Fission and Fusion COPY Flashcards
What is the equation that links mass and energy?
E = mc2
define the binding energy of a nucleus
The binding energy of a nucleus is the energy required to completely separate its constituent protons and neutrons OR the energy released when its formed from its constituent protons and nucleus
Does a nucleus have more mass together or split up?
Separate
Define mass difference
The difference between the mass of a nucleus and the total mass of its separate protons and neutrons
What does u equal in kg?
1.661x10-27Kg
What does u equal to in MeV?
931.5
What does the binding energy equal?
- Calculate the mass difference
- Multiply by 931.5
Define the atomic mass unit, u
it is defined as 1/12 th the mass of a C-12 atom
Describe nuclear fusion
- Two small nuclei fuse together to make a larger nucleus which is more stable and has more binding energy per nucleon then either of the two smaller nuclei
How is energy produced from fusion?
The binding energy per nucleon is greater after fusion. This causes energy to be released in the form of kinetic energy of the larger nucleus.
What is the mass defect?
The mass lost when two smaller nuclei fuse together
Why is it difficult to get nuclear fusion to occur?
- The two smaller nuclei are both positively charged and so would naturally repel each other.
- The nuclei m,ust be moving with vert high speeds in order to overcome this repulsion and get close enough for the strong force to bond them.
Is energy absorbed or released when the bonding energy increases
Energy is released when there is an increase in binding energy
How is energy produced from fission?
When the nuclei splits, the binding energy per nucleon increases
this releases energy as kinetic energy
What is nuclear fission?
A large nuclei splits into two smaller nuclei
The mass defect produced gets turned into kinetic energy which incfeases the binding energy per nucleon
this releases kinetic energy
How do you calculate the binding energy per nucleon?
1) calculate the mass of the protons (number of protons x the atmonic mass of proton, on formula book)
2) Calculate the mass of the neutrons (same way as protons)
3) Calculate the mass of consituents (protons + neutrons)
4) Calculate the mass of the nucleus (mass of atom (given on question) - mass of electons (calculated same way as p + n)
5) Calculate mass defect (3) - (4)
6) multiply mass defect in u by 931.5 to get it in MeV
7) Divide binding energy by number of nucleons
What advantages do both nuclear fission and fusion have over using fossil fuels for energy production? (2)
They do not release greenhouse gases which cause global warming
Nuclear fuels will last longer
How does the sun produce its energy?
Nuclear fusion
What fuels are needed in fusion and where can they be extracted from?
Hydrogen fuels (deuterium and tritium)
deuterium: extracted from sea water
tritium: from lithium metal
Why are deuterium and tritium called ‘hydrogen heavy’ and ‘super hydrogen heavy’ respectively?
They’re isotopes of hydrogen
they have more neutrons than hydrogen so they are ‘heavy’
What are the 3 advantages of using fusion rather then fission for mains electricity?
- Fusion releases more energy PER KG
- Fusion is safer as the plant won’t go into meltdown
- Waste products of fusion are not as radioactive as the waste products of fission
Why is is so difficult for scientists to get fision to work?
It needs a lot of energy to heat it and produce the magnetic field (often more than is released by fusion)
What does the stability of a nucleus depend on?
The mean binding energy per nucleon in the nucleus
What does the size of the mean binding energy sday about the stability of the nucleus?
The greater the mean binding energy, the greater the stabler the nucleus
Draw the graph of the mean binding energy per nucleon against the nucleon number:
- Mark on the position of iron
- Mark on the fusion area
- Mark on the fission area
Which element is the most stable?
Fe
Define induced fission
Where a nucleus has to absorb a neutron before it splits
Define critical mass
The minimum mass required in induced fission that will support a self-sustaining reaction
Sketch and describe a chain reaction showing more than one fission event
Give two points describing the the daughter neclei produced in fission
- They are highly radioactive
- They are beta- and gamma emitters
Define enriched uranium
Has a higher percentage of U-235 than in natural uranium
What is the percentage of U-235 in enriched uranium?
2.3%
What is the percentage of U-235 in natural uranium?
0.7%
What is the other isotope od uranium present in natural and enriched uranium
U-238
Why do reactors use enriched uranium rather than natural uranium.
Enriched uranium has a higher percentage of U-235 than in natural uranium .
This allows for a lower critical mass so the fuel rods are smaller
Why do the tubes that contain the fuel for fission need to be narrow?
This increases the surface area so
more neutrons can pass out of the surface of the rod, through the moderator, lose kinetic energy and then have more chance of causing a fission when they hit a uranium nucleus in anither rod
Define thermal neutrons
Slow moving neutrons with low kinetic energy
Why do we need to turn the neutrons produced in fission to thermal neutrons before other fission reactions occur?
The neutrons in the fission of U-235 are very fast movingand are much less likely to cause fission than thermal neutrons and are more likely to be absorbed by U-238
Give examples of moderators
Graphite and heavy water (deuterium oxide)
Describe low level waste
- Clothing and tools used a the plant
- Shielding not usually required
- Buried in shallow trenches after compaction and incineration
Describe moderators
- must be substances which have a low chance iof absorbing neutrons
- Atoms nust be a similar mass to a neutron, so tha the neutron loses a significant amount of kinetic energy when it hits one of the atoms
- The neutrons typically need to undergo lots of collisions with moderator atoms to lose enough energy to cause fission (e.g. 50)
What do control rods do?
Absorb neutrons
Which materials are often used in control rods?
Boron and cadmium
Why are Boron and Cadmium used as control rods?
There is a high probability of the high energy neutrons being absorbed on collision
How is the rate of fission controlled?
By inserting and removing the coltrol rods
What is a self-sustaining reaction?
When, on average, more than one neutron produces another reaction and may go out of control
Ideally, for a constant reaction rate, only one neutron produced by fission goes on to produce another reaction
How is a fission reaction stopped in an emergency?
The control rods are fully inserted to absorb all the neutrons
What does a coolant do?
Extracts heat from the core of the reactor
What properties does a coolant need to have?
- high specific heat capacity
- Non-flammable
- Non-corrosive
What fluids are usually used as coolants
water and carbon dioxide
How does the coolant extract heat from the core?
It is passed through the core and itno the heat exchange where it turns water into steam
What does shielding do?
- Surrounds the reactor to protect people working near the reactor
- Around 5m of concrete to stop gamma, beta and any neutrons which escape the reactor
What does the pressure vessel do and what is is made of?
- To keep the coolant under pressure
- Made of steel
What is conserves in an elastic collision?
Kinetic energy and momentum
What is assumed when doing calculations (momentum) concerning a neutron and a moderator particle?
The moderator particle is stationary before the collision
Describe high level waste
- Spent nuclear fuel or liquid waste from reprocessing
- Highly radioactive for thousands of years
- Has to be cooled for 20-50 years before transfer
- Currently kept in cooling ponds close to reactors
Describe intermediate level waste
- Requires shielding
- Typically resins and decommisioning equipment
- Waste is usually buried
How are spent fuel rods dealt with?
- They are initially at a high temperature and so are put in cooling ponds where they can stay for years
- The fuel cells are handled remotely by robots when being inserted and taken out of the reactor to avoid humans coming near them
What are the risks to nuclear fuels?
If there is an accident, there would be a risk of highly radioactive material being released which would damage peoples health
- It is expensive and dangerous to decommision a plant
