atomic physics - fission and fusion

Question 1

what is nuclear fission

Answer 1

• fission - act of splitting a nucleus
• a neutron is fired at a large nucleus in order to split it into two smaller nuclei
  –> this releases energy from the nucleus and a few stray neutrons

Question 2

what is often used as the energy source in a nuclear fission reactor

Answer 2

uranium-235

Question 3

what is the nuclear chain reaction

Answer 3

• the neutrons released in a fission reaction begin new fission reactions with other isotopes
• the reaction then continues releasing more energy and gaining more neutrons each time
• the reactor that this takes place in must be built to ensure it is controlled

Question 4

what are 4 main things in a nuclear reactor

Answer 4

fuel rods
control rods
moderator
thick concrete walls

Question 5

fuel rods and nuclear fuel

Answer 5

• fuel rods are packed with pellets of uranium-235
• uranium-238 has a much higher natural abundance but is not fissile enough (doesn’t split easily) to use as a fuel so it undergoes enrichment to become uranium-235
• plutonium-239 can also be used as nuclear fuel but this element is the product of other reactions and is not naturally occurring

the nuclear fuel is put into metal tubes called fuel rods which have a place in the reactor

Question 6

controlling the reaction and control rods

Answer 6

• control rods are made from a substance (like cadmium) that will absorb neutrons
• they arre placed between fuel rods
• when a large amount of energy is required, the control rods are removed to allow neutrons to move between neighbouring fuel rods
• when less energy is required/danger is present, the control rods are fully inserted to allow less reaction

all the way out - full reaction
all the way in - less reaction

Question 7

staying safe and moderators

Answer 7

• the moderator (mainly made out of graphite or ‘heavy water’) surrounds the fuel and control rods
• the moderator slows down the neutrons
  –> makes reaction more efficient as neutrons travelling at the right speed get absorbed
• thick concrete walls surround the reactor core to ensure no radioactivity escapes

Question 8

labels of the nuclear fission reactor (12)
- from right all the way round -

Answer 8

hot gas duct
steam out
heat exchanger
water circulator
cold water in
gas circulator
cold gas duct
concrete shield
moderator
fuel rods
pressure vessel
control rods

Question 9

what is nuclear fusion

Answer 9

act of joining together two small nuclei

exothermic reaction

Question 10

what does the process require

Answer 10

• huge amounts of energy to start (to overcome repulsion energy) but releases even more
• simple reaction = fusing hydrogen-2 and hydrogen-3
  –> this forms a helium nucleus and releases a spare neutron and lots of energy

Question 11

what are the benefits of nuclear fusion

Answer 11

the deuterium-tritium fusion has the following advantages:
- fuels are relatively abundant
- renewable source of energy
- energy density is at least 4x greater than uranium (fusion is more powerful)
- not a chain reaction so it will stop safely if there is a problem

Question 12

why don’t we use fusion power

Answer 12

• reaction requires very large amounts of energy to start
  –> temperatures exceeding 10,000,000 dC are needed for just a plasma
• the process is very inefficient as you get less energy out than what you put in