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
