Physics - Nuclear Fission Flashcards
Define isotope
Elements with the same number of protons, but a different number of neutrons
List isotopes of uranium from most abundant to least abundant giving percentages
Uranium-238 (99.2745%)
Uranium-235 (0.720%)
Uranium-234 (0.0055%)
How can different isotopes be separated? (6 methods)
Centrifuge – heavier elements found at the edge and lighter in the middle
Diffusion – lighter isotopes travel faster and diffuse more readily
Mass spectrometry – heavier elements not bent as much
Laser – laser frequency tuned to ionise one isotope, but not the other
Chemical reaction – although isotopes have similar properties they can sometimes be separated due to the slight differences
Gravity – heavier isotopes of liquid carbon, oxygen and nitrogen sink to bottom
What is meant by a stable nuclei?
One that does not undergo radioactive decay
What is meant by ‘more stable’ nuclei?
Has a longer half-life
Describe nuclear fission
A neutron is absorbed by a heavy parent nuclei. This causes it to become unstable and split into two daughter nuclei and other neutrons
How does U-235 undergo nuclear fission?
U-235 absorbs a neutron to become U-236. This is unstable and splits into two lighter daughter nuclei and several neutrons
Why does nuclear fission of U-235 produce energy and in what form does this energy come?
The mass of U-235 and the addition neutron is greater than the combined mass of the daughter nuclei and the several neutrons produced.
This extra mass is converted into energy in the form of KE of mainly the neutrons
What do the terms in this symbol represent? (_Z^A)X
A=Mass Number
Z=Proton Number
X=Element symbol
What rule is used to balance a nuclear equation?
Sum of the mass number and atomic number to the left of the arrow must equal that to the right of the arrow
Explain how one fission reaction can lead to a chain reaction
During the process of fission several neutrons are released.
These neutrons can go on to cause subsequent fission reactions.
The process is then repeated
Use inequalities to show how the number of neutrons released and also absorbed can lead to a nuclear bomb or a nuclear reactor
If number produced is <=1 then a nuclear reactor forms
If number produced is >1 then a nuclear bomb can be made
What form does the energy in a nuclear reactor come in?
KE of daughter nuclei and neutrons
What can the energy generated in a nuclear reactor be used for?
This can be used to heat water and generate steam to turn turbines and generate electricity
Sketch a simple diagram of a nuclear reactor
(See diagram with: uranium fuel rods, boron control rods, core, concrete shield, graphite moderator, cold water)
Describe the function of the control rods
Absorb excess neutrons
Describe the function of the moderator
Slows down neutrons so they become thermal neutrons and are more easily absorbed by U-235
What safety mechanisms are in place?
Control rods can be dropped to reduce number of neutrons
Concrete shield to absorb neutrons and other radiation
How can a nuclear bomb be created?
Pure fission:
gun assembly - fires one piece of uranium into another therefore reaching critical mass;
implosion – fissile mass surrounded by explosives that compress the fissile material so they reach critical mass
Boosted fission weapon:
fission creates high temperature that fuses hydrogen together to form helium and several neutrons. Fusion creates little energy, but the additional neutrons speeds up the fission reaction
Pure fusion weapons: Not created yet
How much energy does 1kg of nuclear fuel and fossil fuel produce?
Coal: 8kWh
Oil: 12 kWh
Nuclear: 24,000,000 kWh
Describe the results of the alpha particle scattering experiment (Rutherford Experiment) and explain how it gave us the present structure of the atom
Most alpha particles went straight through the gold leaf – atoms are mostly empty space
Small number where deflected by large angles – small very dense nucleus with positive charge
Sketch the structure of the atom and label the protons, neutrons and electrons
(Find a diagram)
Define mass number/atomic mass number/nucleon number (A)
Number of protons and neutrons (nucleons) in an atom
Define atomic number/proton number (Z)
Number of protons in a nucleus
Define relative atomic mass
The ratio of the average mass of one atom of an element to one twelfth of the mass of an atom of carbon-12
Describe how electrons are held in the atom
Electrostatic force between the protons and electrons
Describe how protons and neutrons are held in the nucleus
Strong nuclear force between protons and neutrons
Explain why it takes energy to separate the protons and neutrons in the nucleus
You need to overcome the strong nuclear force
Write down Einstein’s equation for energy*
E=mc^2
By comparing the ranges of different forces, explain why large nuclei are radioactive
Strong nuclear force is short range, while electrostatic is long range. In large nuclei the electrostatic force builds as more protons are added and outer nucleons are repelled - and the strong nuclear force is less effective at keeping them together
What is Alpha radiation? What is its charge/mass? How penetrating is it? How ionising is it?
Helium nucleus (2p+2n), atomic mass 4u, charge +2e, short range (10cm air and paper) and highly ionising
What is Beta radiation? What is its charge/mass? How penetrating is it? How ionising is it?
Electron or positron (e- or e+), atomic mass u/2000, charge -1e, medium range (few mm aluminium) and medium ionising
What is Gamma radiation? What is its charge/mass? How penetrating is it? How ionising is it?
EM radiation, mass 0, charge 0, long range (few cm lead) and weakly ionising
Explain why the different types of radiation are emitted?
Alpha – heavy nucleus
Beta minus – too many neutrons (neutrons are unstable), so neutron converted into a proton and electron and electron is lost
Beta plus* – too many protons (need to reduce electrostatic repulsion), so proton converted into a neutron and positron and positron is lost
Gamma – nucleus is just unstable
Define activity
The number of nuclear decays per second and measured in Becquerel (Bq)
Define half-life
Time taken for half the nuclei to decay or for the activity to drop by a half
Sketch the graph of binding energy per nucleon vs mass number*
(See image on OneNote)
Use the graph of binding energy to explain why nuclear fission produces energy*
The elements closer to Fe have a greater binding energy, therefore less mass per nucleon than the heavier elements. During fission lighter elements are made and the excess mass is converted into energy
Use the graph of binding energy to explain why nuclear fusion produces energy
The elements closer to Fe have a greater binding energy, therefore less mass per nucleon than the lighter elements. During fusion a heaver element is created and the excess mass is converted into energy
What natural source of nuclear fusion do we use?
The sun
Describe the conditions needed to produce nuclear fusion and explain why they are needed
High temperatures so the nuclei can get close enough to overcome the electrostatic repulsion and the strong nuclear force can take over
High density so the probability of a collision is increased
Why is boron used to make the control rods?
It can absorb neutrons
Why is the moderator made of graphite?
It doesn't absorb neutrons It conducts heat well It is chemically stable It is cheap and readily available It doesn't disintegrate much
Describe the role of shielding around a nuclear reactor and what it is made from
High energy neutrons and gamma rays are released during fission, which is absorbed by the shielding. Usually made from thick concrete and possibly lead
What is a chain reaction?
When fission reactions lead to more ones, leading to a continuous chain of reactions.
