core to stop fission by absorbing all free neutrons in the core - emergency shut down

45
Q

What are the 3 main types of nuclear waste

A

Low level
Intermediate level
High level

46
Q

Examples of low level waste and how it is stored

A

Clothing, gloves and tools which are lightly contaminated
Encased in concrete and stored a few m underground till it can disposed with regular waste

47
Q

Examples of intermediate level waste and how it is stored

A

Daily used items to the fuel rods
The waste produced when a nuclear power station is decomissioned and taken apart
Encased in cement and steel drums and stored underground

48
Q

High level waste

A

The unusable fission products from the fission of U-235 or from spent fuel rods
Highly radioactive and extremely hot

49
Q

How is high level waste treated

A

Initially placed in cooling ponds of water for number of years
Mixed with molten glass and made solid
Encased in containers made from steel/lead/concrete
Stored very deep underground

50
Q

Benefits of nuclear power

A

Produce no polluting gases
Highly reliable to produce electricity
Require much less fuel as it provides more energy per kg

51
Q

Risks of nuclear power

A

Production of radioactive waste is very dangerous and expensive to deal with
Nuclear meltdown could have catastrophic consequences to people and environment

AQA A Level Physics (15 decks)

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