Nuclear Energy (Fission and Fusion) Flashcards
When should you treat the neutron and proton as having slightly different masses?
Dealing with fusion or fission
(Calculating mass defects, binding energies or mass difference)
Define mass defect (∆m)
Mass lost when nucleons (protons and neutrons) come together to form nucleus
Define binding energy
Energy released when nucleons (protons and neutrons) come together to form nucleus
NOTE: This is responsible for the mass defect
How is binding energy related to mass defect (∆m)?
Define 1 atomic mass unit
Mass of 1/12 of a carbon-12 atom
(1.661x10-27kg)
What’s the next step here?
Multiply by 931.5MeV
What’s the next step here?
Use E=mc2
Sketch the binding energy per nucleon graph
Label the fusion and fission regions
Why are they there
Energy only released if binding energy per nucleon increases
Define metastable state in radioactive decay
Long-lived excited state of a nucleus
Eventually the nucleus de-excites emitting γ
How many possible decays are there?
2β- and 3γ
What is nuclear fission?
Heavy nucleus splits into two lighter nuclei releasing energy and neutrons
What are the 2 main isotopes used as fuel in nuclear reactors?
U-235, U-238
How do you calculate the energy released in this fission reaction?
Calculate the mass difference (∆mdiff)
How do you calculate if a reaction is possible?
If mass difference is positive reaction is possible
What are the main components of the nuclear reactor?
- Fuel rods - U-235
- Control rods - Boron
- Moderator - water or graphite
- Coolant - CO2 or water
What does the moderator do in a nuclear reactor?
Reduce neutrons’ speeds
(More likely to be absorbed by U-235)
What do the control rods do in a nuclear reactor?
Absorb some neutrons
Stop chain reaction occurring
What does the coolant do in a nuclear reactor?
Transfer heat from fuel rods to the water that spins the turbines
What are the 3 main safety features of the nuclear reactor?
- Reaction happens inside thick walled concrete vessel
- Control rods fully inserted to stop chain reaction if meltdown starts
- Reactor flooded with water to remove thermal energy if meltdown starts
In nuclear reactors how are spent fuel rods disposed?
- Removed remotely from reactor
- Stored in cooling ponds for up to 1 year
- Vitrified by mixing with molten glass
- Sealed in barrels
- Stored in mountains or deep underground
What is nuclear fusion?
Two lighter nuclei are combined to form one heavier nuclei and release energy
Why is nuclear fusion so difficult to achieve?
Requires incredibly high temperatures and pressures
- To ionise the isotopes
- To bring isotopes close enough to overcome electromagnetic repulsion
In fusion how close do the ionised isotopes need to get?
Close enough for the strong force to be larger than electromagnetic
(<1fm)
What are the two main fusion isotopes in stars (in their main sequence)
Deuterium → H-2
Tritium → H-3
How does binding energy per nucleon relate to the stability of an atom?
The higher the binding energy per nucleon, the greater the stability of the atom, as energy has been taken out of the system and so the atom is at a lower energy level
Describe induced fission
- An isotope of uranium 235 is bombarded with slow moving neutrons to form uranium 236
- Uranium 236 is unstable and will undergo fission to split into two lighter nuclei, releasing neutrons and energy
- The neutrons collide with more uranium 235 nuclei and begin a chain reaction
How does fusion work?
- For fusion to occur, nuclei must come within 1fm of each other in order for the strong force to overcome the EM repulsion
- The nuclei will be charged and have like charges meaning they repel
- As a result, the nuclei need a high KE to overcome repulsion so high temp and pressure is needed
- This is why fusion mostly happens in stars
What must be the case after a decay?
The nuclei must be more stable than the nuclei before so binding energy per nucleon must have increased and mass must’ve decreased
