Year 2 Chapter 12- Nuclear energy Flashcards
How are mass and energy related?
E=mc^2,
energy and mass are interchangeable and are related through this equation which can be applied to all energy changes
What is mass defect?
The difference between the mass of a nucleus and the sum of the masses of the individual protons and neutrons
Why is there a mass defect?
Some of the mass of the nucleons is converted into energy and released when they fuse to form the nucleus
What is binding energy?
The energy required to separate a nucleus into its individual nucleons
What is an atomic mass unit?
1/12th the mass of a carbon 12 atom and is used as an estimate of the mass of a proton and neutron
What is the energy equivalent for an atomic mass unit?
1 u = 931.5 MeV
What is nuclear fission?
The splitting of a large nucleus to form two smaller daughter nuclei, neutrons while releasing energy
Why is energy released during nuclear fission?
The smaller daughter nuclei have a greater binding energy per nucleon as they are more stable
What is nuclear fusion?
The joining of two smaller nuclei to form a larger nucleus and release energy
Why is energy released during nuclear fusion?
The larger nucleus has a much greater binding energy per nucleon
What are the conditions required for nuclear fusion and why?
Extremely high temperatures are required as a lot of energy is required to overcome electrostatic repulsion between positive nuclei
What is the shape of the binding energy per nucleon against nucleon number?
Increases very steeply from 1H to 16 O with spikes at 4He and 12C, peak at 56Fe then gradual decline until uranium 238
How is nuclear fission induced?
A thermal neutron is fired into a uranium-235 nucleus causing it to be extremely unstable and split into two daughter nuclei, some neutrons and releasing energy
What is a thermal neutron?
A neutron with low energy meaning it can induce fission as higher energy neutrons rebound off of nuclei after a collision and are not absorbed
How can nuclear fission be a chain reaction?
The neutrons released during fission can be absorbed by other uranium nuclei causing more fission events, a chain reaction occurs where each fission causes at least one other fission event
What is the critical mass?
The smallest mass of fissile material required to maintain a steady chain reaction
What does the moderator do?
Slows down neutrons released in fission to thermal speeds through elastic collisions between the neutrons and moderator nuclei. The closer the nuclei mass to neutrons, the more momentum is transferred
What is often used as a moderator and why?
Water as it is light, not very reactive, inexpensive
Graphite can also be used
What do control rods do and what are they made of?
Absorb neutrons in the reactor to control chain reactions. Made of boron and cadmium which can absorb neutrons to form stable isotopes and so not become unstable themselves
What does the coolant do?
Absorbs heat released in the reactor during fission reactions, which is used to make steam which drives turbines, generating electricity. Water can sometimes be used as the moderator and coolant, molten salt or helium can be used as coolant
What fuel is used in nuclear reactors?
Enriched uranium which is 95% U-238, 5% U-235. 238 doesn’t undergo fission and absorbs neutrons, controlling rate
How is radiation exposure for workers limited?
-Fuel rods inserted remotely into reactor
-Thick concrete shielding around reactor, can be radioactive after long use
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How are nuclear reactors shut down in emergencies?
Control rods dropped entirely into the reactor by absorbing all free neutrons
Why are spent fuel rods dangerous?
Daughter nuclei produced are very unstable and have high activity so they are high-level waste
How is high-level waste (fuel rods) disposed of?
-Removed and handled remotely
-Placed in cooling ponds due to emissions causing heating
-Recyclable materials removed
-Waste encased in glass and stored in thick steel casks surrounded by lead in deep caverns in geologically stable places. Chosen places have minimum impact on environment and locals are consulted
