Nuclear Physics - Mass, Energy, Fusion and Fission

Question 1

Mass defect

Answer 1

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

Question 2

Binding energy

Answer 2

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

Question 3

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

Answer 3

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

Question 4

Units for binding energy

Answer 4

MeV

Question 5

How to calculate binding energy

Answer 5

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)

Question 6

Fusion

Answer 6

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

Question 7

Describe the condition needed for two nuclei to fuse and why

Answer 7

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

Question 8

Fission

Answer 8

The splitting of a large atomic nucleus into smaller nuclei

Question 9

How is fission induced

Answer 9

By firing neutrons at a nucleus

Question 10

Describe the process of fission

Answer 10

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

Question 11

What does a higher binding energy per nucleon indicate

Answer 11

Higher stability

Question 12

What is the most stable element and why

Answer 12

Iron as it has the highest binding energy per nucleon

Question 13

why does fission release energy

Answer 13

The smaller daughter nuclei have a higher binding energy per nucleon

Question 14

Why does fusion release energy

Answer 14

The larger nucleus has a much higher binding energy per nucleon

Question 15

Why does fusion only occur at high temperatures

Answer 15

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.

Question 16

Which elements undergo fusion and fission

Answer 16

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

Question 17

Why are light nuclei stable

Answer 17

Attractive nuclear forces are stronger than repulsive electrostatic force between protons

Question 18

Why are heavy nuclei stable

Answer 18

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

Question 19

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

Answer 19

Helium

Question 20

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

Answer 20

It allows society to use science to inform decision making.

Question 21

When does induced nuclear fission occur

Answer 21

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

Question 22

What are neutrons involved in induced fission called

Answer 22

Thermal neutrons

Question 23

Properties of thermal neutrons

Answer 23

They have low kinetic energy
They are slow moving

Question 24

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

Answer 24

A fast moving neutron will rebound from the nucleus.

Only a slow moving neutron can be absorbed by a U-235 nucleus

Question 25

Thermal neutron

Answer 25

A neutron which is in thermal equilibrium with its surroundings

Question 26

temperature of core reactor

Answer 26

300 degrees

Question 27

How are chain reaction produced as a result of fission

Answer 27

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.

Question 27

Kinetic energy of a thermal neutron =

Answer 27

3/2 x Boltzmann constant x thermodynamic temperature

Question 28

Critical mass

Answer 28

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

Question 29

What happens when you use less than the critical mass

Answer 29

Reaction eventually stops

Question 30

What happens when you use more than the critical mass

Answer 30

Leads to a runaway reaction and eventually an explosion

Question 31

What does a moderator do and how

Answer 31

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.

Question 32

What materials are often used for moderators

Answer 32

Water as its inexpensive and not very reactive Graphite These materials are poor absorbers of neutrons

Question 33

Control rods

Answer 33

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

Question 34

How are control rods used to control chain reactions

Answer 34

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

Question 35

Coolant

Answer 35

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

Question 36

What material is often used as the coolant and why

Answer 36

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

Question 37

What does lowering the rods do to the rate of fission

Answer 37

Decreases the rate of fission as more neutrons are absorbed

Question 38

What does raising the rods do to the rate of fission

Answer 38

Increases the rate as fewer neutrons are absorbed

Question 39

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

Answer 39

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

Question 40

What materials are control rods made from

Answer 40

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

Question 41

How are waste products from nuclear power stations stored

Answer 41

In water tanks or sealed underground

Question 42

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

Answer 42

To prevent damage to people and environment now and later

Question 43

What fuel is used in nuclear reactors and why

Answer 43

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

Question 44

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

Answer 44

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

Question 45

What are the 3 main types of nuclear waste

Answer 45

Low level Intermediate level High level

Question 46

Examples of low level waste and how it is stored

Answer 46

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

Question 47

Examples of intermediate level waste and how it is stored

Answer 47

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

Question 48

High level waste

Answer 48

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

Question 49

How is high level waste treated

Answer 49

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

Question 50

Benefits of nuclear power

Answer 50

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

Question 51

Risks of nuclear power

Answer 51

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