Nuclear Physics - Mass, Energy, Fusion and Fission Flashcards
Mass defect
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
Binding energy
The amount of energy needed to separate a nucleus into its constituent protons and neutrons.
Why do separated nucleons have greater energy and mass than a nucleus
Energy is taken in to break the nucleus into its separated nucleons.
Energy is proportional to mass.
So as energy increases, mass increases
Units for binding energy
MeV
How to calculate binding energy
- Calculate the mass defect.
- Convert mass from u to kg
- E = mc^2 (gives energy in J)
OR
- Calculate mass defect in u
- E = mass defect x 931.5 (gives energy in MeV)
Fusion
The fusing together of two small nuclei to produce a larger nucleus
Describe the condition needed for two nuclei to fuse and why
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
Fission
The splitting of a large atomic nucleus into smaller nuclei
How is fission induced
By firing neutrons at a nucleus
Describe the process of fission
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
What does a higher binding energy per nucleon indicate
Higher stability
What is the most stable element and why
Iron as it has the highest binding energy per nucleon
why does fission release energy
The smaller daughter nuclei have a higher binding energy per nucleon
Why does fusion release energy
The larger nucleus has a much higher binding energy per nucleon
Why does fusion only occur at high temperatures
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.
Which elements undergo fusion and fission
fusion - nuclei smaller than iron
fission - elements larger than iron
Why are light nuclei stable
Attractive nuclear forces are stronger than repulsive electrostatic force between protons
Why are heavy nuclei stable
Electrostatic forces of repulsion between protons is greater than attractive nuclear force.
What is the anomaly in the graph of binding energy per nucleon against nucleon number
Helium
What does the knowledge of the physics of nuclear energy allows society to do
It allows society to use science to inform decision making.
When does induced nuclear fission occur
When a stable nucleus splits into small nuclei due to the absorption of a slow-moving neutron
What are neutrons involved in induced fission called
Thermal neutrons
Properties of thermal neutrons
They have low kinetic energy
They are slow moving
Why can induced fission only occur through absorption of a SLOW-moving neutron and not fast
A fast moving neutron will rebound from the nucleus
Thermal neutron
A neutron which is in thermal equilibrium with its surroundings
temperature of core reactor
300K
How are chain reaction produced as a result of fission
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.
Kinetic energy of a thermal neutron =
3/2 x Boltzmann constant x thermodynamic temperature
Critical mass
The minimum mass of fuel required to maintain a steady chain reaction
What happens when you use less than the critical mass
Reaction eventually stops
What happens when you use more than the critical mass
Leads to a runaway reaction and eventually an explosion
Moderator
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.
What materials are often used for moderators
Water as its inexpensive and not very reactive
Graphite
These materials are poor absorbers of neutrons
Control rods
Absorb neutrons in the reactor in order to control chain reactions.
How are control rods used to control chain reactions
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
Coolant
Absorbs the heat released during fission reactions in the core of the reactor
What material is often used as the coolant and why
Water as it has a high specific heat capacity so can transfer large amounts of thermal energy.
What does lowering the rods do to the rate of fission
Decreases the rate of fission as more neutrons are absorbed
What does raising the rods do to the rate of fission
Increases the rate as fewer neutrons are absorbed
What is the heat energy absorbed by the coolant used to do
Produce steam which is used to power electricity-generating turbines.
What materials are control rods made from
Boron and Cadmium as they must be non-fissionable materials that absorb excess neutrons without decaying themselves
How are waste products from nuclear power stations stored
In water tanks or sealed underground
Why do waste products from nuclear power stations need to be stored safely
To prevent damage to people and environment now and later
What fuel is used in nuclear reactors and why
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
What measures are in place to reduce the worker’s exposure to radiation
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
What are the 3 main types of nuclear waste
Low level
Intermediate level
High level
Examples of low level waste and how it is stored
Clothing, gloves and tools which are lightly contaminated
Encased in concrete and stored a few m underground till it can disposed with regular waste
Examples of intermediate level waste and how it is stored
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
High level waste
The unusable fission products from the fission of U-235 or from spent fuel rods
Highly radioactive and extremely hot
How is high level waste treated
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
Benefits of nuclear power
Produce no polluting gases
Highly reliable to produce electricity
Require much less fuel as it provides more energy per kg
Risks of nuclear power
Production of radioactive waste is very dangerous and expensive to deal with
Nuclear meltdown could have catastrophic consequences to people and environment
