Nuclear physics 2 Flashcards
What is the mass defect
The difference in mass between the constituents (protons + neutrons + electrons) and the mass of the nucleus as constituents mass is greater
What is the reason for the mass defect
Mass is lost when the nucleons fuse to form the nucleus as this mass is converted to energy and released
What is binding energy
The energy required to separate the nucleus into its constituents as it needs to make up for the mass defect energy difference
Therefore, binding energy = mass defect x c^2
Why are atomic mass units used when measuring mass defects
The mass defects are very small so 1u = 1.661 x 10^-27 kg = 931.5 MeV
Means you can convert u to energy by timsing by 931.5 MeV instead of using E = mc^2 (saves time)
What is nuclear fission
When large, unstable nuclei split in to 2 daughter nuclei. It is either spontaneous or induced
Why is energy released during nuclear fission
The more stable daughter nuclei have a higher binding energy per nucleon than the U-235 and energy is released to make up for this difference
Why is the mass of the reactants greater than the mass of the products in nuclear fission
The extra binding energy of the products comes from mass that is lost and this lost mass is the mass defect
What is nuclear fusion
When 2 smaller nuclei join together to form one larger nucleus
Why is energy released during nuclear fusion
The larger nucleus produced has a much higher binding energy per nucleon so energy is released to make up for this difference
Why is nuclear fusion not possible on Earth right now
It can only occur at extremely high temperatures (e.g. in stars) as it requires a massive amount of energy to overcome repulsion forces between nuclei when joining them together
Why do we use binding energy per nucleon instead of just binding energy to compare stability
- If we have 2 atoms, X and Y, and X is more stable:
- Even though X is more stable, if number of nucleons (A) is higher for atom B, it would have a higher binding energy as there are more nucleons to break apart
- Therefore, to compare stability A has to be the same for both so we use A = 1 by finding binding energy per nucleon
Which atom has the highest stability
Iron - 56 as it has the highest binding energy per nucleon
Explain the graph of binding energy per nucleon against nucleon number for nucleon numbers before iron 56
- With lighter nuclei, as the number of nucleons increases, binding energy per nucleon increases very sharply as there are more nucleons surrounding and pulling on the nucleon you are trying to break off
- The repulsive force between protons would decrease the binding energy per nucleon but it is insignificant compared to this other effect
Explain the graph of binding energy per nucleon against nucleon number for nucleon numbers after iron 56
- The pulling forces from the surrounding nucleons are very short ranged
- Therefore, after a point (iron 56), more nucleons aren’t providing any extra forces holding the nucleon that you are trying to break off in place, as they are too far away, so binding energy per nucleon isn’t affected
- However, due to the small repulsive force between protons, the overall binding energy per nucleon decreases slightly with heavier nuclei
How can fission can be shown on a graph of binding energy per nucleon against nucleon number
- Fission is where heavy nuclei breaks into smaller nuclei which have a higher binding energy per nucleon so must be after iron 56
- Before iron 56 fission can’t occur as unstable atoms would be decaying into less stable ones with a lower binding energy per nucleon
How can fusion be shown on a graph of binding energy per nucleon against nucleon number
- Fusion is where smaller nuclei join together to form larger nuclei which have a higher binding energy per nucleon so must be before iron 56
- After iron 56, fusion can’t occur as if nuclei join together, they would be producing less stable nuclei with a lower binding energy per nucleon
How can it be shown that fusion releases far more energy than fission on a graph of binding energy per nucleon against nucleon number
- The difference in binding energy per nucleon is much greater before iron 56 as the gradient of the graph is very steep
- Bigger difference in binding energy means more energy has to be released to make up for it
How can you find the energy released during a fission reaction, given the mass (u) of the reactant and each product
Find the total mass of reactant
Find the total mass of products
Mass defect = mass before - mass after
Energy released (MeV) = mass defect x 931.5
How can you find the energy released during fission using the graph of binding energy per nucleon against nucleon number
Calculate binding energy of reactant= binding energy per nucleon x number of nucleons
Calculate binding energy of reactants by doing same thing and adding the binding energies of each reactant together
Energy released is BE of products - BE of reactant
How can nuclear fission be induced
- Fire thermal neutron into elements like U-235 causing it to become extremely unstable and split into 2 daughter nuclei and at least 1 neutron
- The neutrons released go on to cause more fission reactions causing a chain reaction
Why are thermal neutrons used in induced nuclear fission
They have low energy so are absorbed whereas high energy neutrons rebound away after a collision and do not cause fission
What is a common example of a fission reaction
n- 1 + U-235 –> Ba-141 + Kr- 92 + 3n- 1
Explain the critical mass in an induced fission reaction
- The minimum mass of fuel required to maintain a steady chain reaction
- If exactly the critical mass is used, on average, each fission reaction triggers 1 new fission reaction
- If sub - critical mass is used, the reaction would eventually stop
- Most nuclear reactors use super-critical mass but if mass is too high, the reaction could be uncontrollable
What is the role of the moderator in an induced fission reaction
Slows down neutrons to thermal speeds through elastic collisions between neutrons and the moderator atoms
Why is water often used as the moderator
- Contains hydrogen which is very close in size to a neutron so larger proportional of momentum is transferred per collision, meaning less collisions required to get neutrons to thermal speeds
- Inexpensive
- Not very reactive
What is the role of the control rods in an induced fission reaction
- Absorbs neutrons in the reactor to control chain reactions
- We can control the height of the rods to control the rate at which the reactions occur, which controls the amount of energy produced
- Fully drop control rods to absorb all fission neutrons for an emergency shutdown
Explain the choice of material for the control rods
- Materials which absorb neutrons without undergoing fission themselves
- e.g. boron or cadmium
What is the role of the coolant in an induced fission reaction
- Absorbs the heat released during fissions reactions in the core of the reactor
- The heat is used to make steam which powers electricity generating turbines in a nuclear power station
What materials are used as a coolant
- Materials with a high SHC as it can transfer more thermal energy
- e.g. water, molten salt or gasses like helium
Why is uranium enriched before use
- Natural uranium that is mined has 99% U-238 which can’t undergo fission and only 1% U-235
- Therefore, the uranium is enriched to increase the amount of U-235 to around 5% before being used as fuel in nuclear reactors, boosting the chance of fission
Give 2 safety aspects of the fuel rods in the nuclear reactor
- The U-238 in the fuel absorbs fission neutrons so helps control the rate of fission
- Fuel rods are inserted into the reactor remotely to limit workers exposure to radiation
What is the role of the shielding in the safety of a nuclear reactor
- Thick concreate shielding around the reactor blocks radiation from escaping and affecting workers in the power station
Why might the shielding be dangerous after a while
- The shielding may become radioactive after long term use as neutrons escape the reactor and enter the shielding nuclei
- This causes them to become unstable and start undergoing beta minus decay
What are the 2 types of nuclear waste
- High level waste - most dangerous
- Low level waste - less dangerous
What is an example of high level waste
The daughter nuclei as they are unstable, have a high activity and can stay radioactive for 1000s of years
What is an example of low level waste
- Tools and gloves as they contain only short lived radioactivity
How can low level waste be disposed
Does not take long to stop being radioactive so can be disposed close the surface
How can high level waste be disposed
- The waste is removed remotely to limit exposure
- Place in cooling ponds for up to a year
- Any plutonium or uranium from spent fuel rods is recycled
- Waste is vitrified (encased in glass) and placed in thick steel casks
- It is stored in deep caverns in geologically stable locations so they can’t come free from the casing. These locations are places where the waste will have minimal impact on the environment.
- People in the local area are consulted first
What is the role of the cooling pond in the disposal of nuclear waste
- Waste is extremely hot due to fission reactions and after removal, they may still be producing heat through radioactive emissions so need to be cooled down
- The ponds are on the same site as the reactor so the materials don’t have to be transported long distances, limiting exposure
Give 2 advantages and 2 disadvantages of using nuclear power stations
Advantages:
- Produce power without emitting greenhouse gasses
- Need far less fuel to produce the same amount of power (1 kg of uranium gives same as 25 tonnes of coal)
Disadvantages:
- Daughter nuclei are radioactive and need to be stored for 1000s of years
- Meltdown in power plant is possible and would damage the environment
